Trends of stratospheric BrO (1995 to 2025) derived from ground-based DOAS observations at Kiruna, Northern Sweden

Stratospheric bromine originates partly from natural and partly from anthropogenic sources. After the Montreal protocol and following amendments, anthropogenic emissions (halons and methyl bromide) were largely reduced. This reduction was not only seen in tropospheric in situ measurements, but also in a reduction of stratospheric bromine levels (estimated from BrO measurements) after about 2002.

Here, we report on ground-based observations of stratospheric BrO carried out between 1995 and present in Kiruna (northern Sweden). The (slant) column density of BrO is analysed from UV spectra of zenith scattered sun light at solar zenith angles of 80° and 90°. Our measurements are in agreement with predictions predictions and existing observational data sets for the period before about 2019 for which a steady decline of the BrO levels is found. However, after 2019 the stratospheric BrO levels increased again, in contrast to the expected trend. The reason for this discrepancy is not yet known. Possible explanations might be increasing natural emissions of methyl bromide and/or very short lived bromine containing compounds, perhaps related to climate change (e.g. a warmer sea surface). Also recently again increasing anthropogenic emissions of methyl bromide might contribute. The variation of stratospheric aerosols is unlikely to explain the changed trend after 2019.