Tests of halogen photochemistry using in situ measurements of ClO and BrO in the lower polar stratosphere

In situ observations of the halogen oxides ClO and BrO made from the NASA ER‐2 during the Airborne Arctic Stratospheric Expedition (AASE) I and II missions are used to test current understanding of photochemical parameters. Measurements of ClO obtained during AASE I in the dark perturbed polar vortex are analyzed with respect to temperature to derive the equilibrium expression for the ClO/Cl 2 O 2 system. Assuming photochemical steady state and complete activation of chlorine (ClO + 2Cl 2 O 2 = Cl y ), observations of ClO made during AASE II are used to derive the photolysis rate of Cl 2 O 2 . The photolysis rate derived from atmospheric observations is compared to J values calculated with a photochemical model and various values for the absorption cross section of Cl 2 O 2 . The photolysis rate calculated with the cross section of Huder and DeMore [1995] is shown to be systematically too small, while those of Burkholder et al . [1990] and Cox and Hayman [1988] are too large to be consistent with atmospheric observations. Observations of BrO made during AASE II indicate that our understanding of the inorganic bromine budget in the polar regions is incomplete. A possible role for the adduct BrOOCl is investigated.