Characterization and bioavailability of rainwater dissolved organic matter at the southeast coast of China using absorption spectroscopy and fluorescence EEM-PARAFAC

Abstract

Rainwater brings considerable amounts of dissolved organic matter (DOM) from atmosphere to freshwater and marine environments, but little is known about the chemical composition and bioavailability of rainwater DOM. The quantity, quality, and bioavailability of DOM were investigated for 21 rain events at a coastal site in southeast China, using dissolved organic carbon (DOC) measurements, absorption spectroscopy, and fluorescence excitation-emission matrices-parallel factor analysis (EEM-PARAFAC). The DOC concentration ranged from 35 to 457 μM, which was affected by the prevailing monsoon, rainfall amount and terrestrial/anthropogenic inputs. The volume-weighted average DOC was 118 μM, corresponding to a rainwater DOC flux of 1.98 g m−2 yr−1. Four fluorescent components were identified with EEM-PARAFAC, including three humic-like components (C1-C3) and one tyrosine-like component C4. Absorption coefficient (aCDOM(300)) and fluorescence intensities of C2-C4 strongly correlated with DOC, indicating they can be used for DOC estimation. Rainwater DOM was characterized by low DOC-specific UV absorbance (SUVA254) and humification index (HIX), which indicated a low aromaticity and humification degree. Principal component analysis (PCA) based on DOM indices revealed two principal factors, which were related to the DOM concentration and humification degree respectively. PCA, together with air-mass trajectory analysis, successfully separated different rain events with variable inputs from living plants/local sources, soil organic matter/humified materials, and the marine source. Rainwater DOM generally showed a high bioavailability of 50% ± 19%, with higher degradability of non-chromophoric constituents and C1-C2 than other components. This study demonstrated the applicability of absorption and EEM-PARAFAC combined with PCA and air-mass trajectory analysis in differentiating rain events and tracking organic matter sources, and revealed different effects of microbial degradation on individual PARAFAC components in rainwater.