Atmospheric transport of speciated mercury across southern Lake Michigan: Influence from emission sources in the Chicago/Gary urban area

Abstract

Quantifying the local and regional impacts of speciated mercury (Hg) emissions from major urban and industrial areas is critical for understanding Hg transport and cycling in the environment. The Chicago/Gary urban area is one location where Hg emissions from industrial sources are significant and the regional transport of emissions may contribute to elevated ambient Hg concentrations at downwind locations. From July to November 2007, we collected semi-continuous measurements of gaseous elemental Hg (Hg(0)), fine particulate bound Hg (Hgp), and divalent reactive gaseous Hg (RGM) in Chicago, IL and Holland, MI to characterize the impact of Chicago/Gary source emissions on Hg concentrations in southwest Michigan and to improve our overall understanding of speciated Hg transport and deposition. The mean (and median) concentrations of Hg(0), Hgp, and RGM in Chicago were 2.5ng/m(3) (1.9ng/m(3)), 9pg/m(3) (5pg/m(3)), and 17pg/m(3) (6pg/m(3)), respectively. In Holland the mean (and median) concentrations were 1.3ng/m(3) (1.3ng/m(3)), 6pg/m(3) (6pg/m(3)), and 8pg/m(3) (2pg/m(3)), respectively. Cluster analysis of 24-hour HYSPLIT back-trajectories associated with the semi-continuous Hg measurements indicated that southwest transport from Chicago/Gary to Holland occurred during approximately 27% of the study. In Holland, under this transport regime, we observed a five-fold increase in RGM relative to the median concentration of the other transport clusters. We applied the HYSPLIT dispersion model to two case study periods to further quantify the impact of Chicago/Gary sources on southeast Michigan and investigate the role of direct transport and dispersion of speciated Hg emissions. Results suggested that more than 50% of the maximum RGM concentrations observed in Holland during the selected periods could be attributed to direct transport of primary RGM emissions from Chicago/Gary. The remaining RGM fractions are believed to be associated with Hg(0) oxidation during transport over Lake Michigan.