Quantifying the rate of heterogeneous processing in the Arctic polar vortex with in situ observations of OH

We present simultaneous in situ observations of OH, HO2, ClONO2, HCl, and particle surface area inside a polar stratospheric cloud undergoing rapid heterogeneous processing. A steady‐state analysis constrained by in situ observations is used to show that concentrations of OH calculated during a processing event are extremely sensitive to the assumptions regarding aerosol composition and reactivity. This analysis shows that large perturbations in the abundance of OH are consistent with the heterogeneous production of HOCl via ClONO2 + H2O → HOCl + HNO3 and removal via HOCl + HCl → Cl2 + H2O in a polar stratospheric cloud. If the cloud is composed of supercooled ternary solution (STS) aerosols and solid nitric acid trihydrate (NAT) particles, comparison with observations of OH show that modifications to surface reactivity to account for high HNO3 content in STS aerosols and low HCl coverage on NAT particles are appropriate. These results indicate that with the low HCl levels in this encounter and in a processed polar vortex in general, reactions on STS aerosols dominate the total heterogeneous processing rate. As a consequence, the formation of NAT does not lead to significantly faster reprocessing rates when HCl concentrations are low and STS aerosols are present. Model calculations that include these modifications to uptake coefficients for STS and NAT will lead to significantly slower reprocessing and faster recovery rates of chlorine in the springtime Arctic polar vortex.