1 Introduction

Diesel engines are widely used as the primary power source in transportation and stationary applications, especially where high horsepower output is required. One of the diesel engine peculiarities is that they provide high torque even at low speeds. Also, diesel engines are more fuel-efficient and less prone to catching fire, an edge over gasoline engines when performing heavy-duty jobs (Jones 2011). Diesel engines continue to be used in many heavy-duty applications such as ships, underground mines, and on-road transportation. A chemical reaction occurs inside the diesel engine cylinder where the stoichiometric mixture (with enough air to release the fuel’s chemical energy) is realized, and the temperatures are highest. The regions where stoichiometric mixture conditions are not satisfied give rise to other products such as oxides of nitrogen (NOx), carbon monoxide (CO), HC’s, and mainly particulate matter (PMs) by mass. The PMs in a diesel engine are primarily formed by incomplete combustion of diesel fuel (Wu et al. 2016). Immediately followed by the engine emission, while coming out of the exhaust and dispersion, the gases cool, nucleate, condense, and form further small particles (Heywood 2018). The formation and composition of exhaust emissions depend on several parameters, such as fuel injection technology, type of fuel used, compression ratio, supercharging and exhaust gas recirculation and after-treatment technologies such as diesel oxidation catalysts, particulate reduction systems, NOx after-treatment systems etc. (Fiebig et al. 2014). These pollutants emitted from diesel engines are recognized as environmental and human health hazards. The PMs, i.e., ultrafine particles emitted from diesel engines, have been a serious concern for public health. Numerous studies show that PM emissions threaten human health and the ecosystem (Burtscher 2005; Giechaskiel et al. 2009; Matti Maricq 2007).

It is well established that diesel engines are a significant source of small soot or smoke particles, hydrocarbons, and NOx. Several epidemiological studies have established the adverse health effects of diesel particulate matter (DPM), including Alzheimer’s, Parkinson’s, respiratory health problems, and lung cancer (Attfield et al. 2012; Chio et al. 2014; Englert 2004; Gamble et al. 2012; HEI 2015; Ristovski et al. 2012a, b; Silverman et al. 2012). Hence, the government and industry continuously put strenuous effort into controlling and reducing DPM emissions. As a part of the efforts, automobile standards were introduced first in California and then nationwide in the United States, starting in the early 1960s. Emission standards in Japan, Europe, and other regions have followed US practices. At the beginning of the 1990s, the “Euro” emission standard was introduced, where limits continued to become more stringent in individual emission levels. Recently, the earlier valid legislation level, Euro 5, was replaced by Euro 6 in 2014. The New Technology Diesel Exhaust (NTDE) systems were started in 2007. NTDE refers to current low-emission, advanced-technology diesel engines (both new and retrofitted) incorporating multi-component emissions reduction systems (i.e., wall flow-diesel particulate filters (DPFs), diesel oxidation catalysts (DOCs), and ultra-low sulfur diesel (ULSD) fuel) designed to meet the US EPA 2007 particulate matter (PM) emissions standard of 0.01 g per brake horsepower-hour (g/bhp-hr) (Hesterberg et al. 2012). These contemporary technologies (Dec 2009; Fiebig et al. 2014; Kakaee et al. 2015; Reitz and Duraisamy 2015; Wang et al. 2019a, b), fuel modifications (Bugarski et al. 2015; Crebelli et al. 1995; Pirjola et al. 2019a; Yehliu et al. 2012; Zhang and Balasubramanian 2014a), after treatment technologies (Hauff et al. 2012; Keskin et al. 2020; Russell and Epling 2011; Soltani et al. 2018; Stachulak and Gangal 2013) have paved the pathways for source control of DPM emissions (Winkler et al. 2018). Figure 1 below illustrates how the particulate emissions have changed over the change of standards in the past with the implementation of several new technologies.

Fig. 1
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European Union standards for passenger diesel cars

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Apparently, the modern or new technology diesel engines are lighter and cleaner. However, development in the investigation capabilities of smaller or nanoparticles (Ayres et al. 2008; Bahadar et al. 2016; Handy and Shaw 2007; Oberbek et al. 2019; Ramachandran et al. 2011; Rönkkö and Timonen 2019) and the development of newer and better sampling methods (Back et al. 2020; Giechaskiel et al. 2014; Noll et al. 2020; Noll et al. 2005; Ruusunen et al. 2011) has put DPM under strenuous scrutiny and tightened regulations. Based on sufficient evidence, the International Agency for Research on Cancer (IARC) classified diesel exhaust as a human carcinogen (group 1), and currently, it is considered a contaminant of primary importance to the international community (IARC 2012). During the IARC monograph press release, 2012, the then IARC Director, Dr. Christopher Wild, discussed about the validity of the monograph in the case of newer and cleaner diesel engines, that: ‘the new diesel engines contain far fewer particles and chemicals compared to the older technology engines. In addition to that, there are also qualitative changes, so the composition of the mixture in the exhaust is different’. IARC also encouraged further research in the future by indicating a different direction: ‘what we do not know at this stage is if this composition and the decreased levels of these components translated to a different healthy fact in exposed people and here we should encourage further research in the future (Wild 2012).’ IARC also mentioned the two prominently identified hazardous substances in diesel exhaust; one is diesel particulate matter (DPM), and the other is chemicals that can be attached to these particles in either the vapor phase or gas phase of the exhaust. These substances can even attack DNA and cause direct mutations in the DNA; also, these lead to an inflammatory response in the cells and can cause cells to proliferate faster than the normal rate, leading to cancer development (Wild 2012).

Some key concerns can be derived from the IARC events 2012 that are worth mentioning and demand immediate action. First, DPM is one of the most hazardous substances emitted from the diesel engine. It can even serve as a carrier to other environmental dust or other chemicals emitted from diesel engines. The atmospheric reactivity and bioavailability of these hybrid compounds might have some unknown synergistic and/or antagonistic effects on human body response. Second, the quality and composition of DPM emitted from these engines may differ because of all the changes made to the diesel engine, fuel, and emission control technology. Some chemical, mechanical or physicochemical changes may occur to the surface and bulk of DPM, which might have a different interaction mechanism with the human body, and even smaller or reduced quantities of such DPM might be equally or more harmful.

From both public health and environmental perspectives, there is a lack of agreement between different communities (scientific or industrial) regarding a few critical questions: (1) What physicochemical properties (or unidentified confounding environmental influences) play a major role in impacting human health and the environment? (2) which pathophysiological mechanisms are operative and dominating over others? (3) what regulations and methods should be adopted to deal with them? and (4) how quickly they should be evolved to keep pace with the advancing technologies? Even though there are concerns associated with diesel engine exhaust, trends show that there will be an increase in the diesel engine market in the upcoming years both because of the rising demand for diesel vehicles and growth in the non-automotive diesel engine market (Business Reviews 2020). Moreover, the diesel industry continues to invest substantially in research, innovation, and development to improve engine efficiency and reduce harmful emissions, making diesel a sound investment for motorists for decades to come.

The zero-emission engine innovation is our society’s future goal by developing alternative means, such as electrical or hybrid substitutes for diesel engines. Other technologies, such as battery-electric and fuel cell-electric vehicles, are showing their presence and becoming cost-competitive. However, the customer demand does not show their attractiveness yet. Considering all these, the European Commission (2017) estimated that without new measures, 90% of vehicles driving on EU roads in 2030 would still run on an internal combustion engine (ICE) only. However, that is still full of obstacles and far from being realized because of several constraints. Also, this transition will not be an overnight but a gradual process, depending on how fast we can grow our capabilities. The countries with more resources and technologies might reach that phase earlier than the others. Nevertheless, in many developing countries, this transition to newer-generation diesel engines will take longer than in developed countries. Hence, millions are still under the influence of the exhaust from old diesel engines (Wild 2012). The diesel engine will be with us for at least some time in the future. We must keep looking for better alternatives and improve the presently available resources.

Even though the DPM is a global challenge for human health, its exposure to humans is not even. There are highly impacted industries that involve higher DPM exposure, such as mining and trucking industries. In the case of occupational exposure, especially in mining-related workers, exposure is higher and easier to characterize. The occupational settings that are spatially constrained, such as underground mines, have faced the worst DPM hazards (Bertolatti et al. 2011; Crump and Van Landingham 2012; Debia et al. 2017; Haney 2000; Jones 2015; Utell Chair et al. 2002). The use of diesel-powered equipment in confined spaces (underground mines) can overexpose underground miners to the risk of DPM. Figure 2 shows a schematic representation of how underground miners are affected by DPM and corresponding short- and long-term health effects.’

Fig. 2
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Schematic representation of the impact of DPM exposure to underground mine workers, underlying short- and long-term effects and signs of exposure (CSSE 2017)

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As early as 1998, the US Department of Labor’s Mine Safety and Health Administration (MSHA) documented that this population of workers was exposed to extremely high levels of DPM, and the exposures were associated with severe adverse health effects and recommended feasible controls (Federal Register 58104, 1998). Measured in terms of Elemental Carbon (EC) level, the highest exposure levels were reported for underground mining (27–658 μg/m3), tunnel construction (132–314 μg/m3), and underground mine maintenance workers (53–144 μg/m3). Other maintenance workers (on-road, rail-road, fire-fighters, ship dockworkers etc.) from other fields were exposed to an average of 50 μg/m3 (Pronk et al. 2009). In an industrial hygiene survey of 27 underground metal and nonmetal mines, MSHA recorded 8-h time-weighted average exposures (i.e., personal exposures) ranging from 100 to more than 3500 µg/m3 TC (total carbon) (Federal Register 5756, 2001). The mean full-shift exposure in the production area of these 27 mines was 808 µg/m3 (Federal Register 32868, 2005). Another study correlated ROC (Respirable Organic Carbon), NO, and NO2 concentrations underground with the REC (Respirable Elemental Carbon) levels and found correlation coefficients as r = 0.62, 0.71, and 0.62, respectively, showing reasonable relations between the three concentrations and REC. They found the average ROC level measured for underground miners ranged from 64 to 195 µg/m3, which is approximately twice the level measured for surface workers (38–71 µg/m3). The average NO and NO2 levels underground ranged from 0.20 to 1.49 parts per million (ppm) and from 0.10 to 0.60 ppm, respectively. These values were ∼ 10 times higher than levels on the surface, ranging from 0.02 to 0.11 ppm and from 0.01 to 0.06 ppm, respectively. On average, the REC exposure levels by the facility in this study were 15 to 64 times higher underground than on the surface (Coble et al. 2010). In Australia, the EC levels among miners ranged between 10 and 420 µg/m3 for coal mines and 11 and 117 µg/m3 for underground metal mines (AIOH, 2013; Irving 2006). Another study assessed the trends in DPM exposure and the prevalence of respiratory symptoms among Western Australian miners, using the available secondary data collected from 2006 to 2012 (Rumchev et al. 2020). The measured EC concentrations for 2598 miners ranged between 0.01 mg/m3 and 1.00 mg/m3. The same study showed that the Underground mine workers were exposed to significantly higher median EC concentrations of 0.069 mg/m3 than surface workers (0.038 mg/m3). To compare these data to a general exposure scenario, in 12 southern California communities, mean annual exposures to particulate matter less than 2.5 µm in diameter ranged from 5 to 30 µg/m3 (Gauderman et al. 2004).

Still, in many US underground metal and nonmetal mines, the equipment needed to extract the limestone, gold, silver, salt, or other ore is powered by diesel engines. For more than 10,000 miners working in this confined underground environment, exposure to diesel exhaust and particulate matter is just a part of their job and mostly ignored unless serious complications arise. They work in poorly ventilated environments, and traditionally, this industry has relied on dated, highly polluting engines. Exposed miners complain about acute health effects from the high levels of diesel exhaust, such as headaches and flu-like symptoms (Monforton 2006). It shows the need for extensive and continuous studies to tackle the problem of DPM, especially in crowded occupational settings. Understanding the hazardous nature of DPM is quite important and urgent to push forward the research for emission control and exposure prevention. To comply with the legislative limits or comply with the future limits before the deadlines and simultaneously meet all customer requirements, diesel engines’ systematic further development is necessary. However, it is always a better idea to understand trends and developments. This helps decide the course for further research and development and helps identify the ambiguities in work done so far, along with a quick go-through material and an explicit composition of references to look for researchers and decision-makers.

This review aims to provide a thorough and systematic examination of research conducted on diesel particulate matter (DPM), tracking its journey from emission sources to the consequent health impacts. As previously mentioned, occupational sectors that predominantly use heavy diesel engines are particularly affected by DPM, with the mining industry being a significant contributor due to its reliance on heavy-duty diesel machinery. In the context of underground mining, the confined nature of operations exacerbates the issue of DPM accumulation. Ventilation, whether natural or engineered, remains the primary mitigation strategy to remove DPM from these environments. Within the confines of an underground mine, DPM can persist, posing prolonged hazards and potentially accumulating in areas of limited airflow. This persistence is subject to change due to various operational activities, such as vehicular movement or shifts in ventilation patterns. Moreover, the interaction of DPM with other particulates and environmental moisture in the mine alters its inherent properties, affecting how it is transported and deposited within the subterranean setting. These alterations in the physical properties of DPM have significant implications for both miner exposure and control measures (Azam et al. 2023b).

2 Review procedure and structure

The objective of this review is to delve into the multifaceted research surrounding diesel particulate matter (DPM), with a particular focus on its prevalence and control within the mining sector. Adopting a snowballing methodology, the review commenced with initial searches on electronic databases such as PubMed, OneMine.org, Web of Science, and Google Scholar, following the protocol outlined by Wohlin (2014). The search incorporated a broad array of keywords, including ‘diesel exhaust,’ ‘diesel engine,’ ‘particulate matter,’ and ‘DPM transport,’ among others, to encompass a comprehensive range of studies.

Commencing with a historical perspective, the review progresses to contemporary research on DPM sampling and characterization, along with its associated health impacts. While considerable work has been done in these areas, the specific transport behavior of DPM within the unique confines of underground mines has received less attention and is thus poorly understood. This review integrates analogous research that illuminates this aspect, providing a foundation for future investigative endeavors. The review further evaluates existing engineering controls and preventive technologies for DPM management, highlighting the limitations and applications within the mining industry. It concludes by discussing the implications of current research and the prospective directions for advancement in DPM characterization, monitoring, and mitigation, considering the technological readiness in the field.

3 Background and research history of DPM

The form and working principles of engines we see now originated in the 1800s. During that time, much competitive development took place considering earlier equipment drawbacks. However, during this initial period, the focus was to improve the engines’ efficiency, not on what comes out of the engine after processing the fuel inside these engines. As expected, these early engines were emitting lots of smoke. Still, we found little or no hard evidence of toxicity, epidemiological, or any other relevant work on the exhaust’s potential health or environmental effects until the 1950s. However, carcinogenic substances associated with vehicle exhaust were recognized around the 1930s (Aslew et al. 1937; Badger et al. 1942, 1940; Barry et al. 1935; Cook 1932; Cook et al. 1932). After these pioneer studies, the focus of scientific and research communities shifted towards the hazardous nature of emissions, and several studies were conducted to identify carcinogens in exhaust emissions (Claxton 2015a). The cancerous potential of particulates emitted from gasoline and diesel engines and were carcinogenic in animal bioassays was shown by Kotin et al. in 1954 and 1955 (Kotin et al. 1954, 1955). The public concern and regulations that considered air pollution began in the 1950s. Since then, there has been a tremendous increase in Particulate Matter research, as shown by Fig. 3.

Fig. 3
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Chart showing increase of Particulate Matter research over the time (retrieved from the Pubmed.gov on 24th April, 2021 using keyword ‘particulate matter’)

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Lancet Commission on pollution and health (Landrigan et al. 2018) mentions pollution as the main environmental cause of untimely death and disease worldwide. Among all kinds of pollution, the adverse effect of PM on health are well documented. Still, there is no evidence of a safer exposure level or threshold below which no adverse effects occur (REVIHAAP 2013). Various terminologies have been used to describe airborne PMs, such as aerosol, fog, mist, haze, nanoparticle, smoke, smog etc., and some others describe their shape, structure, origin, and other particle characteristics such as agglomerate, primary and secondary aerosol, mono and polydisperse (Kulkarni et al. 2011). These terms have been used interchangeably in the literature. Exposure to increased PM of respirable size is strongly associated with increased cardiopulmonary morbidity and mortality (González-Flecha 2004). It is the most extensively correlated with human health. PM is categorized based on size and, from a regulatory perspective, includes “inhalable coarse particles” with a diameter of 2.5 to 10 μm and “fine particles” smaller than 2.5 μm in diameter (ISO 2008). Environmental Protection Agency (EPA) estimates that fine particulate matter (or PM2.5) is responsible for over 90% of air pollution-related health damages (EPA 2011). It is classified as primary or secondary, mainly based on its formation and release into the environment. Primary PM is the portion directly emitted into the atmosphere by the sources, whereas secondary PM forms in the atmosphere because of transformation (dispersion and oxidation of precursor gases such as sulfur, nitrogen compounds, and volatile organic compounds (VOCs)) (UNECE 2014). The operations such as grinding or crushing, dust stirred up by vehicles traveling on roads, blasting, and drilling produce larger-sized particulates. They have a short suspension half-life and are mostly filtered out by the nose and upper respiratory airways. Sources of fine particles include combustion-related activities such as industrial processes (Alastuey et al. 2006), motor vehicles (Giechaskiel et al. 2014), wood-burning (Zuk et al. 2007), agricultural or biomass burning (Johnston et al. 2019) etc. The PM size is directly linked to their potential to cause adverse health effects. Small particles less than 10 µm in diameter pose the greatest problems because they can get deep into the lungs, and some may even get into the bloodstream (EPA 2020). Using a two-way fixed effects model, Jorgenson et al. (2020) found a negative relation between state-level average life expectancy with average working hours and fine particulate matter concentration. This negative association between working hours and life expectancy is amplified as the level of PM2.5 concentration increases. The negative relationship between life expectancy and fine particulate matter is magnified when average working hours increase (Jorgenson et al. 2020). Several epidemiological studies conducted globally showed a negative impact of particulate exposure on humans, reviews of which could be found elsewhere (Anderson et al. 2012; Kim et al. 2015; Schwarze et al. 2006).

Another usually classified PM category is ultrafine particles (UFP), widely classified as particles with an aerodynamic diameter of less than 0.1μm (Li et al. 2016; Seigneur 2019). If there is only one property to attribute to the ultrafine particles, describing characteristics, biokinetic, and problems associated with the UFPs, it would probably be their extremely large surface area. A general conception is that ultrafine particles are more dangerous to humans because they have a large surface area per unit mass. Moreover, these UFP’s surface-specific features arise from the differences in adsorbed material. A study found that under similar operating conditions, biodiesel-origin particulates may contain a slightly lower amount of adsorbed trace metals and inorganic ions than mineral diesel, except for nitrates (Shukla et al. 2017). UFPs have a higher capacity to absorb organic pollutants and can reach the deepest part of the human lung and even the brain resulting in more toxic impacts than coarser particles (Karakoti et al. 2006; Kuuluvainen et al. 2016; Laux et al. 2017; Schmid and Stoeger 2016). The degree of toxicity of such fine particles correlates to particle mass and particle number (Schraufnagel 2020).

Biokinetics and endocytosis largely depend upon ultrafine or nanoparticles’ surface chemistry (Oberdörster et al. 2005b). It has also been observed that sometimes physical forces control some cells-UFP’s interaction mechanisms (Chen and Bothun 2014). Nonspecific physical adsorption to the cell surface, which initiates adsorptive endocytosis, is governed by electrostatic forces. This is followed by the invagination of the local plasma membrane to form intracellular vesicles. Adsorptive endocytosis mainly relies on the adsorbed material’s size and surface properties and the available cell surface areas, as specific receptors are not required (Jung et al. 2000). Hence, the greater surface area per unit mass than larger-sized particles of the same chemistry makes ultrafine particles biologically more active. Because of this large surface area of ultrafine particles, they can react severely to adsorb some other atmospheric chemicals, such as hazardous metals and other organic compounds, which leads to harmful biological activity such as oxidative stress arising from an imbalance of oxidants and antioxidants (González-Flecha 2004) or generation of Reactive Oxygen Species (ROS) (Oberdörster et al. 2005b; Schraufnagel 2020; Terzano et al. 2010). In a different in-vitro model study, investigators showed that the positively charged ultrafine particles could penetrate 20–40 times more than negatively charged particles (Yacobi et al. 2010). Although the tests were done on a specific cell set, the conclusions may vary with the experimental cell change. However, it does confirm the importance of the surface charge of the UFPs. Figure 4 summarizes how different properties of particulates interact with the cells.

Fig. 4
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Artistic representation showing impact of physicochemical properties of particulates on their cellular uptake (Foroozandeh and Aziz 2018)

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The international community widely acknowledges the problem of PMs, and the growing awareness has led several countries to implement air quality standards for PMs. Most of these guidelines used PM2.5 and PM10 as reference points for setting particulate standards. Figure 5 shows the change in emissions since the 1970s for the criteria pollutants in the US. It shows that although the PM emissions are continuously decreasing, the numbers still sit on the higher end of the overall emissions. Similarly, Figs. 6 and 7 show the overall PM2.5 and PM10 emissions in the US. These trends show that although the overall PM2.5 and PM10 emissions in the US have not increased, a considerable amount of PM is still emitted into the atmosphere.

Fig. 5
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Chart showing level of criteria pollutants National Tier 1 for the year 1970–2020 (US EPA, n.d.)

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Fig. 6
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Chart showing national emissions of particulate matter (PM2.5) for the year 1970–2020

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Fig. 7
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Chart showing national emissions of particulate matter (PM10) for the year 1970–2020

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World Health Organization (WHO) has developed guidelines that limit annual mean PM2.5 exposure level to 10 µg/m3 and 24 h mean exposure level to 25 µg/m3. WHO has also put guidelines limiting annual mean PM10 exposure level to < 20 µg/m3 and 24-h mean exposure level to 50 µg/m3. Besides the guideline, WHO has also defined three attainable and progressive goals for PM management as Interim Target-1 (IT-1), IT-2, and IT-3 (WHO 2006). Similar guidelines are defined by the European Environmental Agency (EEA) for the EU (EEA 2020). In the US, the Clean Air Act (1990) requires EPA to set National Ambient Air Quality Standards (NAAQS) to regulate PM and five other criteria air pollutants (CO, Pb, NO2, O3, SO2). In 2006, the EPA completed its last review of the PM NAAQS (EPA 2013). Table 1 presents an overview of current regulations controlling PM in some major countries.

Table 1 DPM regulation world-wide (source: (UNECE 2014))
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The above discussion presented regarding various PMs and properties is only summarizing and intended to give an insight into the magnitude of the PM problem and or response so far. Some other recent studies have very well described the problems associated with these PM and the underlying consequences (Anderson et al. 2012; Park et al. 2018; Sang et al. 2020; Schlesinger et al. 2006; J. Sharma et al. 2021) and statutory regulations (Giannadaki et al. 2016; Kuklinska et al. 2015; C. F. Wu et al. 2017a, b).

DPM is an undesirable product of fuel combustion in a diesel engine. Being combustion-originated particles, these are extremely fine. The formation of DPM experiences complicated physical and chemical processes. The final physicochemical features of DPM depend on the relative contributions of different processes such as soot nucleation, growth, oxidation, and aggregation processes occurring both inside the combustion chamber and the environment to which it is released (Kwon et al. 2020). The typical DPM diameter can range from as low as a few nanometers to several 100’s of nanometers depending on the primary production source and dispersion condition (Burtscher 2005; Neer and Koylu 2006). The core of the primary particle is mainly graphitized EC and generally occupies around 70%–90% of the total composition. The rest of the volume is Organic compounds, which consists of other compounds adsorbed on the EC core (Hydrocarbons, PAHs, metals, sulfur, nitrogen compounds, etc). The properties of these phases are different from one another, such as the soot is non-volatile, while the adsorbed HCs are volatile (Ghadikolaei et al. 2020). Numerous studies have been done to study DPM’s formation process inside the IC engine. To summarize the properties of the DPM in the atmosphere depends on both the engine parameters and the atmospheric factors as shown in Fig. 8.

Fig. 8
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Properties affecting the Physico–chemical properties of DPM

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The modern-day advanced filtration system can clean out diesel exhaust from most coarse-sized particles. However, DPM can still be formed by the condensation of Volatile Organic Compounds (VOCs) and Semi-Volatile Organic compounds (SVOCs), which can easily escape particulate filters and other oxidation catalysts. Once these VOCs and SVOCs are released via the tailpipe, they can condense to form nucleation mode particles (Kwon et al. 2020). DPM stay in the source environment stays for very small time and in the that environment the changes are dynamic in nature and mostly dominated by primary particles. These primary particles have much of the feature impacted by the engine parameters as described in the previous paragraph and the effect of the atmosphere is negligible. These nucleation mode particles can coagulate either with each other or other ambient chemical or pollutants and form accumulation mode particles. So as they move away from the source atmospheric factors starts to impact their properties. Rest of the processes such as the condensation, accumulation, and evaporation processes are extremely dependent upon several atmospheric factors such as concentration in the gas phase, pressure, relative humidity, temperature, etc. (Kulmala et al. 2004). Figure 9 explains this whole process of DPM formation and evolution from a diesel engine having several aftertreatment technologies and the impact of engine and atmospheric factors in the whole process.

Fig. 9
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Artistic representation of DPM formation, emission, and transport (Stone et al. 2017)

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Because of this heterogeneous nature of DPM, several studies have shown that its hazardousness can vary depending upon the chemical composition, such as the composition of metals (Chen and Lippmann 2009; Lawal and Fantke 2017), organic, and some other compounds (Mauderly and Chow 2008; Schlesinger et al. 2006). Some of the OC’s in DPM, such as PAHs, are found to be mutagenic and carcinogenic, and others, such as benzo[a]pyrene, are highly carcinogenic (Ghosal et al. 2016; Lawal and Fantke 2017). DPM composition and physicochemical properties are important to adverse health effects and climate impacts, source apportionment, and aerosol modeling. Also, chemical knowledge of DPM can help clarify the origin of adsorption aberrations that interfere with the current regulatory filter-based PM measurement method (Matti Maricq 2007).

The diesel engines have a wide range of on- and off-road industrial applications, especially in occupational settings. Consequently, these occupational environments, such as mining, rail-road construction, transportation, etc., faced elevated diesel exhaust hazards. Pronk et al. (2009) made a literature review of diesel exhaust exposure to occupational workers to summarize personal diesel exhaust exposure levels and determinants of exposure as reported in the published literature. They found a contrast in exposure levels among different occupational settings while identifying several exposure determinants. They used several exposure determinants of personal diesel exhaust exposure from earlier literature, such as elemental carbon (EC), particulate matter (PM), carbon monoxide (CO), nitrogen oxide (NO), and nitrogen dioxide (NO2). For EC, the highest exposure levels were reported for underground mining (27–658 μg/m3), tunnel construction (132–314 μg/m3), and underground mine maintenance workers (53–144 μg/m3). Other maintenance workers (on-road, rail-road, fire-fighters, ship dockworkers etc.) from other fields were exposed to an average of 50 μg/m3 (Pronk et al. 2009). The study shows that workers’ exposure is mainly correlated to the worksite’s constrained nature and the type of diesel equipment used. These settings are constrained environment (underground mining and tunneling etc.), smaller equipment, poorly or intermittently used diesel engines etc.We can see from Fig. 10 that although there is a continuous decrease in the PM2.5 emissions continuously but the Heavy-duty diesel engines are still emitting way more PM2.5 than the other kind of engines. Occupational settings, especially underground mines, mostly use heavy duty diesel engines.

Fig. 10
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Chart showing emissions of Particulate Matter (PM2.5) from different class of gasoline and diesel vehicles for the year 1970–2020 (US EPA, n.d.)

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Some other factors that affect the concentration of DPM in the closed setting are the efficiency of the after-treatment device and ventilation (Grau et al. 2002; Grau and Krog 2008; Morla et al. 2019) of working sites. Among these occupational settings, the mounting concern about diesel exhaust’s cancer-causing potential was based on findings in epidemiological studies on underground miners showing an increased risk of death from lung cancer when working in closed areas (IARC 2012). The mining industry widely uses diesel equipment, whether it be underground or surface. It becomes important to discuss and find a comprehensive and feasible solution to the DPM concern prevalent in the mining environment. Underground mining is a constrained environment that enforces much more complicated DPM hazard management. The federal and state agencies are currently actively involved in protecting mining workers from excessive exposure to DPM. Some of the active study areas include but are not limited to biofuels and after-treatment technologies.

4 Overview of DPM sampling and characterization

A wide range of natural and anthropogenic sources release particulate matter to the atmosphere, as discussed earlier. The primary production source of PM plays a deciding role in their fate and how they behave and regulate atmospheric nucleation and oxidation (Maria et al. 2004). These differences create difficulty in defining a uniform characterization technique for PM. However, knowing the characteristics of a particular kind of aerosol in a particular environment is extremely important because there is a strong association between the chemical composition and biological response. The presence of transition metals such as iron, manganese, and sulfur can produce reactive oxygen species (ROS), causing oxidative stress (Fang et al. 2017; Kuang et al. 2017; Yuan et al. 2019). Surface features of PM also affect their behavior in the atmosphere, such as particle growth by condensation and the photochemical production of ozone and secondary organic aerosols (SOA) (Jang et al. 2002). The surface composition of PM is known to modulate particle oxidation and SOA yield. Sulfate plays a key role in producing highly acidic fine aerosols capable of dissolving primary transition metals, contributing to aerosol oxidative potential (OP) (Fang et al. 2017) and surfaces of acidic aerosol emissions and heterogeneously catalyze and increase SOA production (Jang et al. 2002).

Moreover, PM surface composition can influence their particle size and hygroscopicity, cloud microphysics, the formation of cloud nuclei and fog droplets, and their fragmentation reaction, which play a major role in the lifecycle of atmospheric organic aerosol (Kroll et al. 2015; Wal et al. 2011). Several other studies show that PM cytotoxicity or genotoxicity depends on their heterogeneous chemical composition and can influence toxicological outcomes, which could be attributed to different source environments and emission sources (Longhin et al. 2013). Knowledge of how these processes correlate with DPM composition and mass is essential for predicting the direct and indirect effects of DPM to assess its effect on human health.

It is generally recognized that the exhaust aerosol concentration and accuracy of the further studies depend on the sampling technique and the instrument used. There are several ways to sample and analyze DPM. DPM being ultrafine, can penetrate deep into the lungs, translocate to other generally unreachable parts of the body, and show higher biological activity than its larger counterparts. However, their mass is negligible. With the emergence of nanotechnology and its characterization techniques, there is a growing concern about its health risks. For such analyses, toxicological studies of DPM could require a large amount of particle mass (up to milligram (mg) quantities) (Demokritou et al. 2002). Hence, collecting sufficient DPM for other physicochemical characterization and cellular study becomes difficult most of the time. Also, we have already discussed that DPM is a physiochemically sensitive compound, and its properties can be different if measured under different conditions, which is another constraint to be accounted for while sampling.

Furthermore, the sampling and analysis are prone to be affected by sampling technique, ambient conditions, approach, and substrates. Some cases, such as standardization of emission limits, particle concentration, and number measurement, are desired, guaranteeing air quality standards (Demokritou et al. 2002). At times, characterization techniques may require more than fundamental knowledge to conduct and interpret the analysis, especially when regulations are pressing hard to mitigate PM pollution. So, selecting the best instrument and analysis technique for each application requires a clear measurement objective and an understanding of the instrument’s characteristics being planned to be utilized. This section will briefly discuss some sampling and analysis methods commonly employed for DPM collection and characterization.

4.1 Important points to consider for PM sampling

While coming out of the exhaust, the aerosol properties continuously change after their formation. Several properties determine aerosols’ characteristics and their growth in the environment as described earlier. Consequently, the size and concentration of particles can vary with the nature and place of sampling. Initially, competitive growth (coagulation) and oxidation (shrinking) processes inside the engine determine the ratio of nucleated and agglomerated particles, and this ratio keeps modifying as the particles move away from the exhaust. Nowadays, several after-treatment technologies alter the properties of particulate emissions. The diesel particulate filter (DPF) and Diesel Oxidation Catalyst (DOC) are efficient in reducing PM emissions. DOC oxidizes hydrocarbons (HC) and CO, which leads to PM formation and forms NO2 due to oxidation of NO, which is used as a passive DPF regeneration system and also supports the working of SCR (selective catalytic reduction) catalysts (Fiebig et al. 2014). However, since DOC can oxidize any compound of reducing character, not all oxidation reactions are useful, such as oxidation of sulfur to its oxidized compounds (sulfur trioxide of sulfuric acid) (Shrivastava et al. 2010). Some studies that combine SCR with DPFs are found to increase PM, the total number, and solid > 23 nm number of particles by up to three times (Amanatidis et al. 2014). Several other studies have shown that some other materials, such as wear material, whether from the exhaust of sampling system, can interfere with the desired measurement (Kittelson 1998). These show that it is very important to decide the sample position, e.g., how close to the exhaust we should place our sampler, and can the equipment we want to use effectively take that position and provide the data we are looking for?

Temperature near the exhaust is higher as compared to farther locations. The aerosol characteristics’ changes are rapid closer to the exhaust, and it depends more upon the temperature and exhaust features of a particular engine. Gradually, as the aerosol goes farther away, the rate of change becomes proportional to the environmental factors to which it is emitted. It becomes important to condition the exhaust as per the requirements. So, most of the laboratory scale instruments consider mixing the exhaust with different ratios (as per the requirement) of the clean air in the so-called dilution tunnel to fix the particle size distribution before measurement. Dilution tunnels are quite useful in preserving and modifying the exhaust properties. The sampling system dilutes a small part of the exhaust directly at the tailpipe without exhaust gas transfer lines, leading to sampling artifacts. A dilution tunnel can be advantageous for studying emissions under the same sampling conditions, regardless of engine operation, technology, and emission level provided certain dilution controls can be adopted (temperature, the ratio of air, sampling time) (Ntziachristos and Samaras 2010). Several dilution tunnels and systems are available. Some of them, such as Constant Volume Dilution (CVS), is used to dilute the whole exhaust in a full dilution tunnel.

Complete dilution reduces the fluctuation in measurement because of pressure and temperature by controlling the dilution ratio. Knowing a correct dilution ratio is very important as it causes opposite trends for PM and VOC concentrations because it determines the temperature and concentration (principal partitioning factors for semi-volatile materials between different phases) (Smits et al. 2012). CVS systems can be very costly and difficult to accommodate in laboratory settings; hence Partial Flow dilution (PFD) systems are used. However, they are quite similar in CVS systems but easy to accommodate, less expensive, and similar results can be expected (Maricq et al. 2018; Ntziachristos and Samaras 2010). Some other simplified and commonly used dilution systems are porous diluter tube (Mikkanen et al. 2001), ejector diluter (Giechaskiel et al. 2004), rotating disk diluter (Giechaskiel et al. 2010) and bifurcated flow diluter (Fuchs and Sutugin 1965). The engine exhaust includes particles, gases, condensed/adsorbed material etc. As discussed earlier, the partitioning between gas and solid phase in the soot depends on temperature and pressure. By appropriately selecting these features, we can separate the volatile phase from the non-volatile phase, improving the measurement’s efficiency and repeatability (Giechaskiel et al. 2014). Proper conditioning of the exhaust can be done using systems such as an evaporation tube (Giechaskiel and Drossinos 2010), thermodenuder (Maricq 2014), or catalytic stripper (Amanatidis et al. 2018), we can separate the semi-volatile content from the PM.

Another important factor is the typical concentration in the environment we are interested in. Some places are expected to have larger concentrations of fine or ultrafine particles than others. As discussed earlier, occupational settings (underground tunnels and mines) around heavy traffic areas will typically have a larger concentration of dust, whereas other places such as parks and other green infrastructure zones will have a smaller concentration of dust. Choice of the right instrument will greatly depend upon these factors. The compatibility of the instrument also varies with the size of the environment. For Eg, indoor sampling cannot use large and high-volume samplers as it will create lots of disturbance. A cursory way to understand the concentration levels in an environment will be to look at the level of activities going on, such as around the traffic zones; we can get an idea by looking at the density of traffic, construction going around, vehicles widely used around etc. The exposure in a particular area will be indicated by these parameters, which indirectly references mass concentrations (fine or ultrafine) (Franck et al. 2011) and particle number or surface area concentration (Kumar et al. 2018).

The quantity of samples required is also important, and accordingly, we need to find the technique best suited to collect the desired amount of sample. A toxicological assessment of PM can require several mg of sample (Demokritou et al. 2002). Again this can depend upon the nature and quantity of assays to be tested and increase if additional processing (extraction, sonication etc.) of the sample is required before the core experiments. Generally, for ultrafine particles, the sample requirement is less compared to coarser particulate matter. E.g., Kumar et al. (2018) mentioned in their review that to assay the impact of PM on cellular health, including assays of oxidative stress, inflammation, cell death, and mitochondrial activity, it can require up to 10 mg of sample (Kumar et al. 2018). Figure 11 summarizes the important points which needs to be considered while sampling DPM.

Fig. 11
figure 11

Important points to consider while sampling DPM

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4.2 DPM sampling techniques

For the collection of PM samples, several techniques depend on the end-user requirement. Particulate mass is commonly measured using gravimetric sampling methods. After dilution/conditioning of the exhaust or directly from the engine exhaust, the PM is collected over a filter medium. The general theme is that this filter paper’s weight is measured before and after the test. The difference gives the required mass of PM. However, most of the time, cyclones (Cauda et al. 2014) or impactors (or cascade impactors) (Pennanen et al. 2007) or a combination of both are used to remove particles beyond the desired particle size. Mass sampling equipment is broadly classified based on airflow volume as high volume samplers, mini volume samplers, and cascade impactors (Kumar et al. 2018). Although gravimetric methods are being used for a long time, they have reached their detection limit with the current emissions level. As the PM collected on filter using these techniques has extremely low mass as compared to the mass of filter paper (< 1%), at such levels, it is very easy for artifacts and measurement uncertainties to contribute more than 90% of the actual mass. The next section will briefly discuss the filter paper-related issues in such measurement techniques.

Another set of techniques involves the principle of light scattering, which forms the common basis for most real-time PM monitoring techniques in occupational settings. These optical methods mainly measure and quantify the interaction of PM with the incident light in the form of scattering, absorption, and extinction (Kulkarni et al. 2011). Among these instruments measuring light, scattering is dependent on the particle size and wavelength of the incident light. Extremely small particles compared to the incident light wavelength follow Rayleigh scattering (scattering symmetrical in the forward and backward direction) (Moosmüller et al. 2009). When the size of PM approaches the wavelength’s size, then the Mie scattering (different scattering in forward and backward direction) regime is entered (Lockwood 2016). Some of this equipments are scattering photometers (Sinclair 1967), optical particle counter (OPC) (Vasilatou et al. 2020), condensation particle counters (CPCs) (Saarikoski et al. 2019). OPCs and CPCs are widely used for particle number counting and are based on the intersection of a focused light beam and particle beam, whether uncondensed or condensed, respectively. Some instruments, such as spot meter, aethalometer etc., are based on the light-absorbing tendency of soot particles (DPM). Other instruments are based on PM’s light extinction, such that light extinction is the difference between incident and transmitted light.

Microbalance methods such as TEOM (Tapered Element Oscillation Microbalance) (Barrett et al. 2019) and Quartz Crystal Microbalance (QCM) (Olsson et al. 2013) are also widely used in occupational settings, especially to give real-time measurements. These are based on the change in the resonance frequency of tapered quartz (TEOM) or quartz crystal (QCM) upon loading by the particle. PM can acquire charge because of several processes such as acquiring charged species or other small ions on its surface, thermionic emission, photoemission, static electrification, and thermionic emission (Kulkarni et al. 2011). Although not all of these charges can be measured, some instruments are designed to measure PM concentration by measuring these charges. These are not so widely used as while they can provide real-time PM mass measurement, comparisons to the gravimetric filter method used to measure DPM have shown that water vapor, pressure fluctuations, loading, and other considerations can affect measurement quality (Podsiadlik et al. 2003; Xu et al. 2005). Another reason is that these instruments record cumulative mass instead of instantaneous concentration, hence not good for time-resolved applications (Maricq et al. 2006).

For research purposes, research needs to be carried out according to criteria that truly reflect the nature of exposure; in such case, mass-weighted size distribution or size-fractionated measurement is required. Moreover, in several studies, measurements of number- (or surface area) weighted size distributions have been used as a substitute to approximate the mass size distribution (Khlystov et al. 2004; Shen et al. 2002). Cascade impactors are widely used to measure aerosol particle size distribution in such cases (Marple 2004). Cascade impactors have a long history of their development and usage, and recently there has been a surge of development of such impaction instruments for studying industrial aerosols, especially in mines (Marple 2004). These are instruments with a high-resolution of particle size and can capture the smallest particle size of up to a few nanometers (Arffman et al. 2014; Jiménez and Ballester 2011). Low-pressure impactors classify particles according to their aerodynamic diameter into several stages. Some of the Low-Pressure impactors commonly used are DLPI + (Pagels et al. 2005), ELPI (Järvinen et al. 2014), Aerodynamic Particle Sizer (APS) (Pagels et al. 2005) and NanoMOUDI (Sardar et al. 2005). The Electrical Low-Pressure Impactor (ELPI) uses electrical detection of charged particles with a 14-stage low-pressure cascade impactor to make real-time measurements (Marjamäki et al. 2000). ELPI can be used for particle size distribution measurements and traditional gravimetric impactor measurements. ELPI is used quite often to measure DPM mass (e.g. Ahlvik et al. 1998; Maricq et al. 2006), and quite often, the results are in good agreement with other gravimetric analysis techniques. Substrates are used over the surface of the sampling filter papers in different stages to reduce particle bounce and improve the efficiency of collection plates. These collected samples can be characterized for ultimate and proximate analyses, X-Ray fluorescence, BET surface area, and SEM analysis, and many more. Further, TEM or other small sampling grids can be directly placed on the collecting plates over which the representative particles can be collected.

4.3 DPM characterization techniques

Currently, there is a rapid change going on with emission diesel technology development, regulations, and particle characterization techniques. Particle composition is the central issue for most DPM cases related to health effects, climate change, source apportionment, and aerosol modeling (Matti Maricq 2007). Moreover, because of advances in the engine exhaust after-treatment technologies, the emissions regulations have decreased the net emissions so low that even increasing the DPM measurement instrument sensitivity will not be effective. As discussed earlier, the exhaust after-treatment technologies reduce the DPM emissions and alter the properties. This section will broadly discuss some of the characterization techniques emphasizing modern emerging techniques. It can help us understand the evolution of DPM properties, after-treatment, potential environmental effects, and many other related artifacts. Characterization techniques of DPM can be broadly classified into the following categories:

4.3.1 Chemical and spectrometric analysis

Chemical and other analyses are generally conducted directly on collection substrates or dissolution (Soxhlet extraction). The major classes of chemical analyses include (1) elemental analysis, (2) inorganic ions, (3) hydrocarbons, and (4) polar organic compounds. DPM or soot emission contains significant volumetric and mass fractions of chemical elements adsorbed over carbonous particles. These elements are found to be having a significant contribution to the DPM toxicity. E.g., several studies have found sufficient metal content in the soot, and further, the toxicity of these metal depends on the size distribution, chemical state, and mixing state (Corbin et al. 2018; Mayer et al. 2012). The elemental DPM composition can also be examined via X-Ray fluorescence (XRF), crystalline phases by X-Ray diffraction (XRD). Traditionally, particulate OCs and other molecular compounds’ chemical characterization needs extraction of a sample with organic solvents performed using single or multiple solvent extractions of samples. These are followed GC/MS (gas chromatography/mass spectrometry), GC/FTIR/MS (gas chromatography/Fourier transform infrared spectroscopy/mass spectrometry), HPLC/MS (high-performance liquid chromatography/mass spectrometry and other techniques) (Alves 2008). Schauer et al. identified several organic (C1–C30) and other compounds from the exhaust of medium-duty diesel trucks using techniques such as chromatography and XRF (X-Ray fluorescence) (Schauer et al. 1999). DPM is a mixture of several polar and non-polar organic compounds. Derivatization of such compounds is necessary to identify most polar and non-polar compounds in DPM. This is because universal solvents for both polar and non-polar OC do not exist, high-molecular organics (> C40) and highly polar compounds (particularly multifunctional) do not elute through a GC column, and identified compounds are embedded in an unresolved complex mixture (Alves 2008). Also, because of the long retention time of some of the polar organic compounds present in DPM, derivatization is done. Polar organic compounds may serve as tracers for specific emissions; hence it is important to analyze them (Matti Maricq 2007). Quite often, some variants of diazomethane are used, which improves the sharpness of the peaks and is amenable to analysis by GC/MS (Noya et al. 2008; Schauer et al. 1999). Another method is to convert organic compounds with hydroxyl and carboxyl groups to the corresponding trimethylsilyl ethers and esters using the reagent bis-(trimethylsilyl) trifluoracetamide used by some studies (Nolte et al. 2002). However, the derivatization techniques are compound-class specific, and thus several different methods may be required for a wide-ranging analysis, and also they require a priori knowledge about the particulate matter composition and the properties of derivatizing compounds. A good review of the characterization of solvent extractable organic compounds can be found elsewhere (Alves 2008). Traditionally the extracted organic material is usually differentiated using gas chromatography/mass spectrometry (GC/MS), which characterizes soluble organic fraction. In GC/MS, individual species are distinguished by their retention times, identified by the mass spectrum, and quantified by the total ion count relative to those from reference standards (Matti Maricq 2007). Some other studies conducted using the Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), XRF, and XRD determined traces components of elements such as Na, Mg, Al, K, Ca, Ti, V, Cr, Be, S, Mn, Fe, Co, Cu, Zn, Mo, V, Sr, Ba, Cd, Pt, Pb, and Ni on particle-phase species (Cheung et al. 2010; Lough et al. 2005; Morajkar et al. 2020; Ntziachristos et al. 2007). Others have used ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometer) to study the content of DPM (Gangwar et al. 2012; Ulrich and Wichser 2003). ICP-MS/OES are extremely powerful and attractive techniques to perform elemental analysis. Some studies have used the laser-induced breakdown spectroscopy technique (LIBS) or Time-Resolved LIBS for qualitative and quantitative measurements of the qualitative and quantitative composition of the DPM complex matrix (Lombaert et al. 2004; Viskup et al. 2020). FTIR analysis is also used as a robust analytic technique for evaluating the chemical composition (Popovicheva et al. 2014, 2015). Raman spectrometry is another powerful tool for characterizing carbon material, oxidation reactivity, and nanostructure (Dresselhaus et al. 2010; Escribano et al. 2001; Pawlyta et al. 2015). Nowadays, powerful synchrotron X-Ray absorption techniques are used to characterize DPM. In a recent study, Ouf et al. (2016) colleagues demonstrated the use of surface-sensitive technique NEXAFS (Near Edge X-Ray Absorption Fine Structure) freshly emitted soot particles. Along with that, they found that the highly surface-sensitive XPS experiments of airborne soot are indicative of different oxidation properties at the surface compared to the bulk probed by NEXAFS (Ouf et al. 2016). A good review of the chemical analysis of PAHs has been reviewed extensively in a different paper (Zielinska and Samy 2006).

PAHs are captured on the filter and recovered by solvent extraction techniques (Soxhlet extraction, ultrasonication, microwave extraction) using a combination of several chemicals such as hexane, toluene, dichloromethane etc. Another important analysis technique is Brunauer, Emmett, and Teller (BET) measurement, which provides surface area and porosity measurement of fine particles. In this technique amount of a gas (typically N2, (0.162nm/molecule)) can be adsorbed on the particle surface and used as a measure for the surface area (Gregg and Sing 1995). Physicochemical properties of DPM are well correlated with BET (Ross et al. 1982). It has been shown that particle surface area as determined by the BET method correlates well with pulmonary inflammatory responses (Oberdorster et al. 2002).

In summary, the concentration of ions and elements in DPM, surface functional groups are measured mainly using ion chromatography (IC) based methods, FTIR, calorimetry, X-Ray (XPS, XANES (X-Ray absorption near edge structure spectroscopy), Laser-based methods. These techniques are extremely sensitive to sample collection methods, substrate, and preparation techniques.

4.3.2 Thermal and optical analysis

As it is known that DPM is a complex mixture of elemental and organic carbon, several techniques are used to distinguish between EC and OC. Thermal/optical analysis techniques are generally applied to analyze the carbonaceous content of PM. It is mainly a two-step process. The first stage measures OC (Organic Carbon), defined as the carbon that evolves from the sample during the oxygen-free helium purge gas stage of the analysis plus the pyrolysis carbon in the absence of carbonate carbon. In this step, the sample is heated in an inert atmosphere (typically in stages to ~ 600 °C) to volatilize organic material, which is subsequently oxidized and detected as CO2 or converted to methane (CH4) for flame ionization detection (FID). The second step measures Elemental Carbon (EC), defined as the carbon evolved from the samples after the laser transmittance or reflectance returns to the initial sample values in the absence of carbonate carbon. In this step, oxygen is introduced to the remaining sample and heated to even higher temperatures (up to ~ 900 °C) (Schauer 2003). However, the split between EC and OC as described above might not be as perfect as shown above. Hence, the EC/OC split is operationally defined when thermal/optical analysis is involved (Giechaskiel et al. 2014). There are currently two principal protocols implemented for the analysis of EC/OC from particulate matter, which is the Interagency Monitoring of PRotected Visual Environments (IMPROVE) (Chow et al. 2007) and the National Institute of Occupational Safety and Health (NIOSH), 5040 (Andrews and Fey O’connor 2020). IMPROVE is often referred to as the Total Optical Reflectance (TOR) method. Although the overall process for these processes is the same, some studies have compared the two techniques and some other EC analysis methods and have found some differences (Currie et al. 2002; Schmid et al. 2001). A detailed discussion about these differences and reasons is presented elsewhere (Schauer 2003).

Oxidation behavior is another important feature of DPM. Thermal experiments are commonly used to test DPM oxidation behaviors under a different set of conditions and with the aid of some other techniques if required. Thermogravimetric analysis is also a two-step process: (1) devolatilization: in which volatile substances are removed from the PM in an inert atmosphere (nitrogen or argon); and (2) oxidation: oxidation of soot in an oxidizing atmosphere (air or oxygen). Both steps are crucial in determining the overall content and oxidation properties of DPM. Several parameters are ultimately used to make effective comparisons, such as ignition temperature, activation energy, and pre-exponential factor (the Arrhenius equation). A modified version of the Arrhenius equation is used based on the TG–DTA (TG-Differential Thermal Analysis) in some studies (Stratakis and Stamatelos 2003).

Reactivity of soot is an important parameter for the efficiency of DPF (diesel particulate filter), as the high oxidation reactivity is desired for DPF soot regeneration. Several studies have used thermogravimetric analysis methods to assess the effectiveness of DPF under different soot properties, such as using different fuels (biodiesel blends) (Rodríguez-Fernández et al. 2017; Song et al. 2007; Williams et al. 2010). The results suggest that the form of fuel oxygen, oxygen functional group all are important factors in determining the DPF efficiency measured in terms of regeneration times and soot reactivity (Williams et al. 2010). TGA is used with a multitude of other techniques, and the results are an excellent description of the very fundamental properties of DPM and its reactivity. TGA with the surface and chemical features of DPM (analyzed using BET and SEM (Sharma et al. 2012) or Raman and TEM (Guo et al. 2020)) are found to provide an in-depth explanation of oxidation behavior and kinetics of DPM. TGA is also used to study the effect of fuel properties on toxicological characteristics of diesel particulate emissions (Zhang and Balasubramanian 2014a) and structural changes during the oxidation process of soot (Ishiguro et al. 1991).

Another similar technique is the Temperature-programmed analysis of soot. Temperature programmed analysis is the method of observing the transformation of atoms or molecules from a surface when the surface temperature is varied. Depending on the requirement, these techniques can be called TPD (temperature-programmed desorption), TPR (temperature-programmed reduction), TPSR (Temperature-programmed surface reaction), and TPO (temperature-programmed oxidation). E.g., TPD is used to evaluate the amount and types of oxygen-containing surface groups in carbon materials. The key principle is to heat the material in an inert atmosphere and record spectra of exhaling gases (CO and CO2) by quadrupole mass spectrometry. Recorded spectra may be further deconvoluted into separate peaks attributed to certain functional groups similar to after-experiment exercises performed in XPS (Krasnikova et al. 2019). TPO allows an excellent overview of the soot reactivity and changes in the structure of soot (Knauer et al. 2009) by quantifying the oxidized soot and calculating the reactivity by detecting the emitted carbon-oxides with IR spectroscopy (Messerer et al. 2006) or mass spectrometry (Müller et al. 2005). Several other studies have used TPD analysis and modeling to study the soot’s oxidation reactivity (Du et al. 1990).

In this subsection, we briefly discussed some of the thermal and optical techniques widely used for DPM analysis. Interested readers can find a good review of most of the techniques in some other studies (Stanmore et al. 2001). The next section will overview some of the microscopic analysis techniques.

4.3.3 Microscopic analysis

Microscopic analysis such as SEM, TEM, and AFM provides a detailed investigation of the morphological, microstructural, and mechanical characteristics of DPM up to the nano-scale range and their size variation, phase diversity, and chemical composition. In this subsection, we will briefly look into some of the impactful works and potential of these techniques. The nanostructure features of DPM, such as the morphology, nanostructure parameters, and degree of order, can provide a fundamental understanding of the soot formation and oxidation processes. Nowadays, both transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) are widely used to investigate these features of DPM (Kamatani et al. 2015; Soudagar et al. 2020). Nanostructure parameters such as fringe length, fringe separation distance, and tortuosity are defined based on the HRTEM image analysis (Vander Wal 2005; Wal and Mueller 2006) are used to evaluate the nanostructure of the primary particles quantitatively. A combination of TEM techniques with Raman and XPS is often useful to characterize and provide a fundamental explanation of diesel soot chemical structure and reactivity from atomic-level interactions (Mustafi et al. 2010; Sadezky et al. 2005; Saffaripour et al. 2017). Such a combination of analysis techniques provides detailed structural and chemical information about the surface and the bulk of soot particles. In a recent study, Guo et al.(2020) colleagues used four types of fuels blended with diesel in scaling proportion in a diesel engine to generate 13 different soot samples. They found that the soot samples with more O functional groups and/or C–C bonds on the edge plane are more reactive as they lose more mass at the lower temperature range and require a lower temperature to initiate oxidation, as confirmed by the DSC and TGA analysis (Guo et al. 2020). Similarly, Parent et al. (2016) colleagues used TEM with NEXAFS, XPS, and Raman spectroscopy to characterize aircraft soot. They found graphitic layers arranged in onion-like, turbostratic structures in these particles that too independent of the engine operating regimes. In general, particles were poorly oxidized but slightly more oxidation rate at the very surface. The soot surface also presents a high concentration of unsaturated organic hydrocarbons and structural defects (Parent et al. 2016). Some studies have used TEM and HRTEM to study the effects of Exhaust gas recirculation (EGR) technology on diesel soot properties (Al-Qurashi and Boehman 2008; Rohani and Bae 2017). Others have studied the effect of fuel formulation (Wal and Mueller 2006; Yehliu et al. 2012) and advanced diesel combustion operation (Sun et al. 2020) on diesel soot nanostructure and reactivity. Nowadays, modern computing systems, image analysis software, and algorithms continuously enhance our capability to use TEM instruments (Gaddam et al. 2016; Pfau et al. 2020; Yehliu et al. 2011).

SEM is also quite often used with TEM to characterize soot particles. The SEM gives an intuitive way of identification of a particular matter by its out-looks (Figler et al. 1996). SEM can give a broad overview of PM’s soot morphology and elemental analysis. Such features are extremely helpful and explain the fractal nature of particulates (Baltzopoulou et al. 2018; Guarieiro et al. 2017). Fractal dimension has been widely used for the morphology description of combustion particle agglomerates as it illustrates how fine particles aggregate and grow (Pandey et al. 2007). Such dimensions of particles are found to be different for particles emitted under different conditions (Lee et al. 2001; Wei et al. 2020). Several authors have used electron microscopy techniques to characterize the size, morphology, and fractal geometry of soot particles using SEM and TEM, and other techniques mentioned above under different working and fuel conditions (Karin et al. 2017; LUO et al. 2009; Mustafi and Raine 2009).

Atomic Force Microscopy (AFM) is another important technique mainly for studying the mechanical properties of DPM. Through AFM, interaction force measurements can be made directly in nano-, and micron-scale agglomerated particle systems (Kamp et al. 2014). It can be used to measure features such as the tendency of diesel soot agglomerates to fragment (energy required to do so) and their bond strength (Rothenbacher et al. 2008). In a recent study, AFM has been used to characterize particle dimension and morphology of flame-formed Carbon Nanoparticles, showing that the shape of the particles collected on a sampling plate is never spherical (De Falco et al. 2015). Another group studied the correlation between mechanical and oxidation characteristics of methanol/biodiesel particulate matter using MOUDI for sample collection and AFM, TGA/DSCI for further analysis. The authors found a deep correlation between the mechanical parameters and the oxidation properties of the particles. They found that the mechanical parameters, such as the attraction force, viscous force, the cohesive force, the adhesion energy, and Young’s modulus, are inversely related to the looseness of the particles. Hence, the smaller the mechanical parameters, the smaller the particle hardness and graphitization degree (Liu et al. 2020). Other studies have used AFM to understand the effect of Exhaust Gas Recirculation (EGR) composition and temperature on macroscopic mechanical properties (Zhao et al. 2019), de-agglomeration properties (Ding et al. 2016) of particles emitted from a diesel engine.

4.4 Sampling and analysis of DPM in underground mines

As we already discussed in a previous section that the underground mining has faced the worst hazards of DPM. Still, diesel equipment is widely used in underground mines as it is rugged, mobile, and reliable to improve productivity significantly. The strongest evidence considered by IARC for classifying diesel exhaust to group 1 carcinogen were from studies conducted in underground mining. Underground mining is a constrained environment and needs special treatment of the DPM problem. Hence, underground mining has extensively focused on developing customized measurement, analysis, and control techniques for DPM. Another section will discuss the control techniques widely implemented in underground mines. This section will present a short overview of some common sampling and analysis methods used in underground mines and some discussion about developing real-time DPM monitoring techniques.

Generally, the amount of DPM emitted by a particular engine, with or without an after-treatment device, is sampled and measured by a laboratory test. In the U.S., for compliance determinations made in underground M/NM mines, the regulations require the use of a full-shift personal sampling train to collect the miner’s exposure to DPM in the underground environment. The sampling train consists of a cyclone, an impactor, the filter cassette, a length of tubing, lapel clips, and a constant flow sampling pump. The submicron impactor segregates the mineral dust from the DPM. Samples are collected onto quartz fiber filters. The flow rate required in this case is 1.7 L/min. The sample collected over filter paper for an entire shift is then analyzed for EC and TC using NIOSH 5040 as described in a previous section. Elemental carbon is used as a surrogate for regulating the exposure to DPM in underground metal/nonmetal mines as it is selective of DPM and has the advantage of no sampling artifact (Noll et al. 2002). Besides, EC constitutes a large portion of the particulate mass, and it can be quantified at low levels (Birch and Cary 1996). Also, a strong correlation has been shown between Total carbon (TC) and Elemental Carbon (EC) for samples collected from several US and Australian mines metal/non-metal mines (Noll et al. 2007). Although NIOSH method 5040 is an accurate and extensively used method for determining DPM exposures, it only indicates that overexposure has occurred rather than providing the ability to prevent overexposure or even detect its source. It only provides the average concentration over an entire working shift, and several weeks might pass before results are obtained (Noll et al. 2013).

Nowadays, lots of effort is put into developing real-time or semi-real-time DPM monitoring equipment. Real-time monitoring can provide information that can be acted upon rapidly. Hence, several real-time monitoring techniques are in a continuous development and implementation phase. One simple approach is modifying and implementing a real-time personal dust monitor used mainly in coal mines for dust sampling. It is a portable instrument that uses a tapered element oscillating microbalance (TEOM) technology to measure particulate mass. By improving the airflow volume and cut-off point of these instruments, the PDM seemed to measure DPM well. Light scattering equipment has also been used (Miller et al. 2007). However, these techniques are prone to be influenced by other factors humidity, cigarette smoke, oil mist, composition and particle size of aerosol, and dust at certain concentrations, even when an impactor is used (Chekan et al. 2006; Quintana et al. 2000).

Some techniques for the short term (usually a shift or less), real-time measurement of DPM exposures, is determined using the FLIR/Airtec (Noll 2007). The Airtec is a laser-based technique that measures light extinction, incorporating light absorption and scattering of the particles collected over Teflon membrane filters. With DPM particles only, the absorption will be the dominant effect on light extinction (Noll et al. 2013). Light scattering may have other influences if other scattering aerosols are collected with the DPM (Bond et al. 1999). Some other commonly used real-time monitoring systems are Pinssar READER: a laser-light scattering photometry instrument, the Sunset laboratory Inc., Model 4 OCEC Field Analyzer, and Magee Aethalometer (Pritchard and Hill 2016).

Long-term monitoring can provide insight into overall DPM behavior within an area or after a ventilation change. For long-term monitoring (weeks to months) of DPM in underground mines, Barrett et al. (2019) tested two instruments Magee Scientific AE33 Aethalometer and the Sunset Laboratory Semi-continuous OC-EC Field Analyzer, both in field and laboratory setting. Based on their observation in both field and the laboratory, their study indicated that both the instruments could provide for long-term and autonomous monitoring under certain conditions (e.g., access to consistent power; routine maintenance to clean sampling trains and replace consumables) (Barrett et al. 2019). Some other studies have shown the development of other advanced techniques such as Interdigitated Capacitive Sensor for real-time monitoring of sub-micron and nanoscale particulate matters in the personal sampling device (Back et al. 2020) and carbon particulate sensor’s aethalometer (Volkwein and Hansen 2016); however such techniques are yet to find their way into practical applications in underground mines. To summarize the section, a list of techniques and their relevance with references are provided in Table 2.

Table 2 Different techniques for physicochemical characterization of DPM
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5 Health impacts of DPM

DPM is a complex mixture of solid and liquid particles suspended in a gas with a carbon core. The hazardousness of DPM arises because it tends to attach several compounds, such as polycyclic aromatic hydrocarbon (PAH), sulfur, nitrogen, etc., due to its very small size, large surface area, and hence easy absorption of chemicals to its surface (adsorption). The chemical composition of DPM varies depending on the combustion time, location, operating condition of the engine, and fuel type, and up to 700 compounds have already been identified in diesel emissions studies (U.S. EPA, 2006). The federal EPA has determined that benz[a] anthracene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene, dibenz[a,h]anthracene, and indeno[1,2,3-c,d]pyrene present in the exhaust are probable human carcinogens (CDC,1995). Benzo[a]pyrene is the main lung carcinogen in vehicular exhaust (Armstrong et al. 2004). Both PM and PAHs damage mitochondria and suppress their proper functioning (Armstrong et al. 2004; Jiang et al. 2011).

The physicochemical properties of DPM play a key role in affecting its toxicity. Physical properties of DPM include factors that describe its size and structure, such as the mass, surface area, number/size distribution of particles, morphology, and their physical mixing state (Burtscher 1992). Several studies have found that the surface area (measured using BET) is the most effective dose metric biologically for acute nanoparticle (such as DPM) toxicity in the lung (Stoeger et al. 2009; Waters et al. 2009). The DPM surface area and adsorbed organic compounds play a significant role in manifesting chemical and cellular processes that, if sustained, can lead to adverse respiratory health effects (RISTOVSKI et al. 2012a, b).

Another important criterion for DPM toxicity is the bioavailability of organic or other compounds attached to it. Although the surface of DPM consists of several compounds, ions etc., these species can be highly important for toxicity induced by particles. It is well known within particle toxicology that particles with low solubility can be comparatively more toxic due to a reactive particle surface (Clouter 2001; Schins and Knaapen 2007). The bioavailability of PAHs on the surface of DPM is also investigated and found to vary with the cells it attaches to (Borm et al. 2005; Gerde 2001). As mentioned earlier, these ultrafine particles have a strong potential for adsorbing toxic metals, which may have adverse physiological effects (Li et al. 2013). The bioavailability and toxicity of some metallic ions are also studied using in-vitro assays (Hedberg et al. 2010). Exposure to airborne nanoparticles via the inhalation route leads to deposition in the various compartments of the respiratory tract according to probabilities dependent on three important parameter groups: aerodynamic and thermodynamic nanoparticle properties, breathing pattern, and the three-dimensional geometry and structure of the respiratory tract. Once deposited, insoluble nanoparticles undergo clearance mechanisms specific to the region of the respiratory tract; i.e., at all regions, nanoparticles will interact with proteins of the epithelial lining fluid, potentially forming complexes that are likely to affect their subsequent metabolic fate and biokinetics (Geiser and Kreyling 2010). They can penetrate the deepest parts such as into the epithelial cells of the airways, bloodstream, or lymphatic system, favored by their long retention time (Creutzenberg 2012). It is also not necessary that once inhaled, these PMs will retain their state. They may not be soluble initially, being solid particles. However, they may no longer persist as solid particles and dissolve/disintegrate in cells and body fluids (fully or partially) so that eventually, a bio-persistent core with different particle properties/toxicities is retained (Oberdörster et al. 2005a). Since DPM can attach several elements/ions to its surface and high diversity of emerging studies of nano-sized particles, their bio-persistence and bioavailability should be prioritized. An excellent description of such features of nanoparticles can be looked for in some other studies referenced herein (Geiser and Kreyling 2010; Oberdörster et al. 2005a). These particles can cause significant oxidative damage to whatever tissue or cell they land in (Frikke-Schmidt et al. 2011; Harrison et al. 2011).

DPM is associated with increased mortality (Boffetta et al. 1988; Howe et al. 1983; Laden et al. 2000), cardiovascular (Miller et al. 2015; Mills et al. 2005; Wilson et al. 2018), cancer (Attfield et al. 2012; Crump 2014; Gamble et al. 2012; Kachuri et al. 2016; Moolgavkar et al. 2015; Silverman et al. 2012), respiratory (Rissler et al. 2012; Ristovski et al. 2012a, b), immunological (O’Driscoll et al. 2018; Shaw et al. 2020), neurological impacts (Gerlofs-Nijland et al. 2010; Kim et al. 2020; Reitmayer et al. 2019). Long-term exposures to diesel exhaust are dangerous; however, a short-term exposure with a particulate concentration in the range of 100–300 µg/m3 can also lead to mild pulmonary, bronchial, and respiratory airway inflammatory responses (Behndig et al. 2006; Mudway et al. 2004; Stenfors et al. 2004) and even worse for those already suffering from asthma and allergy (Fujieda et al. 1998). For further details, readers are suggested to check the IARC monograph 2014, and there they can find a summary understanding of most of these studies.

In one of the earlier sections, we discussed the impact of DPM in underground mines and how the international community has widely acknowledged it. A large DPM concentration in confined spaces (underground mines) increases miners’ potential to be overexposed. Underground miners are exposed to the highest DPM levels as it is an enclosed worksite utilizing heavy equipment (Pronk et al. 2009). Workers in underground mines are found to be exposed to exceeded levels of DPM promulgated by MSHA consistently (Cohen et al. 2002), especially at certain locations in underground mines (MSHA, 2019). Diesel exhaust in miners (DEMS) study conducted between 1992 and 2012 concluded that exposure to diesel exhaust caused a statistically significant increased risk of death from lung cancer over that otherwise predicted from cigarette smoking and the natural occurrence rate (confounding parameters) (Attfield et al. 2012; Silverman et al. 2012). Several other studies have documented an elevated lung cancer rate in miners because of diesel exhaust exposure (Boffetta et al. 1988; J. Gamble et al. 1983; J. Guo et al. 2004; Neumeyer-Gromen et al. 2009; Saverin et al. 1999). Although only a few studies are conducted directly on underground miners, all these studies confirm a positive relationship between DPM exposure and response.

Underground mining is different from other industries for obvious reasons. One way to understand it is that the constrained nature of the working environment does not allow particles generated to escape from the environment so easily. A well-designed ventilation system is the only way to eject particles out of the zone. It means that the particles can stay in the underground mines for a long time, and hence, the odds of interaction between the workers and DPM are much more than in other industries. So, whatever health effects we discussed so far, whether that be because of long-term or short-term exposure, will be aggravated in time of response and seriousness. Another difference is that underground mining is an excavation activity and generates lots of other dust of different sizes depending upon the nature of the mining activities (NIOSH 2019). The heat released, pressure, and temperature variation inside an underground mine are important aspects of underground mine flow. However, the point is that there is a lot of interaction between DPM and other dust. Consequently, variations in some of the parameters mentioned in the previous line affect the thermodynamics of particle interaction and growth in such an environment. As DPM are nanoparticles and nanoparticle dust is present in the underground environment, these have a higher surface activity. Under such conditions, the dust generated is potentially different in its reactivity and hazard from the originally mined sources in such contexts. DPM nano-particulates released into the working environment are highly likely to interact with other dust particles being released in the same environment (Fan and Liu 2021). Such interactions will change the overall characteristics of DPM because it is no longer DPM but another hazardous and hybrid PM mixture. However, there is still a lack of understanding about such interactions, whether in terms of characterization or their effect on human health.

Further studies are warranted, focusing on the characterization of DPM nanoparticles and their surface feature modifications under the controlled and uncontrolled transient environment. DPM’s adsorption and porosity feature is another important feature that needs a deeper understanding. Adsorption of moisture and other nanoparticle size dust or ions (such as metallic or hydroxyl ions), which are present abundantly in the specific underground mining environment, can alter the properties of the fundamental particle and can form hybrid entities. These hybrid particles can behave completely differently from their native particles while interacting with human cells. To assess their physicochemical properties, studying their surface energy, particle charge, solvation, and how these change with the thermodynamics of the environment they are released is important (Rabajczyk et al. 2020). The surface of DPM can also serve as a carrier of some of the already proven toxic ions, increase mobility, and further transfer these to human cells. Besides, when discussing ions and surface charge, it is understood that they are quick-acting entities. They can be worst for human cells once able to reach them, and if there is a suitable carrier available, the work gets a lot easier, as suggested by some studies (Corradi and Mutti 2010; Lipfert et al. 2014). Especially in some metal mines, the dust produced contains metals or metal ions such as Zn, Ni, Cu, Co, V, Cr, and Ti. These metals can catalyze the Fenton-type reactions in our body that produce reactive oxygen species (ROS) (Shi et al. 2003). In general, inhaling such nanoparticles can stimulate alveolar macrophages, creating conditions for developing acute systemic inflammatory responses (Farhat et al. 2011). Table 3 references some of the studies done to assess the various health impacts of DPM. To sum it all up, Fig. 12 artistically shows the relation between the health impacts of PM on the human body.

Table 3 Health impacts of DPM
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Fig. 12
figure 12

Health issues related to DPM

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DPM is already well known for its VOCs and SVOCs composition, and it is consistently affected by environmental changes in underground mines, but how much and in what ways is still unknown. Since the toxicological potential of nanoparticles are related to their composition and especially surface features which is the primary point of interaction of these particles to cells or molecules inside our body. There is no evidence of synergistic or antagonistic effects of inhalation of such hybrid mixtures, whether in terms of their target selectivity or transient physicochemical properties.

In Summary, there is a lack of understanding of the transformation behavior of DPM from tailpipe emissions to its exit from the underground environment. After years of research, we might know what comes out of the tailpipe, but we do not know what leaves the underground atmosphere and how much it does. Identifying major contributors in DPM to its toxicity and how these are affected by surrounding conditions are of extreme importance. This will help develop a ‘toxicity reduction’ approach that is more cost-effective than the ‘mass reduction’ approach currently used (Zhao et al. 2021a, b).

6 Integrated approaches to DPM mitigation in mining: engineered solutions and predictive modeling

Although much progress has been made to characterize DPM, several areas are not studied as extensively as others, such as reactive flow modeling of nanoparticles in the closed environment. The focus of the international community on the diesel engine has put the usage of diesel engines in an area of debate between a wide variety of requirements ranging from maximum customer benefit and minimum fuel consumption to minimize emissions (Fiebig et al. 2014). Because of such demands, diesel engine technology has undergone remarkable changes. The technical developments include all issues from fuel to combustion process to exhaust gas after treatment. These include ventilation, fuels and fuel additives (low sulfur and ultra-low sulfur diesel, and Bio-diesel), emission-based maintenance programs, engine type/specification exhaust after treatment (DOCs, DPFs, disposable type filters, SCRs), environmental cabins, administrative controls (limiting engine idle time, limiting the number of equipment allowed in a heading or drift and remote control and automation) and PPE’s (respirators) (Stinette et al. 2019). This section will briefly discuss the DPM engineering control strategies, especially emphasizing those implemented in underground mines.

6.1 Targeted engine design and retrofitting

Extensive research and technological advancements have led to the development of effective methods to mitigate diesel particulate matter (DPM) emissions right at the source. One of the main techniques for controlling diesel exhaust emission is changing the internal engine design. The combustion process in diesel engines is complex and influenced by numerous factors, including engine speed, load, fuel injection timing, turbocharging, and exhaust gas recirculation (EGR). One significant area of focus is the refinement of internal engine design. Fine-tuning parameters like the timing of fuel injection and the initiation of fuel delivery directly into the combustion chamber are crucial for achieving an optimal air–fuel mix, leading to more complete combustion. Advances such as utilizing multiple injection ports with high-pressure delivery systems create smaller fuel droplets that combust more thoroughly, substantially cutting down on particulate emissions (Mathis et al. 2005). Complementary methods, such as implementing EGR, where a portion of the exhaust gas is recirculated back into the combustion chamber, enhance the oxidation process and contribute to further reductions in both particulate and nitrogen oxide emissions. This process effectively enriches the combustion-air mix, facilitating better oxidation and consequent soot reduction (Pischinger 2007). Refer to Table 4 for an illustrative summary of these engine design modifications and their impact on DPM control. Additional insights and comprehensive studies are available in the referenced literature.

Table 4 Different engine design parameters and their significance in controlling DPM emissions
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6.2 Fuel management and alternative fuels

Improvements in diesel fuel quality and composition have played a pivotal role in curbing diesel particulate matter (DPM) emissions, complementing advancements in engine control technologies. Over recent decades, the introduction of cleaner, alternative, and renewable fuel options has significantly lowered particulate emissions from diesel engines. For example, fuels that are devoid of sulfur and aromatics, including certain renewable diesel varieties, have demonstrated a substantial reduction in black carbon emissions upon (Pirjola et al. 2019b). Biodiesel, derived fr, and ultra-low sulfur diesel are also available, which control particle emissions from diesel enginesl, leading to considerable decreases in particle emissions (Huang et al. 2015). To further reduce DPM emissions, the modification of diesel fuel has involved the incorporation of additives designed to improve combustion efficiency and cleanliness. These additives include cetane improvers that enhance fuel ignition properties, detergents that keep fuel injectors clean, and oxidation catalexhaust after-treatment systems are increasingly being used to control particulate emissionsl fuels, which are designed to burn cleaner than conventional diesel, and the blending of diesel with natural gas or hydrogen, are emerging as innovative approaches to reduce particulate emissions. The impact of lubricating oils on engine emissions has also gained attention as a crucial factor in controlling overall engine-out emissions. Using lubricating oils with advanced nano-additives is emerging as a critical strategy. These additives are engineered to optimize the engine’s tribological interface, resulting in improved wear resistance, reduced friction, and consequently, lower particulate emissions from the exhaust (Bojarska et al. 2023, 2021; Zhao et al. 2021a, b). In addition to these modifications, there is growing interest in developing paraffinic fuels, which, due to their high cetane number and low aromatic content, can significantly cut particulate emissions. When used in modern high-efficiency diesel engines, these fuels can further reduce the environmental impact of diesel exhaust. Table 5, as referenced, will provide a concise summary of these innovative fuel technologies and their effectiveness in modern diesel engines to mitigate particulate emissions, detailing the comparative advantages of each fuel type and the associated technological synergies that enable cleaner combustion.

Table 5 Different fuel parameters and their significance in controlling DPM emissions
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6.3 Advanced aftertreatment systems

Exhaust after-treatment technologies have become a critical component in the reduction of particulate emissions from diesel engines. Diesel Oxidation Catalysts (DOCs) are a cornerstone of these technologies. Comprising honeycomb-like structures infused with active noble metals such as platinum (Pt) and/or palladium (Pd), DOCs effectively catalyze the oxidation of hydrocarbons and carbon monoxide into less harmful carbon dioxide and water (Vaaraslahti et al. 2006a). They also play a vital role in breaking down the solid organic fractions (SOFs) associated with diesel particulate matter (DPM), thus diminishing both the chemical complexity and physical dimensions of the DPM.

Additionally, DOCs serve an important dual function by elevating exhaust temperatures, which is essential for the active regeneration of Diesel Particulate Filters (DPFs) (Fiebig et al. 2014). One of the most widely used approaches to remove PM emissions from diesel engines is Diesel Particulate Filter (DPF). DPFs are instrumental in capturing over 90% of DPM, utilizing a combination of mechanisms such as diffusional and inertial deposition, along with flow-line interception, to trap and retain particulate matter within their honeycomb matrices (Wade et al. 1981).

The performance of DPFs, however, is influenced by the amount of soot accumulation, with filtration efficiency typically improving as the soot load increases. Nonetheless, this necessitates periodic or continuous thermal regeneration to prevent clogging and restore filtration capacity. The variability in engine operations and the characteristics of the particulate filters themselves are critical factors affecting the number of particulate emissions. Further refining the after-treatment arsenal, integrating Selective Catalytic Reduction (SCR) systems with DPFs has proven to be an effective strategy for concurrently managing DPM and nitrogen oxides emissions. This combination leverages the strengths of both systems to achieve a more comprehensive reduction in emissions (Czerwinski et al. 2015). Table 6 provides a synthesized overview of these advanced after-treatment systems, elucidating the technological principles and efficiencies that underpin their application in contemporary diesel engines for particulate mitigation. The table will likely present the effectiveness, operational nuances, and comparative benefits of these systems, offering insights into how they can be optimized as part of an integrated emissions control strategy.

Table 6 Different after-treatment technologies employed and their significance in controlling DPM emissions
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6.4 Additional strategies for mitigating DPM exposure in underground mining operations

In occupational environments such as the mining sector, specialized measures are in place to manage diesel particulate matter (DPM) post-emission. Control methods include robust ventilation systems and the use of environmental enclosures (Chang and Xu 2019). Ventilation, a pivotal engineering control, dilutes airborne contaminants through both natural and mechanical means. These systems range from simple arrangements of fans and ducts to complex networks with multiple airways, regulated by a series of controls. Recent advancements have focused on enhancing these ventilation systems, with Computational Fluid Dynamics (CFD) modeling emerging as a key tool in optimizing airflow and fan placement. Proper fan positioning is essential to prevent adverse effects like the recirculation of contaminated air, a challenge that CFD modeling adeptly addresses. Furthermore, DPM transport modeling allows for assessing how DPM behaves and moves through the mine tunnels and shafts. Understanding the transport mechanisms of DPM can aid in the prediction and prevention of high-concentration zones, which are particularly hazardous to workers. Modeling diesel particulate matter (DPM) transport in confined spaces is a complex task that must consider several environmental factors and their interactions with particulate matter. By using predictive modeling, mining operations can proactively implement control measures tailored to the specific conditions of each task and area within the mine.

Especially in the mining industry, a human-centered approach to DPM mitigation recognizes the need to prioritize worker health and safety above all else. The occupations that are most exposed to the DPM should be prioritized. Certain positions within the mining operation, such as equipment operators, maintenance personnel, and those working in areas with poor ventilation, are at greater risk for DPM exposure. Identifying these roles allows for targeted interventions, such as localized exhaust systems, use of low-emission equipment, and the provision of advanced PPE to those most in need. Furthermore, evaluating the levels of DPM that workers are exposed to is a key component of a comprehensive DPM mitigation strategy. Exposure assessments involve both direct measurement of air quality in the mine and biomonitoring of workers for signs of DPM exposure. These assessments are essential for identifying high-risk areas and activities within the mine. Adherence to MSHA recommended DPM exposure limits and implementation of their methodologies for measuring and controlling DPM is a critical aspect of a mining operation’s regulatory compliance and worker protection strategy.

In addition to ventilation, environmental cabs provide miners with reduced DPM concentrations. These cabs, when well-designed as depicted in Fig. 13, maintain a positive pressure environment through a sealed structure and a proficient HVAC system that filters external air before entry. These cabs have proven to be effective, potentially decreasing miner DPM exposure by up to 90% when optimally utilized, though their efficacy is contingent upon factors including the type of air filter used, pressurization levels, cab integrity, and the use of recirculation filters (Cecala et al. 2005; Noll et al. 2008). Administrative controls complement these engineering solutions with strategies encompassing worker training, reducing the diesel-powered equipment fleet, curbing engine idling, and enforcing routine maintenance to minimize DPM exposure (Khan et al. 2016). Despite the array of controls applied, the provision of personal protective equipment remains a recommended safeguard for workers to mitigate health risks further.

Fig. 13
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Environmental cab used to protect underground operators from DPM exposure (NIOSHTIC2 Number: 20036965)

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In this section, we evaluate a multi-faceted approach to mitigating worker exposure to diesel particulate matter (DPM) in throughout the extraction process. While techniques have been developed, no single method is wholly effective for eliminating DPM exposure. Thus, a comprehensive strategy incorporating multiple controls—from fuel composition to combustion technology and exhaust after-treatment—is essential for significant emission reduction. Despite technological advances, complete eradication of the DPM issue remains elusive. Challenges persist with the durability and sustained efficacy of these solutions. For instance, while the design of the engine combustion chamber represents a significant advancement, its performance is inextricably linked to the type of fuel used. Not all fuels yield the same level of effectiveness across different engine designs. Moreover, while state-of-the-art engines and after-treatment systems exhibit high initial efficiency, this efficacy tends to diminish over time. Critical components, such as catalysts within after-treatment systems, necessitate periodic replacement, yet there is a paucity of data regarding the frequency and consistency of such maintenance practices. This gap suggests a disparity between theoretical expectations and operational realities. The field of particle characterization is advancing rapidly, yielding increasingly precise techniques for assessing fine particles and imposing more stringent regulatory standards. This evolution in assessment capabilities indicates that the paradigm for emissions reduction may need to shift. Instead of focusing solely on reducing the overall mass of emissions, a more nuanced understanding and approach to the various sizes and types of particulate emissions is required. Emerging research domains, previously inaccessible to older characterization and control technologies, are now within reach, thanks to technological progress. These advancements open new avenues for exploration and intervention. As such, they warrant the attention of researchers seeking to develop the next generation of control strategies. Future research should not only continue to refine existing methods but also pioneer innovative solutions that address the complexities of DPM mitigation in mining operations.

7 Discussions and research needs

7.1 Field instrumentation and future field sampling needs

The study of diesel particulate matter (DPM) and its impact on occupational health in confined environments as such of underground mining operations remains a pressing concern despite extensive research efforts. As current federal regulations in the U.S. stipulate, a miner’s exposure to DPM should not exceed 160 µg per cubic meter (μg/m3) of total carbon (TC) as an 8 h time-weighted average (TWA), as regulated by gravimetric sampling method. However, the adequacy of these methods is questionable given the ultrafine nature of DPM, which can be prevalent in substantial numbers with significant health implications despite not contributing substantial mass to be detected gravimetrically. As mentioned earlier, the gravimetric method might not be suitable for DPM sampling and exposure assessment. DPM, being ultrafine, might not carry enough mass to show up on a gravimetric scale. Current gravimetric methods may not capture the full scope of DPM exposure, as they tend to underestimate the presence of ultrafine particles. These ultrafine particles, although not significantly contributing to the overall mass in a given volume of air, can be numerous enough to pose severe respiratory and cardiovascular risks to miners due to the active biological interactions.

From engineering control perspective, existing control technologies may effectively filter out larger particles yet allow a substantial number of ultrafine particles to remain suspended in the air. This discrepancy necessitates a paradigm shift in how we monitor and regulate DPM exposure in mining environments. Future field sampling needs should pivot towards the development and implementation of instruments which can be capable of number-based measurements of DPM associated with existing mass based measurents. These instruments should offer real-time monitoring capabilities to detect and quantify the ultrafine particulates that are currently not accounted for by mass-based methods. Conducting thorough research to explore the practicality of quantifying DPM by particle count is crucial. Developing standardized methods for this metric and integrating them into the underground mining industry practices is essential for advancing worker health and safety protocols. Because of the increasing burden of the nano-DPM exposure for future miners, there is a critical need to establish standardized protocols for the use of number-based measurement devices to ensure consistency and comparability of data across different sites and conditions. The development of such instrumentation poses several challenges. The instruments must be sensitive enough to detect particles at the nano-scale level while also distinguishing between different types of particulate matter. For practical application in the field, these devices must be portable, allowing for on-site assessments that can inform immediate adjustments to mining operations to safeguard worker health. Cost is a significant factor for the technology adoption. Affordable instrumentation will ensure that mining operations can deploy these devices extensively without prohibitive expenses. Also, any new measurement technique must gain regulatory acceptance, which involves validation against existing standards and demonstrating its relevance and reliability in reflecting exposure risks.

In conclusion, while the gravimetric standard has served as a benchmark for regulation, its limitations in the context of ultrafine DPM necessitate the exploration of new methodologies for assessment. The future of DPM monitoring and control in underground mining hinges on advancing instruments that can accurately measure particle numbers in real time, facilitating a more comprehensive understanding and management of DPM exposure and its associated health risks.

7.2 Dynamics of DPM transformation: interactions and health implications for miners

A crucial yet underemphasized aspect of DPM research is the time-/environment-dependent particulate transformation process after their post-emission. As previously noted, the transition of DPM from primary to secondary particulates in the environment is influenced by many factors, including the type of engine, fuel properties, and the immediate environmental context of their release. DPM is generated under the harsh conditions of high pressure and temperature within a diesel engine, resulting in particles that are highly reactive in their nascent state (Amann and Siegla 1981; Fiebig et al. 2014). These particles are predisposed to interact and react with any entities they come into contact with. Consequently, the intermediate processes and interactions that transform primary DPM into secondary particles warrant more scientific scrutiny.

From a health perspective, workers are exposed not to the primary particles formed within the diesel engine but rather to the altered DPM that has interacted with various elements within the engine, the exhaust system, and the environment. The physicochemical properties of the inhaled DPM can be substantially different from those of the primary particles, leading to potentially different health outcomes. The transient physicochemical transformations of DPM are particularly pronounced when released into confined environments such as underground mines. Emitted in large quantities by heavy-duty trucks, these particles undergo initial rapid changes within the engine due to high pressure and temperature. Upon exiting the engine, they encounter a drop in pressure and temperature within the exhaust chamber, which induces further, albeit slower, modifications. Subsequent interactions with exhaust control technologies lead to time-evolved property alterations. The DPM that survives these control systems then enters the ambient environment of the mine, where factors like mineral dust content, relative humidity, and temperature variations play significant roles in influencing the DPM’s final properties (Azam et al. 2023a, b).

In underground mines, the interplay between DPM and environmental factors like mineral dust and humidity is pivotal, potentially transforming the DPM in ways that can significantly impact its health effects, as well as its transport and deposition behaviors. These transformations are critical to understanding, as they inform the design of mine ventilation systems—a central aspect of mine operation and worker safety. Ventilation not only distributes clean air but also influences the mine’s microclimate, which in turn affects DPM transport and deposition behavior. The physicochemical characteristics of DPM, such as free radical content, porosity, and surface chemistry, are crucial determinants of their health impacts. Interactions with environmental moisture can modify these properties, influencing the toxicity of DPM to human tissues. Even subtle changes in DPM’s specific gravity due to moisture interactions can alter how they are transported into and deposited within the human respiratory tract. An increase in the particles’ hydrophobic surface properties could lead to deeper lung penetration and reduced clearance, while a rise in hygroscopicity, attributed to the aging of DPM, could enhance deposition both on mine surfaces and within the respiratory system (R et al. 2014; Tritscher et al. 2011).

In summary, understanding the detailed physicochemical dynamics of DPM post-emission is imperative, particularly in the context of confined mining environments, where such knowledge is essential to assess and mitigate health risks to workers. The surface chemistry of the DPM is highly related to the environment. There is a future research need to quantify the radical content, specif surface area alteration with intends of linking toward its toxicity.

7.3 Key factors in modeling DPM transport in underground mines: integrating CFD with environmental dynamics

To understand the DPM transport behavior in confined mine space, thre is a research need to systematically identify and prioritize the factors that influence the intricate transportation behavior by considering the various environmental factors. Therefore, it’s crucial to explore the interplay between air dynamics (turbulent ventilation areaodynamics) and the interactions between particulates and air (particulate and environment interations). The time-/environment-dependent reactive festure of DPM should be explicitly considered for its transport and ultimately on the workers’exposure assessment. In particular, future efforts must focus on modeling the interaction of reactive DPM with water vapor and oxygen in mine air to establish miners cause-effect relationships. At this end, the reactive transport process should be integrated into the air dynamics model to develop comprehensive computational fluid dynamics (CFD) models that include reactive behaviors. This section highlights the key parameters that should be considered in the future to describe the transport of DPM in an underground mine or any enclosed environment.

7.3.1 Moist air dynamics and DPM reative behaviors

In underground mines, the air often contains higher moisture levels due to the enclosed nature of the environment and lack of direct sunlight. Moist air plays a critical role in the transport behavior of DPM within an underground mine. As DPM is emitted from diesel engines, it encounters the ambient moisture present in the mine atmosphere. The hygroscopic properties of the particulate matter can lead to the ab-/ad-sorption of water vapor, causing the particles to grow in size and mass through a process known as coagulation. This growth can alter the stoke’s diameter and specific gravity of DPM which can influence the settling velocity and thus affect their suspension time and transport distance. CFD models that simulate DPM behavior need to account for particulate-air inteaction within the mine. These coupled models should intrinsically consider the implicit particle alteration by water vapor uptake through wetting and diffusion. The changes in particle size distribution due to moist air can significantly impact deposition patterns, which are important for assessing worker exposure and determining the effectiveness of ventilation strategies.

7.3.2 Temperature influences on DPM transport

The temperature in a mine can significantly vary, influenced by factors such as depth (autocompression), rock strata heat emission, as well as the heat emitted from machinery. Temperature affects the buoyancy-driven airflow, which can alter the suspension and settling of DPM. The temperature can change psychrometric behavior of water vapor and mine air which determines the DPM alteration as discussion before. In addition, the higher temperatures, often found near mining equipment, can lead to increased volatility of certain DPM components, potentially enhancing the chemical reactivity and toxicity of the particles. In CFD models, incorporating temperature variations is essential for accurate simulations of airflows and DPM distribution. These models must solve the energy equation alongside the momentum and mass conservation equations to capture the thermodynamic behavior of the air-DPM mixture. Temperature gradients created by mining machinery, as well as geothermal effects, need to be mapped and integrated into the simulation environment.

7.3.3 Ventilation influence on DPM transport

Ventilation is critical in underground mines, providing fresh air and controlling the distribution of contaminants. Ventilation helps to dilute pollutants in the mine atmosphere and control the flow direction of DPM, which is essential for minimizing the inhalation of particulates and ensuring that areas, where miners are working, have the cleanest possible air. Ventilation contributes to the regulation of temperature and humidity within a mine, which can alter the behavior of DPM transport, as discussed earlier. Temperature gradients influence air density and the buoyancy of particles, while humidity levels affect the hygroscopic growth and agglomeration of particulates. Hence, the design of both natural and mechanical ventilation systems must be included in CFD models to determine how DPM moves and disperses. The models need to simulate airflow patterns created by fans, ducts, and the natural pressure differences within the mine.

7.3.4 Complex interplay of DPM and coexistent aerosols in underground mines

The interaction of DPM with other particulates present in the underground mine environment is a crucial consideration in DPM transport modeling. These interactions significantly affect the behavior and fate of DPM, influencing not only the occupational exposure levels but also the strategies employed to control and mitigate DPM in mine atmospheres. The combined effect of DPM and other mine dust on respiratory health is not simply additive; it can be synergistic or antagonistic, depending on the specific characteristics of the particulates involved. DPM can adhere to or coagulate with other particulates, such as dust generated by drilling, blasting, or other mining operations. This process can alter the size, mass, and aerodynamic properties of the particles, affecting their suspension time in the air and their deposition patterns. The presence of other particulates in the mine environment creates competition for deposition sites on mine surfaces and in the respiratory system of workers.

It may seem that modeling the transport and deposition of DPM in underground mines presents a complex challenge due to the multitude of influencing factors, such as temperature, relative humidity, ventilation, and the presence of other aerosols, as mentioned above. These variables introduce a high degree of uncertainty and require sophisticated multi-phase flow simulations within CFD frameworks. Despite these complexities, current advancements in computational power and the refinement of CFD algorithms make this intricate modeling feasible. For instance, modern CFD applications can accurately predict the behavior of particulates taking into account water vapor present in the surrounding atmosphere (Mei and Xu 2021). Thus, while challenging, the accurate modeling of DPM transport in mines is achievable with today’s technology, enabling safer and healthier mining environments.

8 Summary and conclusions

This work evaluates the risks associated with diesel particulate matter (DPM) in the mining industry, synthesizing recent interdisciplinary research advancements. Notably, the conditions for diesel engines in occupational settings like mining diverge significantly from those in non-occupational industries, with mines often relying on continuous operation of machinery. This reliance can lead to prolonged use of diesel machines with suboptimal DPM controls, resulting in increased emissions due to delayed maintenance and clogged after-treatment systems. Additionally, the remote location of many mining operations complicates timely equipment maintenance, as halting operations for repairs or system updates directly affects production and profitability.

Incorporating insights from today’s discussion, the following points underscore critical areas for further exploration and innovation:

8.1 DPM control in mining: tailored strategies for subterranean challenges

It’s essential to recognize how DPM in the mining industry differs from other sectors. The confined spaces, limited airflow, and high levels of particulate generation in mining create a unique set of challenges for DPM dispersion and control. Unlike other industries where open spaces and different operational dynamics prevail, mining requires tailored solutions that address the specific conditions of subterranean environments. Strategies must account for the limited natural ventilation and the potential for DPM to interact with a complex mixture of other aerosols, which can alter its physical and toxicological properties.

8.2 Advancement in DPM measurement instrumentation

Enhanced instrumentation for accurate DPM number measurement is paramount, given the limitations of gravimetric sampling methods that often fail to capture the high number concentration of DPM. Such advancements are critical for assessing actual exposure levels and the potential health risks they pose to miners. The development of advanced, durable sampling equipment that can withstand the harsh mining environment and provide real-time DPM level data is urgently required to manage DPM concentrations effectively.

8.3 Understanding the interaction of DPM with mine environment

A thorough understanding of DPM interactions with key environmental components within the mine is crucial. These interactions, influenced by factors such as moisture, the presence of other particulates, and ventilation dynamics, significantly affect DPM behavior, impacting its transport, deposition, and the health risks o miners. Modern computational modeling capabilities, coupled with the latest advances in environmental dynamics, are instrumental in devising robust mitigation strategies that safeguard miner health while maintaining industry productivity.