Weekday versus Weekend Activity Patterns for Ozone Precursor Emissions in California’s South Coast Air Basin

Abstract

Ambient O₃ concentrations in California’s South Coast Air Basin (SoCAB) can be as much as 55% higher on weekends than on weekdays under comparable meteorological conditions. This is paradoxical because emissions of O₃ precursors (hydrocarbons, CO, and nitrogen oxides [NO_x]) are lower on weekends. Day-of-week emissions activity data were collected and analyzed to investigate the hypothesized causes of the “weekend O₃ effect.” Emission activity data were collected for various mobile, area, and point sources throughout the SoCAB, including on-road vehicles, lawn and garden equipment, barbecues, fireplaces, solvent use, and point sources with continuous emission monitoring data. The results of this study indicate significant differences between weekday and weekend emission activity patterns and emissions. Their combined effect results in a 12–18% decrease in reactive organic gases (ROGs) and a 35–41% decrease in NO_x emissions on Saturdays and Sundays, respectively, relative to weekdays in summer 2000. These changes in emissions result in an increase of more than 30% in the ROG/NO_x ratio on weekends compared with weekdays, which, along with lower NO_x emissions, leads to increased O₃ production on weekends.