The concepts of air-water exchange and mass transfer of organic chemicals across water surfaces have been described in detail elsewhere (Eisenreich etal., 1997; Liss and Duce, 1997). Diffusive air-water exchange refers to the transfer of chemical across an air-water interface and may be visualized as diffusive transfer of a chemical across near-stagnant layers of 0.1 to 1.0 mm thickness. At low wind speeds, insufficientwind energy exists to mix the air and water films or boundary layers, and a stagnant boundary layer is established (Stagnant Two-Film Model). Higher wind speeds generate more turbulence in the boundary layers, parcels of air and water are forced to the surface, and exchange is dependent on the renewal rate of air and water parcels. In highly turbulent seas, gas exchange is enhanced by breaking waves and bubble ejection. Under turbulence and wind conditions normally occurring in estuaries and lakes, the first two models are most applicable although wind extremes may be very important. The gas-phase concentration in the atmosphere (Cg) attempts to reach equilibrium with the concentration of dissolved gas in water (Cw).
When equilibrium is achieved, the ratio of the gas activities in air and water are constant at a given temperature and are represented by Henry's Law constant (H): (H = Cg Cw-1; Pa m3 mol-1). The direction of chemical transfer is from the water to the air when the fugacity in the water exceeds the fugacity (gas phase concentration) in air and is referred to as volatilization. Chemical transfer from the air to the water occurs when the fugacity (that is, activity) in the air (Ca (RT)-1) exceeds the chemical fugacity in water (CwH-1) and is referred to as gas absorption. The processes of gas absorption and volatilization occur simultaneously, and their difference contributes to the net flux. The magnitude of mass transfer is determined by a mass transfer coefficient (K, m d-1) and the concentration difference:
where Fgasjnet is the net flux (ng m-2 d-1), KOL (m d-1) is the overall mass transfer coefficient, and (Cd - Ca/H') describes the fugacity gradient (ng m-3); Cd (ng m-3) is the dissolved phase con-centrationofthecompoundin water; Ca (ngm-3)is the gas phase concentration of the compound in air which is divided by the dimensionless Henry's Law Constant, H', H' = H/RT; R is the universal gas constant (8.315 Pam3 K-1 mol-1); H is the temperature-specific Henry's Law Constant (Pa m3 mol-1); and T is the temperature at the air-water interface (K). For the New York/New Jersey Harbor Estuary, we estimated only the gaseous chemical absorption (Fabs) across the water surface; volatilization was not estimated because it is a loss rather than de-positional term, and spatially- and temporally-distributed dissolved water concentrations were not available. The relevant equation then becomes:
The negative sign on the flux simply means that the direction of transfer is from the air to the water. The mass transfer coefficient is dependent on turbulent mixing in the boundary layers on either side of the air-waxster interface, which is highly correlated with wind speed (Wanninkhof, 1992; Wanninkhof and McGillis, 1999). In addition, the KOL is dependent on the Henry's Law Constant, which is a func tion of temperature and the diffusivity of the compound in air and water. Examples of the application of the calculation can be found in Zhang et al. (1998), Nelson et al. (1998), Bamford et al. (1999), Tottenetal. (2001),and Gigliotti et al. (2001).
It is important that the corresponding volatilization term also be estimated for mass budget calculations but this requires dissolved water concentrations of the target chemical. Later in this chapter, results of intensive field measurements of air and water concentrations measured simultaneously in the estuary in July 1998 will be reported, and the resulting absorption, volatilization, andnetair-water exchange fluxes for PCBs and PAHs. Technically, only gas absorption contributes to atmospheric deposition. In the future, we will estimate the seasonal and annual cycle of air-water exchange fluxes (absorption, volatilization, and net air-water exchange) for PCBs and PAHs utilizing water concentrations measured in all seasons.
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