Evolution of the magnitude and spatial extent of the weekend ozone effect in California's South Coast Air Basin, 1981-2000

Since the mid-1970s, ozone (O-3) levels in portions of California's South Coast Air Basin (SoCAB) on weekends have been as high as or higher than levels on weekdays, even though emissions of O-3 precursors are lower on weekends. Analysis of the ambient data indicates that the intensity and spatial extent of the weekend O-3 effect are correlated with day-of-week variations in the extent of O-3 inhibition caused by titration with nitric oxide (NO), reaction of hydroxyl radical (OH) with nitrogen dioxide (NO2), and rates of O-3 accumulation. Lower NO mixing ratios and higher NO2/oxides of nitrogen (NOx) ratios on weekend mornings allow O-3 to begin accumulating approximately an hour earlier on weekends. The weekday/weekend differences in the duration of O-3 accumulation remained relatively constant from 1981 to 2000. In contrast, the rate of O-3 accumulation decreased by one-third to one-half over the same period; the largest reductions occurred in the central basin on weekdays. Trends in mixing ratios of O-3 precursors show a transition to lower volatile organic compound (VOC)/NOx ratios caused by greater reductions in VOC emissions. Reductions in VOC/NOx ratios were greater on weekdays, resulting in higher VOC/NOx ratios on weekends relative to weekdays. Trends in VOC/NOx ratios parallel the downward trend in peak O-3 levels, a shift in the location of peak O-3 from the central to the eastern portion of the basin, and an increase in the magnitude and spatial extent of the weekend O-3 effect.