The Sensitivity of Regional Ozone Air Pollution Over the United States to Future Global Climate and Anthropogenic Emissions Changes

Pavan Nandan Racherla
Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA

Peter J. Adams
Department of Engineering and Public Policy
Department of Civil and Environmental Engineering
Carnegie Mellon University, Pittsburgh, PA

We examined the relative importance of future changes in climate, anthropogenic emissions, biogenic VOC emissions, CH4, and long-range air pollution transport on U.S. O3 by performing a suite of simulations with an integrated model of global climate, gas-phase chemistry and aerosols. Where applicable we used the A2 2050s climate as a representative future climate, and the A2 2050s (overall U.S. emissions increase) and B1 2050s (overall U.S. emissions decrease) emissions for future emissions. The model simulations show that U.S. O3 is sensitive first and foremost to U.S. anthropogenic emissions changes, best illustrated in the domain-average changes in the average daily maximum 8-hour O3 oncentrations (MDA8-O3) over the eastern U.S. (May- September) due to: 1) climate change with present-day anthropogenic emissions (2.1 ppbv); 2) anthropogenic emissions change alone (-9 to 9 ppbv); 3) climate change with different emissions regimes (0.9 to 3.4 ppbv); 4) increased global CH4 concentration only (2.4 ppbv); and, 5) longrange air pollution transport (1.4 ppbv). The 95th-percentile O3 increase (May-September) due to climate change with B1 and A2 emissions is 1 ppbv and 10 ppbv, respectively. Therefore, the climate change effect on O3 is minimized under an emissions reduction scenario and amplified under an emissions increase scenario. Increased CH4 and long-range transport (A2) together contribute 3.8 ppbv to the domain-average MDA8-O3 (May-September), thereby increasing the O3 background over the U.S. With more stringent O3 standards in the future, this increased O3 background could significantly reduce the benefits of likely drastic U.S. emissions reductions over the next several decades.