Gaussian model for yellow substance absorption spectra

Abstract

Absorption spectra of yellow substance have been measured in the spectral range of 190 to 900 nm for Lake Constance and the river Schussen. After transformation from the length scale (nm) to the energy scale (cm-1) the spectra can be approximated accurately by a sum of three symmetrical peaks with Gaussian line shapes. Between different samples mainly the peak amplitudes are changing, while the centre wavelengths at 205, 233 and 251 nm and the corresponding line widths of 2832, 6979 and 10650 cm-1 remain nearly constant. In the classical exponential model of yellow substance absorption an exponent S is used to parameterise the spectral shape for wavelengths above 300 nm. S is related to the chemical composition, but it depends also on measurement conditions such as the reference wavelength and the data interval. Based on the Gaussian model the dependence of S on Gaussian line shape parameters and on measurement conditions is studied. The accuracy of the exponential approximation is determinded from measurements: For wavelenghts closer than 60 nm to the reference wavelength the maximum relative error of the exponential approximation is below 10 %.