A material balance on the chemical composition of atmospheric fine particle samples typically shows that the fine particle mass consists of a combination of organic compounds, elemental (black) carbon, sulfate ion, nitrate ion, ammonium ion, sea salt, soil dust and water. These chemically distinct particle constituents provide a very strong indication of the sources from which the fine particle mass originated. In addition, small quantities of trace metals and hydrogen ion may be present in fine particles at concentrations too low to account for most of the mass but at concentrations high enough to produce concern over the potential for undesirable effects (e.g. strong acid aerosols; heavy metals toxicity).

Direct emissions of particle-phase material from sources are referred to as primary emissions in order to distinguish them from the secondary fine particles that are produced from gas-phase precursors by atmospheric chemical reactions. Primary fine particles typically arise from combustion, from specialized industrial processes, and from sources of geological dust. Black elemental carbon particles are entirely due to primary emissions from combustion sources. In many cases, elemental carbon particle concentrations are dominated by diesel soot and wood smoke. Elemental carbon particles often act as a good indicator of atmospheric dilution and transport because they are not produced by atmospheric chemical reactions and are generated at particles sizes (typically about 0.2 m particle diameter) that do not deposit out of the atmosphere easily but instead disperse in much the same way as carbon monoxide (CO). Primary organic carbon-containing particles are emitted directly from more than 70 different types of mobile, stationary and fugitive source types. Prominent sources of fine organic aerosols include motor vehicle exhaust, food cooking (e.g. meat charbroliers), wood combustion, the organic carbon content of paved road dust, plus smaller contributions from airborne plant fragments, stationary oil and gas combustion, aircraft engines, fine particle tire dust, brake dust and cigarette smoke, to name but a few of the many possible sources. Fine particle organic compounds plus elemental carbon typically contribute 25-40% of the fine particle mass concentrations measured in cities.

A further contribution to primary fine particle concentrations consists of the fine particle fraction of the geological dust emissions to the atmosphere. While most of the mass of soil and road dust emissions to the atmosphere is found in coarse particles having aerodynamic diameter greater than 2.5 m, a few percent of the crustal dust emissions occur in the fine particle size range. It is not uncommon to find that perhaps 10% of an airborne fine particle sample is composed of geological material, with fugitive paved road dust being the most important source of this type in cities.

Sulfate-containing particles can be emitted directly from combustion sources; typically about 3$ of the sulfur content of fuels is emitted directly as primary particulate sulfate. However, the majority of the sulfates in the atmosphere arise from gas-to-particle conversion processes in which SO2 gas produced by combustion of sulfur bearing fuels is oxidized in the atmosphere to form sulfuric acid, ammonium bisulfate or ammonium sulfate particles. SO2 oxidation can occur by both homogeneous gas phase chemical reactions and heterogeneous processes within cloud or fog droplets. The gas phase homogeneous oxidation of SO2 occurs primarily due to reaction between SO2 and the hydroxyl radical, with additional contributions from ozone-olefin reactions products. The heterogeneous conversion of SO2 to form sulfates with liquid water droplets can occur due to reaction of dissolved SO2 with dissolved hydrogen peroxide, or ozone. In the presence of catalytic metals (Fe, Mn, and others), dissolved SO2 can be oxidized at appreciable rates by dissolved oxygen. Which of the various SO2 oxidation pathways dominates depends on the details of the local accumulation of co-pollutants such as ozone, hydrogen peroxide, hydrocarbons, ammonia, and catalytic metals. The important point to observe is that, given so many competitive pathways, SO2 oxidation usually is ongoing due to at least one or more of the available routes, generally producing overall conversion rates of a fraction of a percent per hour to 10% per hour or more. Over a period of several days, much of the SO2 in the atmosphere will be converted to sulfate if it is not removed first by wet or dry deposition at the earth's surface.

The reaction products of SO2 in the atmosphere are non-volatile and will not return to the gas- phase once formed. The speciation of the particulate sulfates as sulfuric acid, ammonium bisulfate, ammonium sulfate, or some combination of the above, is largely determined by the amount of ambient ammonia available to neutralize the acid sulfate species. In the Eastern United Stated, unneutralized acid sulfate particles are frequently reported, while in California and in other western locations the sulfate aerosols are commonly found to be fully neutralized, with excess ammonia remaining in the gas phase.

Aerosol nitrates are produced by reactions involving atmospheric nitric acid vapor. Nitric acid is formed in the atmosphere as a further reaction product of NO2. During the daytime, NO2 oxidation occurs by reaction with the hydroxyl radical. At night, NO2 can react to form nitric acid by pathways based on formation of the nitrate radical, NO3, followed by production of N2O5, which then can hydrolyze to produce nitric acid. The nitrate radical can also react with certain organic species to produce nitric acid. As nitric acid concentrations build-up in the atmosphere in the presence free ammonia gas, a critical point is passed at which the product of the nitric acid vapor concentration times the ammonia concentration in the gas phase exceeds the value of the equilibrium dissociation constant for ammonium nitrate aerosol. Since ammonium nitrate is a solid or liquid phase species, gas-to-particle conversion occurs at this point. The value of the equilibrium dissociation constant for ammonium nitrate is extremely sensitive to temperature and humidity, with more ammonium nitrate formation from the same quantity of ammonia and nitric acid vapor expected at low temperature and at high relative humidity. While heavy photochemical smog episodes greatly favor rapid nitric acid vapor production, such photochemical smog episodes often occur on days with very high temperatures that do not necessarily favor ammonium nitrate production form that nitric acid vapor. Instead at high temperatures the nitric acid vapor may well stay in the gas phase where it is available for relatively rapid deletion by dry deposition at the ground. Fine particle nitrate concentrations in the vicinity of 100 m/m3 over 24-hour averaging times have been observed in the eastern end of the South Coast Air Basin that surrounds Los Angeles during late October, a time of year with moderate photochemical smog but generally lower values of the equilibrium dissociation constant for ammonium nitrate. Just as ammonium nitrate formation can be encouraged by high nitric acid levels, high ammonia levels, low temperatures, and high relative humidity, ammonium nitrate aerosols once formed can be destroyed (returned to the vapor phase) if one or more of the key factors driving ammonium nitrate formation is removed from an air parcel. Thus ammonium nitrate contained in air parcels transported into the desert downwind of Los Angeles could be expected to evaporate if the gas phase nitric acid or ammonia that surrounds the particles is removed by dry deposition, or if temperatures increase or relative humidities decline, all of which are likely to occur in a desert environment. If the aerosol studied is a mixed salt composed of various proportions of sulfate, nitrate, ammonium, chloride, etc., all found in the same particles, then the above discussion of ammonium nitrate equilibrium with nitric acid and ammonia only needs to be replaced by a more complex description based on multi-component aerosol thermodynamics.

A second common route for aerosol nitrate formation involves nitric acid (and possible NO2) attach on the NaCl in sea salt particles to form NaNO3 aerosol and to release gaseous chloride species to the atmosphere. While most sea salt particles are in the coarse particle mode and are larger than 2.5 m in diameter, one should still look for the formation of some fine particle NaNO3. Further, the corresponding release of gaseous chloride species from the aerosol is another transformation route that can affect fine particle concentration and composition.

The final major route for formation of fine particles by atmospheric chemical reactions involves secondary organic aerosol production. Secondary organic aerosol production is the subject of much ongoing research. Two routes for secondary organics formation seem possible. The traditional view has been that secondary organic aerosol formation will occur when atmospheric chemical reactions produce organic compounds such as aliphatic dicarboxylic acids that have extremely low vapor pressures. Once these low vapor pressure products saturate the gas phase at the level of a few parts per thousand (ppt), any further chemical production will result in aerosol production. More recently, it has been realized that secondary organic aerosol production also can occur if organic gases dissolve into an existing organic liquid phase in the atmospheric particle complex. This organic liquid phase could be present due to primary organic aerosol emissions or due to previous secondary organic aerosol production. Because of the difficulties of distinguishing primary organic aerosols from secondary organic aerosols in an atmospheric sample, and due to the general lack of much organic aerosol data in the Eastern U.S., much less is known about the relative importance of secondary organic aerosol formation than is known about primary particle emissions and sulfate plus nitrate formation. In the Los Angeles area where data are available that can be used to draw some preliminary conclusions, it appears that secondary organic aerosol accounts for 15-30% of the total organic aerosol (hence 5-10% of the entire fine particle mass concentration) on an annual average basis, but can account for the majority of the organic aerosol present over short periods during severe photochemical smog episodes.