Sources of dissolved organic matter in initial rainwater and its photochemical activity

The dissolved organic matter in initial rainwater (RDOM), including soil-dissolved organic matter (SDOM) and water-soluble organic matter (WSOM) in the atmosphere, can potentially mediate the migration and transformation of contaminants in natural water. However, researches on its sources, characteristics, and impact of environmental factors are limited. This study aims to investigate the main sources of RDOM and its properties influenced by environmental factors (such as light exposure and temperature variation). The results revealed that RDOM was mainly composed of SDOM. Temperature (–5°C, −20°C, temperature alternating, 40°C, 50°C, 60°C) induced the variation of SDOM components, altering its fluorescence characteristics, aromatic components, and hydrophobicity. And high temperature showed a more pronounced effect on SDOM components. However, temperature fluctuations had different impacts on the SDOM-mediated photodegradation of bisphenol A (BPA). Among them, only SDOM that had undergone freeze-thaw cycles exhibited a promotive effect on the photodegradation of BPA, with the most significant effect observed at −20°C. Illumination (ultraviolet light (UV) and natural light (NL)) consumed the chromophoric groups of SDOM, disrupted its intricate electron transfer structure, and reduced the production of •OH and Triplet-state dissolved organic matter (³DOM∗). So, illumination, especially UV, significantly weakened its mediating effect on the photodegradation of BPA. Based on these findings above, RDOM through temperature increase and illumination potentially leads to an inhibition of the degradation rate of organic pollutants. Conversely, RDOM may exhibit an enhanced ability following a freeze-thaw process. This study is crucial for elucidating the environmental effects of RDOM.