Measurement and interpretation of cloud effects on the concentrations of hydrogen peroxide and organoperoxides over Ontario, Canada

Measurements of gaseous hydrogen peroxide (H 2O 2) and organoperoxides (ROOH) in clear air and cloudy air were made from a Twin Otter aircraft over central Ontario, Canada, during March and April of 1990 as part of the Eulerian Model Evaluation Field Study (EMEFS). In clear boundary layer air (altitude 500 m to 1750 m), unperturbed by the presence of cloud, the ratio [H 2O 2]/[ROOH] depends on NO 2 mixing ratios, while in the presence of cloud perturbing H 2O 2, the ratio [H 2O 2]/[ROOH] was significantly decreased compared with those in clear air mainly due to the high solubility of H 2O 2. In order to examine the effects of cloud processing on air that was detrained from cloud, we selected measurements made in cloud-free air located between two passes made in air with high liquid water content. Gaseous H 2O 2 concentrations in these detrained air masses were often higher than those in the cloudy air, and sometimes higher than those in the clear air for high NO 2 conditions (> 500 ppt) but lower than clear-sky levels for low NO 2 (< 500 ppt) conditions. This result is in general agreement with that from photochemical modeling, namely, for chemical regimes typical of high-NO 2 (more polluted) conditions, where aqueous-phase reactions of dissolved HO 2 lead to net production of H 2O 2(aq) that can subsequently be detrained from the cloud.