A theoretical model was developed estimating the scattering by seawater that are due to concentration fluctuation. Combining with the model proposed for density fluctuation (Optics Express, 17, 1671, 2009), we evaluated the overall effect of sea salts on the scattering. The variation of seawater scattering with the salinity is a combination of two factors: decreasing contribution due to density fluctuation and increasing contribution due to concentration fluctuation, with the latter effect dominating. The trend is, however, slightly non-linear and the linear adjustment of scattering with salinity that is frequently used would lead to an underestimate by an average of 2%. The results estimated at S = 38.4 per thousand agree with the measurements by Morel (Cahiers Oceanographiques, 20, 157, 1968) with an average difference of 1%, well within his experimental error of 2%. The spectral signature also varies with salinity, with the power-law slope increasing from -4.286 to -4.306 for salinity from 0 to 40 per thousand.
The use of density derivative of the refractive index from the classic Lorentz-Lorenz equation or its variations performed poorly in estimating the scattering by water, leading to the alternative use of pressure derivative instead, which however has been scarcely measured due to its extremely low sensitivity. Recently, density derivative has been deduced directly from theoretical models. Three characterizations of density derivative of the refractive index were evaluated and scattering of water thus calculated converge with each other within 3.5% and agree with the measurement by Morel (Cahiers Oceanographiques, 20, 157, 1968) within 2% (with depolarization ratio = 0.039), all improving over the earlier estimates based on either density or pressure derivatives. Taking into account of uncertainty associated with the depolarization ratio, the prediction based on the model by Proutiere et al. (J. Phy. Chem., 96, 3485, 1992) still agrees with the measurement within the experimental errors (2%).
A new model for seawater scattering was developed, in which Gibbs function was used exclusively to derive the thermodynamic parameters that are associated with density fluctuation. Because Gibbs function was determined empirically from highly accurate measurements of a group of thermodynamic variables and is valid for S(A) up to 120 g kg(-1) (Deep-Sea Research I, 55, 1639, 2008), we expect the model is also valid over the extended range of salinity. The model agrees with the measurements by Morel (Cahiers Oceanographiques, 20, 157, 1968) with an average difference of -0.6% for S = 0 and 2.7% for S = 38.4. The scattering by seawater as predicted increases with salinity in a non-linear fashion, and linear extrapolation of scattering based on Morel's measurements would overestimate by up to 30%. The extrapolation of ZHH09 model (Optics Express, 17, 5698, 2009), which is valid for S(A) up to 40 g kg(-1), however, agrees with the prediction within +/- 2.5% over the entire range of salinity. Even though there are no measurements available for validation, the results suggested that the uncertainty is limited in using the newly developed model in estimating the scattering by seawater of high salinity.
The volume scattering functions for bulk and two submicron size fractions (passing through 0.7 μm GF/F and 0.2 μm membrane filters) were measured in the North Pacific Ocean using a LISST‐VSF to assess the contributions by submicron particles to the overall particle scattering. The contribution by submicron particles increased generally with scattering angle and peaked around 110°. The total backscattering by both submicron fractions did not vary, but that by larger particles increased, with the trophic level. Consequently, the fractional backscattering contribution by submicron particles decreased from approximately 50% to 30% as the chlorophyll‐a concentration increased from <0.1 to 0.3 mg m−3. Despite the experiment covering a limited range of trophic levels, our results confirm that backscattering by submicron particles in clear ocean waters is significant and seems to form a background independent of the backscattering by larger particles.
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