Interactions of air quality and climate: Consequences of US emission controls

This thesis applies global chemical transport (CTM) and general circulation models (GCM), along with chemical and meteorological observations, to investigate the interactions between US air quality and climate.;The frequency of summertime mid-latitude cyclones tracking across eastern North America at 40--50°N is shown to be a strong predictor of ozone pollution days in the eastern US. Analyses of weather maps, assimilated meteorology, and NASA Goddard Institute for Space Studies (GISS) GCM simulations show a long-term decline in the number of summertime cyclones in this track starting in 1980. Using observed correlations between ozone pollution days and cyclone frequency, it is shown that this trend has offset by half the ozone air quality gains expected in the northeastern US from reductions in anthropogenic emissions. Without this trend in cyclones the northeastern US would have been largely compliant with the ozone standard by 2001.;Aerosol distributions derived from the GEOS-Chem CTM using historical and projected emissions are used with the NASA GISS GCM to estimate the climate effects of US anthropogenic aerosols. Aerosol forcing in the eastern US peaked in 1970--1990 (direct effect: -2.0 W m-2; indirect effects: -2.0 W m-2) and has strongly declined since due to air quality regulation. This regional radiative forcing elicits a strong regional climate response, cooling the central and eastern US by 0.5--1.0°C on average during 1970--1990. Observations over the eastern US show a lack of warming in 1960--1980 followed by rapid warming, which is attributed here to trends in US anthropogenic aerosols. It is shown that current US aerosol concentrations are sufficiently low that projected air quality regulations will cause little further warming. Most of the potential warming from US aerosol source controls has thus been realized.;In an additional study, it is shown that anthropogenic emissions of nitrogen oxides (NOx ≡NO + NO2) and carbon monoxide (CO) affect particulate matter (PM) air quality on an intercontinental scale by changing background oxidant levels and thus the production of sulfate and nitrate. Effects are largest (0.3 mug m-3) in receptor regions with large domestic SO2, NOx, and ammonia emissions and hence already high concentrations of PM.