The combined assimilation of MIPAS carbonyl sulfide (COS) and NOAA surface observations in TM5-4DVAR: Consequences for the global COS budget.

The atmospheric budget of carbonyl sulfide (COS, lifetime ~2 years) is primarily determined by emissions from anthropogenic and oceanic sources and uptake by the biosphere. Once the budget of COS is adequately understood, COS could be a suitable tracer to estimate Gross Primary Productivity (GPP), since stomatal uptake by plants is basically a one-way process, unlike the assimilation of CO2. However, currently the global budget of COS is not closed. Here, we will report progress on the use of inverse modelling to better constrain the atmospheric COS budget. To that end, we assimilate data from the MIPAS instrument that flew onboard the ENVISAT satellite (2002–2012). MIPAS is a limb sounder that measures atmospheric emission profiles down to the upper troposphere. Tropospheric COS retrievals are assimilated together with NOAA COS surface observations, and a bias correction scheme is employed to correct for potential calibration differences. Using the 4DVAR-TM5 model, we derive a consistent global COS budget. However, evaluation with independent data reveals that TM5 remains biased low in the free troposphere. We will show that this underestimate may be resolved by accounting for an aqueous-phase oxidation process of the newly discovered HydroPeroxyMethylThioFormate (HPMTF) intermediate in the DMS oxidation chain.