Insight into the abundance and optical characteristics of water-soluble organic compounds (WSOC) in PM_2.5 in urban areas

Airborne particulate matter (PM) poses significant environmental and health challenges, particularly in urban areas. This study investigated the characteristics of water-soluble organic compounds (WSOC) in PM_2.5 (PM with an aerodynamic diameter of 2.5 μm or less) in Singapore, a tropical Asian city-state, over a six-month period. Specifically, we examined the abundance, optical properties, and fluorescence characteristics of WSOC in PM_2.5 collected from various urban locations, employing complementary instrumental techniques, parallel factor (PARAFAC) modeling, and Kohonen's self-organizing map (SOM). Our findings highlight that the differences in WSOC characteristics between sites reflect the influence of primary emissions of PM and secondary PM formation processes. Concentrations of PM_2.5 and WSOC on roadsides are influenced by vehicular traffic composition and volume. Under conducive atmospheric conditions, volatile organic compounds (VOCs) from vehicle emissions are oxidized to form WSOC, with its chemical composition dependent on whether oxidation occurs in the gas or aqueous phase. The findings also indicate that while vegetation barriers (VBs) planted along the roads in this study do not significantly reduce PM_2.5 concentrations, they alter the chemical composition and light absorption properties of WSOC. The capacity of VBs to retain PM less than 1 μm in size shows their potential in reducing exposure to harmful traffic emissions. Moreover, fluorescence analysis revealed the presence of humic-like and protein-like compounds, underscoring the complex chemical nature of WSOC. This study provides a comprehensive insight into the molecular composition and properties of WSOC in PM_2.5.