An investigation of C1O photochemistry in the chemically perturbed arctic vortex

A new lightweight in, situ instrument designed to measure ClO was flown on a balloon launched into the arctic vortex at dawn on February 3, 1995 at Ki runa, Sweden during the Second European Stratospheric Arctic and Mid-latitu de Experiment (SESAME), together with instruments to measure ozone and long -lived tracers. Observations on ascent and descent at different solar zenit h angles are compared to results from Lagrangian and box model calculations that assume the airmasses at similar potential temperatures had comparable photochemical histories. Between 20 and 22 km, in a region where ClO was s ignificantly enhanced, a model constrained by currently recommended rate pa rameters significantly underestimates the abundances of ClO that were obser ved on ascent at High solar zenith angles, whereas the agreement is much be tter if a smaller ClO-Cl2O2 equilibrium constant, one inferred from previou s ER-2 aircraft observations of ClO in the Arctic during nighttime, is assu med. On ascent, ClO is additionally enhanced in a narrow region between 20 and 21 km. We believe the most plausible explanation for this feature is ra pid photolysis of OClO produced by the slow bimolecular reaction ClO + ClO over the 48 hours prior to the observations when the airmass was warmed to 225 K by adiabatic compression while in polar darkness. These results sugge st that under special circumstances, OClO can be produced by a reaction oth er than one involving BrO, and, hence, OClO is not necessarily a universal proxy for BrO abundances in the perturbed polar vortex.