The role of iron and copper in the transformation of photooxidants (HO
2/O-2(-)) and pollutants in clouds was examined at a rural site (Great
Dun Fell, U.K.). The observed concentrations of dissolved iron (60-16
00 nM) and copper (4.2-31 nM) were lower than those previously reporte
d at other locations. Measurement of the oxidation state of dissolved
iron provided insight into the rates and mechanisms of cloudwater tran
sformation reactions. Whenever the iron-containing aerosols were nucle
ated for more than several minutes prior to collection, measured iron
oxidation states agreed with predictions based upon known rate constan
ts for iron redox reactions. During daytime, the reduction of dissolve
d Fe(III) by HO2/O-2(-) and Cu(I) or the photoreduction of Fe(III)-oxa
lato complexes resulted in the establishment of a steady state, with r
espect to iron oxidation states, in which more than 50% of the dissolv
ed iron was present as Fe(II). At night, Fe(II) was slowly oxidized by
H(2)O2 and O-3. (C) 1997 Elsevier Science Ltd.