Airborne hydrogen peroxide measurements in supercooled clouds

Airborne measurements of hydrogen peroxide and its partitioning between the aqueous and gas phases were conducted uring studies of wintertime orographic clouds at temperatures ranging between -9 ø and -24øC. A subset of the derived values of the H202 Henry's law coefficient, based on measurement of gas-phase H202 and the physical properties of the cloud, were found to agree with laboratory measurements extrapolated to the cloud temperature. Disparities between the derived and temperature-extrapolated values were also observed, but this was attributed to overestimates of the in-cloud H202 mixing ratio that result from solute H202 volatilization on the reverse-flow air sample inlet during cloud droplet impaction and freezing. This hypothesis is supported by the fact that the derived Henry's law coefficients which exhibit agreement with the temperature-extrapolated values were conducted using a narrower inlet which intercepts less cloud water. Also discussed are vertical profiles of gaseous H202 within and above a cloud-capped boundary layer and in-cloud measurements which reveal that H202 scavenging by ice hydrometeors is minimal in comparison to uptake by an equivalent mass of liquid cloud droplets.