Photochemical characterization of surface marine waters around Hong Kong

Characteristics of marine water in Hong Kong show seasonal and spatial variations under the influence of both the western pearl river and eastern seawater. The photochemical properties of dissolved organic matter (DOM) in sunlit waters are known to govern the indirect phototransformation of organic pollutants. Yet, little is known about the photochemical properties of DOM in surface marine waters around Hong Kong. In this study, surface marine water samples at eight sites around Hong Kong were collected during two seasons (Sep. and Nov.). The optical properties of water samples from UV-vis and fluorescence spectrometers show significant differences (p < 0.05) between the two seasons. We quantified the apparent quantum yields of reactive intermediates (RIs) including hydroxyl radicals (●OH), singlet oxygen (¹O₂), and excited triplet states of chromophoric dissolved organic matter (³CDOM*) for collected samples. Results showed that the quantum yields of ¹O₂ and ³CDOM* of marine water samples significantly increase from Sep. to Nov. (p < 0.05), and significantly decrease from the west region to the east region (p < 0.05). This suggested the effects of the pearl river plume on the photoreactivity of west marine waters in the wet season. Similar to other studies on freshwater photoreactivity, correlation analysis revealed a strong correlation between optical properties and photochemical properties (spearman |r|> 0.6). Orthogonal partial least squares (OPLS) analysis identified the spectral slope ratio (S_R) and slope (S_275-295 and S_350-400), the ratio of absorbance at 250 nm and 365 nm (E₂:E₃), humification index (HIX), and relative amounts of humic-like components as the effective predictor of quantum yields of RIs among tested parameters (VIP > 1). Overall, this study provides the first characterization of the photoreactivity of surface marine waters around Hong Kong, which is helpful for further understanding and evaluation of the aquatic fate of pollutants in Hong Kong marine waters.