Atmospheric noble gases 1 Solubility equilibrium

NOBLE GAS COMPONENTS IN WATER Atmospheric noble gases 1 Solubility equilibrium 

Gas partitioning at the free air / water interface can be described reasonably well by Henry s law, which assumes that the concentrations in the two phases are directly proportional to each other  

Note that, although dimensionless, the actual value of the Hj implicitly depends on the choice of the concentration units. Therefore caution must be exercised if Henry constants or related measures from different sources are compared. 

Often Equation (1) is formulated for each particular gas in terms of its partial pressure pi (atm, bar,. ..) and corresponding equilibrium concentration C,e, (cm STP g  

instead of Henry constants, the equilibrium concentration for pi = 1 atm is reported. 

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As seen in Figure 4.1, the noble gas solubility in water shows considerable variation with temperature. Therefore, noble gas contents in groundwater, which was in solubility equilibrium with the ambient, can be used to estimate the atmospheric... 

Weiss and co-workers reported solubilities for He, " He, Ne, Ar, Kr as well as for O2 and N2 as a function of temperature and salinity for fresh and ocean waters (Table 1 Weiss 1970, 1971 Weiss and Kyser 1978). As this fundamental piece of work was strongly motivated by practical oceanographic research, the noble gas solubilities were expressed in the form of equilibrium concentrations with moist atmospheric air. For the atmosphere, it is justified to assume that its major elemental composition remains constant over the relevant time scales controlling gas exchange. Hence the gas partial pressure pi can be expressed by the total atmospheric pressure ptot corrected for water vapor content, Cw(T), and the volume or mole fraction Zi of the gas i in dry air (Ozima and Podosek 1983). 

The following sections comprise a brief description of noble-gas isotopes ( He, " He, Ne, and " °Ar), where measured concentrations in ground-waters were found to be in excess of solubility equilibrium with the atmosphere (air-samrated water (ASW)). Such excesses allow the use of these isotopes as natural tracers of groundwater flow (as opposed to those such as °Ne, Ne, Ar, and Ar that exhibit an atmospheric contribution only). 

The temperature dependences of the Henry s Law coefficients of the different gases listed in Table 3.6 are quite variable (Fig. 3.11). Helium, the least soluble noble gas, has very little solubility temperature dependence between 0 and 30 °C. On the other hand, Kr, the second most soluble of the non-radioactive noble gases, is much less soluble at higher temperatures. More details about gas solubilities are presented in the chapter on air-sea gas exchange (Chapter 10). Another notable aspect of the temperature dependence of the gas solubilities is that they are not linear. Thus, mixing between parcels of water of different temperatures at saturation equilibrium with the atmosphere results in a mixture that is supersaturated. This effect has been observed for noble gases in the ocean and may ultimately have a utility as a tracer of mixing across density horizons. 

Atmospheric solubility equilibrium. The dissolved concentrations of the noble gases in equilibrium with the atmosphere can easily be calculated from Henry s law (Eqn. 2), using the noble gas partial pressures in moist air given by Equation (3). The practical problem is to calculate the Henry coefficients Hi(T,S) in appropriate units. Several different expressions for gas solubilities are common in the literature and a variety of units for the concentrations in the two phases are used. In Equation (2), the units atm for Pi and cm STP g for Ci are widely used in noble gas studies. For some calculations it is convenient to use the same concentration units (e.g., mol m ) for both phases, resulting in the dimensionless Henry coefficient Hj, as in Equation (1). The conversion from Hj in volumetric units (e.g., (mol/lgas)(mol/lwater) ) to Hi in units atm (cm STP g ) is a... 

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