Field measurement of air-sea gas transfer: A methodical search

Abstract

In order to obtain field data of ocean-atmosphere gas transfer coefficients, it is preferable to measure the interfacial gas flux in the air rather than in the water. This approach has been reported in the literature for CO,. However, it is shown here that 222Rn and methyliodide, CH,I, may be more suitable gases for air-side flux measurements.

Response [to “Isotopic versus micrometeorologic ocean CO2 fluxes: A serious conflict” by W. Broecker et al.]

“…Micrometeorological techniques applied near the surface are well suited to measure air-sea exchange rates on the short time and spatial scales associated with the transfer processes: about 1 hour and 0.1-10 km. Roether [1985] has made this point but suggests that the very small fluxes of CO2 expected over water inhibit its measurement by micrometeorological methods. This is a reasonable expectation but should not be construed as an argument against doing suitable exploratory, micrometeorological field experiments over the ocean.…”

Evidence for wind‐pumping of air‐sea gas exchange based on direct measurements of CO2 fluxes

“…This failure can be related in part to our observation that the flux depended to a large degree on hourly changes in wind speed rather than on wind speed itself. The exchange coefficient of a "model gas" that does not have a two-way exchange process [e.g., Roether, 1983] should not be expected to completely simulate the exchange of a gas such as CO2 or 02 that has significant atmospheric concentration and that can be pumped downward by pressurization of bubbles generated by breaking waves. In particular, although the radon evasion rate [e.g., Peng et al, 1979] may correctly characterize transfers of nonatmospheric gases from the water, this is an incomplete simulation of C02 flux, since there is no downward pumping of radon in bubbles.…”

“…ß where H is Henry's law constant, with ca/H being the concentration of the gas in the water when it is in equilibrium Mth the concentration of gas in the air. This approach reflects the belief that air-sea gas fluxes are proportional to the bulk mncentration differences (modified by Henry's law), with the constant of proportionality k having dimensions of velocity •d often being referred to as a "piston" velocity [Kanwisher, 1%3;Liss, 1973;Broecker and Peng, 1971;Roether, 1983]. A simple physical model originally proposed by W. G. Whitman more than 50 years ago [Danckwerts, 1970] describes k in terms of diffusion through air and water microlayers at the interface whose thickness (< 1 mm) is determined by turbulent mixing in the adjoining boundary layers.…”

Seasonal variability of methyl iodide in the Kiel Fjord

“…Nevertheless, CH 3 I continues to receive attention both because of its role in iodine transport and chemistry, including transport to higher altitudes [ Solomon et al ., ; Tegtmeier et al ., ], and its potential to be a “model compound” for investigating processes, budgets, and rates related to ocean‐atmosphere transfer [ Roether , ; Richter , ]. The production pathways of CH 3 I and controls on its sea‐to‐air flux remain poorly understood.…”