Abstract
Abstract. Atmospheric dimethylsulfide (DMSa), continually derived from the world's oceans, is a feed gas for the tropospheric production of new sulfate particles, leading to cloud condensation nuclei that influence the formation and properties of marine clouds and ultimately the Earth's radiation budget. Previous studies on the Great Barrier Reef (GBR), Australia, have indicated coral reefs are significant sessile sources of DMSa capable of enhancing the tropospheric DMSa burden mainly derived from phytoplankton in the surface ocean; however, specific environmental evidence of coral reef DMS emissions and their characteristics is lacking. By using on-site automated continuous analysis of DMSa and meteorological parameters at Heron Island in the southern GBR, we show that the coral reef was the source of occasional spikes of DMSa identified above the oceanic DMSa background signal. In most instances, these DMSa spikes were detected at low tide under low wind speeds, indicating they originated from the lagoonal platform reef surrounding the island, although evidence of longer-range transport of DMSa from a 70 km stretch of coral reefs in the southern GBR was also observed. The most intense DMSa spike occurred in the winter dry season at low tide when convective precipitation fell onto the aerially exposed platform reef. This co-occurrence of events appeared to biologically shock the coral resulting in a seasonally aberrant extreme DMSa spike concentration of 45.9 nmol m−3 (1122 ppt). Seasonal DMS emission fluxes for the 2012 wet season and 2013 dry season campaigns at Heron Island were 5.0 and 1.4 µmol m−2 day−1, respectively, of which the coral reef was estimated to contribute 4 % during the wet season and 14 % during the dry season to the dominant oceanic flux.
Highlights
Dimethylsulfide (DMS) is the major volatile sulfur compound released from the global oceans (Andreae and Raemdonck, 1983)
This study has provided environmental evidence that coral reefs in the vicinity of Heron Island are point sources of DMSa, where emissions may at times be detectable as spikes of DMSa above the background oceanic signal
We found the ocean to be the dominant source of DMSa at Heron Island, where the ocean source was supplemented by occasional coral-reef-derived spikes of DMSa that were highly variable irregular events generally occurring at low tide when conditions exist that can stress the reef
Summary
Dimethylsulfide (DMS) is the major volatile sulfur compound released from the global oceans (Andreae and Raemdonck, 1983). The shallow water column over a coral reef has a lower thermal capacity than the open ocean, so it is subject to enhanced heating by incident solar radiation (McGowan et al, 2010) This will lower the diffusivity resistance for mass transfer of DMS through the seawater surface film, as described by the Schmidt number, which is temperature dependent (Saltzman et al, 1993). DMS may be directly exchanged to the atmosphere from the coral surface if aerially exposed at low tide (Hopkins et al, 2016) These particular characteristics of coral reefs suggest that they could be “hotspots” for production of DMSa oxidation products contributing to the sulfate component of new aerosol particles measured from the Great Barrier Reef (GBR) (Modini et al, 2009; Vaattovaara et al, 2013).
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