Development of AC SAF total and tropospheric BrO column retrieval for Sentinel-5/Metop-SGA

Kurzfassung

Bromine compounds play a key role in atmospheric chemistry, particularly in catalytic ozone destruction cycles that reduce ozone levels and change the atmospheric oxidation capacity. Enhanced amounts of BrO are frequently observed in polar regions, volcanic plumes, and over salt lakes/marshes. Continuous satellite-based UV–visible remote sensing of BrO is therefore essential for monitoring the evolution of atmospheric bromine and its interaction with a changing climate. Despite this importance, operational retrieval of global total and tropospheric BrO columns remains challenging due to several issues: (1) spectral fitting challenges caused by low signal-to-noise ratios for the weak BrO absorber, (2) a separation of stratospheric and tropospheric components due to a highly variable stratospheric background affected by chemical-dynamical conditions, (3) limited availability of reliable a priori model profile, and (4) large uncertainties in air mass factor calculations, particularly over high albedo snow/ice-covered surfaces where BrO enhancements frequently occur. Within the framework of the EUMETSAT Satellite Application Facility on Atmospheric Composition Monitoring (AC SAF), an operational retrieval algorithm for total and tropospheric BrO columns is being developed for Sentinel-5 (S5) onboard Metop-SGA. In particular, the S5 tropospheric BrO product will be the first global dataset to be processed operationally. The retrieval approach consists of three main steps: (1) BrO slant column density retrieval using the Differential Optical Absorption Spectroscopy (DOAS) technique with background correction, (2) estimation of the stratospheric BrO vertical column using an improved, observation-based climatology based on an extension of Theys et al. (2009), and (3) air mass factor calculation to derive total and tropospheric vertical columns. The S5 total and tropospheric BrO prototype algorithm has been developed and tested using Sentinel-5P/TROPOMI data, ensuring transfer to the S5 operational processing chain. We present total and tropospheric BrO retrieval results from S5P datasets and discuss the expected performance of S5 BrO product.