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The concentrations of the various chromium species were determined in 89 rainwater samples collected in Wilmington, NC, from October 1, 1999, to December 31, 2001. Volume-weighted annual average concentrations of Cr(total), particulate Cr, Cr(III)(aq), and Cr(VI)(aq) were 4.6, 2.2, 0.8, and 1.2 nM, respectively. There was distinct seasonal and diurnal variability in the concentrations of the various chromium species. Chromium emissions to the global atmosphere by both natural and anthropogenic sources are estimated to be 2.2 x 10(9) mol/yr. Using our rainwater concentration data along with other published rainwater Cr concentrations and an estimate for total global annual rain, the total global flux of chromium removed from the atmosphere via wet deposition is 2.1 x 10(9) mol/yr. This represents complete removal of Cr and indicates that essentially all chromium released into the global atmosphere is removed via rain. About half this chromium is dissolved with roughly equal concentrations of toxic Cr(VI) and relatively harmless Cr(III) species.

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More than 80% of the iron(II) present in a dilute (pH 4.5) H2SO4 solution was oxidized by hydrogen peroxide (3 microM) in 24 h, whereas in rainwater Fe(II) remained stable for days indicating that a complexed form of Fe(II) exists in rainwater that protects it against oxidation. When a rain sample was irradiated for 2 h with simulated sunlight, there was a 57 nM increase in Fe(II) resulting from photoreduction of organic Fe(III) complexes. Once irradiation ceased, the photoproduced Fe(II) rapidly oxidized back to its initial concentration of 32 nM prior to irradiation, but not to zero. These photochemical studies demonstrate that during the daytime when sunlight is present there are dynamic interconversions between complexed and uncomplexed Fe(II) and Fe(III) species in rainwater. During the night, after the photochemically produced Fe(II) is reoxidized to Fe(III), virtually all remaining Fe(II) is complexed by ligands which resist further oxidation. Rain samples oxidized under intense UV light lost their ability to stabilize Fe(II), suggesting the ligands stabilizing Fe(II) are organic compounds destroyed by UV-irradiation. Additional UV-irradiation studies demonstrated that on average 25% of the Fe-complexing ligands in rainwater are extremely strong and cannot be detected by spectrophotometric analysis using ferrozine. The stability of organically complexed Fe(II) has important implications for the bioavailability of rainwater-derived Fe in the surface ocean.

[1] Diurnal and seasonal variations in rainwater iron speciation were investigated during a summer and winter cruise aboard the RV Endeavor at the Bermuda Atlantic Time Series Station. Concentrations of total Fe, Fe(II), and Fe(III) were all higher in rainwater falling at BATS during the summer relative to winter cruise. Iron speciation displayed marked diel variability with a 200-fold higher Fe(II)(aq)/Fe(III)(aq) ratio in rain received between 1200 and 1800 local time (LT) relative to late evening or early morning summer rain. The higher Fe(II)/Fe(III) ratio in the afternoon indicates Fe(II) is produced at the expense of Fe(III) during the course of the day, suggesting photochemical processes are important in controlling iron speciation in marine rains falling over the open ocean. Experiments utilizing authentic rainwater collected at BATS during both summer and winter cruises demonstrate that rainwater Fe(II)(aq) is stabilized against oxidation and precipitation for more than 4 hours after mixing with BATS seawater. Preliminary results suggest this stabilization is the result of organic complexation of Fe in rainwater. Owing to the isolation of BATS from riverine sources of Fe, rainwater is the predominant source of soluble and stable Fe(II) in this region of the North Atlantic.

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