Abstract
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.
Highlights
Light scattering by seawater is an inherent optical property of the ocean, and forms background scattering that is unavoidably measured by scattering sensors and must be corrected to derive the particulate scattering
We investigated the effect of pressure on light scattering by seawater
We updated the parameterization of the Zhang et al (2009) model using the Millard and Seaver (1990) equation for the refractive index of seawater and the Feistel (2008) Gibbs function for seawater thermodynamics, both of which were developed as a function of pressure as well as salinity and temperature
Summary
Light scattering by seawater is an inherent optical property of the ocean, and forms background scattering that is unavoidably measured by scattering sensors and must be corrected to derive the particulate scattering. The presence of dissolved salts in seawater modifies the terms βT, n, and ρ in Eq (1), which, in turn, causes a slight decrease in βd(90) with increasing salinity (Zhang et al, 2009) Another impact of the presence of dissolved salts is the microscopic fluctuations in concentration (i.e., the mixing ratio between water molecules and disassociated sea salt ions) that induce additional fluctuations in the refractive index (Einstein, 1910). While the scattering by seawater is expected to vary with temperature, salinity, and pressure, the parameterization developed in the models (Zhang and Hu, 2009a; Zhang et al, 2009) for n, βT, ρ, and a0 did not consider the effect of pressure (P) These models should only be used for near-surface waters, where the changes in pressure with depth are limited. One immediate application of this theoretical development is to correct the scattering by seawater from in situ measurements of backscattering in deep water
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