How does climate change contribute to surface ozone change over the United States?

[1] The impact of climate change on U. S. surface ozone levels is investigated. We simulated two 10 year periods using the global chemical transport model MOZART-2 ( Model of Ozone and Related chemical Tracers version 2): 1990 - 2000 and 2090 - 2100. In each case, MOZART-2 is driven by meteorology from the National Center for Atmospheric Research (NCAR) coupled Climate Systems Model (CSM) 1.0 forced with the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1 scenario. During both periods the chemical emissions are fixed at 1990s levels, so that only changes in climate are allowed to impact ozone. The simulated surface ozone concentration during the 1990s is compared with observations from the Environmental Protection Agency's Aerometric Information Retrieval System (EPA AIRS) monitoring sites. Model-measurement correlations are high, but MOZART-2 overpredicts ozone especially over the eastern United States. The impact of climate change is calculated separately for background ozone and for the ozone generated through U. S. NOx emissions. Our results show that the response of ozone to climate change in polluted regions is not the same as in remote regions. MOZART-2 predicts a 0 - 2 ppbv decrease in background ozone in the future simulation over the United States but an increase in ozone produced internally within the United States of up to 6 ppbv. The decrease in background ozone is attributed to a future decrease in the lifetime of ozone in regions of low NOx. Over the western United States the decrease in background ozone approximately cancels the increase in locally produced ozone. As a result, the main impact of future climate change on ozone is centered over the eastern United States, where future ozone increases up to 5 ppbv. We predict that in the future over the northeast United States, up to 12 additional days each year will exceed the maximum daily 8-hour averaged ozone limit of 80 ppbv. Various climatic factors are identified which impact the net future increase in ozone over the United States including changes in temperature, water vapor, clouds, transport, and lightning NOx. Significant future changes are generally not found in planetary boundary layer height and precipitation.