Measurements of Trace Metal (Fe, Cu, Mn, Cr) Oxidation States in Fog and Stratus Clouds

Abstract

The oxidation state of four transition metals (Fe, Mn, Cu, and Cr) in cloudwater has been investigated during several cloud events at Whiteface Mountain (NY), one cloud event at San Pedro Hill (CA), and one fog event at Bakersfield (CA). Samples were collected and immediately analyzed for the oxidation states of four transition metals: Fe(II) [44 measurements], Cu(I) [30 measurements], Mn(IV) [27 measurements], and Cr(III) [3 measurements]. Extreme care was taken to minimize contamination and interferences when measuring these oxidation states. Particulate and dissolved concentrations of these metals were also determined. Other measurements performed—relevant to the redox chemistry of these metals—included pH, total elemental concentrations (Fe, Cu, Mn, Cr, Al, K, Ca, Na, and Mg), organic anions (formate, acetate, glycolate, oxalate), inorganic anions (chloride, sulfate, nitrate, sulfite), cations (sodium, calcium, magnesium, potassium), peroxides, and formaldehyde. These measurements were then used in thermodynamic-speciation models to understand the speciation of ambient fog and cloudwater. From this analysis, two different cases were found for Fe(III)soluble speciation. Fe(III) was found to exist either as Fe(OH)_2 ^+ or Fe(Oxalate)_2. However, an unidentified strong chelating ligand with Fe(III) was also suggested by the data. Cu(I) and Cu(II) were calculated to be predominantly Cu^+ and Cu^(2+) (with less than 10% as Cu(II)-oxalate complexes). A chemical kinetic model was also used to investigate the transition-metal chemistry. The model results indicate that Fe(II) should be the predominant chemical form of Fe during daylight conditions. This prediction is in agreement with the field measurements in which the highest ratios of Fe(II)/Fe total were found in samples collected during the day. The model results also indicated that Fe(III) should be the predominant form of Fe during nighttime conditions, which is also in agreement with the field measurements. In the model, Cu(II) and Mn(II) were the predominant oxidation states during daylight and nighttime conditions with Cu(I) and Mn(III) increasing during daylight conditions. Mn(III) concentrations were never high enough to influence the redox chemistry of Cr. Overall, Cr(VI) in cloudwater is predicted to be reduced to Cr(III) if free S(IV) is present.

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