John Dacey and Wade McGillis
Woods Hole Oceanographic Institution
Woods Hole, MA 02543
Program Manager: Lisa billing, Office of Global Programs
One of the biogenic gases present in the atmosphere, dimethylsulfide (DMS), is a major contributor to the sulfur cycle on earth. DMS is also considered to play an important role in climate regulation because its oxidation products may become cloud condensation nuclei that affect the earth s radiative balance. The major source of DMS is the ocean s plankton, There is a large DMS concentration difference between the ocean and the atmosphere, with concentrations in the ocean typically orders of magnitude higher than in the air where it is oxidized by OH-radicals. The high concentration gradient of DMS makes it an ideal tool for the determination of gas transfer velocities: knowing the flux of DMS and knowing the concentration in surface seawater allows calculation of the gas transfer rate.
GasEx 2001 was amulti-disciplinary study focusing on the flux of C02 across the air-sea interface. The primary goal was to measure air-sea C02 fluxes and the surface physical and biogeochemical processes which control C02 fluxes over short (<hour) time scales. This sub-project was directed at measuring the flux of DMS from the ocean to the atmosphere. Unlike CO2, which may move in either direction across the ocean surface depending on biological and physical circumstances, DMS always fluxes to the atmosphere because of its biological source in the surface ocean. The rates we measured can be used to corroborate C02 fluxes since they help characterize the gas transfer rates of the surface layer. Accurate estimates of DMS flux are also important for understanding sulfur chemistry, improving models of atmospheric sulfur, and quantifying the role of DMS in aerosol dynamics.
Sea-to-air DMS fluxes were measured during the GasEx-2001 cruise using the atmospheric Gradient Flux (GF) technique. The approach is designed to make measurements on the temporal and spatial scales which are appropriate far investigating the factors that control fluxes across the ocean surface. By combining these
measurements with measurements of the concentration of DMS in the surface water, gas transfer velocities (kgas) across the air-sea interface can be estimated.
During GasEx-2001 DMS fluxes were measured with the GF technique from the NOAA research Vessel Ronald H. Brown bow boom/mast system (Figure 1 }. Air was sampled at three different elevations for 30-minute intervals at a flow rate of 300 ml min-1. The sampled air was stored in Tedlar bags, and analyzed for DMS using a GC equipped with a Sievers sulfur chemiluminescence detector. These measurements are used to estimate the transfer velocity and will be compared with flux measurements of C02 and wind speed parameterizations. The data products from this effort includes the DMS sea-air flux and gas transfer velocity. Measurements of aqueous DMS were also made regularly during the cruise.
The environmental conditions were fairly constant over the time span of the experiment (Fe brruay 200 1) near 125W 3S. Average Ulo wind speeds varied around 5 m seal, mean water temperature was 26... C and mean air temperature was 25 ... C. Unstable atmospheric conditions prevailed during the experiment. DMS concentrations in the water surrounding the sampling systems varied around 2.6 nM. The profiles measured from the bow of the NOAA research vessel Ronald H. Brown show that the atmospheric profiles of DMS concentration were semi-logarithmic with increasing height. DMS fluxes derived from the profiles ranged up to almost 40 mol m -2 d-l with the highest rates coinciding with higher wind speeds (Figure 2).
These measurements of DMS flux contribute to the development of more accurate estimates of the gas transfer velocity and an improved understanding of the processes
controlling air-sea C02 fluxes.
These results were presented in the National Meeting of the AGU during the February 2002 meeting in Honolulu , and are being compiled in a manuscript to be submitted to the Journal of Geophysical Research.