Tropospheric BrO in Arctic Spring 2019 measured by S5-P/TROPOMI - A statistical analysis of the spatial expansion and shape of tropospheric BrO plumes

Halogen radicals can drastically alter the atmospheric chemistry. In the polar regions, this is made evident by the ozone depletion in the stratosphere (ozone hole) but also by destruction of boundary layer ozone during polar springs. These recurrent episodes of catalytic ozone depletion, better known as “ozone depletion events” (ODEs) are driven by enhanced concentrations of reactive bromine compounds. The proposed mechanism by which these compounds are released into the troposphere is known as “bromine explosion” - reactive bromine is formed autocatalytically from the condensed phase.

In comparison to previous satellite missions, the TROPOspheric Monitoring Instrument (TROPOMI) onboard ESA’s S5-P satellite allows an improved localization and a more precise specification of these events due to its superior spatial resolution of up to 3.5 x 5.5 km². Together with the better than daily coverage over the polar regions, this allows for investigations of the spatio-temporal variability of enhanced BrO levels and their relation to different possible bromine sources and release mechanisms.

We present tropospheric BrO column densities retrieved from TROPOMI measurements using Differential Optical Absorption Spectroscopy (DOAS). One advantage of our retrieval is its independence from any external input data, thereby avoiding systematic biases from external datasets. We used a modified k-means clustering and methods from statistical data analysis to separate tropospheric and stratospheric partial columns, relying only on NO₂ and O₃ columns measured by the same instrument. This ensures in particular that the derived tropospheric BrO data set keeps the same high spatial resolution as the TROPOMI instrument, because no model data with coarse resolution is used. In a second step, the retrieved tropospheric slant column densities (SCDs) are converted to vertical column densities (VCDs).

TROPOMI’s improved spatial resolution is then utilized to study the spatial extent and shape of BrO plumes detected in the Arctic region. For this, a combination of morphological filters and connected component labeling is used to provide a statistical overview of all enhanced BrO plumes detected above certain sensitivity thresholds in the Arctic spring of 2019. This provides a lower limit for the spatial extent of enhanced BrO events (and hence also for ODEs) of around 40-60 km. Additionally, seasonal trends in size and shape of the BrO plumes as well as correlations to relevant meteorological parameters are investigated.