Using Stable Sulfur Isotopes to Quantify the Formation Mechanism and Sources of SO42– in PM2.5 in Haikou City

As an important component of fine atmospheric particles (PM 2.5 ), sulfate (SO 4 2– ) mediates the physicochemical characteristics of PM 2.5 . The δ 34 S–SO 4 2– values and water-soluble inorganic ion concentrations in PM 2.5 were analyzed in Haikou City, Hainan Province, China, from September 2021 to August 2022. SO 4 2– was found to be the most abundant ion in PM 2.5, accounting for 39.0%–58.7% of all water-soluble inorganic ions. The δ 34 S–SO 4 2– values in PM 2.5 ranged from −0.9 to 10.0‰, with low values recorded in autumn (2.3‰ ± 2.3‰) and winter (2.2‰ ± 1.9‰), but higher values were observed in spring (6.4‰ ± 1.9‰) and summer (6.2‰ ± 1.1‰). The calculated total fractionation factors (ε total ) between gas-phase SO 2 and PM 2.5 SO 4 2– were −0.4‰ ± 1.8‰, −0.2‰ ± 1.6‰, 2.9‰ ± 1.4‰, and 3.3‰ ± 1.1‰ in autumn, winter, spring, and summer, respectively. Based on the 34 S fractionation factors, we found that the SO 2 + O 2 (TMI) pathway contributes the highest percentages of secondary SO 4 2– formation in autumn (34.3% ± 6.3%) and winter (38.5% ± 4.8%), while photochemical-derived formation pathways (SO 2 + H 2 O 2 /O 3 and SO 2 + OH) dominated secondary SO 4 2– formation in summer (48.2% ± 2.5%). The estimated source contributions to SO 4 2– in PM 2.5 indicated that ship emissions contributed the highest percentage (48.8% ± 12.0%) in PM 2.5 in Haikou City. The findings of this study indicate that reducing the ship exhaust over the northern South China Sea would be beneficial for mitigating SO 4 2– pollution for the Hainan Free Trade Port.