Tropospheric carbonyl sulfide mass balance based on direct measurements of sulfur isotopes

Abstract

Robust estimates for the rates and trends in terrestrial gross primary production (GPP; plant CO2 uptake) are needed. Carbonyl sulfide (COS) is the major long-lived sulfur-bearing gas in the atmosphere and a promising proxy for GPP. Large uncertainties in estimating the relative magnitude of the COS sources and sinks limit this approach. Sulfur isotope measurements (34S/32S; δ34S) have been suggested as a useful tool to constrain COS sources. Yet such measurements are currently scarce for the atmosphere and absent for the marine source and the plant sink, which are two main fluxes. Here we present sulfur isotopes measurements of marine and atmospheric COS, and of plant-uptake fractionation experiments. These measurements resulted in a complete data-based tropospheric COS isotopic mass balance, which allows improved partition of the sources. We found an isotopic (δ34S ± SE) value of 13.9 ± 0.1‰ for the troposphere, with an isotopic seasonal cycle driven by plant uptake. This seasonality agrees with a fractionation of −1.9 ± 0.3‰ which we measured in plant-chamber experiments. Air samples with strong anthropogenic influence indicated an anthropogenic COS isotopic value of 8 ± 1‰. Samples of seawater-equilibrated-air indicate that the marine COS source has an isotopic value of 14.7 ± 1‰. Using our data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 40 ± 17% for the anthropogenic source and 60 ± 20% for the oceanic source. This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination.

Efficient Production of Carbonyl Sulfide in the Low‐NOx Oxidation of Dimethyl Sulfide

“…Recent isotopic and inverse modeling studies indicate that the marine environment is the dominant source region for OCS and that there is either an over-estimation of the terrestrial OCS sink or an unaccounted-for OCS source that seems to be centered in the tropical oceans based on atmospheric inversion studies (Berry et al, 2013;Davidson et al, 2021;Ma et al, 2021). In response to recent suggestions that the production of OCS from the OH-initiated oxidation of DMS may be a significant source of uncertainty in the OCS budget (Davidson et al, 2021;Ma et al, 2021) and new discoveries of the chemical mechanism of DMS oxidation (Berndt et al, 2019;G. A. Novak et al, 2021;Veres et al, 2020;Wu et al, 2015), we revisit here the chemical mechanism for OCS production in the oxidation of DMS.…”

Global modelling of soil carbonyl sulfide exchanges

“…WIS, MLO, and SMO. These model-observation mismatches have led top-down studies to identify vegetation as an underesti-mated sink in the high latitudes (Ma et al, 2021;Remaud et al, 2022) and the tropical oceanic emissions as the missing source (Berry et al, 2013;Launois et al, 2015;Kuai et al, 2015;Ma et al, 2021;Remaud et al, 2022;Davidson et al, 2021). The present anoxic soil fluxes have little impact on the surface latitudinal distributions and therefore are unlikely to shed new light on the tropical missing source.…”

“…It was recently estimated to account for 432 GgS yr −1 by Ma et al (2021). The hypothesis of a strong tropical oceanic source has not been substantiated by in situ COS and CS 2 measurements in sea waters (Lennartz et al, 2017(Lennartz et al, , 2020(Lennartz et al, , 2021, except by Davidson et al (2021), that invoke an oceanic source of 600 ± 400 GgS yr −1 based on direct measurements of sulfur isotopes. Clearly, an accurate characterization of all flux components of the atmospheric COS budget is still needed.…”

Light and Water Conditions Co-Regulated Stomata and Leaf Relative Uptake Rate (LRU) during Photosynthesis and COS Assimilation: A Meta-Analysis

“…As a trace gas [32], the largest source of the COS atmospheric budget is the oceans of the Southern Hemisphere and the low latitude regions [33,34], as well as anoxic soils [35][36][37][38][39][40], i.e., directly from sulfur-containing organic matter or indirectly transferred from dimethyl sulfide (DMS) and carbon disulfide (CS 2 ) [41][42][43][44]. Meanwhile, anthropogenic activities are the largest influencing factor of regional COS emission [45][46][47][48][49].…”