Provide function to calculate partial pressure of dissolved gas

[jbeaulie]

Dissolved gas concentration is often reported as a partial pressure (i.e. pCO2 = 1200 uatm). One advantage of this expression is that dissolved partial pressures that exceed the partial pressure of the same gas in the atmosphere indicate that the water body is supersaturated and is therefore a source of the gas to the atmosphere. This allows for rapid interpretation as to whether the water body is a source or sink for the gas in question.

Striegl et al (2001) is just one example of an aquatic biogeochemical study reporting partial pressure. Striegl et al. cites Plummer and Busenberg (1982) for the calculation of partial pressure. See full citation below.

Another source of information is page 23 of Colt 2012: "The partial pressure of a dissolved gas is commonly called gas tension and is expressed as mmHg. At equilibrium, the partial pressure of a gas in the gas phase is equal to the partial pressure of the same gas in the liquid phase. The gas tension is equal to the partial pressure in the gas phase that would be in equilibrium with the measured gas concentration."

Striegl, R. G., et al. (2001). "Carbon dioxide partial pressure and 13C content of north temperate and boreal lakes at spring ice melt." Limnology and Oceanography 46(4): 941-945.

Plummer, L. N. and E. Busenberg (1982). "The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O." Geochimica et Cosmochimica Acta 46(6): 1011-1040.

Colt, J. (2012). Dissolved gas concentration in water. Computation as functions of temperature, salinity and pressure. Amsterdam, Elsevier.