Sentinel-5P+ Innovation Project OClO
The stratospheric ozone layer plays an important role for life on Earth as it absorbs a large part of the harmful UV radiation coming from the sun. The amount and vertical distribution of ozone in the stratosphere is determined by transport and by an equilibrium between chemical ozone production on the one hand and catalytic ozone destruction cycles on the other hand. Anthropogenic emissions of long-lived halogen containing substances such as CFCs and halons have disturbed this equilibrium as additional reactive halogens have been released in the stratosphere. This lead to global reductions in ozone columns and the annual appearance of the ozone hole over Antarcica in austral winter / spring. Strong ozone depleteion is also observed in Arctic winter / spring but only in years where the stratosphere is cold enough to facilitate formation of Polar Stratospheric Clouds (PSCs). As a reaction on the rapid loss of stratospheric ozone, the Montreal Protocol was signed in 1987, phasing out the emissions of many long-lived halogen containing substances. Several amendments to this protocol have in the last decades lead to further and more rapid decreases in emissions of of ozone depleting substances, and stratospheric halogen levels are already decreasing. Because of the long lifetimes of the emitted substances, it is expected that return to the ozone levels of the 1980s will take at least until 2050.
Stratospheric chlorine activation can be monitored directly by measuring ClO with microwave radiometry. In the UV/visible spectral range, the OClO molecule can be retrieved as it has a structured absorption spectrum. As the only known formation of OClO is by reaction of ClO and BrO, the amounts of OClO are proportional to the concentrations of these two species. With BrO concentrations being much less variable than those of ClO, OClO can be used as a quantitative measure of chlorine activation at least at solar zenith angles around twilight.
Retrievals of OClO have been performed for all UV/vis heritage instruments (GOME, SCIAMACHY, GOME2, OMI) and the S5P OClO product will act as a continuation of these timeseries. Atmospheric profiles of OClO have also been retrieved from SCIAMACHY, OSIRIS and GOMOS measurements, providing additional information on the vertical distribution of OClO. For the validation of the S5P OClO product, ground-based observations of OClO from instruments in the NDACC network can be used.
Here, data will become available for download in the second year of the project.
In the figure shown above, a preliminary S5p OClO data product (V0.5) has been used to compare the evolution of chlorine activation in the southern hemispheric Polar Vortex in the years 2018 and 2019. As OClO is rapidly photolized, only data taken at 90° solar zenith angle are used. It is important to keep in mind that this selection creates a latitudinal sampling of the vortex which depends on the orbit of the satellite and varies with season. The figure can therefore not directly be compared between instruments operating in different orbits (such as for example the GOME2 instruments which are in morning orbits).
As can be seen from the S5p data, chlorine activation in the southern hemisphere was lower in 2019 than in 2018 from late June, and in particular the stratospheric warming in early September lead to rapid deactivation, much earlier than in other years with the exception of the split vortex event in 2002.