Mathematical Modeling of Urban Organic Aerosol: Properties Measured by High-Resolution Gas Chromatography

Primary fine aerosol emissions from a variety of urban sources have been quantitatively characterized via high-resolution gas chromatography to obtain organic mass distribution fingerprints. To assess the degree of secondary organic aerosol formation in urban areas, a transport model is used to predict the distribution of ambient organic aerosol characteristics that would exist at various sites in the Los Angeles Basin if the primary organic emissions were transported without chemical reaction. Comparisons between the model predictions and ambient measurements show substantial agreement for the nonpolar organics, suggesting that ambient concentrations of this organic fraction result directly from primary emissions. In contrast, ambient concentrations of fine acidic organic aerosols are significantly underpredicted by the model, indicating that secondary formation is important for acidic organics. On the basis of the observed differences between model predictions and measured properties of acidic organics, it is estimated, using monthly averages, that up to 18–27% of the elutable organic aerosol present in the Los Angeles atmosphere may be secondary in origin.