Two models for absorption by coloured dissolved organic matter (CDOM)

During the international SIMBIOS Program (Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies), the standard exponential model for CDOM absorption, applied to data from diverse waters, is presented. Absorption at 440 nm (a sub(g)440) ranged between close to zero and 10/m, and the slope of the semilogarithmic absorption spectrum over minimum range of 400 to 440 nm (s440) ranged between less than or equal to 0.01 and 0.04/nm. No relationship was found between s440 or s440 and salinity. Except in the southern Baltic, s440 was found to have a broad distribution (0.0165 plus or minus 0.0035), suggesting that it should be introduced as an additional variable in bio-optical models when a sub(g)440 is large. An alternative model for CDOM absorption, applied to available high quality UV-visible absorption spectra from the Vistula River (Poland), is presented. It was assumed in this model, that the CDOM absorption spectrum will be comprised distinct Gaussian absorption bands in the UV, similar to those of benzene. Five bands were fit to the data. The mean central energy of all bands was higher in early summer (E similar to 7.2, 6.6, 6.4, 6.2 and 5.5 eV or 172, 188, 194, 200 and 226 nm) than in winter. The higher energy bands were found to decay in both height and width with increasing salinity, while lower energy bands broadened with increasing salinity. s440 was found to be correlated with shape parameters of the bands centred at 6.4 and 5.5 eV. While the exponential model is convenient for optical modelling and remote sensing applications, these results suggest that the Gaussian model offers a deeper understanding of chemical interactions affecting CDOM molecular structure.