Emission of biogenic volatile organic compounds from warm and oligotrophic seawater at the Eastern Mediterranean

Abstract. Biogenic volatile organic compounds (BVOCs) from terrestrial vegetation and marine organisms contribute to photochemical pollution and affect the radiation budget, cloud properties and precipitation via secondary organic aerosol formation. Their emission from both marine and terrestrial ecosystems is substantially affected by climate change in ways that are currently not well characterized. The Eastern Mediterranean Sea was identified as a hotspot for climate change, making it a natural laboratory for investigating the impact of climate change on BVOC emission from both terrestrial and marine vegetation. We quantified the mixing ratios of a suite of volatile organic compounds (VOCs), including isoprene, dimethyl sulfide (DMS), acetone, acetaldehyde and monoterpenes, at a mixed vegetation site ~ 4 km from the southeastern tip of the Levantine Basin, where the sea surface temperature maximizes and ultra-oligotrophic conditions prevail. The measurements were performed between July and October, 2015, using a proton-transfer-reaction–time-of-flight mass spectrometer. The analyses were supported by the Model of Emissions of Gases and Aerosols from Nature (MEGAN 2.1). For isoprene and DMS mixing ratios, we identified a dominant contribution from the seawater. Our analyses further suggest a major contribution at least for monoterpenes from the seawater. Our results indicate that the Levantine Basin greatly contributes to isoprene emission, corresponding with mixing ratios of up to ~ 9 ppbv. This highlights the need to update air-quality and climate models to account for the impact of sea surface temperature (SST) on marine isoprene emission. The DMS mixing ratios were one-to-two orders of magnitude lower than those measured in 1995 in the same area, suggesting a dramatic decrease in emission due to changes in the species composition induced by the rise in SST.