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, as well as changing temperatures in the stratosphere, 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, column amounts of 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 acts as a continuation of these timeseries. Atmospheric profiles of OClO have also been retrieved from limb and occultation measurements of SCIAMACHY, OSIRIS and GOMOS, 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 are used.Andreas Richter for details on data access.
In the figures shown above, the evolution of chlorine activation in the two hemispehres is compared for the years 2018 to 2021. 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 OClO data, chlorine activation in the southern hemisphere is very similar in all four years. However, the timing of the end of activation varies between years and a stratospheric warming in early September lead to rapid deactivation in the year 2019, earlier than in other years with the exception of the split vortex event in 2002. In contrast, the year 2021 was characterized by a longer phase of chlorine activation, leading to one of the largest ozone depeltions on record.
In the northern hemisphere, activation depends strongly on stratospheric temperatures and thus dynamics. While Arctic winters 2018/2019 and 2020/2021 were typical with some chlorine activation in December and Janaury but overall low OClO columns, winter 2019 / 2020 was characterized by large OClO columns until late March. This strong chorine activation lead to the formation of the largest ozone hole observed in the northern hemisphere so far.
For validation, S5P OClO columns are compared to measurements from ground-based zenith-sky observations of instruments affiliated with the NDACC network. The OClO columns used are not an official NDACC producted but were provided on a best effort basis by the contributing institutions (see Team below). As the OClO retrievals from the ground-based instruments are not yet harmonised, a postprocessing has been applied using Langley-plots to estimate offsets in the data. Details on the validation procedure can be found in the validation reports avaialble in the Documents section.
In the figure, a summary validation is shown for S5P OClO columns version v0.97 over polar stations in both hemispheres, focusing on the activated period (July-August-September in the Southern Hemisphere and January-February-March in the Northern Hemisphere). Data from 2018 - 2021 is included as far as available at the time of figure creation (October 2021). As can be seen, excellent agreement is found in terms of correlation (R = 0.97), slope (S = 0.97) and offset (off = -1.6x1012 molec cm-2). More validation results can be found in the validation report available below.
Groundbased zenith-sky data for validation was provided by:
If you are interested in more information on the S5p Innovation project on OClO, please contact Andreas Richter.