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 (SO₄^2–) mediates the physicochemical characteristics of PM_2.5. The δ³⁴S–SO₄^2– 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. SO₄^2– 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 δ³⁴S–SO₄^2– 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 SO₂ and PM_2.5 SO₄^2– 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 ³⁴S fractionation factors, we found that the SO₂ + O₂(TMI) pathway contributes the highest percentages of secondary SO₄^2– formation in autumn (34.3% ± 6.3%) and winter (38.5% ± 4.8%), while photochemical-derived formation pathways (SO₂ + H₂O₂/O₃ and SO₂ + OH) dominated secondary SO₄^2– formation in summer (48.2% ± 2.5%). The estimated source contributions to SO₄^2– 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 SO₄^2– pollution for the Hainan Free Trade Port.