Sulfur isotopic fractionation and its implication: Sulfate formation in PM2.5 and coal combustion under different conditions

In order to exactly explore sulfur source and sulfate formation under highly polluted atmosphere, we determined δ34S values of sulfate in PM2.5 and atmospheric SO2 in Nanjing region from 1 to 23 Jan. 2014. The secondary sulfate formation mechanism was discussed based on sulfur isotopic fractionation in the process of SO2 oxidation. Meanwhile, we synchronously studied δ34S values of raw coals used locally as well as sulfur isotopic fractionation during the combustion under coal burning and smoldering. The results show that δ34S average values of SO2 and sulfate in PM2.5 were 1.5‰ and 5.1‰, respectively. δ34S values of sulfate in PM2.5 were consistent with those of coals widely used in Nanjing region and Northern China, indicating coal combustion was an important sulfur source for PM2.5. Sulfur isotopic fractionation factors ranged from 1.0014 to 1.0075, implying that SO2 heterogeneous and homogeneous oxidation were coexisting during the formation of the secondary sulfate. The contribution of SO2 heterogeneous oxidation to sulfate varied from 40.7% to 64.8% during the observation period. δ34S values of coals presented moderately positive sulfur isotopic signatures due to organic sulfur in low sulfur coals were mainly formed by plant assimilation. Besides, the negative relationship between δ34S values of coals and total sulfur contents was also found. In addition, there existed a significant sulfur isotopic fractionation effect during coal combustion. Sulfate in PM2.5 in flue gas enriched 34S, while SO2 in flue gas enriched 32S. There was presence of the difference of δ34S values in PM2.5 and SO2 in flue gas between coal burning and smoldering, which was related to coal property and combustion temperature.