First systematic review on PM-bound water: exploring the existing knowledge domain using the CiteSpace software

Aerosol water is a master component of atmospheric aerosols and a medium that enables all aqueous-phase reactions occurring in the atmosphere. This integral chemical compound of suspended aerosol particles (PM) has become one of the hottest issues in recent years. To look for scientific productivity in the area of PM-bound water research a bibliometric analysis was performed. Most actual literature regarding aerosol and particulate bound water and implications of the research in this field was downloaded from WOS database using 1996–2018 timespan. Different bibliographic statistics were used to get a general profile of leading authorships, institutions, countries and mainstream journals providing most highly cited articles in the field. Using the CiteSpace software it was possible to identify past trends and possible future directions in measuring aerosol bound water. The search terms used in the database were {“aerosol” AND “water” OR “chemical mass balance”} AND {“particulate matter” OR “PM-bound water” OR “hygroscopic”}. The answers to the following questions were found: which authors, countries, institutions and aerosol journals to the greatest degree influenced PM-bound water research?. The network of co-occurring noun phrases was extracted from the set of publications, followed by co-citation analysis. The network was also clustered by top terms which gave a clear picture of topics most often undertaken. Finally the publication meeting eligibility criteria were looked for chemical compounds most frequently determined in PM-bound water research, which help to indicate works where quantitative assessment of PM-bound water was performed. Obtained results indicate that the paper with the greatest citation burst was Tang and Munkelwitz (J Geophys Res Atmos 99(D9):18801–18808, 1994). The largest number of articles in this specific field was published in Atmospheric Chemistry and Physics. An absolute leader in the quantity of publications among all research institutions is National Aeronautics Space Administration NASA. Meteorology and Atmospheric sciences is the discipline most occupied by highly cited journals in this field. Clustering results indicate that the research has mainly focused on hygroscopic measurement of aerosol, hygroscopic growth of particles; aerosol liquid water, and hygroscopic behavior. Most articles rather points PM-bound water as an artifact in organic carbon and ions measurements without detailed analysis of its contents or probable origin. The number of publications in each cluster of the build network is relatively high, which indicate that scholars have formed a rather consistent studies in the theme of aerosol-bound water. Despite a relevant role played by aerosol-bound water in atmospheric processes a quantitative description of its contents is rather rarely found in the literature (with the total number of only 23 papers concerning PM-bound water contents). In terms of yield, USA, China and Italy ranked highest, playing a propelling role in the research on PM-bound water. Future trends in PM-bound water research should be directed to a quantitative measurements of its contents; source apportionment, chemical composition of PM—modulating its hygroscopicity and therefore cloud formation processes, and the assessment of artefacts influencing the quality of PM-bound water measurements. Those areas should be especially developed in future studies and scientific projects concerning atmospheric water.


Introduction
Coal is the fossil fuel, which to the greatest extent influence the load and number of PM particles of anthropogenic origin. Particles released during combustion processes may bring serious environmental hazards, but also pollute the atmosphere and have adverse impacts on human health. Although the chemical composition of PM is rather well known, one of the PM compound has aroused special interest only in recent years. The latter is the presence of the PM-bound water, both the loosely adsorbed one and chemically bound to the structure of the PM particles (Canepari et al. 2017;Rogula-Kozłowska et al. 2017;Su et al. 2018). The strength of this bounding will vary depending on chemical composition of PM, but also on the location (PM origin) and can be easily delimited when testing thermal behavior of PM particles. The mass ratio between weakly and strongly bound water as well as PM hygroscopicity can be described by two key parameters: the deliquescence relative humidity (DRH) and the crystallization relative humidity (CRH) (Casati et al. 2015), crucial for a correct parameterization of the aerosol hygroscopic growth, used in different practical applications likewise remote sensing (D'Angelo et al. 2016). These parameters defines the relative humidity thresholds at which aqueous solid phase changes occursfrom solid to liquid saturated solution at DRH and CRH, respectively (Casati et al. 2015;Seinfeld and Pandis 2006). As RH increases PM compounds like crystalline soluble salts (NH 4 ) 2 SO 4 or NH 4 NO 3 undergo phase transition and become aqueous solution particles. Under decreasing relative humidity solution particle follows the equilibrium curve to the deliquescence point and remains dissolved in a supersaturated solution. When RH is still decreasing solution particle abruptly loses water vapor and return to initial crystalline form. Particles in the atmosphere are present as a supersaturated solution droplets (liquid particles) as well as solid particles. Generally the particle-bound water consist of weakly-bonded (vapor) water, condensing on aerosol particles when the relative humidity (RH) increases, and bounded-water remaining enclosed in PM compounds. Constitutional water-embedded in the compound undergo removal only under higher temperatures, or under the influence of dehydrating agents. Strongly bound (crystalline) water can be 1 3 released only during heating in a staggered manner; resulting in new solid phases formation (Widziewicz et al. 2018). Significant portions of water can be still present in PM particles even after equilibration, which means that simple conditioning before gravimetric measurements do not guarantee water removal. The amount of PM-bound water can vary from few to several tens of percent of the PM mass (Tang 1979). Many previously conducted studies tried to assess the hygroscopic properties of PM under different humidity conditions, however taking into account variable chemical composition of PM those properties will also vary from site to site. Many works focuses on estimation of thermodynamic properties of model (pure or mixed) aerosol solutions (mostly salts) (Tang 1979(Tang , 1997; much less studies conducted experimental measurements of particulate matter deliquescence and crystallization based on real PM samples (Casati et al. 2015). Another methods used for the assessment of PM-bound water contents are based on mathematical modelling methods (Tsyro 2005;Meng et al. 1995). According to the model estimates made by Tsyro (2005), the fraction of PM 2.5 -bound water at 50% RH (relative humidity) varied across Europe between 20 and 35%. Much lower water contents 10.6% for PM 10 and 13-23% for PM 2.5 were found in Switzerland under 50% RH conditions when using most popularly used mass closure method. Information regarding contribution of retained water to the unaccounted mass of PM is well described in the literature (Hueglin et al. 2005;Ho et al. 2006;Putaud et al. 2010;Perrino et al. 2013Perrino et al. , 2014Taiwo 2016); however its quantitative determination rarely done. The abundance of secondary inorganic components like nitrates and sulfates is one of the most important factors determining aerosol hygroscopicity (Świetlicki et al. 2008). PM-bound water also accelerate formation of secondary inorganic and organic aerosol and under high ambient RH levels could promote haze events frequency . The observational and theoretical analysis of the relationship between particulatebound water and/or secondary aerosol formation under smog episodes or haze events has been infrequently reported (Cheng at al 2016;Wu et al. 2018;Wang and Chen 2019;Ge et al. 2019).
In order to master the characteristics of PM-bound water this study adopts the CiteSpace bibliometric software to analyze the publications panning the time period of 1996-2018 in this field. In recent years many publications has appeared in the scientific market regarding PM-or more generally aerosol-bound water (Perrino et al. 2016;Rogula-Kozłowska et al. 2019). Many of them touch these issues indirectly. This strictly means that information regarding PM-bound water appears for example in articles related to physical and chemical characteristics of particulate matter including water-soluble ions (El-Sayed et al. 2018;Tsai and Kuo 2005) and water soluble carbon (Decesari et al. 2001;Duplissy et al. 2011), chemical mass balance (Tsyro 2005), thermodesorption of aerosol matter (Wittmaack and Keck 2004;Perrino et al. 2012), positive and negative artifacts in particulate organic carbon measurements (Subramanian et al. 2004;Canepari et al. 2013) and others. The least works in a manner remarkably describes a qualitative and quantitative methods for the determination of PM-bound water (Canepari et al. 2013(Canepari et al. , 2017Perrino et al. 2016), forms of water occurrence in PM and its origin (Perrino et al. 2012). At present, there are many software used for bibliometric (scientometric) analysis of bibliographic records of relevant publications. Among those most popular are HistCite, CiteSpace, Pajek, Sci2, BibExcel and Thomson Data Analyzer (TDA). Due to the user-friendliness, fast-computations result, a wide range of graphic tools CiteSpace is probably most often used. CiteSpace was designed to answer the questions about the structure and dynamics of a knowledge domain and its visualization. Through co-occurrence analysis and co-citation analysis on a large number of bibliographic records, it can explore the trends and patterns identified in the 1 3 knowledge domain and found some development trends of a particular study field (Chen 2013(Chen , 2016. Due to the growing public awareness regarding PM, its behavior in the atmosphere, the sources of its origin and health hazards the research on chemical composition of PM, including also its water contents has been growing in recent years (Tan et al. 2017;Canepari et al. 2017;Rogula-Kozłowska et al. 2017;Su et al. 2018). On the one hand it's important to measure the chemical components of atmospheric PM to estimate the climatic effects but also to know to which extent the presence of PM-bound water (non-hazardousness regarding human health) adds to the gravimetric mass of PM particles (Tsyro 2005). Such a knowledge would assist in formulating pollution control strategies for areas not in compliance with the PM standards.
This study aims to investigate current and most actual literature regarding aerosol and particulate bound water and implications of the research in this field from 1996 till 2018 based on the bibliometric technique. Different bibliographic statistics were used to get a general profile of leading authorships, institutions, countries and mainstream journals providing most highly cited articles in the field of PM-bound water.

Data description and CiteSpace analysis
The systematic review presented in this article followed the PRISMA guidelines. The search was performed using all databases in the Web of Science platform, including: "water" OR "chemical mass balance"} AND {"particulate matter" OR "PM-bound water" OR "hygroscopic"}. Research articles: 1488 and reviews: 25 • 1511 publications included after title review (number of papers in the dataset) • After duplicate record excluded • 1511 publications subjected to abstract review eligibility inclusion • study treat water soluble ions instead of PM-bound water • study treat water soluble organic carbon instead of PM-bound water • study treat particle growth or particle nucleation processes instead of aerosol water contents • study treat cloud formation or climate in general Fig. 1 Flow chart illustrating selection process of bibliographic records and selection criteria SciELO, Science Direct, Scopus, and few others databases. WoS is currently the most relevant scientific platform regarding systematic review needs, since it allows for accurate retrieval of records during successive and repeated searches, which means that search results are reproducible and reportable. The search terms used in the databases were {"aerosol" AND "water" OR "chemical mass balance"} AND {"particulate matter" OR "PM-bound water" OR "hygroscopic"} ( Fig. 1). The search was started using TOPIC search, which gave the overall number of 60,121 records. The effectiveness of our search was checked by comparing the number and quality of database outputs under different searching schemes and seeing how much overlaps there were in our findings. The used keywords "filter" in the TOPIC search narrowed the results to 1513 records (subjectively) most relevant to the research question. Records were divided into 6 groups for analysis: "mass closure", "artefacts", "qualitative and quantitative methods for water determination", "water soluble-ions", "Karl Fischer titration" and "other". Only original articles and reviews were included without short reports or letters, case studies, methodologies or books. The search was restricted only to English-language publications. The time span includes 1996-2018 years. Putting restriction regarding time span was based on a clear and strong reasoning. When looking into air quality criteria for Particulate Matter-some kind of gold standard in aerosol literature it can be easily noticed that most influential works regarding experimental measurements of particulate matter hygroscopicity and processes influencing particle hygroscopic growth but also condensational and dissolutional growth equations development were performed in the late 90' (Seinfeld and Pandis 1998;Lee et al. 1999;Ohta et al. 1998). Therefore 1996 was used as a starting point for bibliometric analysis.
There were only 101 records which meet an eligibility criterion ( Fig. 1, Table 8). The final database was created on 14 January 2019 and it includes articles, which can be generally described as those concerning aerosol-bound water topic. This database was generally treated as the output for the scientific domain and for organization purposes will be called "the dataset" or the "network".
The records creating "the network" were subjected to full text review. The aim was to extract from the network works that strictly concerned the issues of quantitative estimation of water contents in PM in the form of a percentage, mass or concentration units. This was started by looking into titles and abstracts before reading full texts of articles. During this process only publications with the term "water" in title, abstract or keywords were selected, which in fact met the final inclusion criteria. Flow chart ( Fig. 1.) illustrates the databases searched in this review, the resulting number of potential studies identified by this search; and the number and reasons for excluding studies based our pre-determined criteria. Final database include only 23 studies concerning the quantitative assessment of PM-bound water contents.
Each piece of finally selected data (from the network) was downloaded into EndNote database and converted into full-record text format. The analyses at the literature level were based on CiteSpace V. The starting parameters were as follows: (1) Time Slicing: 1996Slicing: -2018 (2) Years Per Slice: "1"; (3) Term Source: Title, Abstract, Author Keywords, and Keyword Plus; (4) Node Type: select the corresponding one each time; (5) Selection Criteria: the top 50, (6) Pruning: Minimum Spanning Tree and Pruning Sliced Networks; and (7) Visualization: Cluster View-Static and Show Merged Network.
Few parameters are used in this work in order to present the structure and distribution of scientific knowledge in terms of scientific metrics, visualizations and data summaries. The Centrality parameter is used to find and measure the importance of the term and mark the key point of node in purple circle in the knowledge graph. Burst identifies emergent interest 1 3 in a domain exhibited by the surge of citations and refers to terms, publications, authors, journals which is marked in red in CiteSpace. The TimeLine is an important indicator used to reflect the frontier and the trend in the knowledge domain and finally the publication records are being mapped in the form of Dual-map overlay showing the entire dataset in the context of a global map of major disciplines. Additionally this study conducts some basic and simple analysis on the annual quantity of papers, authors, disciplines, countries, journals, keywords and other research impact metrics related to atmospheric water and specifically PM-bound water topic.

Results and discussion
The starting number of publications in the interesting field sources from the Web of Science database (extracted by TOPIC search) was 60,121. After TITLE review only 1513 were included.

Data description by web of science metrics
The number distribution of publications through the period 1996-2018 is shown in Fig. 2. It is possible to observe how the publishing trend increased from 32 publications in 1996 to 128 publications in 2018, highlighting the actual interest on the topic. Over the past 23 years, it is possible to distinguish two stages. The first took place between 1996 and 2005, which shows rather slow increase reflected by the increase rate equal 1.5 compared to starting year. The second phase shows some acceleration which ran from 2006 up to 2018, with a higher growth rate, indicating the growing interest in this specific research domain. The second increase in the number of generated papers coincides well with the expansion phase in scientific development of mass closure models used for determining inconsistencies between chemical characterization and gravimetric PM mass (Tsyro 2005;Rees et al. 2004).
Approximately 33% of the publications in the dataset were published in the top 7 international aerosol journals (Fig. 3). The greatest number of papers was published in Atmospheric Chemistry and Physics with the total number of papers equal 144 (more than 9% of total papers in the dataset as reported by the red-line trend). This gives an overview on the distribution of potential research interests in the topic of aerosol bound water. Atmospheric Chemistry and Physics is dedicated to the publication of high-quality studies investigating the Earth's atmosphere and the underlying chemical and physical processes occurring in the atmosphere (5-year IF = 5.689). Such great number of studies published in most top and reputable journals (taking into account journal ranking on atmospheric sciences) confirms the importance of aerosol-bound water for both-the scientists but also the scientific audience. It must be however remembered that the influence of journal impact is not directly proportional to the number of published papers, therefore the influence of the journal in the interesting field cannot be judged based on IF factor or number of papers alone. Table 1 presents the top 15 contributing institutes. National Aeronautics Space Administration NASA with the number of records 59 (3.9% of all papers in the dataset) lead the list, followed by University of California System (3.7%) and the Chinese Academy of Sciences (3.6%). Among the top 15 institutions, 9 are American, followed by 4 European and 2 Chinese. However, more than 29% of the total records belong to the USA.
Table 2 ranks countries that contribute the most to the aerosol-bound water research. In terms of publications number USA is at the forefront accounting the 29.8% of the total dataset, with 450 records, followed by China with 205 records (13.6%). Germany, England and Japan occupy, respectively, the 3rd, 4th, and 5th place, with a cumulative number of publication equal to 370. The institutions with more publications in this topic mainly consist on research institutes and universitiesm which ensure high level of study on aerosol bound water.
Every journal covered by Web of Science core collection is assigned to at least one of the subject categories. The top 10 most-cited subject categories for aerosol-bound water topic are reported in Table 3. The most important research fields are Meteorology Atmospheric Sciences; Environmental Sciences and Engineering Chemical with the record count: 61; 50 and 9 respectively.   Engineering Civil 5 10 Engineering Mechanical 5 1 3 The most cited authors by the number of citations (year of publication, number of counts) were Tang and Munkelwitz (1994, counts 223); Seinfeld and Pandis (1998, 207), Saxena and Hildemann (1997, 150), Petters and Kreidenweis (2008, 141), Gysel et al. (2003;140) (Fig. 4). As presented in Fig. 4 there is a strongly cooperative relationship between most cited authors; for example Saxena P. and Seinfeld J.H; Svenningson and Kanakidou. This collaboration is reflected by measuring co-occurrence of pairs of top-terms in the article titles. For example Svenningson is affiliated with Lund University (Sweden) and undertake many research studies regarding particulates hygroscopicity, cloud formation and Po Valley fog, together with the researchers from Italian National Research Council; while Kanakidou works in the Department of Environmental Chemistry, in University of Crete (Greece) and her research interests focus mostly chemical and physical aspects of aerosols composition. Despite the geographical distance that separates Sweden and Greece the cooperative relationship between those authors regarding PM-bound water interests is rather strong. The more two authors are co-cited the more they are related. This is also a simply measure of authors occurrence in the same articles as a co-authors. A red marks on Fig. 4 indicate that all those authors have a burst of terms with relatively great impact and high level of attention from the scientific audience in a certain period of the study process like for example in case of articles: Seinfeld and Pandis (1998) "Atmospheric Chemistry and Physics from air pollution to climate change" and Saxena and Hildemann (1996) "Water-soluble organics in atmospheric particles: A critical review of the literature and application of Fig. 4 The node-link diagram of authors co-occurrence together with marked author burst (red nodes) (different colors on the map indicate different year of the first cooperation between authors, where purple color indicate longer cooperation). Node size represents the overall amount of citations (by author) and colored rings show author citation by time-slice. The network is clustered by top terms founded in the publication titles. The link strength between two nodes refers to the frequency of terms co-occurrence. (Color figure online) thermodynamics to identify candidate compounds". The key nodes in this network are "hygroscopic growth study"; "water aerosol particle"; "liquid aerosol particle"; "water soluble organic aerosol"; "hygroscopic behavior"; "aerosol liquid water"' "soot aerosol particle" and "water vapor", indicating that they have strong influence in the whole network.
When comparing information included in Table 4 and one presented in Fig. 4 we came to the conclusion that the influence of the author on the study on aerosol bound water should not be determined by the number of publications or citations alone.
One of the most important factor reflecting the overall success of the paper is its authorship by author with the strong citation burst (Fig. 5). The list of the most cited authors in terms of citation burst does not necessarily correspond with the list of most references with the highest burst (see Fig. 8). Speaking other words the author with the greatest burst is Zhang (burst = 11.47) (Fig. 5), while the most cited authors (in terms of number of citations) are Saxena and Hildemann (1996) (Table 6).

Categories of co-occurring subject categories founded in the database
In order to compared results presented by WOS citation and journal metrics we created the map of co-citation network at category (discipline) level (Fig. 6). The database includes 76 nodes and 337 links (purple links indicate longer cooperation between disciplines), which generally could be categorized into 5 groups: meteorology and atmospheric-related (on the left); environmental and ecology (plant)-related (left bottom); physical chemistry and physics-related (center); chemistry-related (center); pharmacology and pharmacy related (right bottom) and material science-related (upper right). This categorization corresponds well with WOS metrics presented in Table 3. The biggest nodes correspond to the categories occupied by most cited journals and characterized by greatest centrality-for example soil related journals on the left; atmospheric chemistry and physics journals (center). Other words they are the hottest disciplines in terms of aerosol bound water research. Meteorology and Atmospheric Sciences (cluster #4) together with Environmental Sciences (cluster #9 and #22) and Chemistry (cluster #5 and #6) are a key disciplines in terms of relevant publications about the study on aerosol bound water, and the connections with chemistry and physics indicate that PM-bound water topic will be widely applied among those categories in the future studies, with strong interactivity among different disciplines (Fig. 6). However connections between thermodynamics (cluster #7), optics (cluster #15) and plant sciences (#21) is rather weak. For example articles building cluster #15 ("optics") only points atmospheric water presence as an artifact when measuring PM concentration by means of optical sensors. Figure 7 presents the citation history of Journals. The most cited journals are: Journal of Geophysical Research Atmospheres; Atmospheric Environment and Geophysical Research Letters (most colored nodes in the network). As one can easily noticed, the number of publications in the Journal (Fig. 3) does not perfectly correspond to the number and frequency of their citations (Fig. 7).
Based on the ranks of articles according to WOS statistical metrics the highest cumulated number of citations were assigned to the following papers (Table 5). The most active area of the research: citation burst Table 6 shows top 25 references with strongest citation bursts during the period between 1996 and 2018. The reference characterized by strong citation burst refers to the situation when a sudden increase in the cited frequency of the reference occurs at a time point or time period. This strictly means that citation burst is an indicator of extraordinary degree of attention from the scientific community. Citation burst contains two dimensions: the burst strength and the bursting time. Papers characterized by high values in the "Strength" column can be considered as relevant in the aerosol bound water field. The "Topic" column was filled by hand and it summarizes the content of each publication. "Burst period" column is the period when citation burst evolves. Our analysis found that the article with the strongest citation burst (12.854) is Saxena P. and Hildeemann L.M. "Water-soluble organics in atmospheric particles: A critical review of the literature and application of thermodynamics to identify candidate compounds", 1996, which was one of the few landmark papers published by this author. Its burst lasted for 5 years from 1999 to 2004. Tang In. and Munklewitz H.R. "Water activities, densities, and refractive indices of aqueous sulfates and sodium nitrate droplets of atmospheric importance", 1994, has the second strongest burst of 11.904 from 1996 to 2002. Usually the most recent papers have weaker citation bursts. Several articles like for example Zappoli et al. 1999;Shulman et al. 1996started their citation bursts just 1 year after publication. It is also worth noting that eight over 25  Table 6, are related to the hygroscopic properties of atmospheric aerosols and hygroscopic growth, and this constitutes a major topic.

Networks of co-occurring terms
The word term refers to noun phrases extracted from the text of a bibliographic record or a full text document. We therefore generate a series of networks of co-occurring terms (nouns, keywords, title words); those terms are one appearing in the same document, keywords section, title, respectively and connected in the network. Figure 8 presents the network of co-occurring noun phrases. Only terms related to articles with a number of counts greater than 10 were included. The terms are labeled by crosses, while most important articles by nodes (for better visualization the most hot terms were moved onto right). More "violet" crosses and nodes are those characterized by higher burst. Its therefore easily to observe that the initial "relative humidity"; "hygroscopic Bold font: papers characterized by high citation burst (> 5) and large number of citations 1 3 properties"; "hygroscopic growth" and "aerosol particles" phrases occurs in the paper written by Petters MD and Kreidenweis SM., Atmos. Chem. Phys., 7, 1961-1971, 2007, entitled "A single parameter representation of hygroscopic growth and cloud condensation nucleus activity"; but also in Kanakidou et al. Atmos. Chem. Phys., 5, 1053-1123 "Organic aerosol and global climate modelling: a review", and coincidence with the following phrases "water vapor", "degree", "(m 3 air µg −1 )", "water uptake", "differential mobility analyzer", "hygroscopic behavior" and "hygroscopic growth factor". Hygroscopic properties of sub-micrometer atmospheric aerosol particles measured with HTDMA instruments in various environments-a review published by Świetlicki et al. Tellus (2008), 60B, 432-469 is additionally enriched by "chemical composition" phrase. First studies concerning hygroscopic properties of aerosols were conducted in early 80's (Deliquescence properties and particle-size change of hygroscopic aerosols By: Tang In, ISSN: 0065-7727 Issue: APR, Published: 1979) and in the late 80's they were strongly developed together with the development of devices/methods used for measuring aerosols hygroscopicity and liquid water mass of aerosols like for example: tandem differential mobility analyzers (TDMA), aerodynamic particle seizers (APS), particle into liquid samplers, mist chambers, Karl Fischer titrators, cloud condensation nuclei counters (CCN), Droplet Measurement Technologies (DMT) CCN counters, and finally satellite methods (Table 9). In this last time interval, pollutants with a strong, still active burst are related to the fine particulate matters (PM 2.5 ) and size fractionated PM suggesting the shift of the attention by the academic from the coarse particle nodes to submicrometer one.

Co-citation network
Such network represents a number of references that have been co-cited by a set of papers. A time period is divided into 23 1-year time slices, and an individual co-citation network is derived from each time slice. Every single time-slice network is very complex. In order to reduce the dimension of every slice, the top 50 most cited publications in each year are used to build a network of co-cited references in that particular year. Subsequently, individual networks are combined into single visualization (merged network). The publications in the presented visualization extend from early 70's to the present. Figure 9 presents the resulting network of co-occurring co-citations and additionally the development of the aerosol-bound water topic over time, showing the most important footprints of the related research activities. Each node marked by concentric rings represents a cited reference. The thickness of a ring is proportional to the number of publication citation in a given time slice, while nodes color represents the year of the first co-citation. In presented network the dominant color is yellow corresponding to citations which were first made in the time frame 1996-2018. As can be observed from Fig. 9 the most important publications corresponding to the biggest nodes on the graph with the highest citation frequency from Petters and Kreidenweis (2007) and Kanakidou et al. (2005).
In the next step we make some sense of the nature of major clusters in merged network that may inform us about the stage of the underlying specialties. In the interesting example, a total of 262 clusters of co-cited references are identified. The modularity Q of 0.8025 is rather high, which means that the network is reasonably divided into loosely coupled clusters or in other words the specialties in aerosol bound water domain are clearly defined in terms of co-citation clusters. The mean silhouette score of 0.3102 suggests that the homogeneity of these clusters on average is not very high. It's mainly due to the presence of numerous small clusters. The major clusters that we will focus on in the review are sufficiently high. In this study, we consider a cluster as the embodiment of an underlying specialty. Thus, science mapping consists of multiple specialties that contribute to various aspects of the domain. The areas of different colors indicate the time when co-citation links in those areas appeared for the first time. Areas in violet were generated earlier than areas in yellow. The links depict co-citations. More prominent links are from the original search (Fig. 9). Figure 10 represents the map of the merged network, which in fact reflects the development of aerosol bound water topic over time, showing the most important footprints of the related research activities. This analysis focuses on a network of cited authors connected by co-citation links (more information: Chen et al. 2010). Each node represents a cited reference. To characterize the nature of an identified cluster, we extract noun phrases from the titles. Each term represent some specialties in a scientific field, which nature is a fundamental challenge for gathering information regarding aerosol water science. This analysis indicate that "mixing state"; "internal mixture"; "water-soluble organic carbon" and "inorganic salt aerosol" have largest nodes and therefore are some kind of focuses in this field. This analysis also shown that the study focus has changed over time from "water solubility"; "liquid aerosol particles" and "water vapor" into "hygroscopic behavior"; "water soluble organic carbon" and "hygroscopic growth studies" and broadly understood diverse chemical "mixing state" of aerosols.
For better visual reception the merged network was clusterized (Fig. 11) Top-terms were used to extract information, and the LSI algorithm was used as the calculation method to obtain clustering results; all information regarding single clusters were summarized in Table 7. The table was divided into 5 columns. The "size" column refers to the number of publications within the cluster, the "silhouette" value tells about the homogeneity of a Fig. 10 Landscape view of the resulted network cluster (it ranges between − 1 and 1; higher values indicate meaningful clusters), "the mean year" is the period in which the cluster evolves.
Among 63 clusters found, 13 effective clustering tags were obtained (with silhouette score 0.71-0.99). Cluster #0 (hygroscopic properties) was the largest one, followed by cluster #1 (hygroscopic properties), cluster #2 (critical supersaturation), cluster #3 (aerosol liquid water) and cluster #4 (hygroscopic behavior). The terms used in last column (Table 7) gives us a clear picture of what the cluster is about. The terms find indicate that biggest clusters are about processes of hygroscopic measurement of PM, which is not surprising considering that one of the leading searching quotes during articles acquisition from WoS was keyword: "hygroscopic". To create a better picture of cluster character the last column should be extended by other representative terms with the numbers of LLR (log-likelihood ratio) next to them. Configuration of clusters on Fig. 11 indicate that airborne aerosol size distribution or liposome aerosols creates some thematically (or semantically) separated areas while hygroscopic growth; hygroscopic properties; aerosol liquid water are thematically related areas.
A dual-map of journals was also displayed (Fig. 12), where left map depicts cited journals while the right map-citing ones (the journal in which a source article is published is called a "citing journal; while the journal in which a reference is published   . 12 The dual-map overlay of the science mapping literature regarding aerosol-bound water in time slices 1996-1997 (a); 1998-1999 (b) and 1996-1998 and 2013-2016 (c) is called a "cited journal"). The lines, which start from left to right, are citation links. This dual map overlay indicate that most articles on PM-bound water were published in physics, materials, chemistry, ecology, earth, marine, veterinary, animal science, mathematics, systems, mathematical journals and they mostly cited works from chemistry, material, physics, environmental, toxicology, nutrition, plant, ecology, geology, geophysics, systems, computer, computing. Dividing time span into single-slice overlays (Fig. 12a-c) indicate that citation years are rather similar regarding disciplines most engaged into PM-bound water research. Two parallel trends can be distinguished when speaking about development of the PM-bound water field-the first concern environmental-chemical approach, the second can be described as health trend (health, nursing, medicine disciplines). The dataset including 101 publications were subjected to full text review. This analysis outlined the past and present in PM-bound water research. Water content was monitored in 24 studies, although only 23 among them presents results in the form of mass, percentage or concentration values. Among 20 different pollutants selected (Fig. 13), the concentrations of organic carbon/organic matter were most investigated with the number of studies-44 (Table 8) and had the longest history of investigation (1973). Water soluble ions (for example Cl − , NO 3 − , SO 4 2− ) take the second place in this ranking with the number of studies-42. PM 10 and PM 2.5 were determined in 32 and 29 studies, respectively). Water content treated as unidentified mass of PM but also determined as aerosol water uptake or hygroscopic growth measurement was investigated in 20 studies (water associated with inorganic ions or organic carbon wasn't included in this summary) (Table 8, Fig. 13). Articles regarding quantitative measurements of PMbound water were reviewed and used to gather information regarding methodology used for PM-bound water determinations (Table 9). Most of those studies (22) used experimental methods for the assessment of water contents. Since PM-pollution is not randomly assigned across locations, presented experiments do not adequately control for a number of potential confounding determinants of PM-bound water and therefore they cannot be treated as semi (quasi) experiments. Only two works used data from EMEP  Table 8. Note All articles taken into bibliometric analysis were published in the time-span 1996-2018, however some of them presents data collected before this period; therefore 1987 and 1992 investigations appeared on the graph When the paper is based on modelling results or experiments performed in laboratory conditions (without any field measurements) usually the column "period" cannot be filled              1 3 measurements and therefore they were classified as descriptive one. When speaking about method design-10 studies used modeling methods, mostly mass closure. There is rather no diversity in pollutants investigated since 1987. However we can easily observe that among PM 2.5 and PM 10 most often investigated pollutants are inorganic ions together with EC and OC. Humidity and temperature are rather rarely found parameters (Fig. 13). What is also interesting more intensive PM-bound water measurements started in early 20's and before this times only few single publications were found regarding aerosol water contents. It's probably due to analytical limitations.

Study focuses and frontiers
When comparing the results of highly cited publications (Table 5) in the aerosol water domain with those having strongest citation burst (Table 6) it was possible to distinguished few papers which can be treated as a reference for research focuses and trends. Papers characterized by striking study results and high level of attention were marked by bold font (Table 6). It was found that they are generally groupable into two main categories: chemical compounds contributing to water soluble PM ("Compounds that are likely to contribute to the water-soluble PM fraction" section) and source apportionment of PM-bound water ("Source apportionment of the PM-bound water" section).

Compounds that are likely to contribute to the water-soluble PM fraction
Molecular composition of the water-soluble fraction of PM is rather sparse and incomplete (Saxena et al. 1995). There is whole spectrum of compounds which are likely to contribute to the water-soluble fraction of PM. Compared to inorganic compounds of PM, like sulfates, nitrates or ammonia, the concentrations, composition, and formation mechanisms of its organic compounds are not well understood. It's mostly because no single analytical method is capable of analyzing the entire mirage of those compounds. Since water-soluble organic compounds (WSOC) could account for 20-90% of the total carbon (depending on PM origin and sampling locations) (Karthikeyan and Balasubramanian 2005;Wozniak et al. 2008) proper determination of the relative contribution of individual water soluble organic compounds to the total WSOC mass is very important. To get an in depth information regarding WSOC, these compounds should be isolated from PM. It must be however remembered that not all WSOC compounds could be extracted using single extraction solvent or same extraction method. Gas chromatography-mass spectrometry (GC-MS; Mayol-Bracero et al. 2002;Wang et al. 2006) and a combination of ion chromatography and high performance liquid chromatography (HPLC; Yang et al. 2004) characterized less than 10% and 20% of WSOC, respectively. Therefore only small part of WSOC could be analyzed at the molecular level (Wei et al. 2019). The most frontier research in this area is to know to which extent the water content of atmospheric particles is influenced by the presence of organics and how the aggregate hygroscopic properties of inorganic particles are altered when organics are also present (Saxena et al. 1995). Along with the development of this research area, the 1 3 new PM-bound organic compounds are gradually discovered. In terms of water soluble PM fraction, the majority of studies are about the Water-soluble Ions (Haywood et al. 2011;Guo et al. 2010;Hsieh et al. 2011).

Source apportionment of the PM-bound water
Generally water solubility of the different classes of aerosol components changes along with the aerosol origin. The percentage of water soluble species with respect to the total soluble aerosol mass is much higher at the locations of air stagnation also influenced by strong anthropogenic emission of gaseous precursors like SO 2 and NO x . In such conditions a very high fraction (over 70%) of aerosol compounds consisted of polar species (Zappoli et al. 1999;Majewski et al. 2018). Knowing that fact, particle size could be increased by several times through water uptake therefore influencing aerosol formation mechanism, its interaction with radiation, or the human health effects both: the liquid water content of size-resolved aerosol together with its source apportionment are a study focus today. Studies of source apportionment (SA) for PM-bound water have enhanced understanding of dominant pollution sources mostly influencing PM water uptake and quantification of their contribution to overall PM hydrophilicity. The contribution of single emission source to the water mass concentration of PM can be now determined by thermal ramp technique (Canepari et al. 2013).

Factors confounding PM-bound water measurements
There are few unsolved problems regarding PM-bound water measurements reflected by the number of articles focusing on artifacts. It's hard to answer the question which measurements methods and which conditions favors those measuring uncertainties. Among probable sources of uncertainties aerosol scientist lists the following one: water content differs from filter to filter significantly. Therefore, in order to determine water content of a PM sample one must know the water content of this particular filter. Mass closure methods most often used for the quantitative measurements of PM bound water usually underestimates or overestimates the reconstructed mass. The reason for this phenomenon might be attributed to non-inclusion of strongly water-bound component and also the adopted conversion factors for estimating organic matter and crustal material (Harrison et al. 2003;Terzi et al. 2010). The problem of the model overestimation may be directly link to positive sampling artifacts. According to Turpin and Lim (2001), overestimation of particulate OC may result from adsorption of organic vapor onto quartz-fibre filter material. Also approximations in the determination of the OC/OM and metal/metal oxide conversion factors can be a significant sources of uncertainty (Terzi et al. 2010). Estimation of water contents by Karl Fischer titration methods is also non free from generating artifacts. KF method suffers from the interference of some classes of compounds, both organic and inorganic (EPA 2007, Method 9000), some of which are likely found in PM samples (aldehydes, ketones, carbohydrates, Fe(III) and Cu(II) salts). Another confounding factor could be filter material. There is no certainty which material is best for water predictions in PM (Rogula-Kozłowska et al. 2017). In terms of gravimetric analysis with beta attenuation method glass or quartz fiber filters are generally preferred, while Teflon filters are preferred for gravimetric determinations because of their higher insensitivity to relative humidity during the procedure (Brown et al. 2006).

Methods used for the measurement of PM-bound water
Till day no organization has established any guidelines for air quality regarding atmospheric water. Since water compound is treated as non-criteria pollutant its rather obvious. Less understandable is the lack of preferred methodology for determining the water content in PM particles. Water content in PM mass could be on significant level-even 40% of its mass. Therefore to improve the accuracy of the gravimetric analysis it's of great importance to create technical guideline for the measurement of water contents. It's also very important to established most preferable conditions for PM collection, preventing the condensation of liquid in the form of mist or droplets on PM material. In the final network there were 23 no. of studies in the subject of PM-bound water measurement.
In 80's and 90's PM-bound water measurements were almost completely focused on simple thermo-gravimetric methods. Based on prepared summary (Table 9) in 9 papers the quantitative determination of water was done by means of mass closure method, same number of papers presents aerosol water contents determined by Karl Fischer titration method, 4 papers used well known thermodynamic models like: ISORPIA, EQUSAM or SCAPE. The hygroscopic tandem differential mobility analyzer (H-TDMA) was used in only 3 from 23 found papers.
To better understand the PM-bound water topic an analysis of co-occurring keywords in the final network was performed. This analysis relied on final database including 23 papers. To understand how it is related to other topics a timeline view was used (Fig. 14). It was shown that in addition to top-terms (Fig. 10) the keywords that have the highest centrality and additionally biggest nodes (square shape) were: PM 2.5 ; chemical composition, aerosol; atmospheric aerosol; mass closure, PM 10 and urban. According to the evolution of keywords urban air and aerosol appeared in 1996; while hygroscopic growth in 2018; aerosol water content in 2009; deliquescence in 2004; liquid water in 2018; chemical mass closure 2006. None among these works poses strong burst. It was also evident that 2003-2009 period had a high concentration of nodes.
In next step the keywords were grouped in the synonym categories. For example hygroscopicity/hygroscopic or Karl Fischer titration/analysis/measurement and so on and the final frequency of keyword categories occurrence was calculated (Fig. 14).
The co-occurring keywords reflect research hotspots in PM-bound water research field. As shown in Fig. 15, a simple analysis of co-occurring keyword frequencies was obtained by counting the frequency of specific key word occurrences in the group of all analyzed key words. The keywords with the highest importance were those directly 1 3

Fig. 14
The timeline of keywords occurrence in the topic of PM-bound water connected to water ("PM-bound water/water content/atmospheric water") (50%); "particulate matter" occupied the second place in this classification (43%). Mass balance and mass closure were also widely studied in PM-bound water research studies (36%).

Conclusions
Presented study provides a systematic bibliographic review for aerosol researchers to achieve a good understanding of how PM-bound water scientific field evolve over last 23 years. Based on visually encoded signs and tabular summaries it recognize potentially insightful patterns concerning atmospheric water, and synthesize various information regarding its presence in the atmosphere, different chemical reactions responsible for particle nucleation and identify past trends and possible future directions in quantitative measurements of aerosol bound water. This study adopts the CiteSpace bibliometric analysis to discuss the most important focuses and trends of aerosol and PM-bound water in terms of publications, authors, countries, institutions, disciplines and type of journals. By using appropriate tools we indicate that the study on aerosol-bound water in this time span has experienced a rather steady and slow development trend and the attention in this field began to rise more rapidly in 2005. In terms of total publications number USA and China have the highest productivity in this field. Zhang XQ.; Andrews E.; Hansson HC.; Ferron GA.; McMurry PH have made comparatively great contributions to the field of study on aerosol-bound water with strong influence reflected by high author citation burst. The most influential article (reflected by the overall number of citations) in the time span 1996-2018 is Świetlicki et al. (2008), which concern the hygroscopic properties of submicron atmospheric aerosol particles measured with H-TDMA instruments. Based on social network analysis (SNA) National Aeronautics Space Administration NASA (USA) together with University of California (USA) and Chinese Academy of Sciences (China) are the most influential institutions in this field, which statement could be reflected by the largest quantity of publications in terms of study on aerosol-bound water and thus they take the leading position. In spite of the relevant role played by aerosol-bound water and its contribution to atmospheric visibility, aerosol optical depth (AOD), climate and health, a quantitative determination of adsorbed water was attempted only in 23 papers. Through the bibliometric analysis and visualization analysis in the field of PM-bound water from 1996 to 2018, Fig. 15 The frequency of keyword occurrences in the final database regarding PM-bound water measurement it is found that most of these papers are drawn in Italy and China, but there is less cooperation among researchers and among research institutions from USA. In case of atmospheric water this situation is opposite-USA institutions contribute most to the research and encourage domestic research institutions to strengthen the research investment in this discipline. At the same time this means that we should also encourage the cooperation of Polish research institutions to undertake this important topic, as well as try to strengthen a cross regional and transnational cooperation between Italy, China and USA, which are now more advanced in the field of aerosol and PM-bound water. Researchers in Poland should firmly grasp the frontier and hot spot of PM-bound water research, and carry out in-depth research in this advantageous field. The relevant studies on aerosol and PM-bound water mainly focus the disciplines of Meteorology and Atmospheric Sciences together with Environmental Sciences and Ecology and involve Chemistry, Physics and other disciplines, with strong interdisciplinary characteristic. Journal of Geophysical Research Atmospheres; Atmospheric Environment and Geophysical Research Letters are three major journals with the most prominent scientific achievements, largest quantity of publications and highest citation number in the field of PM-bound water. According to the analysis of relevant study indicators (high citation burst and most actual topic) water-soluble organics in atmospheric particles and source apportionment of the PM-bound water are a study focuses today.
There are two main contributions behind this study. The first is information visualization based on CiteSpace, which is an important tool for tracking new advances in the PMbound water research; the second one is that this article is a good base for future comparative reviews. The contents of atmospheric water understood as both water vapor as well as water bound to solid atmospheric particles has always been a public concern, since its presence in the atmosphere influence the climate and by changing air humidity and particle size it directly affects human health. Analysis of most important keywords or top-terms occurrence indicate that there is still lack of works regarding PM-bound water relations with smog or haze events. Future trends in the discipline of PM-bound water will probably developed in few directions: proper quantitative measurements of its contents; humidity conditions that particles experience in the atmosphere determining their behavior, the chemical composition of aerosol particles determines their ability to take up water, positive or negative errors affecting PM-bound water measurement. Inevitably, the weak points of the study is the data source strongly depending on initial searching criteria selected arbitrary by authors. More number/or different types of keywords might be selected in future research.