Ambient air and seawater were immediately analyzed aboard ship for dimethylsulfide (DMS) concentrations using the same automated collection/purge and trap system. Air samples were pulled through a Teflon filter and Teflon tubing which ran approximately 50 m from the top of the aerosol sampling mast (18 m above sea level, forward of the ship’s bridge) to the analytical system. One hundred ml/min of the 4 L/min flow were pulled through a KI solution at the analytical system to eliminate oxidant interferences. The air sample volume ranged from 0.5 to 1.5 L depending on the DMS concentration. Seawater samples were collected from the ship's seawater pumping system which had an inlet located near the ship’s bow at a depth of approximately 4 m. The seawater line ran to the analytical system where 5.1 ml of sample were valved into a Teflon gas stripper. The samples were purged with hydrogen at 80 ml/min for 5 min. Water vapor in either the air or purged seawater sample stream was removed by passing the flow through a -25C Teflon tube filled with silanized glass wool. DMS was then trapped in a -25C Teflon tube filled with Tenax. During the sample trapping period, 6.2 pmole of methylethyl sulfide (MES) were valved into the hydrogen stream as in internal standard. At the end of the sampling/purge period the coolant was pushed away from the trap and the trap was electrically heated. DMS was desorbed onto a DB-1 mega-bore fused silica column where the sulfur compounds were separated isothermally at 50C and quantified with a sulfur chemiluminesence detector. The detection limit during ACE-Asia was approximately 0.8 pmole. The system was calibrated using gravimetrically calibrated DMS and MES permeation tubes. The precision of the analysis, based on both replicate analyses of a single water sample and replicate analyses of a standard introduced at the inlet of the air sample line, was typically +- 8%. The performance of the system was monitored regularly by running blanks and standards through the entire analytical and sampling system (including the Teflon filter and sampling line). Values reported here have been corrected for recovery losses (0-5%). System blanks were below detection limit. Water samples are reported in units of nanomoles per liter. Air samples are reported in units of parts-per-trillion by volume (ppt). The mixing ratios were calculated at standard temperature (25C) and pressure (1013 mbar) such that 1 nmole/m3 equals 24.5 ppt. The flux of DMS from the ocean to the atmosphere in micromoles/square meter/day was calculated for each seawater DMS measurement using the exchange coefficients of Wanninkhof (1992) and Liss and Merlivat (1986). The calculated DMS flux includes the seawater DMS concentration, wind speed and sea water temperature. The true wind speeds and sea water temperatures were measured aboard the Brown and are described in detail in separate data files.
Liss, P.S. and L. Merlivat, Air-sea gas exchange rates: Introduction and synthesis, in The Role of Air-Sea Exchange in Geochemical Cycling, edited by P. Buat-Menard, pp. 113-127, D. Reidel, Norwess, Mass., 1986.
Wanninkhof, R.H., Relationship between wind speed and gas exchange over the ocean, J. Geophys. Res., 97, 7373-7382, 1992.
Data can be downloaded in ACF format by following the ASCII link, or in binary netCDF file format by following the netCDF link