[1] Ground-based zenith sky UV-visible measurements of stratospheric bromine monoxide (BrO) slant column densities are compared with simulations from the SLIMCAT three-dimensional chemical transport model. The observations have been obtained from a network of 11 sites, covering high and midlatitudes of both hemispheres. This data set gives for the first time a near-global picture of the distribution of stratospheric BrO from ground-based observations and is used to test our current understanding of stratospheric bromine chemistry. In order to allow a direct comparison between observations and model calculations, a radiative transfer model has been coupled to the chemical model to calculate simulated slant column densities. The model reproduces the observations in general very well. The absolute amount of the BrO slant columns is consistent with a total stratospheric bromine loading of 20 ± 4 ppt for the period 1998-2000, in agreement with previous estimates. The seasonal and latitudinal variations of BrO are well reproduced by the model. In particular, the good agreement between the observed and modeled diurnal variation provides strong evidence that the BrO-related bromine chemistry is correctly modeled. A discrepancy between observed and modeled BrO at high latitudes during events of chlorine activation can be resolved by increasing the rate constant for the reaction BrO + ClO ! BrCl + O 2 to the upper limit of current recommendations. However, other possible causes of the discrepancy at high latitudes cannot be ruled out.

When sighted from the ground in clear weather, stratospheric clouds make large changes in the sky color during twilight. Spectrometric measurements performed from the ground during CHEOPS show that the color changes can be either large reddenings or blueings. A radiative transfer model demonstrates that the first are caused by thin hazes above 22 km while the second are related to thick clouds below, and that the color change during twilight is little sensitive to tropospheric clouds. Statistics of presence of PSC's above Kiruna during CHEOPS which shows that reddenings are correlated with temperature below NAT condensation at high altitude (30 hPa, 21.5 km) and blueings at low altitude (50 hPa, 18.5km), support this interpretation, as also the consistency with PSC measurements with in‐situ aerosol counters, balloon radiometer and from satellite observations. Spectrometric observations of the zenith sky at twilight is therefore thought to be a powerful method for identifying the presence and the altitude of PSCs above a station in all tropospheric weather conditions.

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Abstract.We have used a three-dimensional stratospheric model to investigate 1996/1997, the mean difference is negative and the amplitude is less than 15%. In winter 1995/1996 the mean difference is negative and the amplitude reaches 43%.The differences in 1995/1996 and 1996/1997 result despite good agreement between model and observations in midwinter, and they develop following the large chemical losses which occurred in those winters. We suggest that a major contribution to the model/observation differences for those two winters is a model underestimation of the chemical ozone loss.