Urban aerosols are strongly anthropogenic in origin, with a definite combustion signature. Number concentrations usually exceed 105/cm3, and size distributions typically exhibit three modes: named nuclei, accumulation, and coarse. The constituents of urban aerosol comprise the full spectrum of compounds possible in the atmospheric aerosol: sulfate, nitrate, ammonium, elemental and organic carbon (EC and OC), and crustal compounds (silicon, aluminum, calcium, and iron oxides). These aerosols result from primary emissions (EC, OC, soil material) and gas-to-particle transformation of the oxides of nitrogen, hydrocarbons, ammonia, and S02.
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