North Atlantic Air-sea CO2 and Dimethylsulfide Exchange Measured by Eddy Covariance

We measured the exchange of CO₂ and dimethylsulfide (DMS) between the ocean and atmosphere during a 24-day campaign in the North Atlantic in Jun 2011 using the eddy covariance (EC) technique. In situ measurements of the air-sea partial pressure differences of both gases (dpCO₂ and pDMS) allowed for calculation of their piston velocities, k_CO2 and k_DMS. Whitecap fraction was quantified using sea surface images collected at 1 Hz during daylight conditions. For CO₂, nominally open-path gas analyzers (LiCor LI7500) were converted a closed-path configuration to reduce sensitivity to ship motion, the contamination of the CO₂ flux due to water vapor fluctuations in the sample volume was minimized by drying the air sample upstream of the gas analyzer, and the entire flux package was mounted on the bow mast to reduce tubing lengths and minimize the loss of turbulent fluctuations as air was drawn from the sample inlet to the gas analyzers. Fast-response DMS was measured using an atmospheric pressure chemical ionization mass spectrometer (API-CIMS). During the cruise, dpCO₂ ranged from -120 ppm (ocean CO₂ sink) to +40 (ocean CO₂ source) and 15-minute averaged wind speeds ranged from near zero to 17 m/s. Data from a broad range of atmospheric and oceanographic conditions were used to assess the accuracy/precision of shipboard air-sea flux measurements by EC using our techniques, and to examine the dependence of k_CO2 and k_DMS on physical processes that affect air-sea gas exchange.