Weekday Versus Weekend Activity Patterns for Ozone Precurser Emissions in California's South Coast Air Basin

Ambient ozone 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 ozone precursors (hydrocarbons, carbon monoxide, and; nitrogen oxides) are lower on weekends. Day-of-week emissions activity data were collected and analyzed to investigate thehypothesized causes of the 'weekend ozone 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 weekendemission activity patterns and emissions. Their combined effect results in a 12%-18% decrease in reactive organic gases (ROG) and a 35%-41% decrease in nitrogen oxide (NOx) emissions on Saturdays and Sundays, respectively, relative to weekdays in the summer of 2000. These changes in emissions result in an increase of more than 30% in the ROG/NOx ratio on weekends compared to weekdays, which,along with lower NOx emissions, leads to increased ozone production on weekends.