Atmosphere-water exchange

Weiss (1974) produced the following equation to express concentration of a particular gas in seawater  

By combining Henry s law (described earlier) with Fick s first law described by the following equation  

Implicit in this model is the assumption that molecular diffusivity and Henry s Law constant are directly and inversely proportional, respectively, to the gas flux across the atmosphere-water interface. Molecular diffusion coefficients typically range from 1 x 10-5 to 4 x 10-5 cm2 s-1 and typically increase with temperature and decreasing molecular weight (table 5.3). Other factors such as thickness of the thin layer and wind also have important effects on gas flux. For example, wind creates shear that results in a decrease in the thickness of the thin layer. The sea surface microlayer has been shown to consist of films 50-100 pm in thickness (Libes, 1992). Other work has referred to this layer as the mass boundary layer (MBL) where a similar range of film thicknesses has been 

Modified from Broecker and Peng (1974) and Broecker and Peng (1982). 

A more simplified equation used to examine the flux (F) of gas transfer across the atmosphere-water interface is as follows  

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The surface s role in land-atmosphere water exchange deals with the subdivision of the phase space into at least two levels soil and ground water. The soil level plays the role of a buffer zone between precipitation and ground water. The simplest parameterization of fluxes between these levels is reduced to their linear dependences WsH(t,i,j) = XijWs(t,iJ) and wHS(t,i,j) = However, a more strict... 

To assess the relative importance of the volatilisation removal process of APs from estuarine water, Van Ry et al. constructed a box model to estimate the input and removal fluxes for the Hudson estuary. Inputs of NPs to the bay are advection by the Hudson river and air-water exchange (atmospheric deposition, absorption). Removal processes are advection out, volatilisation, sedimentation and biodegradation. Most of these processes could be estimated only the biodegradation rate was obtained indirectly by closing the mass balance. The calculations reveal that volatilisation is the most important removal process from the estuary, accounting for 37% of the removal. Degradation and advection out of the estuary account for 24 and 29% of the total removal. However, the actual importance of degradation is quite uncertain, as no real environmental data were used to quantify this process. The residence time of NP in the Hudson estuary, as calculated from the box model, is 9 days, while the residence time of the water in the estuary is 35 days [16]. 

By comparison with many other silicate minerals, isotope studies of natural clays are complicated by a number of special problems related to their small particle size and, hence, much larger specific surface area and the presence of interlayer water in certain clays. Surfaces of clays are characterized by 1 or 2 layers of adsorbed water. Savin and Epstein (1970a) demonstrated that adsorbed and interlayer water can exchange its isotopes with atmospheric water vapor in hours. Complete removal of interlayer water for analysis with the total absence of isotopic exchange between it and the hydroxyl group, may not be possible in all instances (Lawrence and Taylor 1971). 

In the seventies, the growing interest in global geochemical cycles and in the fate of man-made pollutants in the environment triggered numerous studies of air-water exchange in natural systems, especially between the ocean and the atmosphere. In micrometeorology the study of heat and momentum transfer at water surfaces led to the development of detailed models of the structure of turbulence and momentum transfer close to the interface. The best-known outcome of these efforts, Deacon s (1977) boundary layer model, is similar to Whitman s film model. Yet, Deacon replaced the step-like drop in diffusivity (see Fig. 19.8a) by a continuous profile as shown in Fig. 19.8 b. As a result the transfer velocity loses the simple form of Eq. 19-4. Since the turbulence structure close to the interface also depends on the viscosity of the fluid, the model becomes more complex but also more powerful (see below). 

The external processes (boundary fluxes) can be combined into four pairs of generalized exchange fluxes that is (a) input/output by streams, rivers, or ground-water, (b) air-water exchange, (c) sediment-water exchange, (d) exchange with adjacent water compartments. If the box represents a pond or lake as a whole, flux (d) does not exist. The fluxes into the system are controlled by external parameters such as the concentration in the inlets, the atmospheric and the sedimentaiy concentrations. These concentrations can be constant or variable with time. 

Figure 23.1 General view of a linear one-box model of a well-mixed pond, a lake, or part of a lake or ocean. See Box 23.1 for definitions. If the box represents the complete water body of a lake, the terms with vex (water exchange with adjacent boxes) do not exist. Similarly, if the box is not in contact with the atmosphere, the air-water exchange flux (va,w) is absent.

Given the PCBs inputs, 7t, and the atmospheric concentrations, Ca, we can now calculate the total steady-state concentration in the lake for the two congeners from Eq. 5 of Box 23.1. Note that among the input processes (nominator of Eq. 5) only the input from the rivers and the atmosphere are different from zero, whereas among the removal processes (denominator of Eq. 5) flushing, air/water exchange, and sedimentation are relevant. Thus, we can formulate the steady-state explicitly for the case of the two PCB congeners ... 

Ca concentration in the atmosphere K nondimensional Henry s law constant kajv, =Va/w / h air-water exchange rate h = AI w mean depth of river (Table 24.1)... 

The distribution of Hg within seepage lakes is a net result of the processes that control Hg transport between the atmosphere, water column, seston, sediments, and groundwater. This discussion focuses on the processes that control the exchange of Hg between the sediments and lake water. We first present data on spatial and temporal concentrations in the water column, sediments, pore water, and groundwater. These data set the context for a subsequent discussion of the chemical and physical processes responsible for the transport of mercury across the sediment-water interface and are necessary for assessing transport rates. 

Eisenreich, S.J., K.C. Hornbuckle, and D.A. Achman. 1997. Air-water exchange of semivolatile organic chemicals (SOCs) in the Great Lakes. In J.E. Baker, Ed. Atmospheric Deposition of Contaminants to the Great Lakes and Coastal Waters, Society of Environmental Toxicology and Chemistry, SETAC Press, Pensacola, Florida, 109-135. 

The role of the World Ocean in the global cycle of C02 is mainly manifested through the process of its exchange at the atmosphere-ocean boundary. The intensity of ocean-atmosphere gas exchange is determined by the dynamic and diffusive behavior of the turbulent layers of water and air near the interface. Here numerous physical schemes appear which reflect the situations of wave formation, their collapse, and the... 

Water exchange processes in the atmosphere-ocean system... 

The processes of transport at the atmosphere-water surface border have been well studied. The transport of moisture from the surface of a water body into the atmosphere is one of the complicated physical processes of mass and energy exchange across the water-air interface (Figure 4.12). These processes are functions of many climatic parameters and, to a large extent, are regulated by eddy motions in the surface layer of the atmosphere determined by the wind field. 

Let us consider a block scheme of global water exchange and write respective equations for it. The basic regularity of global water exchange is the invariability of water supplies on Earth over time periods of hundreds of years (i.e., we can reliably write the balance equation WE -f Ws I W0, where WE, Ws, and WQ are water supplies on Earth, on land and in the oceans, respectively). A compartment of the atmosphere is related to the respective region of water basin. Such a relationship is valid... 

The circulation of water in the Arctic Basin is a complex system of cycles and currents with different scales. Block HB simulates the dynamics of Arctic Basin water by the system of sub-blocks presented in Figure 6.2. The water dynamics in 2 is presented by flows between compartments Eijk. The directions of water exchanges are represented on every level zk = z0 + (k — 1 )A k according to Aota et al. (1992) in conformity with the current maps assigned as SSMAE input. The external boundary of 2 is determined by the coastline, the sea bottom, the Bering Strait, the southern boundary of the Norwegian Sea, and the water-atmosphere interface. 

Figure 5.2 The most common kinetic model used to estimate rates of gas exchange across the atmosphere-water boundary is the Stagnant Film Model. This model essentially has the following three regions of importance (1) a well-mixed turbulent atmospheric zone (PG) (2) a well-mixed thin-film liquid zone (PG) and (3) a laminar zone (A-B) separating the two turbulent regions. The thin-film is considered permanent with a thickness defined as z- (From Broecker and Peng, 1974, with permission.)...

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