The cloudwater chemistry of iron and copper at Great Dun Fell, U.K.

The role of iron and copper in the transformation of photooxidants (HO2/O2−) and pollutants in clouds was examined at a rural site (Great Dun Fell, U.K.). The observed concentrations of dissolved iron (60–1600 nM) and copper (4.2–31 nM) were lower than those previously reported at other locations. Measurement of the oxidation state of dissolved iron provided insight into the rates and mechanisms of cloudwater transformation reactions. Whenever the iron-containing aerosols were nucleated for more than several minutes prior to collection, measured iron oxidation states agreed with predictions based upon known rate constants for iron redox reactions. During daytime, the reduction of dissolved Fe(III) by HO2/O2− and Cu(I) or the photoreduction of Fe(Ill)-oxalato complexes resulted in the establishment of a steady state, with respect to iron oxidation states, in which more than 50% of the dissolved iron was present as Fe(II). At night, Fe(II) was slowly oxidized by H2O2 and O3.