Observations of BrO and its vertical distribution during surface ozone depletion at Alert

During the ALERT2000 polar sunrise experiment at Alert, Nunavut, Canada, we performed measurements of boundary layer bromine oxide radicals (BrO) by differential optical absorption spectroscopy (DOAS) using scattered sunlight in the spectral range from 320 to 400 nm. For the first time the Multi-Axis-(MAX)-DOAS method was applied to derive vertical profile information of BrO. BrO was observed at slant column densities (SCD) of up to 1015 molecules/cm2 during a 10-day period of complete surface ozone depletion. The largest BrO column densities were found by observing scattered sunlight from 5° above the horizon, and SCDs were decreasing with increasing elevation angles of the light-receiving telescope. For zenith scattered light the lowest absorption was recorded. Radiative transfer modelling and the calculation of air mass factors show that in most cases the bulk of the observed BrO was present in a layer of 1±0.5 km thickness above the surface (in the boundary layer). The inferred extent of the BrO layer agrees very well with the observed height of the ozone depletion layer (Bottenheim et al., Atmos. Environ., 2002) from ozone sonde data. Assuming that BrO layer is well-mixed, volume mixing ratios reached levels of 20–30 ppt BrO. These values are consistent with previous measurements of BrO during low ozone events in the Arctic boundary layer.