Wet deposition of mercury in Michigan: meteorological associations and spatial variability

TitleWet deposition of mercury in Michigan: meteorological associations and spatial variability
Publication TypeThesis
Year of Publication1995
AuthorsHoyer MEsther
DegreeDoctor of Philosophy
Number of Pages265 pp.
UniversityUniversity of Michigan
CityAnn Arbor, MI

Over the past ten years, mercury studies have identified the atmosphere as a major source of Hg to remote lakes and that modest increases in the deposition of Hg from the atmosphere can lead to levels of Hg in fish that are of toxicological concern. More recently, studies in the Great Lakes region have identified an increase in the atmospheric deposition of Hg and Hg deposition via precipitation is recognized as an important fraction of the total atmospheric Hg deposition. This study was initiated to assess the sources, transport and deposition of Hg in Michigan. The main objectives of this work were 1) to develop and implement procedures for collection and analysis of part-per-trillion (ppt) amounts of mercury in event precipitation using automatic collectors, 2) to investigate the seasonal and spatial differences in the precipitation concentration and wet deposition of mercury in Michigan, 3) to describe the seasonal and spatial variability in Hg concentrations in precipitation attributable to variability in meteorological parameters, and 4) to investigate the source-receptor relationships for Hg in Michigan. The two hypotheses tested in this study were: 1. The seasonal variation in Hg concentration and wet deposition can be attributed to seasonal variation in meteorological factors such as the upwind path of the air mass, precipitation volume, precipitation type, upwind stagnation, and upwind precipitation. 2. A gradient in the concentration and wet deposition of Hg exists in Michigan which can be related to the location and magnitude of emissions from anthropogenic sources. The results from this study indicate that seasonal variations in the concentration of Hg are due to 1) seasonal variation in the frequency of transport from areas of high emission density, 2) seasonal variation in the frequency of transport from areas of low emission density, 3) the occurrence of precipitatin as snow, and 4) the occurrence of stagnant conditions in spring and summer. This study provided the first direct evidence for a gradient in Hg concentrations and wet deposition in the Great Lakes Basin with higher concentrations and wet deposition observed at sites in closer proximity to major source regions. Mercury sources in the southern Lake Michigan basin and sources in the upwind transport sector southwest of southwestern Michigan had the largest impact on Hg deposition at South Haven. Mercury sources in the Detroit Metropolitan area contributed to elevated Hg concentrations and wet deposition in southeastern Michigan, potentially due to emissions of divalent Hg species, and emissions or atmospheric reactions leading to elevated particulate Hg.