Constraining the budget of atmospheric carbonyl sulfide using a 3-D chemical transport model
Authors: Cartwright Michael P. , Pope Richard J. , Harrison Jeremy J. , Chipperfield Martyn P. , Wilson Chris , Feng Wuhu , Moore David P. , Suntharalingam Parvadha 
Published: September 2023

Abstract. Carbonyl sulfide (OCS) has emerged as a valuable proxy for photosynthetic
uptake of carbon dioxide (CO2) and is known to be important in the
formation of aerosols in the stratosphere. However, uncertainties in the
global OCS budget remain large. This is mainly due to the following three flux terms: vegetation uptake, soil uptake and oceanic emissions. Bottom-up estimates do not yield a
closed budget, which is thought to be due to tropical emissions of
OCS that are not accounted for. Here we present a simulation of atmospheric OCS over the period
2004–2018 using the TOMCAT 3-D chemical transport model that is aimed at better
constraining some terms in the OCS budget. Vegetative uptake of OCS is
estimated by scaling gross primary productivity (GPP) output from the Joint
UK Land Environment Simulator (JULES) using the leaf relative uptake (LRU)
approach. The remaining surface budget terms are taken from available
literature flux inventories and adequately scaled to bring the budget into
balance. The model is compared with limb-sounding satellite observations made by the
Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) and surface flask measurements from 14 National Oceanic and Atmospheric Administration – Earth System Research Laboratory (NOAA-ESRL) sites worldwide. We find that calculating vegetative uptake using the LRU underestimates the
surface seasonal cycle amplitude (SCA) in the Northern Hemisphere (NH) mid-latitudes and high latitudes by
approximately 37 ppt (35 %). The inclusion of a large tropical source is able to balance the global budget, but further improvement to the SCA and phasing would likely require a flux inversion scheme. Compared to co-located ACE-FTS OCS profiles between 5 and 30 km, TOMCAT
remains within 25 ppt (approximately 5 % of mean tropospheric
concentration) of the measurements throughout the majority of this region
and lies within the standard deviation of these measurements. This provides
confidence in the representation of atmospheric loss and surface fluxes of
OCS in the model. Atmospheric sinks account for 154 Gg S of the annual
budget, which is 10 %–50 % larger than previous studies. Comparing the surface monthly anomalies from the NOAA-ESRL flask data to the model
simulations shows a root-mean-square error range of 3.3–25.8 ppt. We
estimate the total biosphere uptake to be 951 Gg S, which is in the range of recent inversion studies (893–1053 Gg S), but our terrestrial vegetation flux accounts for 629 Gg S of the annual budget, which is lower than other recent studies (657–756 Gg S). However, to close the budget, we compensate for this with a large annual oceanic emission term of 689 Gg S focused over the tropics, which is much larger than bottom-up estimates (285 Gg S). Hence, we agree with recent findings that missing OCS sources likely originate from the tropical region. This work shows that satellite OCS profiles offer a good constraint on
atmospheric sinks of OCS through the troposphere and stratosphere and are
therefore useful for helping to improve surface budget terms. This work also shows that the LRU approach is an adequate representation of the OCS
vegetative uptake, but this method could be improved by various means, such
as using a higher-resolution GPP product or plant-functional-type-dependent
LRU. Future work will utilise TOMCAT in a formal inversion scheme to better
quantify the OCS budget.