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Organic species are important constituents of tropospheric particulate matter in remote, rural, and urban areas. Such aerosol can be primary (emitted in the particle phase as solids or liquids) or secondary (formed in situ as condensable vapors) in nature. Secondary organic aerosol (SOA) is formed when products resulting from the gas-phase oxidation of a parent organic species partition to the particle phase. This partitioning can occur via condensation onto existing inorganic aerosol (heterogeneous-heteromolecular nucleation), absorption into an existing organic aerosol, dissolution to the aerosol aqueous phase, or homogeneous-heteromolecular nucleation.

SOA yield is defined as the amount of SOA formed per the amount of a parent organic species that is oxidized. This yield depends functionally on stoichiometric and partitioning coefficients for each of the oxidation products formed and the total amount of organic aerosol mass available to act as absorptive media. Appropriate yield parameters are developed for a series of parent organics using smog chamber experiments. The effects of parent organic structure and the oxidizing species on SOA yield are also examined during the smog chamber experiments. Such yield parameters are used to model SOA formation from the oxidation of biogenic organic species on a global and annual scale. Yield parameters can also be used to define a new concept, the incremental aerosol reactivity for parent organic species, which is a convenient way of ranking parent organics in terms of their SOA-forming potentials.

Efforts to improve the simulation of SOA formation in the California Institute of Technology three-dimensional air quality model are also described. The Caltech Atmospheric Chemistry Mechanism was designed to predict concentrations of the highly functionalized secondary organic oxidation products capable of leading to SOA. A module that treats formation of SOA thermodynamically is used to predict the distribution of these products between the gas- and aerosol-phases. The new mechanism and thermodynamic module will used to simulate a smog episode that occurred in 1993 in the South Coast Air Basin of California.

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