Development of Ozone Reactivity Scales for Volatile Organic Compounds

Methods for ranking photochemical ozone formation reactivities of volatile organic compounds (VOCs) are discussed. Photochemical mechanisms for the atmospheric reactions of 118 VOCs were used to calculate their effects on ozone formation under various NOx conditions in model scenarios representing 39 different urban areas. Their effects on ozone were used to derive 18 different ozone reactivity scales, one of which is the Maximum Incremental Reactivity (MIR) scale used in the new California Low Emission Vehicle and Clean Fuel Regulations. These scales are based on 3 different methods for quantifying ozone impacts and on 6 different approaches for dealing with the dependencies of reactivity on NOx. The predictions of the scales are compared, the reasons for their similarities and differences are discussed, and the sensitivities of the scales to NOx and other scenario conditions are examined. Scales based on peak ozone levels were highly dependent on NOx, but those based on integrated ozone were less sensitive to NOx and tended to be similar to the MIR scale. It is concluded that the MIR scale or one based on integrated ozone is appropriate for applications requiring use of a single reactivity scale.