Source attribution for mercury deposition in the contiguous United States: Regional difference and seasonal variation

Abstract

Quantifying the contribution of emission sources responsible for mercury deposition in specific receptor regions helps develop emission control strategies that alleviate the impact on ecosystem and human health. In light of the maximum available control technology (MACT) rules proposed by U.S. Environmental Protection Agency (EPA) and the ongoing intergovernmental negotiation coordinated by United Nations Environmental Programme (UNEP) for mercury, the Community Multiscale Air Quality Modeling System (CMAQ-Hg) was applied to estimate the source contribution in six subregions of the contiguous United States (CONUS). The considered source categories include electric generating units (EGU), iron and steel industry (IRST), other industrial point sources excluding EGU and IRST (OIPM), the remaining anthropogenic sources (RA), natural processes (NAT), and out-of-boundary transport (BC). It is found that, on an annual basis, dry deposition accounts for two-thirds of total annual deposition in CONUS (474 Mg yr⁻¹), mainly contributed by reactive gaseous mercury (about 60% of total deposition). The contribution from large point sources can be as high as 75% near the emission sources (<100 km), indicating that emission reduction may result in direct deposition decrease near the source locations. Out-of-boundary transport contributes from 68% (Northeast) to 91% (West Central) of total deposition. Excluding the contribution from out-of boundary transport, EGU contributes to about 50% of deposition in the Northeast, Southeast, and East Central regions, whereas emissions from natural processes are more important in the Pacific and West Central regions (contributing up to 40% of deposition). This suggests that the implementation of the new EPA MACT standards will significantly benefit only these three regions. Emission speciation is a key factor for local deposition. The source contribution exhibits strong seasonal variation. Deposition is greater in warm seasons due to stronger Hg⁰ oxidation. However, the contribution from anthropogenic sources is smaller in warm seasons because of larger emissions from natural processes and stronger vertical mixing that facilitates transport.

Implications:

In the United States, electric generation, industrial combustion, and waste incineration are the most important anthropogenic emission sources of mercury. These sources can contribute up to 75% of total annual mercury deposition near the source locations. Controlling emissions from these sources will benefit more significantly to the eastern United States. Speciation of mercury emissions plays an important role in local deposition. Total annual mercury deposition caused by foreign anthropogenic emissions is significant compared to that from domestic sources on a national basis, suggesting that a coordinated global emission reduction effort is important for effectively reducing mercury deposition in the United States.

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Corrigendum

Acknowledgments

This work is supported in part by Texas Air Research Center (TARC) and the EPA. The funding support is gratefully acknowledged. The authors also want to thank Kirk Baker at the EPA for his assistance in obtaining the emission inventory and meteorological data used in this modeling assessment.