Hydrogen peroxide in natural cloud water: Sources and photoreactivity

Abstract The photochemical behaviour of H 2 O 2 in cloud aqueous phase was investigated from a large amount of in situ cloud droplet sampling data. Then we performed laboratory irradiation experiments of real cloud samples, free from gaseous chemistry, in order to factorise between gas phase and liquid phase photochemical reactions. H 2 O 2 was revealed to be sensitive to solar radiation, with higher concentrations during the day. H 2 O 2 was dependent on the degree of air mass pollution with anthropogenic influences leading to lower values of aqueous H 2 O 2 . We observed higher differences between day and night of H 2 O 2 concentrations for air masses influenced by anthropogenic activities (mainly Northern direction) compared to those form remote areas (oceanic). Contrary to this field diurnal trend, during laboratory irradiation experiments of the cloud aqueous phase, the concentration showed a regular linear decrease leading to the conclusion that photolysis is more important than photoproduction in cloud aqueous phase. So the diurnal cycle observed during field measurements is due to the high mass transfer from gas phase to liquid phase favoured by the high dispersion of liquid phase and the high solubility of hydrogen peroxide. As a consequence, H 2 O 2 can act as a photochemical source of OH radicals in cloud aqueous phase.