| Summary: | This paper, part 3 of the description of vertical pigment distributions in the Baltic Sea, discusses the mathematical expression enabling the vertical distributions of the non-photosynthetic pigment absorption factor <i>f<sub>a</sub></i> to be estimated. The factor <i>f<sub>a</sub></i> is directly related to concentrations of the several groups of phytoplankton pigments and describes quantitatively the ratio of the light energy absorbed at given depths by photosynthetic pigments to the light energy absorbed by all the phytoplankton pigments together (photosynthetic and photoprotecting). Knowledge of this factor is highly desirable in the construction of state-of-the-art "light-photosynthesis" models for remote-sensing purposes.<br> The expression enables <i>f<sub>a</sub></i> to be estimated with considerable precision on the basis of two surface parameters (available from satellite observations): the total chlorophyll <i>a</i> concentration at the surface<i>C<sub>a</sub></i>(0) and the spectral downward irradiance <i>E<sub>d</sub></i>(λ, 0) just below the sea surface. The expression is applicable to Baltic waters from the surface down to an optical depth of τ ≈ 5.<br> The verification of the model description of <i>f<sub>a</sub></i> was based on 400 quasi-empirical values of this factor which were calculated on the basis of empirical values of the following parameters measured at the same depths: <i>E<sub>d</sub></i>(λ, <i>z</i>) (or also <i>PAR</i>(<i>z</i>)), <i>a<sub>pl</sub></i>(λ, <i>z</i>),and the concentrations of all the groups of phytoplankton pigments <i>C<sub>a</sub></i>(<i>z</i>) and <i>C<sub>j</sub></i>(<i>z</i>) (where <i>j</i> denotes in turn chl <i>b</i>, chl <i>c</i>, <i>PSC</i>, <i>phyc</i>, <i>PPC</i>). The verification shows that the errors in the values of the non-photosynthetic pigment absorption factor <i>f<sub>a</sub></i> estimated using the modeldeveloped in this work are small: in practice they do not exceed 4%.<br> Besides the mathematical description of the vertical distribution of <i>f<sub>a</sub></i>, this paper also discusses the range of variation of its values measured in the Baltic and its dependence on the trophic index of a basin and depth in the sea. In addition, the similarities and differences in the behaviour of <i>f<sub>a</sub></i> in Baltic and oceanic basins are compared.
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