Quantitative Relationships between Physicochemical Properties of Organic Carbon from Coal Combustion and Heterogeneous Photooxidation of SO2 to Sulfates

Coal combustion significantly contributes to atmospheric organic carbon (OC), which can act as photosensitive sites for the conversion of SO₂ to sulfates. Light can significantly enhance the sulfate formation by SO₂ uptake on OC from coal combustion. These OC owned variable photo-reactivity, as shown by distinct SO₂ uptake coefficients and sulfate production. It was confirmed that the photo-reactivity of OC towards SO₂ greatly relied on its physicochemical properties, including hydrophilicity, light absorption capacity and •OH generation ability. The linear correlations of these properties with SO₂ uptake and sulfate production have been well built to explain the variability of OC photo-reactivity. The hydrophilicity of OC was largely influenced through the content of hydrophilic functional groups such as aromatic carbonyls (Ar–C=O) and OH, while the light absorption capacity and •OH generation ability of OC were determined by aromatic C–H (Ar–H) and Ar–C=O groups. The sulfate formation rate deriving from the photoreaction of SO₂ with OC was estimated to be 0.43–1.33 µg m⁻³ h⁻¹, suggesting an importance role of OC photochemistry as a key sulfate source.