Photooxidation of dimethylsulfide (DMS) in the Canadian Arctic

A Taalba,H Xie,M G Scarratt,M Levasseur,S Bélanger

doi:10.5194/bg-10-6793-2013

A Taalba, H Xie + Show 3 more

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https://doi.org/10.5194/bg-10-6793-2013

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Abstract

Abstract. Photolysis of dimethylsulfide (DMS), a secondary photochemical process mediated by chromophoric dissolved organic matter (CDOM), has previously been demonstrated to be an important loss term of DMS in the surface layer of warm seas and the Southern Ocean. The role of photolysis in regulating the DMS dynamics in northern polar seas remains, however, less clear. This study for the first time determined the apparent quantum yield (AQY) spectra of DMS photooxidation in Canadian Arctic seas covering Baffin Bay, the Mackenzie estuary and shelf, and the Canada Basin. The DMS AQY was fairly invariant at salinities < 25 but rose rapidly with further increasing salinity in an exponential manner. Salinity can therefore be used as a quantitative indicator of the DMS AQY. The DMS AQY in the ultraviolet (UV) wavelengths was linearly and positively correlated with the spectral slope coefficient (275–295 nm) of the CDOM absorption spectrum, suggesting that marine CDOM photosensitizes the degradation of DMS more efficiently than does terrestrial CDOM or that coastal waters contain higher concentrations of substrates (most likely dissolved organic matter and redox metals) that compete for DMS-oxidizing radical intermediates. High concentrations of nitrate (~ 12 μmol L−1) in deep water samples boosted DMS photooxidation by 70–80%, due likely to radical chemistry of nitrate photolysis. Coupled optical-photochemical modeling, based on the obtained DMS AQY spectra, shows that UV-A (320–400 nm) accounted for 60–75% of the DMS photolysis in the sunlit surface layer and that photochemistry degraded DMS on an e-folding time from 9 to 100 d (mean: 29 d). The photooxidation term on average accounted for 21% of the DMS gross loss rate and was comparable to the atmospheric DMS ventilation rate estimated for the same geographic regions. The methodology adopted here to study the relationship between CDOM quality/origin and DMS AQYs, if applicable to other ocean areas, may bring results of global significance for DMS cycling and might have implications for probing other CDOM-driven photochemical processes.

Highlights

Dimethylsulfide (DMS) is the most abundant volatile sulfur compound in seawater and its egress from the ocean accounts for ∼ 50 % of the total biogenic sulfur flux to the atmosphere (Bates et al, 1992; Liss et al, 1997)
We modeled the photochemical DMS turnover rate constants based on the obtained apparent quantum yield (AQY) spectra and discussed the implication of photooxidation for DMS cycling in northern marine systems
The temperature and salinity profiles of Sta. 1562 indicate that the 5 m sample was within the upper polar mixed layer characterized by relatively low salinities caused by ice melting and runoff, the 160 m sample was located in the upper halocline formed by Pacific winter water, and the 950 m sample originated from the North Atlantic (Matsuoka et al, 2012)

Summary

Introduction

Dimethylsulfide (DMS) is the most abundant volatile sulfur compound in seawater and its egress from the ocean accounts for ∼ 50 % of the total biogenic sulfur flux to the atmosphere (Bates et al, 1992; Liss et al, 1997). Upon entering the troposphere from the sea, DMS is rapidly oxidized to sulfate aerosols, potentially contributing to the formation of cloud condensation nuclei (CCN) (Lana et al, 2012; Kulmala et al, 2013). Marine DMS, along with other oceanic precursors (e.g., sea salts and organics) of atmospheric aerosols, may moderate climate warming (Charlson et al, 1987; Quinn and Bates, 2011). In contrast to the enormous progress made in mapping the concentrations and air–sea fluxes of DMS in major ocean basins (e.g., Kettle et al, 1999; Lana et al, 2011; Yang et al, 2011), our knowledge of DMS distributions and fluxes

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