Atmospheric mercury speciation in urban air: Identifying the relative importance of local anthropogenic sources in Detroit, Michigan.

摘要

Mercury (Hg) in the atmosphere exists mainly in three forms: gaseous elemental mercury (GEM), reactive gaseous mercury (RGM), and particulate-bound mercury (HgP). The relative contributions from local versus regional and global sources to the levels of atmospheric Hg and the subsequent deposition is a topic of debate, which can in turn influence regulatory policies aiming to control anthropogenic Hg source emissions to reduce bioaccumulation of this toxic element in the ecosystem.; The central hypothesis of this dissertation is that local anthropogenic sources contribute more to the temporal variability in the ambient Hg form and to Hg deposition in Detroit than transported Hg from regional and global sources. To evaluate this hypothesis, measurements of ambient speciated Hg were conducted at an urban site in Detroit and at a rural site in Dexter in southeast Michigan. Multivariate analyses, source-receptor modeling, and detailed case study analyses show that Hg speciation at both sites shared similar impacts from meteorology, regional and global source transport, and physiochemical transformations. Concentrations of all three Hg species were significantly (p0.001) higher in Detroit than in Dexter. The limited variations in Hg levels observed at Dexter, particularly in RGM and HgP both of which are short-lived and primarily anthropogenic in origin, confirmed that the more episodic and variable trends in the hourly Hg speciation data in Detroit was largely due to localized anthropogenic sources.; The large variations observed in ambient speciated Hg levels had direct impacts on the dry deposition of Hg in Detroit based on measurements from two intensive campaigns. The urban-rural differences in ambient Hg also agreed well with the urban-rural differences seen in wet deposition of Hg measured in 2004. These results provide insights into the underlying relationships between Hg in the air and Hg in dry and wet deposition; highlighting the importance of urban/industrial atmospheric environment as a major source, transport medium, and receptor of mercury. The results of this work also indicate that mercury reduction measures that are intended to control local Hg emissions in major urban/industrial areas are efficient and necessary to reduce Hg deposition into their surrounding ecosystems.