The response of surface ozone to climate change over the Eastern United States
We investigate the response of surface ozone (O3) to future climate change in the eastern United States by per- forming simulations corresponding to present (1990s) and future (2050s) climates using an integrated model of global climate, tropospheric gas-phase chemistry, and aerosols. A future climate has been imposed using ocean boundary con- ditions corresponding to the IPCC SRES A2 scenario for the 2050s decade. Present-day anthropogenic emissions and CO2/CH4 mixing ratios have been used in both simulations while climate-sensitive emissions were allowed to vary with the simulated climate. The severity and frequency of O3 episodes in the eastern U.S. increased due to future climate change, primarily as a result of increased O3 chemical pro- duction. The 95th percentile O3 mixing ratio increased by 5 ppbv and the largest frequency increase occured in the 80– 90 ppbv range; the US EPA’s current 8-h ozone primary stan- dard is 80 ppbv. The increased O3 chemical production is due to increases in: 1) natural isoprene emissions; 2) hy- droperoxy radical concentrations resulting from increased water vapor concentrations; and, 3) NOx concentrations re- sulting from reduced PAN. The most substantial and statis- tically significant (p<0.05) increases in episode frequency occurred over the southeast and midatlantic U.S., largely as a result of 20% higher annual-average natural isoprene emis- sions. These results suggest a lengthening of the O3 sea- son over the eastern U.S. in a future climate to include late spring and early fall months. Increased chemical production and shorter average lifetime are two consistent features of the seasonal response of surface O3, with increased dry deposi- tion loss rates contributing most to the reduced lifetime in all seasons except summer. Significant interannual variability is observed in the frequency of O3 episodes and we find that it is necessary to utilize 5 years or more of simulation data in order to separate the effects of interannual variability and climate change on O3 episodes in the eastern United States.