Abstract
Abstract. Hourly total gaseous mercury (TGM) concentrations at three monitoring sites (receptors) in Ontario were predicted for four selected periods at different seasons in 2002 using the Stochastic Time-Inverted Lagrangian Transport (STILT) model, which transports Lagrangian air parcels backward in time from the receptors to provide linkages to the source region in the upwind area. The STILT model was modified to deal with Hg deposition and high stack Hg emissions. The model-predicted Hg concentrations were compared with observations at three monitoring sites. Estimates of transport errors (uncertainties in simulated concentrations due to errors in wind fields) are also provided that suggest such errors can reach approximately 10% of simulated concentrations. Results from a CMAQ chemical transport model (CTM) simulation in which the same emission and meteorology inputs were used are also reported. The comparisons show that STILT-predicted Hg concentrations usually agree better with observations than CMAQ except for a subset of cases that are subject to biases in the coarsely resolved boundary conditions. In these comparisons STILT captures high frequency concentration variations better than the Eulerian CTM, likely due to its ability to account for the sub-grid scale position of the receptor site and to minimize numerical diffusion. Thus it is particularly valuable for the interpretation of plumes (short-term concentration variations) that require the use of finer mesh sizes or controls on numerical diffusion in Eulerian models. We report quantitative assessments of the relative importance of different upstream sources for the selected episodes, based on emission fluxes and STILT footprints. The STILT simulations indicate that natural sources (which include re-emission from historical anthropogenic activities) contribute much more than current-day anthropogenic emissions to the Hg concentrations observed at the three sites.
Highlights
Mercury was one of the first priority PBT (Persistent, Bioaccumulative and Toxic) pollutants identified by the US EPA, due to its significant health influences, especially damage to the nervous and reproductive systems. Human activities, such as mining and burning of fossil fuels, are important sources of atmospheric mercury, but there are many “natural” sources, including soil, minerals and water. (Here, “natural” sources include mercury that is released into the soil from primary mineral sources and re-emission of mercury which was deposited from historical anthropogenic activities.)
total gaseous mercury (TGM) measurements are available from the Canadian National Atmospheric Chemistry (NAtChem) database (Environment Canada, 2002)
Hg concentrations at three monitoring sites were simulated using the Stochastic Time-Inverted Lagrangian Transport (STILT) Lagrangian particle transport model, which was modified for this study to deal with Hg deposition and high stack Hg emissions
Summary
Mercury was one of the first priority PBT (Persistent, Bioaccumulative and Toxic) pollutants identified by the US EPA, due to its significant health influences, especially damage to the nervous and reproductive systems Human activities, such as mining and burning of fossil fuels, are important sources of atmospheric mercury, but there are many “natural” sources, including soil, minerals and water. To assess which source regions and types are responsible for observed mercury pollution, it is necessary to use numerical models (Cheng and Schroeder, 2000; Cohen et al, 2004; Lim et al, 2001; Lynam and Keeler, 2006) In this context, STILT, a receptororiented model, has proven to be especially useful for identifying transport pathways and estimating surface emission fluxes.
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