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

• Main Authors: J. Ma, L. M. J. Kooijmans, A. Cho, S. A. Montzka, N. Glatthor, J. R. Worden, L. Kuai, E. L. Atlas, M. C. Krol
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-03-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/3507/2021/acp-21-3507-2021.pdf
• ISSN: 1680-7316; 1680-7324

<p>Carbonyl sulfide (<span class="inline-formula">COS</span>) has the potential to be used as a climate diagnostic due to its close coupling to the biospheric uptake of <span class="inline-formula">CO₂</span> and its role in the formation of stratospheric aerosol. The current understanding of the <span class="inline-formula">COS</span> budget, however, lacks <span class="inline-formula">COS</span> sources, which have previously been allocated to the tropical ocean. This paper presents a first attempt at global inverse modelling of <span class="inline-formula">COS</span> within the 4-dimensional variational data-assimilation system of the TM5 chemistry transport model (TM5-4DVAR) and a comparison of the results with various <span class="inline-formula">COS</span> observations. We focus on the global <span class="inline-formula">COS</span> budget, including <span class="inline-formula">COS</span> production from its precursors carbon disulfide (<span class="inline-formula">CS₂</span>) and dimethyl sulfide (<span class="inline-formula">DMS</span>). To this end, we implemented <span class="inline-formula">COS</span> 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<span class="inline-formula">⁻¹</span> was assumed. We also used new inventories for anthropogenic and biomass burning emissions. The model framework is capable of closing the <span class="inline-formula">COS</span> budget by optimizing for missing emissions using NOAA observations in the period 2000–2012. The addition of 432 Gg a<span class="inline-formula">⁻¹</span> (as S equivalents) of <span class="inline-formula">COS</span> 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 <span class="inline-formula">COS</span> 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 <span class="inline-formula">COS</span> 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 <span class="inline-formula">COS</span>. We finally find that the biosphere flux dependency on the surface <span class="inline-formula">COS</span> 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 <span class="inline-formula">COS</span> mole fractions from our inversion, the prior biosphere flux reduces from 1053 to 851 Gg a<span class="inline-formula">⁻¹</span>, which is closer to 738 Gg a<span class="inline-formula">⁻¹</span> as was found by <span class="cit" id="xref_text.1"><a href="#bib1.bibx10">Berry et al.</a> (<a href="#bib1.bibx10">2013</a>)</span>. 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 <span class="inline-formula">COS</span> budget.</p>