Modeling Weekday/Weekend Ozone Differences in the Los Angeles Region for 1997

Abstract

Numerous studies of ambient ozone (O₃) in the Los Angeles (LA) area have found both increases and decreases in elevated O₃ levels on weekends, depending on location and year. Since the mid-1990s, average daily maximum O₃ levels have been higher on weekends than on week-days throughout most of the area. We used the Comprehensive Air-Quality Model with extensions to investigate causes of weekday/weekend O₃ differences in the LA area for August 3–7, 1997, from the Southern California Ozone Study. Weekday/weekend emission changes were estimated, because explicit weekend inventories are not yet available from regulatory agencies. Changes to on-road motor vehicle (MV) emissions were derived from observed weekday/weekend traffic differences. The estimated changes in MV emissions of nitrogen oxides (NO_x) were a 5% increase on Friday, a 27% decrease on Saturday, and a 37% decrease on Sunday, relative to Monday–Thursday levels. The corresponding changes in MV volatile organic carbon (VOC) emissions were an 8% increase on Friday, an 8% decrease on Saturday, and a 15% decrease on Sunday. Modeling these MV emissions changes explained the observed weekend O₃ effect very well. Furthermore, changes to the mass of MV NO_x emissions were the main contributor to O₃ differences rather than changes to the timing of MV emissions. Ozone increases on weekends were caused by NO_x emission decreases, because O₃ formation is strongly VOC-limited throughout most of the LA area.