Light Scattering By Pure Seawater: Effect of Pressure

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Marine Science


Ocean Science and Engineering


The Zhang et al. model [Optics Express, 17, 5698-5710 (2009)] for calculating light scattering by seawater does not account for pressure, which should, theoretically, affect molecular scattering. While negligible in near surface waters, the error associated with this approximation could be significant when backscattering is measured directly in the deep ocean, by deep CTD casts or biogeochemical-Argo floats, for example. We updated the parameterization in the Zhang et al. model using (1) the Millard and Seaver equation for the refractive index of seawater [Deep Sea Research Part A, 37, 1909-1926 (1990)] and (2) the Feistel equation for Gibbs free energy for seawater thermodynamics [Deep-Sea Research I, 55, 1639-1671 (2008)]. As these equations include the effect of pressure as well as salinity and temperature, our new parameterization allows us to investigate the potential effect of pressure on scattering. Increasing pressure suppresses the random motion of molecules, reducing the fluctuations in both density and concentration, which in turn causes an overall decrease in light scattering by seawater. For pure water and seawater with a salinity of 34 PSU, the decreases are approximately 13% and 12%, respectively, with a 100-MPa (approximately the pressure of seawater at 10000 m) increase in pressure. Below the thermocline and/or halocline where temperature and salinity change slowly, the steady increase of pressure is the dominant factor affecting the light scattering by seawater. At depths where backscattering is typically dominated by molecular scattering by seawater, particulate backscattering would be underestimated if the effect of pressure on molecular scattering were not considered.

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Deep Sea Research Part I: Oceanographic Research Papers



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