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Description
Bromine monoxide (BrO) is a halogen radical altering – inter alia – the atmospheric ozone and radical chemistry. High BrO/SO$_2$ ratio was seen at the outer part of the plume, while in the center region of the plume BrO/SO$_2$ ratios were found to be lower. These measurements obtained from ground-based as well as airborne instruments suggest that the formation of BrO – a secondary product of volcanic gas – might be hindered in the center region of the plume, where it is limited by the mixing in of ambient ozone.
Here, we present BrO and SO$_2$ column densities of the volcanic plume of Mount Etna on Christmas 2018 measured by the TROPOspheric Monitoring Instrument (TROPOMI) onboard ESA’s Sentinel-5P satellite using the Differential Optical Absorption Spectroscopy (DOAS) technique. The high spatial resolution of S5-P/TROPOMI (up to 3.5x5.5km²) allows to resolve spatial variation of the BrO/SO$_2$ ratio in the volcanic plume and thus to differentiate between different plume parts. Dense, i. e. SO$_2$ rich parts in the center of the early plume (several 100kms distance from volcanic vent) show low BrO/SO$_2$ ratios in the order of several 10$^{-5}$. The BrO/SO$_2$ ratio increases for the 8-20h older , more diluted plume, where it reaches several 10$^{-4}$ in the center (for distances up to 1500km from the vent). Throughout the whole plume we observe an increase in the BrO/SO$_2$ ratio towards the edges of the plume, where the BrO/SO2 molar ratio is 3-6x10$^{-4}$. These data indicates the relevance of ambient ozone (and free radical) intrusion also for older volcanic plumes thousand kilometer downwind from the volcanic vent.
