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For decades, reactive halogen species (RHSs) have been the subject of detailed scientific research due to their influence on the oxidizing capacity of the atmosphere and on the climate. From the RHSs, those containing bromine are of particular interest in the polar troposphere as a result of their link to ozone-depletion events (ODEs) and to the perturbation of the cycle of toxic mercury, for example. Given its remoteness and related limited accessibility compared to the Arctic region, the RHSs in the Antarctic troposphere are still poorly characterized. This work presents ground-based observations of tropospheric BrO from two different Antarctic locations: Marambio Base (64°13′ S, 56°37′ W) and Belgrano II Base (77°52′ S, 34°7′ W) during the sunlit period of 2015. By means of MAX-DOAS (Multi-axis Differential Optical Absorption Spectroscopy) measurements of BrO performed from the two research sites, the seasonal variation in this reactive trace gas is described along with its vertical and geographical distribution in the Antarctic environment. Results show an overall vertical profile of BrO mixing ratio decreasing with altitude, with a median value of 1.6 pmol mol−1 in the lowest layers of the troposphere. Additionally, observations show that the polar sunrise triggers a geographical heterogeneous increase in bromine content in the Antarctic troposphere yielding a maximum BrO at Marambio (26 pmol mol−1), amounting to 3-fold the values observed at Belgrano at dawn. Data presented herein are combined with previous studies and ancillary data to update and expand our knowledge of the geographical and vertical distribution of BrO in the Antarctic troposphere, revealing Marambio as one of the locations with the highest BrO reported so far in Antarctica. Furthermore, the observations gathered during 2015 serve as a proxy to investigate the budget of reactive bromine (BrOx = Br + BrO) and the bromine-mediated ozone loss rate in the Antarctic troposphere.

Three years of multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements (2011–2013) have been used for estimating the NO2 mixing ratio along a horizontal line of sight from the high mountain subtropical observatory of Izaña, at 2370 m a.s.l. (NDACC station, 28.3° N, 16.5° W). The method is based on horizontal path calculation from the O2–O2 collisional complex at the 477 nm absorption band which is measured simultaneously to the NO2 column density, and is applicable under low aerosol-loading conditions. The MAXDOAS technique, applied in horizontal mode in the free troposphere, minimizes the impact of the NO2 contamination resulting from the arrival of marine boundary layer (MBL) air masses from thermally forced upwelling breeze during middle hours of the day. Comparisons with in situ observations show that during most of the measuring period, the MAXDOAS is insensitive or very slightly sensitive to the upwelling breeze. Exceptions are found for pollution events during southern wind conditions. On these occasions, evidence of fast, efficient and irreversible transport from the surface to the free troposphere is found. Background NO2 volume mixing ratio (vmr), representative of the remote free troposphere, is in the range of 20–45 pptv. The observed seasonal evolution shows an annual wave where the peak is in phase with the solar radiation. Model simulations with the chemistry–climate CAM-Chem model are in good agreement with the NO2 measurements, and are used to further investigate the possible drivers of the NO2 seasonality observed at Izaña.