The response of surface ozone to climate change over the Eastern United States

We investigate the response of surface ozone (O-3) to future climate change in the eastern United States by performing 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 conditions 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 O-3 episodes in the eastern U.S. increased due to future climate change, primarily as a result of increased O-3 chemical production. The 95th percentile O-3 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 standard is 80 ppbv. The increased O-3 chemical production is due to increases in: 1) natural isoprene emissions; 2) hydroperoxy radical concentrations resulting from increased water vapor concentrations; and, 3) NOx concentrations resulting from reduced PAN. The most substantial and statistically 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 emissions. These results suggest a lengthening of the O-3 season 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 O-3, with increased dry deposition loss rates contributing most to the reduced lifetime in all seasons except summer. Significant interannual variability is observed in the frequency of O-3 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 O-3 episodes in the eastern United States.