Observed and modeled HOCl profiles in the midlatitude stratosphere: Implication for ozone loss

Vertical profiles of stratospheric HOCl calculated with a diurnal steady-state photochemical model that uses currently recommended reaction rates and photolysis cross sections underestimate observed profiles of HOCl obtained by two balloon-borne instruments, FIRS-2 (a far-infrared emission spectrometer) and MkIV (a mid-infrared, solar absorption spectrometer). Considerable uncertainty (a factor of two) persists in laboratory measurements of the rate constant (k1) for the reaction ClO + HO2 ! HOCl + O2. Agreement between modeled and measured HOCl can be attained using a value of k1 from Stimpfle et al. (1979) that is about a factor-of-two faster than the currently recommended rate constant. Comparison of modeled and measured HOCl suggests that models using the currently recommended value for k1 may underestimate the role of the HOCl catalytic cycle for ozone depletion, important in the midlatitude lower stratosphere.