Air-sea gas transfer models

Molecular processes at interfaces dominate the exchange mechanisms of momentum, heat, and gases. Turbulence in the fluid on either side of the air-sea interface is determined by eddies of many sizes (Fig. 10.1). As one approaches the interface firom the 

A schematic representation of the boundary layers for momentum, heat and mass near the air—water interface. The velocity of the water and the size of eddies in the water decrease as the air—water interface is approached. The larger eddies have greater velocity, which is indicated here by the length of the arrow in the eddy. Because random molecular motions of momentum, heat and mass are characterized by molecular diffusion coefficients of different magnitude (0.01 cm s for momentum, 0.001 cm s for heat and lO cm s for mass), there are three different distances from the wall where molecular motions become as important as eddy motions for transport. The scales are called the viscous (momentum), thermal (heat) and diffusive (molecular) boundary layers near the interface. 

G is an empirical constant, with units of length per unit time, determined experimentally, and the exponent, n, is predicted to vary between V2 and 1. The ratio of the kinematic viscosity to diffusion coefficient is called the Schmidt number. Sc (Table 10.1)  

Gc is proportional to Scd . Comparison of transfer coefficients of the same gas at different temperatures, and/or in different liquids, requires knowledge of the correct dependence of the gas exchange rate on the kinematic viscosity. However, when comparing the gas transfer coefficients of different gases in the same solution at the same temperature, the dependence on kinematic viscosity plays no role because it is a property of the fluid and cancels. 

Different gas exchange models suggest a variety of dependencies of the mass transfer coefficient, Gc, on the Schmidt number. We present three different models of the mechanism of gas exchange along with evidence for their importance in nature. 

---

The simplest model of air-sea gas transfer is the two-hlm model (Liss and Slater, 1974 ... 

Despite the central role that air-sea gas exchange plays in studies of marine productivity, biogeochemical cycles, atmospheric chemistry, and climate, it has proved extremely difficult to measure air-sea gas fluxes in situ. Only in 2001 were believable direct measurements of oceanic CO2 fluxes reported in the literature (McGillis et al., 2001a). In this section we examine the various models that have been proposed to understand the basic processes that control gas exchange mechanisms, describe results from laboratory experiments, and discuss the various techniques that have been developed to try to measure gas transfer rates in situ. Finally, we describe the development of wind speed (U) based para-metrizations and assess their impact on computation of air-sea gas fluxes. 

Tamburrino, A., Aravena, C., and Gulliver, J. S. 2007. Visualization of 2-D divergence on the free surface and its relation to gas transfer. In Transport at the Air Sea Interface— Measurements, Models and Parameterizations, C. S. Garbe, R. A. Handler, B. Jahne, (Eds). Springer-Verlag, New York. 

Donelan, M., Drennan, W., Monehan, E., and Wanninkhof, R., Eds. 2002. Gas Transfer at Water Surfaces. Geophysical Monograph 127, American Geophysical Union, Washington, DC. Garbe, C. S., Handler, R. A., and Jahne, B., Eds. 2007. Tramport at the Air Sea Interface -Measurements, Models and Parameterizations, Springer-Verlag, New York. 

A critical parameter in all models of DMS chemistry in the marine atmosphere is tne sea-to-air flux. Although the sea surface concentrations of DMS nave been measured in a wide variety of environments (14). the flux has never been measured directly. Instead, it has been calculated using observed concentrations and various models of gas transfer across the air sea interface. All of the models parameterize die transfer as a first order loss, as follows... 

Transfer of DMS Across the Sea-Air Interface into the Atmosphere. At this time there is no empirical evidence for isotope discrimination during the sea-air transfer of DMS. Theoretically, the transfer of DMS is estimated from gas exchange models (Equation 3)... 

A classical two-layer model, which has been previously apphed to the air-sea exchange of SVOCs (Achman et al. 1993 Zhang et al. 2007 Li et al. 2009), assumes that the rate of gas transfer is controlled by the pollutant s ability to diffuse across the air layer and sea surface water on either side of the air-water interface. The molecular diflusivity of the pollutant, dependent on the amount of resistance encountered in the hquid and gas films, describes the rate of transfer while the concentration gradient drives the direction of transfer (Totten et al. 2001). The flux F (ng m day ) is calculated by... 

---