1 Introduction

The expansion of urban areas worldwide presents complex issues for urban studies. Since the 1950s, the growth of suburbs and the development of metropolitan areas have been identified as significant challenges for geographers and urban planners. Initially observed in developed nations, this trend has now spread to developing countries, with particularly severe consequences. Numerous studies have examined the impact of suburban development on transportation patterns and air pollution in metropolitan regions, with some notable examples [1, 2]. Research has also shown that changes in land values can contribute to air pollution [3]. Additionally, studies have highlighted the role of land use changes and transportation infrastructure in influencing urban air quality [4, 5]. These findings underscore the urgent need to address the issue of increased vehicle emissions, which pose significant risks to the environment and public health. The use of inefficient small vehicles with outdated engines results in higher carbon emissions, further exacerbating greenhouse gas levels in the atmosphere [6]. Consequently, this rise in emissions amplifies the negative impacts of air pollution, releasing harmful pollutants such as particulates, nitrogen oxides, and sulfur oxides [7, 8].

The increasing problem of rising energy consumption caused by urban sprawl is a pressing and urgent issue. The expansion of development into sprawling patterns leads to higher energy usage for heating, cooling, and transportation, putting a significant strain on already limited resources. This excessive demand for energy often results in increased air pollution, especially when coming from fossil fuel sources, perpetuating environmental harm and risking public health [9]. The impacts of urban sprawl are severe, including worsened traffic congestion, longer commutes, inefficient energy use, and heightened air pollution [10, 11]. The critical problem of vegetation loss due to urban sprawl has serious environmental consequences. As urban areas expand, forests and green spaces are cleared for development, leading to the loss of essential trees and plants that help clean the air by absorbing carbon dioxide and releasing oxygen [12]. The lack of vegetation worsens air pollution as the natural air purification process diminishes. Additionally, the removal of vegetation contributes to the urban heat island effect, raising surface temperatures and increasing the formation of ground-level ozone—a harmful air pollutant [13, 14]. Infrastructure development associated with urban sprawl significantly contributes to air pollution, as construction activities produce dust and airborne pollutants that further degrade local air quality, exacerbating the negative effects of rapid population growth, uncontrolled urban expansion, and other local development and infrastructure projects [15, 16]. Industrial growth driven by urban sprawl exacerbates air pollution as the need for more industries and commercial establishments leads to higher emissions of pollutants, such as sulfur dioxide (SO2) from industrial operations and increased vehicle traffic. Moreover, the expansion of industrial and commercial land uses due to urban sprawl compounds the air quality issue, underscoring the importance of addressing these challenges and exploring sustainable solutions to mitigate the adverse impacts on public health and the environment [17,18,19].

The consequences of urban sprawl in metropolitan areas are significant as they contribute to regional climate change in urbanized areas [20]. These emerging issues, such as nocturnal surface urban heat island intensity (SUHII) [21], fiscal over-concentration, and air pollution [22], have worsened the situation in Global South cities due to a lack of understanding of urban contemporary needs. Rapid urbanization, informality, social exclusion, poverty, unemployment, urban sustainability, and climate and environmental change are among the new urban challenges in African cities [23, 24]. Suburban development patterns are linked to increased reliance on vehicles and traffic congestion outside the main city in many metropolitan areas with scattered development characteristics [25, 26] Car dependency is characterized by a high per capita vehicle travel rate, car-based land-use patterns, and limited transportation options [27, 28]. Statistics show that over three-quarters of trips in metropolitan areas are made by car, with the United States accounting for over 88%. Consequently, the transportation sector consumes more than 80% of urban energy, leading to urban air pollution and posing a significant sustainability challenge globally [29, 30].

Between 1983 and 2004, rapid urbanization and land use transformation in Shanghai's urban, suburban, and rural areas led to significant water and air pollution. Long-term monitoring of air and water quality identified transportation systems as the main sources of pollutants (SO2, TSP, and NOx). Spatial analysis showed that pollution sources extended beyond Shanghai city limits, with varying frequencies of acid rain [31]. A study of 45 US metropolitan areas found that urban spatial structure played a crucial role in mitigation policies. Over a 13 year period, the study examined annual ozone exceedances, precursor emissions, regional climate, and the relationship between urban decentralization and ozone exceedances, emissions, and temperature. The study highlighted the impact of sprawl on air pollution, with population shifts to the outskirts increasing traffic volume and pollution exposure. While the relocation of residents reduced exposure to air pollution by 13%, it also increased the risk for those who did not relocate [32, 33].

In recent years, there has been a growing number of studies focusing on the impact of metropolitan expansion on air pollution in Iran, particularly in major metropolitan areas like Tehran [20, 34, 35]. These studies have linked urban sprawl and changes in land use and land cover to environmental issues such as increasing land surface temperatures and air pollution [36,37,38]. The Tehran Metropolitan Area (TMA) has experienced significant population growth, with over 15.98 million people residing in the region in 2016, of which approximately 8.73 million (54.6%) lived in Tehran city while the rest lived outside its boundaries. The rapid urbanization has led to a rise in the number of cities from 5 in 1957 to 59 in 2020 [39], indicating a shift of population and activity from the center to the periphery, including suburbs, small towns, and villages. The daily functional connectivity between these dispersed centers and Tehran, often facilitated by private vehicles, has resulted in serious environmental challenges, with air pollution emerging as a major concern in the TMA. According to the Tehran Air Quality Report (2016), the primary air pollutants in Tehran are suspended particles measuring less than 2.5 microns and 10 microns.

Understanding the causes and impacts of air pollution over time requires precise data to accurately depict spatial and temporal changes and interactions. Satellite products are effective tools for studying environmental issues, including air pollution. Various methods have been developed using advanced radiative transfer models or the time-intensive Look-Up-Table approach. A combination of two simplified algorithms for retrieving Aerosol Optical Depth (AOD) from Landsat 8 images has been utilized: the Simplified and Robust Surface Reflectance Estimation Method (SREM) and the Simplified Aerosol Retrieval Algorithm (SARA). SREM is used to estimate Land Surface Reflectance (LSR) from Top of Atmosphere (TOA) data with geolocation information, which is a crucial input for SARA [40]. Therefore, remote sensing techniques play a vital role in understanding complex urban environmental systems and their impact on urban environmental quality [40]. Numerous studies have been conducted in recent years using satellite measurements to analyze air quality and pollution. Predicting particle concentration (PM) at the Earth's surface in areas or times without measurements is challenging but important for forecasting PM-related changes. One drawback is that PM is typically measured at ground level. Additionally, satellite data, such as AOD, may struggle to estimate particulate data due to factors like cloud cover, snow, or water pollution, as well as data calibration issues. To address these challenges, researchers have employed various techniques like kriging, weighted geographic regression, and artificial neural networks [42]. Comparing PM2.5 concentrations during orange alert periods to corresponding pollution episodes can provide valuable insights, especially in stable atmospheric conditions with high humidity and low wind speed. This comparison can help evaluate the impact of specific policies on reducing PM2.5 levels [43].

In ref [44] conducted a study titled "Study of geographical factors in air pollution in Tehran" to explore the impact of geographical structures on the deterioration of air quality in Tehran. The study identified topography, climate (south and west winds), population density (metropolitan area), industrial distribution (30% in the west, 54% in the south, and 16% in the east of Tehran), and the urban transportation network as the key factors influencing air pollution in Tehran. In a study titled "Pollution Emission List of Tehran," [45] investigated pollution levels and the contribution of different sectors to pollution in Tehran. The study revealed that mobile sources accounted for 618 thousand tons of the production of five air pollutants in Tehran in 2011, compared to 108 thousand tons from all fixed sources combined, indicating that 85.1% of pollution in Tehran is attributed to mobile sources, while 14.9% is from stationary sources. The environmental burden in Tehran is significant, with over 7000 deaths or 100,000 years of life lost and an economic cost of approximately USD 3 billion in 2017. Implementing effective air pollution reduction strategies could lead to substantial cost savings. It is crucial to develop and prioritize strong public policies to enhance air quality improvements [46]. A study on persistent convergent cross mapping exposure in Tehran from 2012 to 2017 revealed that annual PM2.5, PM10, and NO2 levels in Tehran consistently exceeded WHO air quality guidelines by 3.0–4.5 times, 3.5–4.5 times, and 1.5–2.5 times, respectively. Except for O3, all air pollutants showed lower concentrations in summer and higher concentrations in winter. Approximately 45 to 65% of the Air Quality Index (AQI) values in Tehran fell into the unhealthy for sensitive groups category, with PM2.5 being the primary pollutant responsible. Temperature was found to be the most influential meteorological factor affecting PM2.5 and PM10 concentrations, while cloud cover and solar radiation significantly influenced ambient SO2 and O3 levels. Additionally, there was a moderate relationship between wind speed and NO2 and CO concentrations [47].

The research background analysis in relation to the current article indicates that urban dispersion is a significant factor in environmental pollution, particularly air pollution, in foreign research. It has been found that urban dispersion leads to long distances between activity and residential spaces, necessitating personal transportation in metropolitan areas and increasing travel between these areas. Studies have shown that over 85 percent of air pollution in Tehran is caused by private vehicles, primarily within metropolitan boundaries and on a micro-scale. The current article focuses on air pollution in Tehran in the context of urban dispersion in the metropolitan area, specifically examining intra-city and inter-city transportation, distinguishing it from previous research. Data from the Tehran Air Quality Control Company in 2016 revealed a nearly triple increase in air pollution levels and a doubling of pollutant concentrations over the decade from 2006 to 2016. Environmental reports have shown that air pollution in Tehran has exceeded the world standard by 8.2 times in certain years, such as 2009. The main issue addressed in the study is the impact of scattered spatial development on air pollution in Tehran, resulting from high volumes of traffic between activity and residential centers within the Tehran Metropolitan Area. The study aims to answer the question: ‘‘What are the effects of suburban expansion on urban and suburban traffic flow in the Tehran metropolitan area, and how does this relate to air pollution?’’.

2 Research methods

2.1 Introduction of the study area

This study focuses on the Tehran metropolitan area, which encompasses the two provinces of Tehran and Alborz, functioning as a cohesive unit. As per the 2016 census, this area comprises two provinces, 18 cities, 59 municipalities, and 97 villages, spanning 18,814 square kilometers, representing 1.14% of the country's total land area. The Tehran metropolitan area (TMA) stands as the primary population hub, hosting 19.99% of the nation's population, as per the latest official census data from 2016. Analysis of the population distribution within TMA reveals a decline in Tehran’s population share from 85.64% in 1975 to 54.61% in 2016, while peripheral settlements have seen an increase from 14.36% to 45.4% during the same period [39]. The rapid expansion of suburban areas is evident through the establishment of 59 official cities and numerous rural centers, shaping a dispersed spatial layout in the surrounding region. A prominent characteristic of this geographical area is the daily commuting and traffic flow between "suburb to center," "center to suburb," and "suburb to suburb," predominantly reliant on private vehicles, leading to environmental pollution. Figure 1 illustrates these trends.

  1. 1.

    Data Collection and Preparation:

    • The current study is an analytical type that was conducted using official secondary data and quantitative research methods.

    • To analyze the spatial patterns of suburban transportation, raw traffic data from the Road Transport Organization (107 suburban axes) and raw data from the Transport and Traffic Organization of Tehran Municipality (22 districts of Tehran) were used in 2016.

    • Raw data from air pollution measuring stations and remote sensing technology in the ArcGIS software were also utilized.

  2. 2.

    Identification of Polluted Days:

    • Data from pollution monitoring stations in the metropolitan area were obtained.

    • Contaminated days were identified based on the available data.

    • These stations served as checkpoints in the analysis.

  3. 3.

    Selection of Study Days:

    • The study days (at 7 a.m.) were chosen based on the availability of Landsat (OLI) satellite images.

  4. 4.

    Geometric and Radiometric Correction:

    • Following geometric and radiometric correction, programming methods and multivariate regression algorithms were applied to the images.

  5. 5.

    Application of Algorithms:

    • Programming methods and multivariate regression algorithms were applied to the images.

  6. 6.

    Generation of Air Pollution Maps:

    • Air pollution maps were created using the Inverse Distance Intermediation (IDW) model.

    • The IDW model had a low (about 3.29 m) average model error rate (RMSE).

    • IDW is recognized as a model for effective air pollution assessment because it does not extrapolate values beyond the range of the observed data points.

    • This prevents the generation of unrealistic pollution values in areas where no monitoring data exists.

    • Therefore, IDW is a spatial interpolation method commonly used in GIS for various uses, including air pollution assessment.

Fig. 1
figure 1

Tehran Metropolitan Area

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In this paper, circles based on the distance from the Tehran metropolis's center of gravity as the focus of economic activity and population (CBD) in three major areas were determined. The first circle includes Tehran, which is located 17 km from the city’s geographical center to the city’s outskirts (the final limit of the city). The second ring, defined as the suburban area, is located 40 km from the first ring's outer edge. In this region, metropolitan areas have the highest concentration of employment and population. The third ring is defined as the suburban (Exurban) area, which is located 30 km from the outer edge of the second area [48] (Fig. 2).

Fig. 2
figure 2

Urban development rings (influential areas)

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2.2 Findings and analysis

2.2.1 Expansion of TMA and production of travel flow

Due to a combination of historical and socioeconomic factors, Tehran's rapid and unpredictable growth and uneven development in the metropolitan area have led to numerous environmental issues and their negative impacts. These issues extend both within and beyond the designated boundaries. The haphazard clustering of urban elements and transportation systems has created a situation where the natural connections between ‘‘man and the environment’’ and ‘‘man and man’’ are out of sync and unstable. According to Saeidnia, the primary sources of environmental challenges and constraints in Tehran are (1) the city’s sheer size and scope; (2) an imbalanced, scattered, and irregular physical layout in land use and networking; and (3) a centralized structure and the emergence of new hubs along transportation routes. The repercussions of Tehran's sprawling expansion on travel generation and absorption within the city and between cities in the metropolitan area are explored in more detail below.

2.2.2 A: Travels and traffic within the city of Tehran

The physical expansion of Tehran and increased travel between its regions have led to updated statistics on travel production and consumption in the city's 22 regions compared to a decade ago. According to data from the Deputy Transportation and Traffic Organization of Tehran Municipality (2015), the number of daily trips in Tehran rose from 14.6 million in 2004 to 18.3 million in 2015, marking a 25% increase in intra-city trips. The daily turnover reached 23.6 million. Public transportation accounted for about 60% of transfers, with 18% by metro, 20% by bus, 22% by taxi, and 7% by services. Reports on daily motor vehicle traffic show around 3 million vehicles traveling in Tehran daily, with areas 4, 12, 2, 5, and 6 experiencing the most traffic. Each region varies in terms of production and travel attraction, with regions 12 and 6 having the highest travel attraction, while regions 4, 5, and 2 have the highest travel production.

2.2.3 B: Travel and road traffic in the metropolitan area

Research conducted in 2016 on traffic volume and numbers in 107 main roads of Tehran Metropolitan Area (TMA) revealed that the western and southwestern axes experience higher traffic compared to other axes in terms of both volume and number of vehicles. The concentration of large populations and the development of satellite cities around the main city are particularly noticeable in the western part of Tehran's metropolitan area, especially along the main axes. The total registered traffic in the metropolitan area and the roads leading to Tehran in 2016 was 909,091,900. Furthermore, a study on the annual traffic volume in Tehran in 2016 indicated that over 1,000,093,000 trips were taken within the city. Comparing the total traffic, approximately 54.58% was within Tehran, while 45.42% was in the metropolitan area. It is important to highlight that the analysis of traffic distribution reflects a significant portion of Tehran's population within the overall population of the metropolitan area (Fig. 3).

Fig. 3
figure 3

The annual density of car traffic in the axes of TMA

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The analysis of high-density travel zones and pendulum traffic in relation to the three loops in this study shows that the central business district (CBD) has the highest concentration of employment and population, accounting for over 54% of traffic. The suburban area comes in second, with more than 95% of the population and employment located in these two areas. The Exurban area, the third ring, experiences the lowest traffic volume and density due to its lower population and employment concentration compared to the first two rings. Hot spots in the western axis of Tehran's metropolitan area and the third ring are linked to the Qazvin-Karaj freeway, connecting the country’s center to the west (Table 1).

Table 1 Landsat (OLI) satellite images
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2.3 Spatial monitoring of air pollution in the metropolitan area of Tehran

There are numerous studies on air pollution in Tehran, with data from various reports such as the Urban Collection Plan (2002), Tehran City Master Plan (2006), Tehran City Air Pollution Release Report (2013), and Tehran Air Quality Report (2011–2016). These reports indicate that over 85% of the city's air pollution is attributed to mobile sources like cars, taxis, motorcycles, vans, minibuses, buses, and trucks, while fixed domestic and industrial sources contribute only 15%. The total amount of pollutants produced in Tehran by these sources is estimated to be around 727,000 tons per year. The main pollutants from mobile sources are carbon monoxide (98%), nitrogen oxides (47%), sulfur oxides (6%), volatile compounds (86%), and particulate matter (70%). Additionally, heavy metal substances like Cu, As, Ni, Cr, Pb, and Mn, as well as carbonaceous components, are found in PM2.5 (Fig. 4).

Fig. 4
figure 4

Zoning of PM10 (right) and PM2.5 (left) pollution samples in 2014, 2015 and 2016 samples (7:00 AM)

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A study conducted on the PM2.5 index of particulate pollutants in Tehran from 2010 to 2016 indicates that PM2.5 has been the most prominent and challenging pollutant in recent years. (Table 2) illustrates the air quality in previous years in relation to suspended particles smaller than 2.5 microns in diameter. This pollutant consistently ranks as the primary contributor to air pollution in Tehran throughout the year, particularly from November to March.

Table 2 Status of (PM2.5) in Tehran between 2010 and 2016
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Table 3 shows the state of the air quality index in terms of particulate pollutants with diameters less than 10 microns (PM10) from 2006 to 2017. As can be seen, the number of infected days due to this pollutant was higher in 2008 than in previous years (24 days). In addition, 2011 had three very unhealthy days, and 2009 had one dangerous day. This is despite the fact that the year 2006, with 3 days exceeding the maximum allowable, had better conditions in terms of this pollutant than the other years under consideration (Fig. 4).

Table 3 Condition of suspended particles (PM10) during 2006–2017 (in terms of number of days)
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The zoning of air pollution intensity using satellite imagery (RS) and GIS yielded similar distribution patterns for both types of pollution in the three-time periods studied. The pollution extends from west to east on a wide axis, increasing sharply in the center of TMA before decreasing.

The analysis of contamination zones using triple loops reveals that the CBD and Suburban areas have the highest concentration of particulate matter less than 2.5 microns. In the Exurban area, pollution is also present due to the Qazvin-Karaj freeway axis, although at a lower level compared to the other zones. The primary cause of this pollution in urban and suburban regions is the increased traffic of fossil fuel-powered private vehicles. While efforts have been made to reduce this pollution over the past five years through measures such as improving fuel quality, particulate matter less than 2.5 microns remains a significant pollutant in Tehran. The spatial distribution of particulate matter less than 10 microns follows a similar pattern to that of particulate matter less than 2.5 microns, with higher concentrations in central and suburban areas.

A study conducted on three different days (February 23, 2014, February 26, 2015, and December 28, 2016) analyzed the traffic situation at 7 a.m. using three loops. The study found that the majority of traffic flows in one direction within the city of Tehran (CBD), accounting for over 54% of total traffic in the Tehran metropolitan area. The suburban area experiences the highest traffic density. The ring covering 40 km of the metropolitan area, located after the first ring in the western axis, has the highest population concentration (37.83%), employment (36.4%), and transportation, leading to the highest road traffic flows towards Tehran. Areas like Robat Karim, Andisheh, Mohammadshahr, Kamalshahr, Nasimshahr, Varamin, Qarchak, Pakdasht, Golestan, Mallard, Quds, Shahriyar, and Islamshahr have the highest population and density within this circle. The suburbs beyond the third ring have lower population density (2.22%) and employment density (2.31%), with main traffic flows coming from trans-regional axes such as Qazvin-Karaj, Saveh road, Qom highway-Tehran, and others.

The spatial correlation between transport index and air pollution in the two particulate matter indices of less than 2.5 and 10 microns for the three selected days previously mentioned indicates a strong correlation (over 75%). However, with the increase in transportation, there is also an increase in pollution. Table 1 illustrates the level of spatial correlation between the transport factor and the occurrence of air pollution (4). According to the table, the spatial correlation in the particulate matter index of less than 2.5 microns is above 78% on all selected days, showing a significant correlation compared to the particulate matter index of less than 10 microns (Table 4).

Table 4 Spatial correlation between transportation and air pollution (7:00 AM)
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3 Conclusion

Air pollution is a significant issue in many large cities, especially in the Tehran metropolitan area, with transportation accounting for over 85% of air pollution emissions. Understanding the key factors influencing air pollution is contingent on grasping the dynamics and spatial patterns of urban expansion. The theoretical urban literature suggests that urban sprawl leads to increased travel, including commuting between the city center and suburbs, as well as within suburban areas. It is worth noting that suburban development in developing countries differs from the traditional model seen in developed Western European nations. This study focused on air pollution in the Tehran metropolitan area within the framework of these theoretical concepts, yielding the following results:

  • (1) Today, the Tehran metropolitan area has a population of 15.98 million people, 54.61% of whom live in Tehran, and the rest in 58 cities and hundreds of settlements. The large expansion of the main city and the rupture of the surrounding areas, which have emerged at a rapid pace in the last 5 decades, is the main factor shaping inner-city and extra-urban travel; according to official statistics, from 2003 to 2014, the share of travel increased by 25%, with 23.6 million daily transfers. The daily traffic of over three million personal cars in Tehran, primarily in areas 4, 12, 2, 5, and 6, demonstrates the city's reliance on private cars for intercity travel. In addition, a traffic study conducted in 2016 on the TMA’s 107 main roads revealed that 909 million and 919 thousand vehicles traveled on these roads, with the main destination being Tehran.

  • (2) In the next order contract, more than 54% of traffic is done within the main metropolis and the suburban area, according to the study of dense travel zones and pendulum traffic in relation to the three rings of analysis in this study. Due to the obvious lower concentration of population and employment compared to the first and second rings, the third ring has the lowest number and density of traffic. The Qazvin-Karaj freeway, which connects the country’s northwest to the center, is responsible for the presence of hot spots in the western axis of Tehran’s metropolitan area and the third ring. Were relatively similar; in this way, they stretched from west to east in a relatively wide axis, and when they reached the center of Tehran's metropolitan area, their intensity increased, and they began to decline from there. The findings of this paper also demonstrated that the patterns of distribution and spatial accumulation of air pollution in the three analysis loops follow the spatial pattern of traffic flow, and the intensity of correlation in all cases is greater than 75%.

  • (3) The findings and results of this study align with those of [31, 45, 49] in relation to urban development, sprawl, and the expansion of air pollution over time and space. These studies highlight the impact of rapid urbanization on environmental challenges, with the loss of urban vegetation and green spaces due to the construction of large buildings. The shift towards car-based transportation and increased commuting between residential and work areas contributes significantly to air pollution, diminishing the ecological benefits provided by urban greenery.

The following recommendations can be made to solve the problem based on the findings of the current study:

  • It is suggested that transportation policies be developed and implemented in a continuous set in terms of infrastructure and the needs of the entire metropolitan area.

  • Since air pollution is formed in a unique natural context (western winds and topography) and at the city and regional scales, the suburbs, along with the main city (Tehran), play a role in the production of travel flow and air pollution, the proposal To solve this problem, integrated environmental policies and management can be developed and implemented at the metropolitan area scale.