Abstract
We use a global chemical transport model (GEOS-Chem) with 1° × 1° horizontal resolution to quantify the effects of anthropogenic emissions from Canada, Mexico, and outside North America on daily maximum 8-hour average ozone concentrations in US surface air. Simulations for summer 2001 indicate mean North American and US background concentrations of 26 ± 8 ppb and 30 ± 8 ppb, as obtained by eliminating anthropogenic emissions in North America vs. in the US only. The US background never exceeds 60 ppb in the model. The Canadian and Mexican pollution enhancement averages 3 ± 4 ppb in the US in summer but can be occasionally much higher in downwind regions of the northeast and southwest, peaking at 33 ppb in upstate New York (on a day with 75 ppb total ozone) and 18 ppb in southern California (on a day with 68 ppb total ozone). The model is successful in reproducing the observed variability of ozone in these regions, including the occurrence and magnitude of high-ozone episodes influenced by transboundary pollution. We find that exceedances of the 75 ppb US air quality standard in eastern Michigan, western New York, New Jersey, and southern California are often associated with Canadian and Mexican pollution enhancements in excess of 10 ppb. Sensitivity simulations with 2020 emission projections suggest that Canadian pollution influence in the Northeast US will become comparable in magnitude to that from domestic power plants.
Highlights
Air quality standards to protect public health from the harmful effects of surface ozone are becoming increasingly stringent
We examined the frequency of occurrence of Canadian/Mexican pollution influences greater than 10 ppb under model conditions when surface ozone exceeds the US air quality standard (75 ppb)
The US Environmental Protection Agency (EPA) (2006) defines a policy-relevant background (PRB) in its standard-setting process as the ozone concentration that would be present in US surface air in the absence of North American anthropogenic emissions
Summary
Air quality standards to protect public health from the harmful effects of surface ozone are becoming increasingly stringent. The US Environmental Protection Agency (EPA) (2006) uses the Policy-Relevant-Background (PRB) in its standard-setting process as the surface ozone concentration that would be present in the US in the absence of anthropogenic emissions from North America (defined as the ensemble of the US, Canada, and Mexico). The PRB represents the ozone concentration that is not amenable to reduction under current policy frameworks It is important for regulatory decisions as it sets the maximum ozone reduction and relative health benefits that can be achieved through North American emission controls. We use a high-resolution version of GEOS-Chem (1 latitude  1 longitude) to resolve transport from Canada and Mexico This version was previously used by Park et al (2006) to quantify transboundary pollution influences on aerosol concentrations in the US in the context of the Regional Haze Rule (US EPA, 1999)
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