Jeremy Avise

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 (RRFE), 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 RRFE. 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 (CAFC or CAFCB 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 RRFEC was calculated (RRFEC = RRFE × CAFC). In general, RRFEC > RRFE, 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 RRFE 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 (NOX = NO + NO2). Regions that are generally NOX limited show a decrease in ozone and RRFEC, while VOC-limited regions show an increase in ozone and RRFEC. Comparing results to a previous study using different climate assumptions and models showed large variability in the CAFCB. Implications: 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 NOX emissions, the impact of climate change is somewhat reduced, while the opposite is true in regions of high anthropogenic NOX emissions relative to biogenic VOC emissions. Further, different future climate realizations are shown to impact ozone in different ways.

Attribution of projected changes in US ozone and PM 2.5 concentrations to global changes

The impact that changes in future climate, anthropogenic U.S. emissions, background tropospheric composition, and land-use have on regional U.S. ozone and PM2.5 concentrations is examined through a matrix of downscaled regional air quality simulations using the Community Multi-scale Air Quality (CMAQ) model. Projected regional scale changes in meteorology due to climate change under the Intergovernmental Panel on Climate Change (IPCC) A2 scenario are derived through the downscaling of Parallel Climate Model (PCM) output with the MM5 meteorological model. Future chemical boundary conditions are obtained through downscaling of MOZART-2 (Model for Ozone and Related Chemical Tracers, version 2.4) global chemical model simulations based on the IPCC Special Report on Emissions Scenarios (SRES) A2 emissions scenario. Projected changes in U.S. anthropogenic emissions are estimated using the EPA Economic Growth Analysis System (EGAS), and changes in land-use are projected using data from the Community Land Model (CLM) and the Spatially Explicit Regional Growth Model (SERGOM). For July conditions, changes in chemical boundary conditions are found to have the largest impact (+5 ppbv) on average daily