Combined assimilation of NOAA surface and MIPAS satellite observations to constrain the global budget of carbonyl sulfide

Abstract. Carbonyl sulfide (COS), a trace gas in our atmosphere that leads to the formation of aerosols in the stratosphere, is taken up by terrestrial ecosystems. Quantifying the biosphere uptake of (COS) could provide a useful quantity to estimate Gross Primary Productivity. Some COS sources and sinks still contain large uncertainties, and several top down estimates of the COS budget point to an underestimation of sources especially in the tropics. We extended the inverse model TM5-4DVAR to assimilate MIPAS satellite data, in addition to NOAA surface data as used in a previous study. To resolve possible discrepancies among the two observational datasets, a bias correction scheme was implemented. A set of inversions is presented that explores the influence of the different measurement instruments and the settings of the prior fluxes. To evaluate the performance of the inverse system, the HIAPER Pole-to-Pole Observations (HIPPO) aircraft observations and NOAA airborne profiles are used. All inversions reduce the (COS) biosphere uptake from a prior value of 1053 GgS a^-1 to much smaller values, depending on the inversion settings. These large adjustments of the biosphere uptake often turn parts of the Amazonia into a (COS) source. Only inversions that exclusively use MIPAS observations, or strongly reduce the prior errors on the biosphere flux maintain the Amazonia as a COS sink. Assimilating both NOAA surface data and MIPAS data requires a small bias correction for MIPAS data, mostly at higher latitudes, to correct for inconsistencies in the observational data and/or transport model errors. Analysis of the error reduction and posterior correlation between land and ocean fluxes indicates that co-assimilation of NOAA surface observations and MIPAS data better constrains the (COS) budget than assimilation of one individual dataset alone. Our inversions with bias corrections reduce the global biosphere uptake to respectively 570 and 687 GgS a^-1, depending on the prior biosphere error. Over the Amazonia, these inversions reduce the biosphere uptake from roughly 300 to 100 GgS a^-1, indicating a strongly overestimated prior uptake over the Amazonia. Although a recent study also reported reduced (COS) uptake over the Amazonia, we emphasise that a careful construction of prior fluxes and their associated errors remains important. For instance, an inversion that gives large freedom to adjust the anthropogenic and ocean fluxes of CS₂, an important (COS) precursor, also closes the budget satisfactorily with much smaller adjustments to the biosphere. Thus, a better characterisation of biosphere and ocean fluxes by observations is urgently needed, especially over the data-poor tropics.