Using Stable Sulfur Isotope to Trace Sulfur Oxidation Pathways during the Winter of 2017–2019 in Tianjin, North China

After the implementation of the Coal Replacing Project (CRP) in the northern parts of China in 2017, its effect on PM(2.5) composition is still unclear. In the study, water-soluble ionic components (WSICs) and stable sulfur isotope ratios (δ(34)S) of SO(4)(2−) in PM(2.5) collected during the domestic heating period before and after the implementation of CRP in Tianjin were analyzed. Results showed that the average concentrations of both PM(2.5) and WSICs have dropped dramatically after the CRP, especially for the SO(4)(2−) (by approximately 57–60%). After the CRP, the range of δ(34)S(sulfate) was significantly narrowed to 4.1–7.5‰ in January 2018 and 1.4–6.1‰ in January 2019, which suggested that the sulfur source was becoming simple. It was interesting that the δ(34)S(sulfate) value in the pollution period before the CRP was higher than that in the clean period, whereas it showed the opposite tendency after the CRP, which implied that the contribution of sea salt was high during the pollution period before the CRP. The MIXSIAR model calculated that the contributions of the transition-metal ion (TMI) oxidation and NO(2) oxidation pathways in the three sampling stages were higher than those of the OH radical oxidation and H(2)O(2)/O(3) oxidation pathways, indicating that the formation pathway of sulfate was mainly dominated by heterogeneous oxidation. Before the CRP, the NO(2) oxidation pathway was the dominant sulfate oxidation pathway during a haze episode, and the TMI oxidation pathway dominated the formation of sulfates after the CRP.