Direct oceanic emissions unlikely to account for the missing source of atmospheric carbonyl sulfide

Abstract

Abstract. The climate active trace-gas carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere. A missing source in its atmospheric budget is currently suggested, resulting from an upward revision of the vegetation sink. Tropical oceanic emissions have been proposed to close the resulting gap in the atmospheric budget. We present a bottom-up approach including (i) new observations of OCS in surface waters of the tropical Atlantic, Pacific and Indian oceans and (ii) a further improved global box model to show that direct OCS emissions are unlikely to account for the missing source. The box model suggests an undersaturation of the surface water with respect to OCS integrated over the entire tropical ocean area and, further, global annual direct emissions of OCS well below that suggested by top-down estimates. In addition, we discuss the potential of indirect emission from CS 2 and dimethylsulfide (DMS) to account for the gap in the atmospheric budget. This bottom-up estimate of oceanic emissions has implications for using OCS as a proxy for global terrestrial CO 2 uptake, which is currently impeded by the inadequate quantification of atmospheric OCS sources and sinks.

Global carbonyl sulfide (OCS) measured by MIPAS/Envisat during 2002–2012

“…Apparently, these enhancements are mainly caused by oceanic release. This conclusion is supported by various budget estimations, but Blake et al (2004) and Lennartz et al (2017) assume a larger contribution of anthropogenic Asian emissions. A striking feature is a region of low OCS amounts between Brazil and southern Africa, which is most distinct during austral summer.…”

“…Kettle et al, 2002;Berry et al, 2013) as well as by a model study of Launois et al (2015). In addition to natural sources, Blake et al (2004) and Lennartz et al (2017) suggest a larger contribution of anthropogenic Asian emissions instead. During September to November the OCS amounts measured at low latitudes are reduced by 20-30 pptv.…”

Biomass Burning Unlikely to Account for Missing Source of Carbonyl Sulfide

“…A constant, supplemental source is added to maintain a balanced budget. Previous works (Berry et al, 2013;Campbell et al, 2017;Suntharalingam et al, 2008) ascribe this flux to a missing ocean source; however, more recent estimates by Lennartz et al (2017Lennartz et al ( , 2019 find that ocean emissions are insufficient to close the budget and other fluxes must be involved. The model assumes that the atmosphere is well mixed at every time step, which is reasonable because the atmospheric lifetime of OCS is approximately 2 years Khalil & Rasmussen, 1984;Montzka et al, 2007).…”

The influence of dissolved organic matter on the marine production of carbonyl sulfide (OCS) and carbon disulfide (CS<sub>2</sub>) in the Eastern Tropical South Pacific

“…The oceanic emissions of both gases have recently gained interest because they are suggested to account for a missing source of atmospheric OCS (Berry et al, 2013;Kuai et al, 2015;Glatthor et al, 2015;Launois et al, 2015). In-situ measurements of OCS in surface seawater are still limited, but those 10 available suggest that oceanic emissions are too low to fill the proposed gap of 400-600 Gg S yr -1 in the atmospheric budget (Lennartz et al, 2017). Still, oceanic emission estimates are associated with high uncertainties (ca.…”

“…On larger spatial scales, the CDOM absorption coefficient at 350 5 nm (a 350 ) can serve as a proxy for both photoexcitable carbonyl-groups and organic sulfur precursors, making the overall photoproduction rate second-order dependent on a 350 (von Hobe et al, 2003). Accordingly, a global parameterization for photochemical production was developed based on a 350 , by integrating data from the Atlantic, Pacific and Indian ocean (Lennartz et al, 2017). To improve this parameterization on a regional scale, we tested whether the precursors can be further specified by an easily measurable fraction of the DOM pool (FDOM components, DOS SPE ), without performing costly and 10 potentially incomplete analysis on the molecular level.…”