Aerosol generation in the urban environment (Invited Presentation)

Atmospheric particulate matter consists of directly emitted particles (soot, dust, sea-salt, etc.) and material formed in the atmosphere by gas-to-particle conversion processes (secondary aerosol). Atmospheric oxidation of sulfur dioxide, nitrogen oxides, and organic vapors produces aerosol sulfates, nitrates, and secondary organic aerosol respectively. The current understanding of the role of these processes in urban environments and the remaining challenges are discussed.

High sulfate concentrations in urban areas in Southern and Central California are shown to be the result of heterogeneous sulfur dioxide oxidation in low clouds and fogs. Mathematical models assuming that all cloud droplets have the same chemical composition tend to underpredict the sulfate formation in these environments. Size-resolved cloud chemistry models can explain the additional sulfate formation.

The formation and evaporation of aerosol nitrate remains one of the most challenging urban aerosol problems. Laboratory studies indicate that the reversible nitrate mass transfer is quite rapid, with timescales less than 20 minutes for submicron particles. The latest thermodynamic models are quite successful in reproducing the phase transitions and liquid water content of atmospheric inorganic particles. A combination of thermodynamic and dynamic modeling approaches is necessary for the simulation of the atmospheric aerosol size/composition distribution.

Formation of secondary organic aerosol is a two-step process. First, atmospheric oxidation of the gaseous organic precursors (mainly aromatics, but also biogenic terpenes, large alkanes, etc.) and production of low volatility vapors. The second step is the dissolution of these products in the organic aerosol phase. Recent advances in understanding these processes will be summarized and the current ability to reproduce the ambient observed organic aerosol concentrations will be discussed.