Abstract
We use a global three‐dimensional coupled oxidant‐aerosol model (GEOS‐CHEM) to estimate natural and transboundary pollution influences on sulfate‐nitrate‐ammonium aerosol concentrations in the United States. This work is motivated in part by the Regional Haze Rule of the U.S. Environmental Protection Agency (EPA), which requires immediate action to improve visibility in U.S. wilderness areas along a linear trajectory toward an endpoint of “natural visibility conditions” by 2064. We present full‐year simulations for 1998 and 2001 and evaluate them with nationwide networks of observations in the United States and Europe (Interagency Monitoring of Protected Visual Environments (IMPROVE), Clean Air Status and Trends Network (CASTNET), National Atmospheric Deposition Program (NADP), European Monitoring and Evaluation Programme (EMEP)) and with Asian outflow observations from the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) aircraft mission. Shutting off U.S. anthropogenic emissions in the model defines “background” aerosol concentrations representing contributions from both natural and transboundary pollution sources. We find that transboundary transport of pollution from Canada, Mexico, and Asia dominates over natural influences for both sulfate and nitrate. Trans‐Pacific transport of Asian pollution accounts for 30% of background sulfate in both the western and eastern United States. Our best estimates of natural concentrations for ammonium sulfate and ammonium nitrate in the United States are either consistent with or lower than the default values recommended by EPA for natural visibility calculations. However, the large transboundary pollution influence in our calculation suggests that a natural visibility objective cannot be approached without international emission controls.
Highlights
Introduction[2] Visibility degradation in the United States is mostly due to fine aerosols [Malm et al, 2000] including carbonaceous (elemental and organic), sulfate, nitrate, and soil dust components
Aerosol in the United States: Transboundary Pollution Influence [32] We apply our model simulations to quantify background sulfate-nitrate-ammonium aerosol concentrations in the United States and to separate the contributions to this background from natural sources and from transboundary pollution
We find that ammonium nitrate formation in most of North America is generally limited by the supply of ammonia (GR < 1)
Summary
[2] Visibility degradation in the United States is mostly due to fine aerosols [Malm et al, 2000] including carbonaceous (elemental and organic), sulfate, nitrate, and soil dust components These aerosols originate from both anthropogenic and natural sources. Defining this natural visibility endpoint requires better information on natural aerosol concentrations and on the perturbing effects from transboundary transport of anthropogenic pollution We previously examined this issue in a global three-dimensional (3-D) model simulation of carbonaceous aerosols [Park et al, 2003] and found that quantification of wildfire emissions was of critical importance. We define here an aerosol ‘‘background’’ following U.S EPA (manuscript in preparation, 2003) as the aerosol concentration that would be present over the United States in the absence of domestic anthropogenic emissions It includes contributions from natural sources and from transboundary transport of pollution. Our model results suggest that trans-Pacific Asian pollution is a major contributor to the sulfate background over the United States on an annual average basis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
