Sulfate (SO4
2–) is an important chemical
species in atmospheric aerosols, which strongly impacts atmospheric
chemistry processes and climate change. Stable sulfur isotopes (δ34S) of sulfate aerosols in PM2.5 were measured
in Beijing from November 13 to December 2, 2018, to investigate the
pathways of formation of sulfate aerosols. The results showed that
SO4
2– constituted a major fraction (18%)
of water-soluble ions and significant enhancement of sulfate was observed
during the haze period. The δ34S-SO4
2– values averaged at 4.4 ± 1.4‰ during
the full period, exhibiting a downward trend with an increase in sulfate
concentration. The change in sulfur isotope values could not be explained
by the changes in emission sources. Significant correlations were
found between observed δ34S-SO4
2– values and SO2 oxidation ratios (R =
−0.88; p < 0.01), indicating the changes
in sulfur isotopes were attributed to the SO2 oxidation
processes. On the basis of Rayleigh distillation, the average fractionation
factor between SO2 and SO4
2– was 4.0 ± 1.2‰. Combining sulfur isotopes and the Bayesian
model, we quantified the contributions of primary sulfate, OH, H2O2/O3, NO2, and O2 [catalyzed by transition metal ions (TMIs)] oxidation pathways to
sulfate formation were 7%, 20%, 16%, 27%, and 30%, respectively. The
contributions of TMI and NO2 pathways increased from 24%
and 20% during the clean period to 38% and 29% during the haze period,
respectively. Our results highlighted that sulfur dioxide oxidized
by TMI-catalyzed O2 and NO2 were the dominant
pathways of sulfate formation in Beijing under haze pollution during
the heating seasons.