Background of PM
Particulate matter (PM) is often comprised of organic and inorganic matter (Perez, et al. 129). Experts often use their mass and composition to categorize them into two main groups – fine and coarse particles (The University of Texas at El Paso 4). Generally, fine particles are PM2.5, while coarse particles are PM10 (the numbers refer to the diameter sizes of the particles). The particles often vary in size and chemical composition, depending on the type of source. Keuken, et al. (26-35) and Lenschow, et al. (31-32) say that PM consists of a complex blend of airborne material, such as inorganic salts and mineral dust. Some studies point out that PM could consist of water suspended in air, elemental carbon, trace elements, and organic matter (World Health Organization 262).
PM is a major cause of air pollution. The presence of smog in major world capitals, such as London, Beijing, and Los Angeles in the 1940s and 1950s, from the use of oil and coal, explains this concern (Cao, et al. 1499-1507). As such, legislators and researchers have expressed their concerns regarding the effects of PM and air pollution in the world. Following such concerns, since the early 1970s, there has been a global attempt to reinforce the concept or air quality and establish air quality standards. For example, in the late 1980s, the World Health Organization (243) was among the first global organizations to set personal exposure guidelines for different types of PM, including sulfur dioxide (SO2), nitrogen dioxide (NO2), and ozone (O3). Other organizations have used the same guidelines to enhance theirs. Epidemiology studies have supported these changes by demonstrating that high mortality rates are associated with high PM exposure (Perez, et al. 129-130). Indeed, the inorganic nature of some PM constituents and their possible toxic chemical properties are major causes of concern for health experts who highlight the negative health effects of PM (Weinstein, et al. 980-988).
Indoor air pollution comes from two main sources – indoor sources and outdoor sources. Road transport is the main source of air pollution in urban centers. However, different factors affect the concentration of pollutants. Some of them include meteorological conditions, traffic conditions and the topography of a region (Keuken, et al. 26-35). For example, canyon sites often have high emission levels, while plateau regions have low levels of emission. Indoor sources of emission often come from ordinary domestic activities, such as cooking and cleaning. However, their effects do not last more than a few hours (Pipal, et al. 3621-3630).
According to a report prepared by an Environmental Audit Committee (cited in the World Health Organization 262), industrial emissions of nitrogen oxides are 1.5 times greater than road transport emissions. The same measure is true for PM that has a diameter of up to 10 μm. Relying on UK-based studies, the Environmental Audit Committee says we cannot ignore the health effects of PM because it affects more than 50,000 people annually (World Health Organization 262; Harrison and Yin 17). For example, some mutagens and carcinogenic components of some PM are responsible for major respiratory diseases and ailments in human beings (Cao, et al. 1499-1507; Fuzzi, et al. 8221). Therefore, air pollution is a huge cause of concern for many scientists and environmentalists around the world (Calvo, et al. 1-28; Poschl 7521; AQEG 1-5). Relative to this fact, Marcazzan, et al. (4639–4650) say researchers have often explored the impact of different forms of pollution on human health and the environment to find unique ways to minimize their effects on the population. Part of this process involves understanding the origin of PM.
Origin of PM
PM could originate from different types of sources. According to Calvo et al., the main sources of PM are anthropogenic sources and natural sources. Anthropogenic sources of PM mostly come from human activities (Calvo, et al. 4). Lenschow, et al. (31-32) say there are three main sources of PM10 – combustion, natural sources, and traffic (which accounts for half of the emission sources of PM10). Some scientists believe that aerosol contaminants from artificial sources are more harmful to human health compared to contaminants from natural sources (Rao 42; Calvo, et al. 4; Pipal, et al. 3621-3630). Natural sources of PM may include sea spray and biogenic sources of contamination. Researchers say the latter often includes pollen fragments and particulate products of abrasion that often come from leaf surfaces and other parts of a plant’s anatomy (Rao 43-45).
Some studies describe the sources of particulate matter as either being mobile or stationery (Kong, et al. 155-165). Mobile sources of PM are often from moving sources, such as cars or movable machines. Stationary sources of PM come from burning fossil fuels for industrial, commercial, or residential purposes (Weinstein, et al. 980-988). Such emissions could produce secondary PM. Rao (42) provides a more accurate understanding of this statement by saying secondary PM often comes from chemical reactions of fossil fuels. According to the Committee on Research Priorities for Airborne Particulate Matter (62), the primary precursors of secondary PM include the combustion of fossil fuel from motor vehicle use and the reliance on industrial materials, which are products of fuel use as well. Pipal, et al. (3621-3630) point out that secondary PM also comes from residential wood combustion and the process of cooking meat. The PM that comes from these sources often contains complex mixtures of organic and inorganic substances.
Physical Characteristics of PM
Samara (99-100) assumes an abstract understanding of the physical characteristics of PM by saying they are solid or liquid particles suspended in the air. Depending on the origin and composition of PM, experts have given it unique names that symbolize different characteristics (Samara 99-100). Different researchers have also explained the varying physical properties of PM and found that they contain endless variability in space and time (Committee on Research Priorities for Airborne Particulate Matter 59). Although we analyze the chemical compositions of PM2.5 and PM10 in later sections of this paper, it is important to point out that their variability in shape and size stretches to their chemical composition as well (Pipal, et al. 3621-3630). Nonetheless, if we focus on the physical properties of particulate matter alone, we find that many researchers use particle size and shape as the main criteria for defining the physical properties of PM (Rao 42; Hueglin, et al. 637-640; Calvo, et al. 4; Pipal, et al. 3621-3630). The particle size often refers to the length of the PM’s diameter, while the particle shape refers to whether the crystal shape is regular or irregular. The particle size also refers to whether they are in liquid, semi-liquid, or in solid forms (Hinds 4).
In line with the quest to understand the physical characteristics of PM 2.5 and PM 10, Harrison and Yin investigated the measurements of both PM particles and said “Particulate matter can be classified as PM10 (50% cutoff aerodynamic diameters of 10 μm), PM2.5 (fine particles, 50% cutoff diameters of 2.5μm), coarse particles (PM2.5-10)” (17). Researchers have also used different methods of PM formation to classify the physical characteristics of PM. In this regard, they have come up with three size modes that include “nucleation mode (0.01-0.1 μm), accumulation mode (0.1-2μm) and coarse mode (>2μm)” (Harrison and Yin 17). Nucleation mode particles are usually small (about 1-2μm in diameter). However, coagulation and condensation allow them to grow in size. Consequently, the nucleation mode particles could grow to 20-30μm in diameter. Webb (15) says that particles in the accumulation mode cannot grow more than the above specifications through coagulation because their low concentrations do not allow them to do so. Their low rates of loss (because of wet and dry depositions) also limit them in this regard.
Coarse particles (PM10) have a shorter atmospheric time compared to finer particles (PM2.5). Cao, et al. (1499-1507) examined the optical properties of PM and found that their varying properties influence how they radiate. For example, the scattering efficiency for PM is highest for fine particles, thereby explaining why 10ug of fine particles (0.2<D<1 μm) scatter across a wider distance as opposed to PM that have a wider diameter, say D>2.5 μm (Weinstein, et al. 980-988). Comprehensively, it is important to point out that the physical characteristics of PM2.5 and PM10 vary because of different factors, including the type of source and the geographic sample region. Meteorological conditions also influence the physical properties of the PM particles.
Chemical Characteristics of PM
A study done by Mkoma, et al. (109-117) to investigate the chemical properties of PM2.5 and PM10 found that metals and ions were the main chemical properties of PM. The metals and ions were 21% and 16% of the sample (Mkoma, et al. 109-117; Alharbi and Shareef 88). Weather patterns affect the concentration of the metals and ions because the researchers found that PM concentrations were 84% higher in summer, compared to winter (Marcazzan, et al. 4639–4650; Mkoma et al. 109-117; Alharbi and Shareef 88). Some crustal matter species such as Fe, Mn, Ti, Ca+2, Mg+2 were also found to be higher during summer months, as opposed to winter months (Alharbi and Shareef 88).
A comparative study of the chemical properties of PM in residential and industrial locations reveals a substantial increase in Zn, Mn, B, Mg, Fe, and Al and the ions K+, SO4—, and Cl– at industrial locations, as opposed to residential locations (Alharbi and Shareef 88). Research studies have also revealed a strong correlation of Al, Fe, Mg, K and Mn compounds in many PM compounds, implying that they could be coming from a common source (Mkoma, et al. 109-117). Researchers that have done a deeper research on the same issue indicate that the source could be crustal mineral aerosols (Harrison and Yin 17). There is also a strong correlation between cations and anions, which are some common chemical properties of PM (Pipal, et al. 3621-3630). Alharbi and Shareef (88) explain this association by suggesting that it may imply the presence of CaSO4, (NH4)2SO4, KCl, and KSO4 in PM. Hinds (16-18) adds to this debate by saying that, to some extent, the association could also imply the presence of MgSO4.
Querol et al. (6547-6555) say that differences in ionic ratios could explain different chemical concentrations of PM. Particularly, the researchers draw our attention to the ionic ratios of SO4-2/NO3–, Ca+2/K+, and Ca+2/Na+ because they believe that they could best explain the differences in PM concentrations, depending on weather patterns and geographical differences of study (Querol, et al. 6547-6555). Relative to this assertion, X. Zhou, et al. (518-526) say that understanding the chemical composition of PM2.5 and PM10 should also involve understanding the fraction of carbon and sulfates in PM. More importantly, they draw our attention to the need to understand the percentages of elemental carbon and sulfates (X. Zhou, et al. 518-526; Shen 13). Relative to this line of analysis, H. Zhou, et al. (102-113) add to this debate by saying the ratio of organic carbon is often higher than that of elemental carbon during the summer periods. According to Pipal, et al. (3621-3630), the ratio of elemental carbon to organic carbon could be useful in estimating the secondary formation of organic carbon in the air.
Generally, the principal chemical properties of PM include sulfates, nitrates, and ammonium. The sulfate usually occurs as a product of atmospheric oxidation of SO2. Comparatively, the nitrate compounds emerge as products of the neutralization of nitric acid vapor (Kong, et al. 155-165). Usually, this process occurs through the form of ammonia, in the form of ammonium nitrate (NH4NO3) (Kong et al. 155-165). It could also occur through the displacement of hydrogen chloride from sodium chloride (Hueglin, et al. 637-651). The ammonium in PM does not occur in its natural form, but, rather, in the form of ammonium sulfate ((NH4)2SO4). It could also emerge in the form of a nitrate (NH4NO3) (Hueglin, et al. 637-651).
Sodium and chloride in PM are commonly found in seaside areas or coastal regions. This chemical component usually comes from sea salt. Elemental carbon is found in PM through the high temperature combustion of fossil fuel (Lenschow, et al. 31-32). Organic carbon usually emerges in PM as organic compounds. It could also occur from primary sources of pollution, such as industrial processes, motor vehicle emissions, or through secondary processes, such as the oxidation of volatile organic compounds (Cao, et al. 1499-1507). Water-soluble components in PM could also contain water as a chemical component. For example, ammonia, sulfate, ammonium nitrate, and sodium chloride contain water because they are water-soluble (Cao, et al. 1499-1507). These chemical compounds have the ability to absorb more water in highly humid environments, as opposed to less humid ones. By doing so, they turn from crystalline solids to liquid droplets. Generally, water is responsible for the unaccounted mass of PM2.5 and PM10 (Hueglin, et al. 637). In addition to the main chemical properties of PM described above, other minor chemical properties of PM exist in small proportions. Trace metals and trace organic compounds are the main components that fit this category (Committee on Research Priorities for Airborne Particulate Matter 59). Although they may be in small concentrations, experts say their chemical properties make them interesting to study as well (Hueglin, et al. 637-651).
Biological Aerosol Particles of PM
Biological aerosol particles are distinct from the other types of PM mentioned in this report because living organisms emit them. These particles are also smaller than other types of PM. Weinstein, et al. (980-988) explain that they can be less than one micrometer (0.00004″). To explain the different types of biological aerosol particles, Fuzzi, et al. say “PBAPs contain a large range of different biological components, including microorganisms (bacteria, archaea, algae and fungi) and dispersal material such as fungal spores, pollen, viruses and biological fragments that are directly emitted to the atmosphere from their sources” (8220). Because of their small size, these particles easily react to air currents. However, their rate of movement depends on meteorological conditions. Collectively, gravity, size and air density play a role in their movement.
Some common sources of biological aerosol particles are sewage matter, water, and soil, which, in turn, convey microbial pathogens, allergens, and endotoxins (Poschl 7521; AQEG 1-5). The concentration of these particles could change, because of dust storms or leaf blowers. The change in concentration may lead to different allergic reactions or infections. For example, some environmentalists have blamed high concentrations of Stachybotrys chartarum (a type of toxic mold) have for some deaths in certain parts of the world (Samara 99-100). According to Fuzzi, et al. (8221), the toxicity associated with biological aerosol particles often comes from their ability to act as disease transmitting vectors for human and animal diseases. Atmospheric processes could lead to the degradation of viruses (Webb 15-20). Photochemistry and reaction with radicals are examples of these processes, thereby leading to a lower toxic effect of biological aerosols away from their emission source. Researchers have identified CO2 concentrations and carbon concentrations as other variables affecting the toxicity of biological aerosol particles because they have found that high concentrations of CO2 and carbon compounds mean a high level of toxicity for biological PM (Mkoma, et al. 109-117; Fuzzi, et al. 8221). Nonetheless, differences in measurement and identification techniques have made it difficult for experts to estimate atmospheric concentrations of biological aerosol particles.
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