Methods for developing a numerical scale ranking reactivities of volatile organic compounds (VOCs) towards ozone formation were investigated. Effects of small VOC additions on ozone formation (incremental reactivities) were calculated for 140 types of VOCs in model scenarios representing a variety of single-day pollution episodes. Relative reactivities determined from effects of the VOCs on maximum ozone concentrations (ozone yields) varied widely among the scenarios, but relative reactivities determined from effects on integrated ozone levels were less variable. A 'maximum reactivity' scale was derived from ozone yield reactivities in scenarios where NOx inputs were adjusted so the VOCs had the greatest effect on ozone, and a 'maximum ozone' scale was derived from scenarios where NOx inputs gave maximum ozone concentrations. These scales gave different relative reactivities for many VOCs, particularly aromatics. Several 'multi-scenario' scales were derived from the ozone yield and integrated ozone reactivities in the unadjusted scenarios. The maximum ozone scale was more consistent with averages of ratios of ozone yield reactivities, but the maximum reactivity scale was more consistent with ratios of integrated ozone reactivities and also corresponded best to multi-scenario scales developed to minimize the total error in ozone predictions. Information concerning effects of NOx levels, of the composition of base case VOC emissions, and of other scenario conditions on reactivities was also obtained.

"September 1991." "EPA/600/3-91-050." "Cooperative agreement no. CR-814396-01-0." "Project officer, Joseph J. Bufalini." "Chemical Processes and Characterization Division." "Atmospheric Research and Exposure Assessment Laboratory."