Abstract. We report on time-dependent O₃, NO₂ and BrO profiles measured by limb observations of scattered skylight in the stratosphere over Kiruna (67.9° N, 22.1° E) on 7 and 8 September 2009 during the autumn circulation turn-over. The observations are complemented by simultaneous direct solar occultation measurements around sunset and sunrise performed aboard the same stratospheric balloon payload. Supporting radiative transfer and photochemical modelling indicate that the measurements can be used to constrain the ratio J(BrONO₂)/k_BrO+NO2, for which at T = 220 ± 5 K an overall 1.7 (+0.4 −0.2) larger ratio is found than recommended by the most recent Jet Propulsion Laboratory (JPL) compilation (Sander et al., 2011). Sensitivity studies reveal the major reasons are likely to be (1) a larger BrONO₂ absorption cross-section σ_BrONO2, primarily for wavelengths larger than 300 nm, and (2) a smaller k_BrO+NO2 at 220 K than given by Sander et al. (2011). Other factors, e.g. the actinic flux and quantum yield for the dissociation of BrONO₂, can be ruled out.

The observations also have consequences for total inorganic stratospheric bromine (Br_y) estimated from stratospheric BrO measurements at high NO_x loadings, since the ratio J(BrONO₂)/k_BrO+NO2 largely determines the stratospheric BrO/Br_y ratio during daylight. Using the revised J(BrONO₂)/k_BrO+NO2 ratio, total stratospheric Br_y is likely to be 1.4 ppt smaller than previously estimated from BrO profile measurements at high NO_x loadings. This would bring estimates of Br_y inferred from organic source gas measurements (e.g. CH₃Br, the halons, CH₂Br₂, CHBr₃, etc.) into closer agreement with estimates based on BrO observations (inorganic method). The consequences for stratospheric ozone due to the revised J(BrONO₂)/k_BrO+NO2 ratio are small (maximum −0.8%), since at high NO_x (for which most Br_y assessments are made) the enhanced ozone loss by overestimating Br_y is compensated for by the suppressed ozone loss due to the underestimation of BrO/Br_y with a smaller J(BrONO₂)/k_BrO+NO2 ratio.