Measurement Report: A new coupled method of pH titration and size-resolved analysis to identify the structure, aging, and source of water-soluble organic carbon

Abstract. Measuring water-soluble organic carbon (WSOC) in aerosols is critical, as WSOC is involved in many key particle-associated chemical reactions. Here, the coupled effects of pH and particle size on the chemical structures (functional groups) and optical properties (UV/fluorescence properties) of WSOC were investigated to explore the structure, aging, and source of these materials. The results showed that the specific UV absorbance at a wavelength of 254 nm (SUVA254) and mass absorption efficiency (MAE365) were higher in smaller particle sizes, revealing the relatively higher aromaticity and more freshness of WSOC in smaller particles. The carboxylic groups tend to be enriched in larger particles, whereas the contribution of phenolic groups was generally higher in smaller particles than that in larger particles. Overall, the chromophores in smaller particles showed a more pronounced pH dependence, revealing that the chromophores in smaller particles were more pH-sensitive. The change in the fluorescence peak position suggested that hydroxyl groups play a leading role in pH-responsive fluorescence in summer, while carboxylic groups in winter. WSOC with sizes of < 0.77 μm mainly originate from primary combustion emissions, and those with sizes of 1.40–2.50 μm primarily originate from secondary formations. Overall, the pH-dependent and particle size-dependent behaviors of aerosols WSOC provide insights into the structure, aging, and source of WSOC, and ultimately contribute to improving the accuracy of assessing the climate effects of WSOC.