Insights into the Associations between Optical and Molecular Signatures of Dissolved Organic Matter from Urban Stormwater Runoff

Abstract

Modern fluorescence techniques are widely employed to determine the dissolved organic matter (DOM) fluorescent components and quality in urban stormwater runoff. However, its molecular basis is still poorly defined, leading to unclear and unspecific interpretations of fluorescence characterization. This research focused on the correlation analysis between optical and molecular signatures of DOM. The optical parameters are effectively correlated with the individual molecules detected by mass spectrometry. Moreover, molecular characterization parameters are also associated with three-dimensional fluorescence spectroscopy in reverse. Through nonmetric multidimensional scaling (NMDS), the fluorescence indices tracing the source of DOM are effectively categorized and verified by molecular characteristics. Meanwhile, Spearman rank correlation analysis indicated that humic-like components derived from microbial- and tryptophan-like compositions are associated with aromaticity; however, terrestrial humic-like constituents are closely correlated with H/C, O/C, and N/C ratios. In addition, the molecular mechanism of four fluorescent components indicated that the nitrogen (N)-containing components, reduced constituents, and aliphatic compounds are resistant to degradation, while the oxidized and aromatic components are preferentially removed. Those signatures observed for individual molecules are consistent with the bulk characteristics of DOM, as reflected by optical properties.