S 180655 Cl

Defining salinity in terms of chlorinity alleviates the practical difficulties of measuring salinity through evaporating water samples to dryness. For calibration purposes, artificial water with salinity almost equal to 35%, known as Copenhagen water, is manufactured to serve as a reference. Copenhagen water has a chlorinity of 19.381%. This approach requires the chemical titration of water samples usually obtained by Nansen bottles, named after Fridtjof Bedel-Jarlsberg Nansen, a Norwegian explorer and scientist (18611930), which are self-closing containers that collect water from different depths.

Pure water is a poor electrical conductor. However, the presence of dissolved salts greatly increases its conductivity, which, in fact, is a function of pressure, temperature, and the degree of ionization of the dissolved salts. In the second half of the 20th century, technical improvements in the measurement of the electrical conductivity of seawater led to the development of the so-called salinometers. Conductive sali-nometers measure the ratio between the conductivity of the sample against that of a reference sample of known salinity. Researchers using conductivity salinometers in the beginning were giving a salinity value of 35% to any sample having the same conductivity as the Copenhagen water, even though the mass of salt per kilogram of water was not guaranteed to be the same in both cases. This was because conductivity depends on the degree of ionization of the dissolved salts and not on the absolute mass of salt. In 1978, the salinity scale was redefined in terms of the conductivity ratio, K15, between any given sample and the reference solution:

S (psu) = 0.0080 - 0.1692 K1« + 25.3851 K15 + 14.0941 K - 7.0261 IK25+ 2.7081 K5f5 K15 = C(S,15,0)/C(KCl,15,0),

where C(S,15,0) is the conductivity of the water sample at a temperature of 15°C and atmospheric pressure, and C(KCl,15,0) is the conductivity of a standard solution that contains 32.4356 g. potassium chloride (KCl) at the same temperature and pressure. The practical salinity unit is thus defined as a ratio of conductivities and has no physical units.

An alternative to conductivity salinometers are refractive salinometers, based on the fact that the speed of light through a medium depends on its density. In a refractive salinometer, a drop of sample water is placed on a prism. Because the water and the prism have different densities, light passing through the system is refracted at an angle that depends on the density (i.e., the temperature and salinity). On average, conductive salinometers have a precision of about 0.001 psu, whereas laboratory refractive salinometers have a precision of 0.06 psu, and handheld refractometers have a precision of 0.2 psu. Today's standard instrument for measuring both temperature and salinity is the Conductivity-Temperature-Depth profiler, which allows a quasi-continuous vertical sampling and a precision of 0.005 psu. Based on the same principle, conductivity/ temperature instruments are mounted on autonomous profilers (e.g., Argo floats) and the thermosalinographs that use near-surface water intakes of ships to continuously measure temperature and salinity.

A promising approach for remote sensing of the salinity is the microwave radiometry measuring the emissivity or brightness temperature of the sea surface, because the dielectric constant of seawater depends on temperature and salinity. The largest sensitivity of the surface emissivity to salinity has been observed in the L-band (1.40-1.43 GHz). The Soil Moisture and Ocean Salinity of the European Space Agency and the Aquarius-SAC/D mission of NASA-Argentine Space Agency are the first two space missions designed to provide global, synoptic estimates of the sea surface salinity with an accuracy of about 0.1 psu every 30 days with a 100- to 200-km. spatial resolution.

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