Practical salinity unit

Figure 1 The solubility of the principal atmospheric gases in seawater, as a function of temperature. Units are millilitres of gas contained in a litre of seawater of salinity 35 psu, assuming an overlying atmosphere purely of each gas. Note that salinity is defined in terms of a conductivity ratio of seawater to a standard KC1 solution and so is dimensionless. The term practical salinity unit , or psu, is often used to define salinity values, however. It is numerically practically identical to the old style unit of parts per thousand by weight...

PSU (practical salinity units) are a unit of measurement of salinity, i.e. of the total amount of dissolved salts in water. If sea water has a salinity of 5 psu, 5 g of salt are dissolved in 1000 g of water. 

The PSS-78 is based on the measurement of a conductivity ratio and, hence, is technically unitless. Nevertheless, some oceanographers use a psu designation to represent a practical salinity unit and others report salinity in units of parts per thousand (%o). The latter convention has been adopted in this text. In any event, it is important to appreciate that the practical salinity is no longer directly traceable to the theoretical definition given in Eq. 3.1. 

Salinity A measure of the salt content of seawater now expressed in practical salinity units related to measurement via conductivity. 

Fig. 3 Vertical profiles of the water potential temperature (T , degrees Celsius), water salinity (S, practical salinity unit), and water specific potential density (or , kgm-3) a in upper layer of the Black Sea central area in August 1995 and b in deep layer (mean values based on high vertical resolution CTD measurements). 1 Upper mixed layer, 2 seasonal pycnocline (thermocline), 3 cool intermediate layer, 4 main pycnocline (halocline), 5 deep pycnocline, 6 bottom mixed layer...

Fig. 4 Vertical sections of (a,b) climatic water temperature (degrees Celsius) and (c,d) climatic water salinity (practical salinity units) in the Black Sea along 36° E in March (a,c) textbfa,c) and September (b,d)...

Table 1 Parameters of the climatic annual cycle of variability of water temperature (degrees Celsius) and water salinity (practical salinity units) at standard levels of main baro-clinic layer in the Black Sea central area (offshore end of standard section 3, see Fig. 1). Tmin. Tmax.Smin. and Smax annual extremes of climatic monthly mean temperature and salinity mon corresponding months of extremes DT and DS ranges of temperature and salinity annual cycles...

Fig. 7 a Climatic fields of the Black Sea upper mixed layer thickness (m) in area with sea depth >50 m. b Sea surface temperature (degrees Celsius) and c sea surface salinity (practical salinity units), both superimposed on the areas of water subduction with velocities 1 < 40 m month-1, 2 40-80 m month-1, 3 > 80 m month-1 in March... 

Fig. 8 Climatic monthly fields of the water salinity (practical salinity units) of the Black Sea at a depth of 100 m in a February, b May, and c August and the salinity difference between August and February (d)...

Fig. 10 Climatic monthly fields of the Black Sea water salinity anomaly (practical salinity units) at a depth of 100 m received by subtraction of the annual mean and first EOF fields from the climatic monthly salinity field in a January, b February, c March, d April, e May, and f June. Dashed lines negative anomaly...

Fig. 13 Multiannual variability and quadratic trends of a sea surface temperature (degrees Celsius), b minimal temperature, and c sea surface salinity (practical salinity units) in the Black Sea in August at offshore ends of standard sections 1 from Sebastopol southwest-ward (see line 1 in Fig. 1) and 2 from Gelendjic southwestward (see line 3 in Fig. 1)...

The third category of salinity methodologies was based on conductometry, as the conductivity of a solution is proportional to the total salt content. Standard Seawater, now also certified with respect to conductivity, provides the appropriate calibrant solution. The conductivity of a sample is measured relative to the standard and converted to salinity in practical salinity units (psu). Note that although psu has replaced the outmoded %o, usually units are ignored altogether in modern usage. These techniques continue to be the most widely used methods because conductivity measurements can provide salinity values with a precision of 0.001 psu. Highly precise determinations require temperature control of samples and standards to within 0.001 °C. Application of a non-specific technique like conductometry relies upon the assumption that the sea-salt... 

In Table 20.4, the abbreviation psu, short for practical salinity unit, indicates salinity expressed in the Practical Salinity Scale of 1978 (PSS-78) as a dimensionless quantity. The term psu is not an official unit (Unesco, 1985 Siedler, 1998) but is in widespread use and is particularly helpful to distinguish, say, a given salinity value from absolute salinity in g/kg. Before 1978, salinity was computed from chlorinity, CL by the Cox scale, 5= 1,80655 xCZ (Mamayev et al, 1991), The recommended numerical conversion factor between the PSS-78 salinity and the Cox salinity is 1, Cox salinity is usually expressed in parts per thousand, ppt, %o, or g/kg. None the less, it is lower by about 0.5% than the absolute salinity of seawater in grams of dissolved substance per kilogram of seawater, which in turn is not exactly known but can be estimated sufficiently well (Millero et al., 2008), see Section 20.2.1. 

Methane can be removed from estuarine waters by microbial methane oxidation and emission to the atmosphere. Methane oxidation within estuaries can be quite rapid, with methane turnover times of < 2 h to several days. Methane oxidation appears to be most rapid at salinities of less than about 6 (on the practical salinity units scale) and is strongly dependent on temperature, with highest oxidation rates occurring during the summer, when water temperatures are highest. Methane oxidation rates decrease rapidly with higher salinities. 

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