Aqueous-phase photoproduction of hydrogen peroxide in authentic cloud waters: Wavelength dependence, and the effects of filtration and freeze-thaw cycles

Quantum yields (Φλ) of hydrogen peroxide (HOOH) photoproduction are reported at 313, 334, and 366 nm for authentic cloud waters collected over a three-year period (1991–1993) at Whiteface Mountain, New York. Quantum yields of HOOH production (based on the total sample absorbance) in these cloud waters ranged up to 0.014. For each sample the HOOH quantum yield decreased with increasing wavelength: Φ313 > Φ334 > Φ366. Action spectra for HOOH photoproduction in these cloud waters indicate that light between 290 and 380 nm is primarily responsible for the aqueous-phase HOOH photoproduction in tropospheric water drops. Quantum yields at 334 and 366 nm were each highly correlated with quantum yields at 313 nm. The cloud water absorbances per centimeter at 334 and 313 nm were also highly correlated. This suggests that similar classes; of chromophores were responsible for the aqueous-phase HOOH photoformation in the cloud waters from different events over a three-year period. The chromophores have not been identified, thus absolute quantum yields for HOOH formation from specific chromophores in an authentic sample could not be calculated. Comparisons of wavelength-dependent relative quantum yields in a given authentic cloud water sample (assuming Fe(oxalate)2− was the primary source of HOOH) with those for Fe(oxalate)2− in well-defined aqueous systems indicate that photolysis of Fe(oxalate)2− was not the primary source of HOOH photoproduction in any of the nine authentic cloud water samples studied. Filtration (0.5 μm Teflon) caused only small differences ( 15% increase) in rates of HOOH photoformation versus unfiltered controls. Filtration through a 0.05 μm polycarbonate membrane filter did not cause distinguishable differences in the ultraviolet-visible absorption spectra of a cloud water sample previously filtered with a 0.5 μm Teflon filter. These observations suggest that particles were neither the dominant source nor the dominant sink of the photoproduced HOOH. Freezing (up to 403 d) caused only minor changes in HOOH photoproduction rates (−9 to + 12%), and in the ultraviolet-visible absorbance spectra and pH values of the cloud water studied. These observations indicate that freeze-thaw cycles in clouds will have only minor influences on the aqueous-phase photoformation of HOOH.