Photochemical modeling of the Southern California air quality study

Abstract

The Southern California Air Quality Study (SCAQS) provides detailed experimental observations that can be used to explore the causes of the Los Angeles smog problem. In the present study, the CIT photochemical airshed model is updated and then applied to the August 27-29, 1987, SCAQS intensive monitoring period. Using measured meteorological parameters, measured initial and boundary conditions, and the official emission inventory prepared by the government, ozone concentrations are underpredicted by 23% on average. Recent field experiments suggest that emissions of organic gases and carbon monoxide are understated in the official emission inventory. When the organic gas emissions from on-road vehicle engine exhaust are increased as suggested by emission rate measurements made in a Los Angeles area roadway tunnel during SCAQS, ozone predictions match the observed concentrations more closely (the mean normalized bias in ozone predictions for this case is +1%). Reactive hydrocarbon concentrations are underpredicted by 35% in the base case calculation, but are underpredicted by only 12% in the increased emission case. Results of this study thus support prior estimates that the organic gas emissions in Los Angeles have been understated in recent years.

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