Aerosol is condensed-phase material suspended as discrete particles in a gas. Atmospheric aerosol, termed particulate matter (PM), is critically important because of its impact on climate, visibility, biogeochemical cycling, atmospheric reactivity, and human health. PM diameter ranges from several nanometers to several micrometers (Willeke & Whitby, 1975; McMurry et al., 2000); larger particles settle by gravity too rapidly to be considered aerosol. Smaller groups of molecules are described as gas clusters. PM size is described by a continuum size distribution in which dN/d(lnD_p) (# cm^-3) represents the number concentration of particles in a size bin between lnD_p and lnD_p + dlnD_p, where D_p is particle diameter (assumed spherical) (Whitby & Sverdrup, 1980). By adding up the particles in all bins, the total number of particles per cm³ is found. Number concentrations of particles range from tens of particles (remote) to millions of particles (urban) per cm³, indicating the importance of proximity to sources in determining particle number concentrations (Pandis et al., 1995).

Particle sizes are determined by the mechanism through which the particles enter the atmosphere. For example, homogeneous nucleation, in which a new particle is formed directly from the gas phase, creates particles on the order of a nanometer in diameter (Hirsikko et al., 2011). These particles grow via coagulation or gas-to-particle conversion to hundreds of nanometers. Nucleation and gas-to-particle conversion of material are examples of secondary aerosol formation processes. In contrast, aerosol material emitted to the atmosphere directly is known as primary. Generally, particles with diameters smaller than 100 nm are known as ultrafine or Aitken-mode particles (Seinfeld & Pandis, 2006). Collectively, all particles smaller than 2.5 micrometers (PM_2.5) are known as fine aerosols. Particles with diameters between 2.5 and 10 micrometers are correspondingly known as coarse and typically are primary particles generated by physical-mechanical processes (Seinfeld & Pandis, 2006). Aitken-mode particles quickly coagulate and grow due to condensation of vapor species. Because coarse particles settle from the atmosphere relatively rapidly, particles accumulate in the diameter range of hundreds of nanometers and reside in what is known as the accumulation mode (Seinfeld & Pandis, 2006).

Globally, natural sources of primary aerosol dominate the mass emissions of PM. Because approximately 75% of the Earth's surface is ocean, sea salt is the biggest contributor (Lewis & Schwartz 2004). There are two types of ‘sea salt' particles. Jet drops consist of a brine core (dominated by sodium and chloride) surrounded by an organic film (Lewis & Schwartz 2004). These particles are formed from wave crashing and other mechanical processes. In contrast, film drops are primarily organic and formed from less harsh mechanical processes such as bubble bursting through the sea surface microlayer (Lewis & Schwartz 2004). Another natural source of primary aerosol is soil dust, generated primarily through wind action in arid regions (Mahowald et al., 2005). Soil dust particles consist of mineral materials. Primary biological aerosol particles that consist of plant waxes, pollens, spores, and similar material also are natural examples of PM (Despres et al. 2012).

Anthropogenic sources of primary PM include industrial and combustion processes. The nature of industrial particles depends on the process, but combustion particles generally are dominated by black or elemental carbon and heavy organic material such as polycyclic aromatic hydrocarbons (Bond et al. 2007).

Nucleation of new particles is one formation route for secondary particles. It is believed that sulfuric acid (H₂SO₄), ammonia (NH₃) and other amines, and water are involved in this process (Benson et al., 2011; Kirkby et al. 2011), with speculation that organics also participate (Vaattovaara et al. 2006).

Gas-to-particle conversion (partitioning to existing PM via adsorption or absorption) is another pathway through which secondary PM is formed (Pankow, 1994). Species such as H₂SO₄ that have a low vapor pressure can relatively easily undergo such a phase transition. Secondary inorganic PM also can form thermodynamically (salt formation). For example, NH₃ can combine with H₂SO₄, nitric acid (HNO₃), or hydrochloric acid to form ammonium salts when the concentrations of these precursor gases are large enough (Tang & Munkelwitz 1994). Because of the hygroscopicity of many inorganic species, water is often taken up, increasing particle size and mass (Petters & Kreidenweis 2007).

Secondary organic aerosol (SOA) is more complex. It is assumed that a mixture of liquid organics provides a medium into which SOA species are absorbed (Pankow 1994). In general, it is believed that the species that undergo this process are functionalized (e.g., carbonyls, carboxylic acids, alcohols, nitrates) oxidation products of primary volatile organic compounds that are present in the atmosphere as a result of both natural and anthropogenic processes (Hallquist et al. 2009). Oligomerization of species in the condensed phase leads to humic-like substances (Kalberer et al. 2004). In addition, certain species heterogeneously react at the surface of a particle and are converted into material that remains in the condensed phase (reactive uptake) (Kroll et al. 2005).

Secondary PM also forms through cloud processing. Clouds continuously undergo evaporation/condensation cycles. During a period in which clouds have significant water content, (partially) soluble species are absorbed from the gas phase into the droplets. Once there, they can be oxidized into species with decreased volatility, such that they remain as particles once the cloud droplets evaporate. This route is believed to be relevant for SOA (Carlton et al. 2006) and secondary sulfate formation, the latter of which is catalyzed by the presence of certain metals in cloud water (Hoffmann & Calvert 1985). An overview of the chemical and physical characteristics, growth and loss mechanisms, and sources of particles in the troposphere is given in Figure 1.

Particles cause direct and indirect climate effects. The direct effect on climate results from altering the global radiation balance through the scattering and absorptive properties of particles in the atmosphere (Charlson et al. 1992). PM also influences cloud formation and lifetime. In the upper troposphere, particles serve as ice nuclei (IN) through both homogeneous freezing (direct freezing of a particulate liquid solution) and heterogeneous freezing (freezing caused by the contact of separate liquid and solid phases) mechanisms. Larger, insoluble particles tend to be the most efficient IN, as they provide a lattice upon which ice forms. In the lower troposphere, soluble particles of the appropriate size activate to cloud droplets by behaving as cloud condensation nuclei (CCN) (Twomey 1974; Albrecht 1989; DeMott et al.2010). Activation occurs when particles are exposed to supersaturated water vapor (relative humidity in excess of 100%) and take up enough water to grow to a stable cloud droplet. As the number of CCN increases, the size distribution of the cloud droplets shifts to smaller sizes, increasing cloud reflectivity and lifetime. These cloud-particle connections generally increase Earth's albedo, leading to a change in the planetary radiation balance. Although uncertain, it is believed that this overall aerosol indirect effect (~-0.7 W m^-2) combined with forcing associated with the direct effect (~-0.5 W m^-2) is on the same order of magnitude as, but opposite in sign, to the climate forcing caused by greenhouse gases (~+1.66 W m^-2), as shown in Figure 2 (Intergovernmental Panel on Climate Change, 2007).

Because of their optical properties, particles significantly impair visibility (White & Roberts, 1977). Accumulation mode particles typically reside in a size domain of approximately 300-400 nm. This corresponds to the wavelength of the visible portion of the solar spectrum. According to Mie theory, particle scattering is enhanced when particle size and light wavelength are similar (Bohren & Huffman 1983). Scattering prevents light from reaching an object in the distance and getting reflected back to a viewer's eye, leading to visibility degradation. Visibility degradation is quantified through an extinction coefficient (b_ext). In the Beer-Lambert law, b_ext and a distance determine the decrease in intensity of light of a given wavelength over that distance.

Because of suspension, transport, deposition, and resuspension, particles provide a mechanism by which material is transported between various environmental media and locations. For example, a small part of the nitrogen cycle includes emission of nitric oxide (NO) from soil (Kulmala & Petaja, 2011). The NO is oxidized in the atmosphere to HNO₃. As described above, HNO₃ can combine with NH₃ to form ammonium nitrate salt. This particle deposits to the surface at some location downwind, completing the cycle between the atmosphere and the Earth's surface.

The presence of particles in the atmosphere also affects chemical processes that can occur. For example, in an urban area into which marine air is transported, HNO₃ displaces chloride from sea salt, leading to the liberation of chlorine and the formation of sodium nitrate (Gard et al. 1998). The presence of particles provides a surface upon which heterogeneous reactions occur (George & Abbatt 2010). The curvature of particles also enhances photolytic reactions relative to homogeneous photolysis reactions (Nissenson et al. 2006).

The primary motivation behind the establishment by the United States Environmental Protection Agency (USEPA) of National Ambient Air Quality Standards (NAAQS) for both PM_2.5 and PM₁₀ (based on mass of particles per volume of air) is the human health consequences suffered by the exposed population. A positive correlation between statistical rates of morbidity and mortality and ambient PM mass concentrations exists (Schwartz et al. 1996).

Inhalation and deposition to the lungs of PM cause or exacerbate cardiovascular issues. Particles that are small enough also cross tissue barriers into the bloodstream and are transported throughout the body. Questions remain regarding the link between human health impacts and specific constituents because of the complex mixture typically associated with PM. It is also possible that the USEPA will move toward a number concentration-based PM NAAQS (Reche et al., 2011). The smallest particles dominate the number concentration of particles but contribute very little to the total particle mass concentration. Therefore, any standard based solely on mass concentration may not protect the population from health impacts of the numerous particles that do not contribute significantly to mass concentration.

Significant research over the past decades has elucidated much about the chemistry, physics, and impacts of tropospheric aerosols. However, as the possibility of a number-based NAAQS is considered, it is imperative that processes that greatly influence particle number, such as homogeneous nucleation, continue to be investigated. The probability of more stringent mass-based NAAQS for PM indicates the need to continually focus on emission control of primary PM, the chemistry leading to secondary PM, and best practices for secondary PM precursor control. In addition, it is critical that scientific uncertainty in understanding the roles that aerosols play in climate forcing and human health be reduced.

Aerosol is condensed-phase material suspended as discrete particles in a gas. Atmospheric aerosol, termed particulate matter (PM), is critically important because of its impact on climate, visibility, biogeochemical cycling, atmospheric reactivity, and human health. PM diameter ranges from several nanometers to several micrometers (Willeke & Whitby, 1975; McMurry et al., 2000); larger particles settle by gravity too rapidly to be considered aerosol. Smaller groups of molecules are described as gas clusters. PM size is described by a continuum size distribution in which dN/d(lnD_p) (# cm^-3) represents the number concentration of particles in a size bin between lnD_p and lnD_p + dlnD_p, where D_p is particle diameter (assumed spherical) (Whitby & Sverdrup, 1980). By adding up the particles in all bins, the total number of particles per cm³ is found. Number concentrations of particles range from tens of particles (remote) to millions of particles (urban) per cm³, indicating the importance of proximity to sources in determining particle number concentrations (Pandis et al., 1995).

Particle sizes are determined by the mechanism through which the particles enter the atmosphere. For example, homogeneous nucleation, in which a new particle is formed directly from the gas phase, creates particles on the order of a nanometer in diameter (Hirsikko et al., 2011). These particles grow via coagulation or gas-to-particle conversion to hundreds of nanometers. Nucleation and gas-to-particle conversion of material are examples of secondary aerosol formation processes. In contrast, aerosol material emitted to the atmosphere directly is known as primary. Generally, particles with diameters smaller than 100 nm are known as ultrafine or Aitken-mode particles (Seinfeld & Pandis, 2006). Collectively, all particles smaller than 2.5 micrometers (PM_2.5) are known as fine aerosols. Particles with diameters between 2.5 and 10 micrometers are correspondingly known as coarse and typically are primary particles generated by physical-mechanical processes (Seinfeld & Pandis, 2006). Aitken-mode particles quickly coagulate and grow due to condensation of vapor species. Because coarse particles settle from the atmosphere relatively rapidly, particles accumulate in the diameter range of hundreds of nanometers and reside in what is known as the accumulation mode (Seinfeld & Pandis, 2006).

Globally, natural sources of primary aerosol dominate the mass emissions of PM. Because approximately 75% of the Earth's surface is ocean, sea salt is the biggest contributor (Lewis & Schwartz 2004). There are two types of ‘sea salt' particles. Jet drops consist of a brine core (dominated by sodium and chloride) surrounded by an organic film (Lewis & Schwartz 2004). These particles are formed from wave crashing and other mechanical processes. In contrast, film drops are primarily organic and formed from less harsh mechanical processes such as bubble bursting through the sea surface microlayer (Lewis & Schwartz 2004). Another natural source of primary aerosol is soil dust, generated primarily through wind action in arid regions (Mahowald et al., 2005). Soil dust particles consist of mineral materials. Primary biological aerosol particles that consist of plant waxes, pollens, spores, and similar material also are natural examples of PM (Despres et al. 2012).

Anthropogenic sources of primary PM include industrial and combustion processes. The nature of industrial particles depends on the process, but combustion particles generally are dominated by black or elemental carbon and heavy organic material such as polycyclic aromatic hydrocarbons (Bond et al. 2007).

Nucleation of new particles is one formation route for secondary particles. It is believed that sulfuric acid (H₂SO₄), ammonia (NH₃) and other amines, and water are involved in this process (Benson et al., 2011; Kirkby et al. 2011), with speculation that organics also participate (Vaattovaara et al. 2006).

Gas-to-particle conversion (partitioning to existing PM via adsorption or absorption) is another pathway through which secondary PM is formed (Pankow, 1994). Species such as H₂SO₄ that have a low vapor pressure can relatively easily undergo such a phase transition. Secondary inorganic PM also can form thermodynamically (salt formation). For example, NH₃ can combine with H₂SO₄, nitric acid (HNO₃), or hydrochloric acid to form ammonium salts when the concentrations of these precursor gases are large enough (Tang & Munkelwitz 1994). Because of the hygroscopicity of many inorganic species, water is often taken up, increasing particle size and mass (Petters & Kreidenweis 2007).

Secondary organic aerosol (SOA) is more complex. It is assumed that a mixture of liquid organics provides a medium into which SOA species are absorbed (Pankow 1994). In general, it is believed that the species that undergo this process are functionalized (e.g., carbonyls, carboxylic acids, alcohols, nitrates) oxidation products of primary volatile organic compounds that are present in the atmosphere as a result of both natural and anthropogenic processes (Hallquist et al. 2009). Oligomerization of species in the condensed phase leads to humic-like substances (Kalberer et al. 2004). In addition, certain species heterogeneously react at the surface of a particle and are converted into material that remains in the condensed phase (reactive uptake) (Kroll et al. 2005).

Secondary PM also forms through cloud processing. Clouds continuously undergo evaporation/condensation cycles. During a period in which clouds have significant water content, (partially) soluble species are absorbed from the gas phase into the droplets. Once there, they can be oxidized into species with decreased volatility, such that they remain as particles once the cloud droplets evaporate. This route is believed to be relevant for SOA (Carlton et al. 2006) and secondary sulfate formation, the latter of which is catalyzed by the presence of certain metals in cloud water (Hoffmann & Calvert 1985). An overview of the chemical and physical characteristics, growth and loss mechanisms, and sources of particles in the troposphere is given in Figure 1.

Particles cause direct and indirect climate effects. The direct effect on climate results from altering the global radiation balance through the scattering and absorptive properties of particles in the atmosphere (Charlson et al. 1992). PM also influences cloud formation and lifetime. In the upper troposphere, particles serve as ice nuclei (IN) through both homogeneous freezing (direct freezing of a particulate liquid solution) and heterogeneous freezing (freezing caused by the contact of separate liquid and solid phases) mechanisms. Larger, insoluble particles tend to be the most efficient IN, as they provide a lattice upon which ice forms. In the lower troposphere, soluble particles of the appropriate size activate to cloud droplets by behaving as cloud condensation nuclei (CCN) (Twomey 1974; Albrecht 1989; DeMott et al.2010). Activation occurs when particles are exposed to supersaturated water vapor (relative humidity in excess of 100%) and take up enough water to grow to a stable cloud droplet. As the number of CCN increases, the size distribution of the cloud droplets shifts to smaller sizes, increasing cloud reflectivity and lifetime. These cloud-particle connections generally increase Earth's albedo, leading to a change in the planetary radiation balance. Although uncertain, it is believed that this overall aerosol indirect effect (~-0.7 W m^-2) combined with forcing associated with the direct effect (~-0.5 W m^-2) is on the same order of magnitude as, but opposite in sign, to the climate forcing caused by greenhouse gases (~+1.66 W m^-2), as shown in Figure 2 (Intergovernmental Panel on Climate Change, 2007).

Because of their optical properties, particles significantly impair visibility (White & Roberts, 1977). Accumulation mode particles typically reside in a size domain of approximately 300-400 nm. This corresponds to the wavelength of the visible portion of the solar spectrum. According to Mie theory, particle scattering is enhanced when particle size and light wavelength are similar (Bohren & Huffman 1983). Scattering prevents light from reaching an object in the distance and getting reflected back to a viewer's eye, leading to visibility degradation. Visibility degradation is quantified through an extinction coefficient (b_ext). In the Beer-Lambert law, b_ext and a distance determine the decrease in intensity of light of a given wavelength over that distance.

Because of suspension, transport, deposition, and resuspension, particles provide a mechanism by which material is transported between various environmental media and locations. For example, a small part of the nitrogen cycle includes emission of nitric oxide (NO) from soil (Kulmala & Petaja, 2011). The NO is oxidized in the atmosphere to HNO₃. As described above, HNO₃ can combine with NH₃ to form ammonium nitrate salt. This particle deposits to the surface at some location downwind, completing the cycle between the atmosphere and the Earth's surface.

The presence of particles in the atmosphere also affects chemical processes that can occur. For example, in an urban area into which marine air is transported, HNO₃ displaces chloride from sea salt, leading to the liberation of chlorine and the formation of sodium nitrate (Gard et al. 1998). The presence of particles provides a surface upon which heterogeneous reactions occur (George & Abbatt 2010). The curvature of particles also enhances photolytic reactions relative to homogeneous photolysis reactions (Nissenson et al. 2006).

The primary motivation behind the establishment by the United States Environmental Protection Agency (USEPA) of National Ambient Air Quality Standards (NAAQS) for both PM_2.5 and PM₁₀ (based on mass of particles per volume of air) is the human health consequences suffered by the exposed population. A positive correlation between statistical rates of morbidity and mortality and ambient PM mass concentrations exists (Schwartz et al. 1996).

Inhalation and deposition to the lungs of PM cause or exacerbate cardiovascular issues. Particles that are small enough also cross tissue barriers into the bloodstream and are transported throughout the body. Questions remain regarding the link between human health impacts and specific constituents because of the complex mixture typically associated with PM. It is also possible that the USEPA will move toward a number concentration-based PM NAAQS (Reche et al., 2011). The smallest particles dominate the number concentration of particles but contribute very little to the total particle mass concentration. Therefore, any standard based solely on mass concentration may not protect the population from health impacts of the numerous particles that do not contribute significantly to mass concentration.

Significant research over the past decades has elucidated much about the chemistry, physics, and impacts of tropospheric aerosols. However, as the possibility of a number-based NAAQS is considered, it is imperative that processes that greatly influence particle number, such as homogeneous nucleation, continue to be investigated. The probability of more stringent mass-based NAAQS for PM indicates the need to continually focus on emission control of primary PM, the chemistry leading to secondary PM, and best practices for secondary PM precursor control. In addition, it is critical that scientific uncertainty in understanding the roles that aerosols play in climate forcing and human health be reduced.