Observations of the balloon-borne LPMA/DOAS remote sensing instruments performed in the Arctic stratosphere in February 1999 are used to constrain a photochemical model. Measurements of all relevant nitrogen, chlorine, and bromine species indicate that moderate heterogeneous chlorine activation occurred in a filament of the Arctic vortex. Model-measurement comparisons for OClO serve as an indicator of how well various scenarios of the involved reaction kinetics, in particular of the ClO-BrO and the ClO-ClO cycles, reproduce the observations. Recent suggestions for the photolysis rate of the ClO dimer, the equilibrium constant between ClO dimer and monomer, the rate of the ClO-ClO association reaction, and the branching ratio of the ClO-BrO reaction are consistent with the observations. Formation of an unstable isomer of ClONO2 cannot be reconciled with the observations. Modeled odd oxygen loss rates can be larger by 10% to 20% for the updated reaction kinetics compared to standard recommendations.