Photoproduction of reactive intermediates from dissolved organic matter in coastal seawater around an urban metropolis in South China: Characterization and predictive modeling

Chromophoric dissolved organic matter (CDOM) is an important photochemical precursor to reactive intermediates (RIs) (e.g., excited triplet states of chromophoric dissolved organic matter ((CDOM)-C-3*), hydroxyl radicals (center dot OH), and singlet oxygen (O-1(2))) in aquatic systems to drive the photodegradation of contaminants. There have been limited studies on the photoproduction of RIs in coastal seawater CDOM in Asia, which impedes our ability to model the lifetimes and fates of contaminants in these coastal seawater systems. Hong Kong is an urban metropolis in South China, whose coastal seawater is susceptible to anthropogenic activities from the surrounding areas and the nearby Pearl River. We investigated the photoproduction of RIs in seawater around Hong Kong during the wet vs. dry season. Higher intensities of fluorescent components, dissolved organic carbon concentration ([DOC]), apparent quantum yields of RIs (Phi(RIs)), and steady-state concentrations of photogenerated RIs ([RIs](ss)) were observed for samples collected in the areas closest to the Pearl River during the wet season. Lower humification degrees and Phi(RIs) but higher intensities of fluorescent components and [RIs](ss) were generally observed for the wet season samples compared to the dry season samples. Statistical analysis revealed strong significant correlations (Spearman |r| > 0.6, p < 0.05) between Phi(RIs) and the absorbance properties (including the absorbance ratio E-2:E-3, spectral slope coefficients S350-400, and spectral slope ratio S-R) of CDOM, and between [RIs](ss) and the quantity-reflected properties (including the fluorescence intensity of humic-like components) of CDOM. Our modeling analyses combining orthogonal partial least squares and stepwise multiple linear regression showed excellent prediction strengths for [O-1(2)](ss) and [(CDOM)-C-3*](ss) (R-adj(2) > 0.7) when [DOC] and the chemical and optical properties of CDOM were used as predictor variables. These modeling results demonstrate the feasibility of predicting the concentrations and quantum yields of RIs in seawater around Hong Kong, and potentially other coastal cities in South China, from easily measurable chemical and optical properties.