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Automated speciated mercury measurements were made at a rural (Dexter, MI) and an urban (Detroit, MI) site in Michigan during selected times from 1999 to 2002 to assess the concentrations of elemental (Hg0), reactive gaseous (RGM), and particulate mercury (Hgp) in these environments. Here we present the first-ever reported values for RGM in Michigan. Median RGM concentrations were 2.21-2.93 pg m(-3) at Dexter and were 3-11 times higher in Detroit at 6.41-22.0 pg m(-3). Maximum RGM concentrations of 38.7 pg m(-3) and 270 pg m(-3) were observed in Dexter and Detroit, respectively. Measured RGM/Hg0 ratios were in the range of 0.04-11.60% indicating that at times RGM comprises greater than the currently held view of 5% of total gaseous mercury in the air. Well-pronounced diurnal patterns of RGM were observed at the rural site, whereas the urban site exhibited patterns that were influenced by nighttime emissions and regional transport. An analysis of RGM/Hgp ratios at the urban site when combined with trajectory analysis suggests that the site receives mercury inputs from both local and regional sources. Episodes of elevated ozone concentrations which were accompanied by increases in RGM concentrations were observed to occur in the late afternoon and overnight. These may be evidence of advection of ozone and RGM over long distances to the site.

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