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July 2003

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


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Eric M. Fujita, Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada
William R. Stockwell, Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada
David E. Campbell, Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada
Robert E. Keislar, Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada
Douglas R. Lawson, National Renewable Energy Laboratory, Golden, Colorado

Abstract

Since the mid-1970s, ozone (O3) 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 O3 precursors are lower on weekends. Analysis of the ambient data indicates that the intensity and spatial extent of the weekend O3 effect are correlated with day-of-week variations in the extent of O3 inhibition caused by titration with nitric oxide (NO), reaction of hydroxyl radical (OH) with nitrogen dioxide (NO2), and rates of O3 accumulation. Lower NO mixing ratios and higher NO2/oxides of nitrogen (NOx) ratios on weekend mornings allow O3 to begin accumulating approximately an hour earlier on weekends. The weekday/ weekend differences in the duration of O3 accumulation remained relatively constant from 1981 to 2000. In contrast, the rate of O3 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 O3 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 O3 levels, a shift in the location of peak O3 from the central to the eastern portion of the basin, and an increase in the magnitude and spatial extent of the weekend O3 effect.

Implications

The decrease in weekend O3 precursor emissions of VOC and NOx provides a natural test case for investigating the response of O3 levels to specific changes in precursor emissions. Our analysis demonstrates that peak O3 will increase in the urbanized areas of the SoCAB if NOx emissions are decreased at current levels of VOC. For this level of VOC, modeling suggests that peak O3 will increase unless NOx mixing ratios are decreased by approximately 90% from current weekend levels. The study also shows that weekend O3 would be even higher were it not for concomitant weekend emission reductions of VOCs. The development of accurate day-of-week emissions inventories is required to assess the effects of current and future O3 control regulations.


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