The dynamic chemistry of transition metals in the troposphere
The redox chemistry of Fe, Cu, Mn, and Cr in fog and cloudwater, ambient aerosols, and surface microlayer has been investigated at coastal and inland locations in the Los Angeles basin, in Bakersfield, California, at Whiteface Mt., New York, and on Lake Erie and Lake Ontario. Samples were collected and analyzed for Fe(II), Fe(III), Fe-_(tot), Cu(I), Cu-_(tot), Mn(III+IV), Mn_(tot), Cr(III), Cr(VI), S(IV), S(VI), organic ligands (formate, acetate, oxalate). TOC (total organic carbon), pH, major cations (Na^+, Ca^(2+), NH_4^+, Mg_2^+, K^+), chloride, sulfate, nitrate, peroxides (RO_2H), and aldehydes (RCHO); the amount of sunlight was also measured. The concentrations and the ratios between the measured oxidation states and the total metal concentrations are as follows: Fe(II)(μM) Fe(II)/Fe_(tot) Cu(I)(μM) Cu(I)/Cu_(tot) 0.1-5 0.02-0.55 0.02-0.27 0.3-0.96 Mn(IV)(μM) Mn(IV)/Mn_(tot) Cr(III)(μM) Cr(III)/Cr_(tot) 0.026-0.21 0.25-0.97 0.002-0.051 0.09-0.74 The atmospheric redox cycle of Fe involves both dissolved and aerosol surface species and appears to be related to the presence of organic compounds which act as electron donors for the reduction of Fe(III). Fe(III) reduction is enhanced by light, but significant Fe(II) levels were observed in the dark. We suggest that reduction of Fe(III) species by organic electron donors may be an important pathway that affects the speciation of Fe in both urban and rural atmospheres. We found that 40-70% of the total chromium (-20 nM) was Cr(VI) in three cloudwater samples at Whiteface Mt. In the surface micro layer samples, a significant amount of Cr(III) was detected when the concentration of TOC was elevated. The concentrations of Cu(I) and Mn(III+IV) are below detection limit in most samples. In the few samples with measurable concentrations of Mn, Mn is in the reduced, Mn(II), state. In addition to field observations, the photolytic reduction of amorphous iron hydroxide (am-Fe(OH)_3), lepidocrocite (y-FeOOH), goethite (a-FeOOH), hematite (α-Fe_2O_3), and natural iron containing aerosol particles in the presence of formaldehyde, formate, acetate, oxalate, and butyrate has been investigated in the laboratory. Important parameters in the photoreduction experiments are pH, wavelength of the irradiating light, the nature of the electron donor, and the characteristics of the iron phase. The results show that the fastest rates of photoreduclion of Fe( III) to Fe(II) are achieved with am-Fe(OH)_3 as the electron acceptor and formate as the electron donor. Ambient iron-containing aerosol particles with oxalate as the electron donor resulted in a significant photochemical production of H_2O_2.
© 1994 Laser Pages Publishing. The authors want to thank Drs. J.O. Nriagu and Y.K. Chau and the staff aboard the CSS Limnos of Canadian Center for Inland Waters, Burlington, Ontario for the opportunity to participate in sampling in the Great Lakes. The assistance of the staff at Whiteface Mt. Field Station at SUNY Albany, including Ken Dernerjian , Sheema B. Roychowdhury, and Scoll Wolfe is greatly appreciated. Cliff Weinman and his staff at the FAA radar facility in San Pedro have been helpful during our sampling in southern California. The authors also wish to thank George Luther, Torn Church, Alain Veron, and the crew of RV Cape Henlopen of the College of Marine studies at the University of Delaware, and Ted Murphy and Mel Dutton of the California State University, Bakersfield.