Retrieval of tropospheric BrO columns from TROPOMI and their validation using MAX-DOAS measurements in Ny-Ålesund

Bromine compounds play an important role in atmospheric chemistry with respect to ozone chemistry and the resulting oxidation capacity. Large amounts of reactive bromine can be released by an autocatalytic heterogeneous mechanism called "bromine explosion", and plumes of enhanced bromine monoxide (BrO) have been observed over polar sea ice regions by satellite measurements in spring. These enhancements of BrO columns result from increases in stratospheric or tropospheric bromine columns or both. As nadir-viewing UV-visible spectrometers have limited vertical resolution, it is not straight-forward to separate total BrO columns into tropospheric and stratospheric partial columns using satellite data.In this study, an algorithm for tropospheric BrO retrieval from satellite measurements including TROPOMI, which provides much improved spatial resolution, was developed. The retrieval algorithm is based on the Differential Optical Absorption Spectroscopy (DOAS) technique and three different stratospheric correction methods were tested based on: output from a 3D atmospheric chemistry model, a climatology of stratospheric BrO profiles, and an empirical multiple linear regression model to separate the tropospheric partial column from the total column.Retrieved tropospheric BrO columns from satellite measurements were compared with ground-based MAX-DOAS BrO observations at the NDACC station in Ny-Ålesund. The comparisons between ground-based and satellite measurements of tropospheric BrO show good agreement in both time-series and scatter plots, demonstrating the satellite retrieval algorithm is valid and applicable to study bromine release in the tropospheric layer. In particular, TROPOMI shows improved validation results for short distance collection compared to previous satellite data, which suggests the applicability of high-resolution satellite data on small-scale bromine explosion events observed during the MOSAiC campaign.