Measured and modeled HOCl profiles in the mid-latitude stratosphere: implications for ozone loss

The HOCl catalytic cycle for ozone loss is important in the mid-latitude stratosphere. We use a diurnal steady-state photochemical model to calculate profiles of HOCl for conditions sampled by two high-altitude balloon-borne instruments, MkIV and FIRS-2. To assess how well this model represents ozone loss by the HOCl cycle, we compare our calculations of HOCl and its precursors, ClO and HO2, with balloon-borne measurements obtained by an FTIR solar absorption spectrometer (MkIV), a far-infrared emission spectrometer (FIRS-2), and a submillimeterwave limb sounder (SLS), and satellite-borne measurements obtained by a microwave limb sounder (MLS on Aura). We then evaluate these comparisons in light of a number of recent laboratory studies of the main formation mechanism of HOCl, the reaction of ClO + HO2. Comparisons between model and observations suggest better agreement might be achieved with a faster rate constant. Aura MLS retrievals of HOCl are discussed in the context of the balloon measurements and model results.