Evaluating the effects of climate change on summertime ozone using a relative response factor approach for policymakers

The impact of climate change on surface-level ozone is examined through a multiscale modeling effort that linked global and regional climate models to drive air quality model simulations. Results are quantified in terms of the relative response factor (RRF_E), which estimates the relative change in peak ozone concentration for a given change in pollutant emissions (the subscript E is added to RRF to remind the reader that the RRF is due to emission changes only). A matrix of model simulations was conducted to examine the individual and combined effects of future anthropogenic emissions, biogenic emissions, and climate on the RRF_E. For each member in the matrix of simulations the warmest and coolest summers were modeled for the present-day (1995-2004) and future (2045-2054) decades. A climate adjustment factor (CAF_C or CAF_CB when biogenic emissions are allowed to change with the future climate) was defined as the ratio of the average daily maximum 8-hr ozone simulated under a future climate to that simulated under the present-day climate, and a climate-adjusted RRF_EC was calculated (RRF_EC = RRF_E × CAF_C). In general, RRF_EC > RRF_E, which suggests additional emission controls will be required to achieve the same reduction in ozone that would have been achieved in the absence of climate change. Changes in biogenic emissions generally have a smaller impact on the RRF_E than does future climate change itself. The direction of the biogenic effect appears closely linked to organic-nitrate chemistry and whether ozone formation is limited by volatile organic compounds (VOC) or oxides of nitrogen (NO_X = NO + NO₂). Regions that are generally NO_X limited show a decrease in ozone and RRF_EC, while VOC-limited regions show an increase in ozone and RRF_EC. Comparing results to a previous study using different climate assumptions and models showed large variability in the CAF_CB.We present a methodology for adjusting the RRF to account for the influence of climate change on ozone. The findings of this work suggest that in some geographic regions, climate change has the potential to negate decreases in surface ozone concentrations that would otherwise be achieved through ozone mitigation strategies. In regions of high biogenic VOC emissions relative to anthropogenic NO_X emissions, the impact of climate change is somewhat reduced, while the opposite is true in regions of high anthropogenic NO_X emissions relative to biogenic VOC emissions. Further, different future climate realizations are shown to impact ozone in different ways.