Thermodynamic properties of sea air

Abstract

Abstract. Very accurate thermodynamic potential functions are available for fluid water, ice, seawater and humid air covering wide ranges of temperature and pressure conditions. They permit the consistent computation of all equilibrium properties as, for example, required for coupled atmosphereocean models or the analysis of observational or experimental data. With the exception of humid air, these potential functions are already formulated as international standards released by the International Association for the Properties of Water and Steam (IAPWS), and have been adopted in 2009 for oceanography by IOC/UNESCO.In this paper, we derive a collection of formulas for important quantities expressed in terms of the thermodynamic potentials, valid for typical phase transitions and composite systems of humid air and water/ice/seawater. Particular attention is given to equilibria between seawater and humid air, referred to as "sea air" here. In a related initiative, these formulas will soon be implemented in a source-code library for easy practical use. The library is primarily aimed at oceanographic applications but will be relevant to air-sea interaction and meteorology as well.The formulas provided are valid for any consistent set of suitable thermodynamic potential functions. Here we adopt potential functions from previous publications in which they are constructed from theoretical laws and empirical data; they are briefly summarized in the appendix. The formulas make use of the full accuracy of these thermodynamic potentials, without additional approximations or empirical coefficients. They are expressed in the temperature scale ITS-90 and the 2008 Reference-Composition Salinity Scale.

Thermophysical property anomalies of Baltic seawater

“…The proper thermodynamic potential for FREZCHEM is a function which takes chemical potentials rather than particle numbers as independent variables, such as the Landau potential, = pV , where p and V are pressure and volume (Landau and Lifschitz, 1987;Goodstein, 1975). The Landau potential is related to the Gibbs potential by a Legendre transform (Alberty, 2001;Feistel et al, 2010c). The chemical potential of water in seawater expressed in terms of the Gibbs function is an example for such a Legendre transform.…”

Metrological challenges for measurements of key climatological observables: oceanic salinity and pH, and atmospheric humidity. Part 1: overview

“…the specific Helmholtz energy of humid air, f AV ( A , T , ρ ), as a function of dry-air mass fraction, A , temperature and mass density (IAPWS, 2010; Feistel et al, 2010a). …”

“…These differences can be exactly expressed in terms of the relative fugacity (see Appendix C in the supplement) of water vapour in the atmosphere, which is one of the options for defining relative humidity. To a reasonable approximation, the spatial distribution of the relative fugacity of water vapour can be described by that of the relative humidity in the WMO definition (Erikson, 1965; Kraus, 1972; Hansen and Takahashi, 1984; IOC et al, 2010; Feistel et al, 2010a; Feistel and Ebeling, 2011; Li and Chylek, 2012; Li et al, 2014). At the sea surface, the thermodynamic driving force for evaporation is the difference between the chemical potentials of water in the ocean and in the atmosphere (Kraus and Businger, 1994; IOC et al, 2010).…”

Metrological challenges for measurements of key climatological observables. Part 4: atmospheric relative humidity

“…To a reasonable approximation, see Sec. 5, the spatial distribution of the relative fugacity of water vapour can be described by that of the RH in the standard definition (Erikson, 1965; Kraus, 1972; Hansen and Takahashi, 1984; IOC et al, 2010; Feistel et al, 2010a; Feistel and Ebeling, 2011; Li and Chylek, 2012; Li et al, 2014). At the sea surface, the thermodynamic driving force for evaporation is the difference between the chemical potentials of water in the ocean and in the atmosphere (Kraus and Businger, 1994; IOC et al, 2010).…”

“…The mistake possibly originated in a WMO training manual (Retallack, 1973) and continues to propagate (Gill, 1982; Rogers and Yau, 1989; Pruppacher and Klett, 1997; Katsaros, 2001; Jacobson, 2005; Dai, 2006; Pierrehumbert, 2010), including in some background articles of TEOS-10 (Feistel et al, 2010a, 2010b) and online 11 . On the other hand, ASHRAE (1994) and BS 1339-1 (2002) identify the ratio of mixing ratios, r / r sat , as the “degree of saturation” and “percentage saturation”, respectively, thus clearly distinguishing it from the standard definition.…”