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emission rate ratios that imply rich on-road operation. Issues such as super-emitters, speed-dependent emission rates, and off-cycle operation must be considered.

Biogenic VOC emissions appear to be of comparable magnitude to anthropogenic VOC emissions in the United States as a whole. Biogenic emissions can also be a significant source of VOCs in urban airsheds. For yearly or seasonal periods, these emissions are not well quantified. Moreover, because of the large variability in emissions that can occur over the growing season, much larger errors can be incurred when annual or seasonal inventories are applied to a given single- or multiple-day episode of high ozone concentrations. Because natural VOC emissions tend to be highly reactive and to increase during the day, past measurements of these emissions may have understated their importance relative to anthropogenic VOC emissions. Much research is needed to improve the methods used to calculate biogenic VOC emissions.

If VOC emissions have been underestimated as much as the studies discussed in this chapter suggest, then VOC emission reductions in many areas of the United States will be less effective than was previously believed (see Chapters 6 and 11). Hence a major upward revision in VOC emissions inventories could force a fundamental change in the nation's ozone reduction strategy, which has been based primarily on VOC control.

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