Evolution of the Magnitude and Spatial Extent of the Weekend Ozone Effect in California's South Coast Air Basin, 1981-2000

摘要

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 (NO_2), and rates of O_3 accumulation. Lower NO mixing ratios and higher NO_2/oxides of nitrogen (NO_x) 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)/NO_x ratios caused by greater reductions in VOC emissions. Reductions in VOC/ NO_x ratios were greater on weekdays, resulting in higher VOC/NO_x ratios on weekends relative to weekdays. Trends in VOC/NO_x 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.