Factors influencing the retention of hydrogen peroxide and molecular oxygen in rime ice

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Abstract
Vertical transport of many atmospheric trace gases is modulated by retention in solid (ice phase) precipitation during the impaction and freezing (i.e., riming) of supercooled cloud droplets. Here we report field measurements of the concentrations of both hydrogen peroxide (H2O2) and molecular oxygen (O2) in rime ice. Gas retention efficiencies, defined as the ratio of the observed concentration divided by the equilibrium concentration predicted by Henry's law, are presented. Rime ice collections were conducted in a wind tunnel using 3.2‐ and 9.5‐mm impaction cylinders. Velocities were varied between 9 and 20 m/s and ice substrate temperatures were colder than −3.5°C. Averaged values of the retention efficiencies for H2O2 and O2 were 0.05 and 0.32, respectively. Also discussed are direct measurements of H2O2 volatilization obtained by monitoring increases in gaseous H2O2 associated with droplet impingement and freezing on an impaction grid placed upstream of a gas sampling inlet. The observed relationships between retention and the time interval between droplet impaction suggest that H2O2 and O2 are volatilized subsequent to droplet freezing and prior to burial by continued riming. A comparison of the timescales that are expected to govern retention indicates that solute diffusivities in the solidified droplets are ∼10−12 m2 s−1.