the emitting sources or particle precursors and also on atmospheric conditions. Some PM components—Including nitrate species, organic species, and water—are semivolatile and repartition between the gas and particle phase depending on environmental factors such as temperature, relative humidity, or the composition of the PM. For modeling and monitoring purposes the composition of dry atmospheric PM is generally reduced to a few major categories. Commonly identified components include sulfates, nitrates, organic carbon, elemental or black carbon, sea salt, soil or crustal material, and specific elements of interest, such as Pb. The health effects associated with exposure to these individual components are not well characterized and are likely to vary signficantly.

Primary and Secondary Sources PM is emitted from both natural and anthropogenic sources, and its components are both primary (directly emitted) and secondary (formed in the atmosphere). Direct natural emissions come from wildfires, sea spray, and resuspension of organic matter such as leaf litter. The first of these produces primarily PM2.5, while the latter two are mainly PM10. Mineral dust has both natural and anthropogenic origins: it is lofted from arid and semiarid regions and can be mobilized by agricultural or construction activities. Its emission rates are especially susceptible to climate conditions. Combustion of fossil fuels and biofuels is a large primary anthropogenic source. Combustion processes are the only sources of black carbon, which together with “brown” carbon1 has an important role in PM light absorption. Sources of secondary PM precursors (gases leading to particulate matter through atmospheric reactions) include gaseous vegetative emissions, motor vehicle emissions, and wood-smoke emissions. Reduced sulfur and nitrogen compounds are oxidized to the particulate components sulfate and nitrate, respectively. Ammonium is a common cation (positively charged ion) incorporated from the gas phase into the particle phase to neutralize these acid secondary species, although sodium, calcium, and other cations derived from sea salt or minerals are also often present.

Mass or Number? Regulations and many measurement strategies have focused on characterizing the mass concentrations of PM, but for some health and welfare effects size or number concentrations may be the more relevant characteristic. This question is still unresolved. As discussed above PM differs from gaseous pollutants because it is a complex mixture with


“Black carbon” refers to combustion-generated carbonacous aerosol that strongly absorbs visible light. “Brown carbon” refers to carbonaceous particles that are optically in between strongly absorbing black carbon and nonabsorbing organic carbon, formed largely by inefficient combustion of hydrocarbons.

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