Abstract
Although the multiphase chemistry of SO2 in aerosol particles is of great importance to air quality under polluted haze conditions, a fundamental understanding of the pertinent mechanisms and kinetics is lacking. In particular, there is considerable debate on the importance of NO2 in the oxidation of SO2 in aerosol particles. Here experiments with atmospherically relevant deliquesced particles at buffered pH values of 4–5 show that the effective rate constant for the reaction of NO2 with SO32− ((1.4 ± 0.5) × 1010 M−1 s−1) is more than three orders of magnitude larger than the value in dilute solutions. An interfacial reaction at the surface of aerosol particles probably drives the very fast kinetics. Our results indicate that oxidation of SO2 by NO2 at aerosol surfaces may be an important source of sulfate aerosols under polluted haze conditions.
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All data are available in the main paper and the Supplementary Information files. Source data files are provided for Figs. 1, 2 and 4 and for Extended Data Figs. 1–6. Source data are provided with this paper.
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Acknowledgements
T.L. thanks the National Natural Science Foundation of China projects (92044301, 21806108), the National Key R&D Program of China (2016YFC0202000) and Dengfeng Project of Nanjing University for funding. T.L. and J.P.D.A. thank the Natural Sciences and Engineering Research Council (RPGIN-05972, BCPIR-537926) for funding, and J.G. Murphy and M. Davis for the loan of the TILDAS NH3 monitor and helpful discussions.
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T.L. and J.P.D.A. designed the research project. T.L. performed the research. T.L. and J.P.D.A. analysed data. T.L. and J.P.D.A. wrote the paper.
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Extended data
Extended Data Fig. 1 Time evolution of gas-phase and aerosol-phase species for a typical ammonium nitrate seed aerosol experiment.
Concentrations of wall-loss corrected NO3- (wNO3-) and SO42- (wSO42-) were derived by correcting SO42- concentrations using the wall-loss rate of aerosol NO3- before adding NH3 into the chamber.
Extended Data Fig. 2 Time evolution of gas-phase and aerosol-phase species for the HONO control experiment.
Concentrations of wall-loss corrected SO42- were derived by correcting SO42- concentrations using a wall-loss rate of NO3- before adding NH3 into the chamber. HONO was added as an oxidant instead of NO2 by bubbling a mixed solution of 10 ml 10 mM NaNO2 and 25 ml 50 mM H2SO4 for 15 min.
Extended Data Fig. 3 The fit of kexp vs 1/aH+ displayed in log-scale.
AN and MA represent ammonium nitrate and malonic acid experiments, respectively. The fit is based on equation (2) in the main text.
Extended Data Fig. 4 Dependence of kexp on the activity of hydrogen ion.
The grey line represents a regression fitting with equation (2) in the main text, resulting in a negligible value of \({{{\mathrm{k}}}}_{{{{\mathrm{NO}}}}_2 + {{{\mathrm{HSO}}}}_3^ - }\). The black and red dashed lines represent the results for \({{{\mathrm{k}}}}_{{{{\mathrm{NO}}}}_2 + {{{\mathrm{HSO}}}}_3^ - }\) of 3.0 × 106 M-1 s-1 and 1.5 × 107 M-1 s-1, respectively.
Extended Data Fig. 5 Time evolution of gas-phase and aerosol-phase species for control experiments without adding SO2.
a, malonic acid, b, ammonium nitrate. Concentrations of wall-loss corrected NO3- for malonic acid and ammonium nitrate experiments were derived by correcting NO3- concentrations using wall-loss rates of organics and NO3- before adding NH3 into the chamber, respectively.
Extended Data Fig. 6 Average mass-based size distributions.
Average mass-based size distributions of organics, nitrate, and sulfate for typical a malonic acid and b ammonium nitrate seed aerosol experiments. The average mass-based size distributions are shown because the time series data are quite noisy. Nevertheless, the distributions appear similar throughout the course of the experiment.
Supplementary information
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Liu, T., Abbatt, J.P.D. Oxidation of sulfur dioxide by nitrogen dioxide accelerated at the interface of deliquesced aerosol particles. Nat. Chem. 13, 1173–1177 (2021). https://doi.org/10.1038/s41557-021-00777-0
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DOI: https://doi.org/10.1038/s41557-021-00777-0
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