Abstract
Abstract. Gaseous elemental mercury is a global pollutant that can lead to serious health concerns via deposition to the biosphere and bio-accumulation in the food chain. Hourly measurements between June 2004 and May 2005 in an urban site (Milwaukee, WI) show elevated levels of mercury in the atmosphere with numerous short-lived peaks as well as longer-lived episodes. The measurements are analyzed with an inverse model to obtain information about mercury emissions. The model is based on high resolution meteorological simulations (WRF), hourly back-trajectories (WRF-FLEXPART) and a chemical transport model (CAMx). The hybrid formulation combining back-trajectories and Eulerian simulations is used to identify potential source regions as well as the impacts of forest fires and lake surface emissions. Uncertainty bounds are estimated using a bootstrap method on the inversions. Comparison with the US Environmental Protection Agency's National Emission Inventory (NEI) and Toxic Release Inventory (TRI) shows that emissions from coal-fired power plants are properly characterized, but emissions from local urban sources, waste incineration and metal processing could be significantly under-estimated. Emissions from the lake surface and from forest fires were found to have significant impacts on mercury levels in Milwaukee, and to be underestimated by a factor of two or more.
Highlights
Elemental mercury emitted to the atmosphere has a lifetime ranging from one half to two years (Lindberg et al, 2007; Schroeder and Munthe, 1998) making it a global pollutant
Because gaseous elemental mercury is a long-lived species, we can assume a linear relationship between an emissions vector x and the measurements y given by the sensitivity matrix H (Rigby et al, 2011; Brioude et al, 2011; Stohl et al, 2009; Lauvaux et al, 2008): y = Hx + residual
This paper developed a hybrid inversion scheme based on particle back-trajectories and forward Eulerian modeling to evaluate sources of elemental mercury using atmospheric measurements in Milwaukee
Summary
Elemental mercury emitted to the atmosphere has a lifetime ranging from one half to two years (Lindberg et al, 2007; Schroeder and Munthe, 1998) making it a global pollutant. Manolopoulos et al (2007a) made year long measurements at a rural site and found significant impacts from a local coal-fired power plant on reactive gaseous mercury, but not elemental mercury They recommend the use of receptor-based monitoring to account for small-scale sources and processes that cannot be represented in large-scale Eulerian models. We develop a method that is a variant of the inversion method described in Stohl et al (2009) with elements of the two-step approach of Rigby et al (2011) and the varying resolution method of Manning et al (2011) This hybrid inverse model is used to evaluate a range of sources of mercury: local, regional and distant sources, forest fires and lake surface emissions. We use a time scale analysis (Section 4.2) to evaluate the limits in temporal resolution of the inverse method and thereby improve the emissions estimate of local sources
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