Relationships Between DOC and CDOM Based on the Total Carbon‐Specific Fluorescence Intensities for River Waters Across China

Abstract

Abstract Weak correlations between chromophoric dissolved organic matter (CDOM) absorption coefficient a(440) and dissolved organic carbon (DOC) were observed due to weak absorption for river waters especially in the Qinghai‐Tibetan Plateau, and thus, it is impossible to estimate DOC concentrations by CDOM absorption across China. Therefore, it is necessary to estimate DOC concentrations through the fluorescent fraction of CDOM (i.e., FDOM). FDOM was proposed as a new method for estimation of large‐scale DOC concentrations in river waters across China. A total of 301 water samples from eight river basins across China were selected to assess FDOM by excitation‐emission matrix (EEM) fluorescence. Five fluorescent regions were obtained by EEM coupled with fluorescence regional integration (FRI) (EEM‐FRI) method. However, weak correlations between the fluorescence intensities F R(1–5) , F SUM , and DOC concentrations were observed for all water samples, respectively. Therefore, the total carbon‐specific fluorescence intensities F SUM /DOC were used to establish relationships between DOC and FDOM. All 301 water samples excluding five outliers with F SUM /DOC higher than 8,000 nm/(mg/L)) were divided into four groups based on the threshold values for F SUM /DOC, and strong positive correlations between F SUM and DOC were observed in each group ( R 2 = 0.843, F SUM /DOC < 1,000 nm/(mg/L); R 2 = 0.928, 1,000 < F SUM /DOC < 2,000 nm/(mg/L); R 2 = 0.964, 2,000 < F SUM /DOC < 3,000 nm/(mg/L); and R 2 = 0.953, 3,000 < F SUM /DOC < 8,000 nm/(mg/L)), respectively. This result indicated that the riverine DOC concentrations on large‐scale across China can be estimated directly by FDOM properties.