DMS measurements:
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