Secondary sulfate aerosols played an important role in aerosol formation and aging processes, especially during haze episodes in China. Secondary sulfate was formed via atmospheric oxidation of SO2 by OH, O3, H2O2, and transition-metal-catalyzed (TMI) O2. However, the relative importance of these oxidants in haze episodes was strongly debated. Here, we use stable sulfur isotopes (δ34S) of sulfate aerosols and a Rayleigh distillation model to quantify the contributions of each oxidant during a haze episode in Nanjing, a megacity in China. The observed δ34S values of sulfate aerosols showed a negative correlation with sulfur oxidation ratios, which was attributed to the sulfur isotopic fractionations during the sulfate formation processes. Using the average fractionation factor calculated from our observations and zero-dimensional (0-D) atmospheric chemistry modeling estimations, we suggest that OH oxidation was trivial during the haze episode, while the TMI pathway contributed 49 ± 10% of the total sulfate production and O3/H2O2 oxidations accounted for the rest. Our results displayed good agreement with several atmospheric chemistry models that carry aqueous and heterogeneous TMI oxidation pathways, suggesting the role of the TMI pathway was significant during haze episodes.

Nitrate (NO3−) is a major contributing species to haze formation in Northern China. So far, formation processes and source apportionments of nitrate aerosols during haze pollution have not yet been well understood. In this study, the PM2.5 samples were collected in Beijing from 13 November to 24 December 2018. In addition to water‐soluble ions, oxygen (δ18O‐NO3−) and nitrogen (δ15N‐NO3−) isotopes in particulate NO3− were also determined, in order to investigate the formation pathways and potential sources of NO3− aerosols. The results showed that NO3− was a dominant species (43%) of secondary inorganic aerosols during the sampling period. The δ18O‐NO3− and δ15N‐NO3− values averaged at 83.8 ± 13.4 and 11.5 ± 5.0‰, respectively. Combining isotope compositions and Bayesian isotope mixing model, we found that heterogeneous reaction and gas phase oxidation contributed equally to nitrate formation during the sampling period. However, the contribution of heterogeneous processes to nitrate increased from 39% during the clean period to 64% during the haze period. On average, coal combustion, biomass burning, vehicle emissions, and soil emission contributed 50%, 26%, 20%, and 4%, respectively, to nitrate aerosols during the sampling period. Compared to the result in 2013, the significant decrease (~21%) of relative contribution of coal combustion to nitrate was due to strict reduction of coal consumption in Beijing. Finally, the relative contribution of traffic emissions to nitrate increased from 18% during the clean period to 30% during the haze period, suggesting that control of traffic emissions would be an important way to decrease nitrate concentrations and improve the air quality in Beijing.

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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.

Inorganic nitrate production is critical in atmospheric chemistry that reflects the oxidation capacity and the acidity of the atmosphere. Here we use the oxygen anomaly of nitrate (Δ17O($$\rm{NO}_{3}^{-}$$ NO 3 − )) in high-time-resolved (3 h) aerosols to explore the chemical mechanisms of nitrate evolution in fine particles during the winter in Nanjing, a megacity of China. The continuous Δ17O($$\rm{NO}_{3}^{-}$$ NO 3 − ) observation suggested the dominance of nocturnal chemistry (NO3 + HC/H2O and N2O5 + H2O/Cl−) in nitrate formation in the wintertime. Significant diurnal variations of nitrate formation pathways were found. The contribution of nocturnal chemistry increased at night and peaked (72%) at midnight. Particularly, nocturnal pathways became more important for the formation of nitrate in the process of air pollution aggravation. In contrast, the contribution of daytime chemistry (NO2 + OH/H2O) increased with the sunrise and showed a highest fraction (48%) around noon. The hydrolysis of N2O5 on particle surfaces played an important role in the daytime nitrate production on haze days. In addition, the reaction of NO2 with OH radicals was found to dominate the nitrate production after nitrate chemistry was reset by the precipitation events. These results suggest the importance of high-time-resolved observations of Δ17O($$\rm{NO}_{3}^{-}$$ NO 3 − ) for exploring dynamic variations in reactive nitrogen chemistry.