Recent Research on Voc Ozone Reactivity

Control policies that take into account differences among VOCs in their relative effects on O3 formation have been adopted in California and are being considered by the United States EPA. These require a means to quantitatively rank VOCs by reactivity, which has policy and scientific difficulties. Among the policy issues is the fact that relative effects of VOCs on O3 depends on ambient conditions, and decisions must be made on how take this into account when a single scale is required. The scientific difficulties come from the fact that the predictions of the models needed to estimate effects of VOCs on ambient ozone are no more reliable than the chemical mechanisms and the model for airshed conditions they employ. The mechanisms of the hundreds of types of emitted VOCs are complex and uncertain, and environmental chamber data, which have a number of limitations, are necessary to test their predictive capabilities. The large grid-based models that are considered to be the most accurate for simulating ozone formation over wide regions have employed highly condensed mechanisms, and current reactivity scales, and until recently most studies of VOC reactivity effects, are based on use of trajectory or box models designed to represent urban conditions. There is a concern that such models may not accurately predict relative VOC reactivities in multi-day or long-range transport scenarios, where O3 formation is also of concern.