Hydrogen Peroxide Retention in Rime Ice

The extent to which H202 dissolved in cloud droplets is trapped in rime ice affects the composition of precipitation and the rate of H202 removal from the atmosphere. Measurements were conducted in winter stratiform clouds at a remote mountain-top site in southeastern Wyoming, thus avoiding the difficulties of preparing laboratory clouds whose chemical and physical properties are similar to natural clouds. Quantities directly observed were H202 concentrations in cloud water collected as rime, air temperature, gaseous H202, 03, and SO2, and cloud liquid water concentration. Values of the retention coefficient, F, defined as the ratio of the H202 concentration in the melted rime sample divided by the equilibrium concentration in the supercooled droplets, were always less than unity (F = 0.24 •: 0.07). Corrections to account for the rapid reaction between dissolved H202 and sulfur(IV) increase the average value of the retention coefficient o only 0.30. An observed correlation between F and riming rate suggests that H202 is released to the gas phase during riming. These field measurements do not agree with laboratory determinations of F.