Surface Ocean and Atmospheric Measurements of Volatile Organic Compounds in the Southern Ocean

Volatile organic compounds are present ubiquitously throughout the atmosphere. Particularly over the remote marine atmosphere, they play a role in ozone and particle formation. For gases such as methanol, acetone and acetaldehyde it is unclear whether the ocean acts as a source or a sink. For isoprene and dimethyl sulfide, the oceanic source strength is poorly constrained. This is in part due to a paucity of in situ measurements, especially in remote oceanic regions such as the Southern Ocean.

In this work we present shipborne underway air and water measurements of methanol, acetone, acetaldehyde, isoprene and dimethyl sulfide. The ship transected the Atlantic sector of the Southern Ocean at approximately 60° S during the transition from late summer into autumn. The measurements were taken using a recently developed Segmented Flow Coil Equilibrator coupled to Proton Transfer Reaction-Mass Spectrometer. A figure of the segmented flow coil equilibrator used to take these measurements is shown here. Using air and water measurements, hourly fluxes and saturations are computed. The underway measurements allowed mapping of a patch of high biological activity and up to 60 nmol dm^-3 dimethyl sulfide near the South Sandwich Islands. Additionally, binning of the data in 24 hourly bins revealed a diel cycle in acetaldehyde air and water concentrations. A smaller diel cycle was observed for isoprene and acetone surface water concentrations.

The underway measurements suggest that the ocean acts as a net sink for methanol, despite an episode of outgassing observed in a patch of high biological activity. Largely due to very low air concentrations, the Southern Ocean appears to act as a weak source for acetone. On the other hand, the ocean is calculated to be a sink for acetaldehyde over the cruise track. Underway measurements reveal episodic high concentrations of dimethyl sulfide which suggest that the global climatology underestimates the average flux and concentration of DMS in this area. Isoprene is largely supersaturated in the surface ocean. In this dataset, surface isoprene and methanol concentrations correlate, likely due to a common biological origin. Surface acetone, methanol and isoprene concentrations were found to correlate negatively with surface underway fCO₂, suggesting a role of biology in the production of these compounds.