Recently, a large number of laboratory studies have focused on the aqueous-phase photochemistry of single organic compound in atmospheric condensed phases, yet few studies have been conducted on the aqueous-phase photochemical oxidation of real-world complex dissolved organic matter (DOM). Therefore, in this work, we report experimental results for the photochemical oxidation of DOM extracts from ambient fine aerosol samples upon direct photolysis or against OH oxidation, under both simulated sunlight and ultraviolet irradiation conditions. The products at different stages of photolysis were analyzed via UV-vis and spectroscopy and soot-particle aerosol mass spectrometry (SP-AMS) to investigate their optical and chemical characteristics. The results demonstrate the effective degradation of DOM under UV irradiation, and the f44 values of the corresponding products aremuch lower than under sunlight irradiation. A variety of carboxylic acids were generated during liquid-phase photolysis, and oxalic acid was found to be the most abundant. The light absorbance and concentration of HULIS did not change significantly under sunlight illumination; however, under UV and UV+·OH conditions, the concentration of HULIS increased continuously with reaction time. The HULIS concentration at 23 h was approximately four times the initial value, indicating the formation of brown carbon species with carboxyl, hydroxyl, and aromatic and other functional groups. Our results show that the increase in light absorptivity and formation rate of brown carbon from DOM are limited when aqueous-phase oxidation occurs under sunlight illumination. In comparison, DOM can constantly decompose into HULIS or small molecules under ultraviolet light illumination, and the light absorptivity of the remaining organic matter may be relatively high, resulting in final products with a high unit mass absorption efficiency (MAE). We have investigated the aqueous-phase oxidation of actual filter extracts for the first time, and our results provide valuable insights to the formation of air pollution complexes.
Keywords: air complex pollution; aqueous-phase photochemistry; brown carbon; dissolved organic matter (DOM); oxidation characteristic.