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Carbonyl Sulphide: new ways of Observing the Climate System

Periodic Reporting for period 2 - COS-OCS (Carbonyl Sulphide: new ways of Observing the Climate System)

Reporting period: 2019-03-01 to 2020-08-31

Carbonyl sulphide (COS) is the most abundant sulphur compound in our atmosphere with a mixing ratio of about 500 parts per trillion (ppt). Surprisingly, its global budget is poorly known, with recent updates of the dominant budget terms of more than 100%. The budget of COS is dominated by emissions from (tropical) oceans and uptake by the terrestrial biosphere, but significant uncertainties exist. Due to its long atmospheric lifetime of about 2 years, COS is transported to the stratosphere where its photochemical breakdown contributes to the formation of Stratospheric Sulphate Aerosols (SSA). Since these aerosols cool our climate, this is a first link between the sulphur cycle and climate.

A second link to climate is the uptake of COS by the terrestrial biosphere. COS has been suggested as a possible new constraint on the atmospheric carbon cycle, but only if some large caveats in our knowledge of its budget are resolved. If the COS budget is better understood, there are good prospects to use COS to constrain gross primary productivity (GPP) of terrestrial vegetation. Recent analyses of COS stored in ice cores link observed changes in COS to variations in GPP during the Holocene. Without a drastic reduction in COS source and sink uncertainties and their relation with climate, these claims remain however rather speculative.

The two big scientific questions that are addressed in this reseach are:

1) What is the contribution of COS to the stratospheric sulphate aerosol layer?
2) Can the global budgets of COS and CO2 be reconciled, and what are the implications for terrestrial gross primary productivity?

This is important for society, because the impact of ongoing climate change on the carbon cycle is uncertain. Will large carbon reservoirs become unstable in a warming climate (positive feedback) or will the biosphere take up more carbon (negative feedback)?
Fundamental knowledge about these processes advances our understanding on how these intriguing mechanisms work.

The objectives of the proposed research are to:

1) Perform the first world-wide characterisation of COS isotopologues by measuring seasonal, latitudinal, and altitude variations in troposphere and stratosphere
2) Measure COS gradients over the troposphere–stratosphere transition up to 30 km altitude using innovative AirCore sampling techniques
3) Investigate fractionation effects during soil and plant uptake in laboratory experiments
4) Develop the first model with capabilities to simulate COS-isotopologues and the coupled COS and CO2 cycles
5) Pioneer the use of satellite observations of COS and its isotopologues
6) Constrain the budgets of COS and CO2 using inverse modelling techniques employing surface measurements, satellite data, and new AirCore and isotopic COS measurements
Results of the project can be found on the project website:

Highlights include:
a) we performed a first successful campaign with the novel AirCore technique. With a balloon a thin tube is brought to the stratosphere and on descent air is sampled. This resulted in the first COS profiles in the stratosphere.
b) we developed a model for COS. Using surface observations we are able to optimise the surface fluxes such that observations are almost perfectly fitted world-wide (see illustration)
c) we have been able to simulate and evaluate exchange of COS with the biosphere. We use a mechanistic model called SiB4. This model is being further developed in cooperation with the scientific community
d) we developed a technique to measure the S-isotopologues of COS. We measured COS (& isotopologues) in air sampled from the stratosphere that we had available in our lab. Results are currently being analysed.
e) we compared our model results to satellite observations of COS.
1) We now have a first version of data-assimilation techniques available. Together with other EU groups that work on COS (France, UK) we currently conduct a model inter comparison. COS is interesting to constrain the carbon cycle. That's why we are involved in the Copernicus-related project CoCo2, that is currently being evaluated for funding.
2) We are the first that are able to conduct AirCore measurements of COS in the stratosphere. We also managed to embark our equipment on HEMERA, a balloon experiment that will sample stratospheric air. We are determined to get air samples from the stratosphere from this campaign.

Currently we have the first publications in the writing phase. We expect 4 PhDs to graduate on COS-OCS research. (1) COS inverse modelling and isotope modelling (2) COS-CO2 coupled inverse modelling (3) AirCore analysis of COS (4) isotopologue measurements of COS.