The influence of dissolved organic matter on the marine production of carbonyl sulfide (OCS) and carbon disulfide (CS&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;) in the Peruvian upwelling

Sinikka T Lennartz,Boris P Koch,Henry C Bittig,Rüdiger Röttgers,Kerstin B Ksionzek,Tim Fischer,Marc Von Hobe,Dennis Booge,Rafael Gonçalves-Araujo,Birgit Quack,Astrid Bracher,Christa A Marandino

doi:10.5194/os-15-1071-2019

Abstract

Abstract. Oceanic emissions of the climate-relevant trace gases carbonyl sulfide (OCS) and carbon disulfide (CS2) are a major source to their atmospheric budget. Their current and future emission estimates are still uncertain due to incomplete process understanding and therefore inexact quantification across different biogeochemical regimes. Here we present the first concurrent measurements of both gases together with related fractions of the dissolved organic matter (DOM) pool, i.e., solid-phase extractable dissolved organic sulfur (DOSSPE, n=24, 0.16±0.04 µmol L−1), chromophoric (CDOM, n=76, 0.152±0.03), and fluorescent dissolved organic matter (FDOM, n=35), from the Peruvian upwelling region (Guayaquil, Ecuador to Antofagasta, Chile, October 2015). OCS was measured continuously with an equilibrator connected to an off-axis integrated cavity output spectrometer at the surface (29.8±19.8 pmol L−1) and at four profiles ranging down to 136 m. CS2 was measured at the surface (n=143, 17.8±9.0 pmol L−1) and below, ranging down to 1000 m (24 profiles). These observations were used to estimate in situ production rates and identify their drivers. We find different limiting factors of marine photoproduction: while OCS production is limited by the humic-like DOM fraction that can act as a photosensitizer, high CS2 production coincides with high DOSSPE concentration. Quantifying OCS photoproduction using a specific humic-like FDOM component as proxy, together with an updated parameterization for dark production, improves agreement with observations in a 1-D biogeochemical model. Our results will help to better predict oceanic concentrations and emissions of both gases on regional and, potentially, global scales.

Highlights

Oceanic emissions play a dominant role in the atmospheric budget of the climate-relevant trace gases carbonyl sulfide (OCS) and carbon disulfide (CS2) (Chin and Davis, 1993; Kremser et al, 2016)
We find different limiting factors of marine photoproduction: while OCS production is limited by the humiclike dissolved organic matter (DOM) fraction that can act as a photosensitizer, high CS2 production coincides with high DOSSPE concentration
Our results show that Fluorescent dissolved organic matter (FDOM) C2 is a good candidate as a proxy for OCS photoproduction, but its sampling coverage is insufficient for global model approaches at the moment

Summary

Introduction

Oceanic emissions play a dominant role in the atmospheric budget of the climate-relevant trace gases carbonyl sulfide (OCS) and carbon disulfide (CS2) (Chin and Davis, 1993; Kremser et al, 2016). OCS is the most abundant sulfur gas in the atmosphere, and CS2 is its most important precursor. Both gases influence the climate directly (OCS) or indirectly (CS2 by oxidation to OCS in the atmosphere), as OCS is a major supplier of stratospheric aerosols (Brühl et al, 2012; Crutzen, 1976), which exert a cooling effect on the atmosphere and can foster ozone depletion (Junge et al, 1961; Kremser et al, 2016). 50 %) (Kremser et al, 2016; Whelan et al, 2018) Reducing these uncertainties for present and future emission estimates requires (i) increasing the existing field data across various biogeochemical regimes and (ii) increasing process understanding and quantification in the whole water column to facilitate model approaches

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