Effects of 2000--2050 global change on ozone air quality in the United States

I investigate the effects of 2000--2050 global change in climate and anthropogenic emissions of ozone precursors on U.S. ozone air quality by using a global chemical transport model (GEOS-Chem CTM) driven by meteorological fields from a general circulation model (NASA/GISS GCM). I begin by evaluating the model simulations and exploring the important factors controlling tropospheric ozone budgets. Global CTMs in the literature show large differences in the tropospheric ozone production rate P(Ox) (2300--5300 Tg yr-1). Simulations conducted with the GEOS-Chem model using three different meteorological data sets show differences in P(Ox) by about 10%. Multivariate statistical analysis of model budgets in the literature indicates that 74% of the variance in P(Ox) can be explained by differences in NOx emissions, inclusion of non-methane volatile organic compounds, and stratospheric ozone influx. My 2000--2050 global change analysis follows the IPCC A1B scenario and separates the effects from changes in climate and anthropogenic ozone precursor emissions through sensitivity simulations. 2000--2050 changes in anthropogenic emissions increase the tropospheric ozone burden by 17%. 2000--2050 climate change increases this burden by 3%, mostly due to lightning in the upper troposphere; in the lower troposphere, ozone decreases except over polluted continental regions. 2000--2050 anthropogenic emission changes reduce the U.S. summer daily maximum 8-hour ozone by 2--15 ppb, but climate change causes a 2--5 ppb positive offset over the Midwest and Northeast, driven in part by decreased ventilation from convection and frontal passages. Ozone pollution episodes are far more affected by climate change than mean values, with effects exceeding 10 ppb in the Midwest and Northeast. Ozone air quality in the Southeast is insensitive to climate change, reflecting compensating effects from changes in isoprene emission and air pollution meteorology. A 'climate change penalty' is defined as the additional emission controls necessary to meet a given ozone air quality target. I find that a 50% reduction in U.S. NOx emissions is needed in the 2050 climate to reach the same target in the Midwest as a 40% reduction in the 2000 climate. Emission controls reduce the magnitude of this penalty and can even turn it into a climate benefit in some regions.