7.5
Generating Upper and Lower limits of DMS air-sea transfer
Penny Vlahos, University of Connecticut, Groton, CT; and E. C. Monahan, B. W. Blomquist, B. J. Huebert, and J. Edson
The air-sea flux of DMS is dependent on the rate of formation in seawater, rates of biotransformation and photolysis, the partial pressure of DMS in seawater and the physical environment (salinity, temperature and wind speed). Here we consider reported rates from field studies and couple these with known physical parameters to generate ranges in DMS air-sea fluxes for different oceanic zones in order to help improve parameterizations for global circulation models. It has long been recognized that the air-sea transfer, or “piston”, velocities of most gases increase with increasing wind speed, and that at higher wind speeds bubbles play an important role in this exchange. Rising bubbles in plumes beneath whitecaps form “low impedance vents” for gas evasion or invasion (Monahan and Spillane, 1984) in addition to numerous non-rising bubbles. The rapid expansion of whitecapping, and hence bubble plume numbers, with freshening winds has been described in increasing detail over the years (see e.g., Toba and Chen,1973; Monahan and Lu, 1990). The role of the individual bubbles in serving as transport capsules, and hence the significance of the diverse molecular diffusivities of the gases moving between the bubbles and the surrounding water, has been the focus of much work (e.g., Asher, et al, 1991; Wanninkhof et al, 1995). Field data from Blomquist et al, 2006 suggest that DMS transfer rates may diverge from those predicted by traditional wind-based models. Vlahos and Monahan (2009) developed a DMS transfer model that takes into account the role of the non-rising bubble surfaces in attracting such molecules and thus impeding to a degree their exchange across the air-water interface. Using an octanol-water partition coefficient of 0.9 for DMS, one may calculate the potential change in the activity of DMS in a mixed water-bubble plume. For a surface active molecule the effective Henry's Law constant can be predicted using; Heff = H/ (1+ (Cmix/Cw) ΦB), where H is the dimensionless Henry's Law constant, Cmix/Cw is a dynamic solubility enhancement of DMS due to bubbles and ΦB is the fraction of bubble surface area per m2 surface ocean. Heff may be substituted in gas transfer models to predict the DMS air-sea gas flux over a range of wind speeds for comparisons with field data, as shown in figure 1. Keiber et al (1996) found that photo-degradation, biotransformation and evasion to the atmosphere all occurred at comparable rates in the equatorial Pacific however the dominant process depended on the water depth and the physical environment. Additional rates from Bates et al (1994) in the Northeast Pacific, Burkhill et al (2002) in the North Sea and Bailey et al., (2008) in the Sargasso Sea will be used to couple with wind speeds and the physical environment.
Session 7, Air-Sea Flux Estimation and Parameterization: 3. Models
Wednesday, 29 September 2010, 10:30 AM-12:00 PM, Capitol AB
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