Sources of hydrogen peroxide in cloudwater

Abstract

Results of a theoretical investigation of H 2O 2 formation in cloud droplets arising from gaseous HO 2 radical scavenging are presented. It is shown that this process is pH dependent with the maximum rate of H 2O 2 production occurring below pH 3. This dependence arises as a result of the dissociation of HO 2 in water ( pK a = 4.9) and the subsequent disproportionation reaction of HO 2 and O 2 − to form hydrogen peroxide. O 2 − is also removed by reaction with O 3 to produce OH radicals and this process becomes more competitive as both the pH and O 2 − HO 2 ratio increase. The presence of soluble organic species, such as aldehydes, in cloudwater counteracts the effect of ozone by converting OH back to HO 2. For low pHs (< 3) the net contribution of organic solutes of H 2O 2 production is predicted to be relatively small, being limited by the availability of OH radicals scavenged from the gas phase. Existing cloud chemistry models may overestimate the rate of aqueous oxidation of formaldehyde by OH radicals. Under conditions where scavenging of gas-phase free radicals by cloud droplets is efficient, uptake of HO 2 radicals may be reversible. The aqueous concentration of OH is unlikely to approach thermodynamic equilibrium with the gas phase ( H ∼-30 M atm −1 and can be treated as irreversible. In clouds with a small mean droplet radius, efficient scavenging of precursor OH radicals should result in a decrease in gas-phase HO 2 production with a reduction in the yield of aqueous H 2O 2, although this is offset by the presence of soluble organic species. A similar effect is predicted for clouds with a high liquid water content. The supply of HO 2 and OH radicals to cloud droplets is controlled by gas-phase ozone chemistry which is in turn dependent on the solar u.v. radiation intensity. The u.v. density in clouds may be higher than in clear air when the solar zenith angle is small, thus enhancing H 2O 2 production, but falls off markedly as the solar zenith angle becomes larger. Predicted rates of H 2O 2 formation in clouds based on midday conditions are likely to be considerably higher than the average daytime value, particularly in summer. Diurnal and seasonal effects on H 2O 2 generation are expected to be more marked in clouds than in clear air.