Mercury exists in mainly two oxidation states in the atmosphere, Hg0 and Hg(II).
Inorganic divalent mercury, Hg(II), has a greater solubility; therefore is in higher
concentration in rainwater, than Hg0. The toxic species, methylmercury is an organic
form of Hg(II) and is present in low concentration. Mercury is released into the
atmosphere by natural and anthropogenic sources. Rainwater is thought to be a main
removal mechanism for atmospheric mercury. The concentration and speciation of
mercury were determined in rainwater from Wilmington, NC, from September 1, 2003 to
September 30, 2005. Volume weighted averages for total Hg in unfiltered rainwater,
total dissolved Hg, particulate Hg, dissolved gaseous Hg (Hg0) and methyl-Hg were 52.9
± 4.7 pM, 40.6 ± 4.0 pM, 13.7± 1.5 pM, 4.3 ± 0.9 pM and 1.1 ± 0.1 pM, respectively.
All mercury species were present in all seasons with no significant difference in
concentrations between summer and winter, except dissolved gaseous mercury
concentration was higher in the winter, with a higher ratio of Hg(II)/Hg(0) in summer
relative to winter events. Diurnal variation was seen where Hg(II) decreased during the
day into the night, suggesting photochemical reduction of Hg(II). All Hg concentrations
were higher in continental storms relative to coastal rain events. Both total mercury
species (UFHg and TDHg) were positively correlated with particulate mercury. Total
mercury species were washed out of the atmosphere by rainwater with lower
concentrations for larger rain events. A weak positive correlation was observed between
TDHg and NO3
-, TDHg and SO4
2-, DGHg and Cl-, and Hgpart and DOC.
The photochemistry of mercury from Wilmington was also investigated. UFHg,
TDHg, and DGHg were generally produced upon irradiation of rainwater samples by simulated sunlight. Particulate Hg concentrations generally declined upon irradiation and
MMHg concentrations showed no pattern, in some instances, increasing, decreasing or
remaining the same. Positive correlation was observed between, production of UFHg and
Hgpart and a negative correlation was observed between production of TDHg and Hgpart.
Continental events increased in Hgpart while decreasing in TDHg, whereas coastal events
increased in TDHg while decreasing in Hgpart after irradiation. Seasonal differences
between Hg species were similar with an increase in TDHg and DGHg, while decreasing
in Hgpart and greater changes were observed during the winter. Diurnal variations of
Hg(II)/Hg(0) ratio increased during the afternoon and decreased there after.
Atmospheric global inputs by natural and anthropogenic sources, 4.1 x 106 kg or
2.0 x 107 mol per year, were in good agreement with calculated total flux of mercury
removed via wet deposition, 3.8 x 106 kg or 1.9 x 107 mol per year, suggesting that
essentially all mercury released into the atmosphere is removed via rain
