Inverse modelling of carbonyl sulfide: implementation, evaluation and implications for the global budget

Abstract

Abstract. Carbonyl sulfide (COS) has the potential to be used as a climate diagnostic due to its close coupling to the biospheric uptake of CO2 and its role in the formation of stratospheric aerosol. The current understanding of the COS budget, however, lacks COS sources, which have previously been allocated to the tropical ocean. This paper presents a first attempt at global inverse modelling of COS within the 4-dimensional variational data-assimilation system of the TM5 chemistry transport model (TM5-4DVAR) and a comparison of the results with various COS observations. We focus on the global COS budget, including COS production from its precursors carbon disulfide (CS2) and dimethyl sulfide (DMS). To this end, we implemented COS uptake by soil and vegetation from an updated biosphere model (Simple Biosphere Model – SiB4). In the calculation of these fluxes, a fixed atmospheric mole fraction of 500 pmol mol−1 was assumed. We also used new inventories for anthropogenic and biomass burning emissions. The model framework is capable of closing the COS budget by optimizing for missing emissions using NOAA observations in the period 2000–2012. The addition of 432 Gg a−1 (as S equivalents) of COS is required to obtain a good fit with NOAA observations. This missing source shows few year-to-year variations but considerable seasonal variations. We found that the missing sources are likely located in the tropical regions, and an overestimated biospheric sink in the tropics cannot be ruled out due to missing observations in the tropical continental boundary layer. Moreover, high latitudes in the Northern Hemisphere require extra COS uptake or reduced emissions. HIPPO (HIAPER Pole-to-Pole Observations) aircraft observations, NOAA airborne profiles from an ongoing monitoring programme and several satellite data sources are used to evaluate the optimized model results. This evaluation indicates that COS mole fractions in the free troposphere remain underestimated after optimization. Assimilation of HIPPO observations slightly improves this model bias, which implies that additional observations are urgently required to constrain sources and sinks of COS. We finally find that the biosphere flux dependency on the surface COS mole fraction (which was not accounted for in this study) may substantially lower the fluxes of the SiB4 biosphere model over strong-uptake regions. Using COS mole fractions from our inversion, the prior biosphere flux reduces from 1053 to 851 Gg a−1, which is closer to 738 Gg a−1 as was found by Berry et al. (2013). In planned further studies we will implement this biosphere dependency and additionally assimilate satellite data with the aim of better separating the role of the oceans and the biosphere in the global COS budget.

Plant gross primary production, plant respiration and carbonyl sulfide emissions over the globe inferred by atmospheric inverse modelling

“…The improvement is also noticeable in the COS and CO 2 vertical profiles over Northern America (see Supplementary material). In contrast to theMa et al (2021) top-down study, there is no significant negative bias in the COS vertical profiles here (see Fig.S6-10).…”

“…For COS, this means a vegetation sink of around -620 GgS.yr −1 , which is in the lower range of recent estimates based on topdown approaches (Launois et al (2015b): -663-772 GgS.yr −1 , Ma et al (2021): 557-1053 GgS.yr −1 ). The soil sink and the anthropogenic sources have both decreased and amount to -210 and 335 GgS.yr −1 , respectively.…”

“…The atmospheric concentrations vary on seasonal and interannual timescales but have been indeed considered constant per latitudinal band. Such a feedback might have significant impact on the total vegetation sink (see for example the discussion in Ma et al (2021)), in particular over the Amazon. Thus, refining our inverse system would require including the feedback between the atmospheric concentrations and the COS vegetation sink (first order approach).…”

“…These emissions have been all computed using box models calibrated with ship-borne measurements made in different parts of the globe (Lennartz et al, 2017(Lennartz et al, , 2020a. The We have not considered DMS and CS 2 as separate tracers as done in Ma et al (2021). CS 2 has a lifetime estimated between 4 days (Khan et al, 2017) and 12 days (Khalil and Rasmussen, 1984) and DMS has a lifetime of 1.2 days.…”

“…CC BY 4.0 License. that CS 2 is emitted as COS. Ma et al (2021) showed that implementing the CS 2 oxidation process into the atmosphere leads to a decrease in surface COS concentrations of 40 ppt in the vicinity of Japan. Also, there is an oceanic hot spot located in the footprint of the site (see Figure S9) which might not be reliable.…”