The response of surface ozone to climate change over the Eastern United States
- ISSN: 1680-7324
- DOI: 10.5194/acp-8-871-2008
We investigate the response of surface ozone (O-3) to future climate\nchange in the eastern United States by performing simulations corresponding\nto present (1990s) and future (2050s) climates using an integrated\nmodel of global climate, tropospheric gas-phase chemistry, and aerosols.\nA future climate has been imposed using ocean boundary conditions\ncorresponding to the IPCC SRES A2 scenario for the 2050s decade.\nPresent-day anthropogenic emissions and CO2/CH4 mixing ratios have\nbeen used in both simulations while climate-sensitive emissions were\nallowed to vary with the simulated climate. The severity and frequency\nof O-3 episodes in the eastern U.S. increased due to future climate\nchange, primarily as a result of increased O-3 chemical production.\nThe 95th percentile O-3 mixing ratio increased by 5 ppbv and the\nlargest frequency increase occured in the 80-90 ppbv range; the US\nEPA's current 8-h ozone primary standard is 80 ppbv. The increased\nO-3 chemical production is due to increases in: 1) natural isoprene\nemissions; 2) hydroperoxy radical concentrations resulting from increased\nwater vapor concentrations; and, 3) NOx concentrations resulting\nfrom reduced PAN. The most substantial and statistically significant\n(p < 0.05) increases in episode frequency occurred over the southeast\nand midatlantic U.S., largely as a result of 20% higher annual-average\nnatural isoprene emissions. These results suggest a lengthening of\nthe O-3 season over the eastern U.S. in a future climate to include\nlate spring and early fall months. Increased chemical production\nand shorter average lifetime are two consistent features of the seasonal\nresponse of surface O-3, with increased dry deposition loss rates\ncontributing most to the reduced lifetime in all seasons except summer.\nSignificant interannual variability is observed in the frequency\nof O-3 episodes and we find that it is necessary to utilize 5 years\nor more of simulation data in order to separate the effects of interannual\nvariability and climate change on O-3 episodes in the eastern United\nStates.