Chlorine activation as seen by TROPOMI OClO 2017 – 2025 measurements

Chlorine dioxide (OClO) is a by-product of the ozone depleting halogen chemistry in the stratosphere. Although being rapidly photolysed during daytime, it plays an important role as an indicator of the chlorine activation in polar regions during polar winter and spring under twilight conditions because of the nearly linear dependence of its formation on chlorine oxide (ClO).

The TROPOspheric Monitoring Instrument (TROPOMI) is an UV-VIS-NIR-SWIR instrument on board the Sentinel-5P satellite developed for monitoring the composition of the Earth’s atmosphere. Launched on 13 October 2017 in a near polar orbit, it provides    continuous monitoring possibilities for many constituents including the observations of OClO at an unprecedented spatial resolution.

We analyze the time series (2017 – 2025) of slant column densities (SCDs) of chlorine dioxide (OClO) at polar regions. Especially we focus on the higly variable conditions in the NH polar region by comparing the OClO timeseries with meteorological data. This allows us to investigate the conditions under which the chlorine activation starts and ends.