Cloud processing of soluble gases

Experimental data from the Great Dun Fell Cloud Experiment 1993 were used to investigate interactions between soluble gases and cloud droplets. Concentrations of H2O2, SO2, CH3COOOH, HCOOH, and HCHO were monitored at different sites within and downwind of a hill cap cloud and their temporal and spatial evolution during several cloud events was investigated. Significant differences were found between in-cloud and out-of-cloud concentrations, most of which could not be explained by simple dissolution into cloud droplets. Concentration patterns were analysed in relation to the chemistry of cloud droplets and the gas/liquid equilibrium. Soluble gases do not undergo similar behaviour: CH3COOH simply dissolves in the aqueous phase and is outgassed upon cloud dissipation; instead, SO2 is consumed by its reaction with H2O2. The behaviour of HCOOH is more complex because there is evidence for in-cloud chemical production. The formation of HCOOH interferes with the odd hydrogen cycle by enhancing the liquid-phase production of H2O2. The H2O2 concentration in cloud therefore results from the balance of consumption by oxidation of SO2 in-cloud production, and the rate by which it is supplied to the system by entrainment of new air into the clouds.