Absorptivity Inversely Proportional to Spectral Slope in CDOM

The application of the Bouguer-Lambert-Beer (BLB) Law is limited to dissolved organic matter (DOM) in the environment due to the wide distribution of overlapping chromophores with different molar absorptivities (ε). Here, we report our findings demonstrating the existence of an inverse relationship between ε and the spectral slope (S) in the absorption spectra of chromophoric DOM (CDOM). Specifically, ε values at 275 and 380 nm are directly proportional to the square of the root-mean-square width of the Gaussian peak (W²) located at 4.80 eV (258 nm) and 3.79 eV (327 nm), and W² is inversely proportional to the values of S of the right half of log-transformed spectra of the Gaussian peak with respect to wavelength, corresponding to S at 275-295 and 380-443 nm in spectra, respectively. This finding expands the applicability of the BLB Law to uniformly quantify DOC concentrations in inland, coastal, and pelagic waters (30-3000 μmol·L^-1, n = 7745, R² = 0.97). Furthermore, the relationship between ε and S aids in interpreting the absorbance spectra of DOM, providing valuable insights into its composition and properties. This finding represents a significant advancement in expanding the applications of UV-vis spectroscopy in CDOM monitoring from micro to global scales.