On Sep 30, 2021, Nature Chemistry published a paper titled “Oxidation of sulfur dioxide by nitrogen dioxide accelerated at the interface of deliquesced aerosol particles” by associate Professor Tengyu Liu. Experiments show that the effective rate constant for the reaction of NO2 with SO32− is more than three orders of magnitude larger than the value in dilute solutions. Results indicate that SO2 oxidation by NO2 in aerosol particles may be an important source of sulfate aerosol under polluted haze conditions.
Atmospheric sulfate aerosol formation from multiphase oxidation of SO2 plays important roles in the chemistry of the atmosphere, impacting air quality, climate, human health and ecosystem health. However, atmospheric air-quality models that include gas-phase and in-cloud multiphase oxidation of SO2 cannot capture observed extremely high levels of sulfate aerosols. SO2 oxidation by NO2 in aerosol particles has been previously proposed to drive sulfate formation during haze events, but the aerosol pH values are required to be ~6 or higher for this chemistry to be environmentally important when literature values for the kinetics are used. Beyond aerosol pH, the fundamental mechanisms and kinetics remain to be elucidated for this process.
The authors study the oxidation of SO2 by NO2 in atmospherically relevant NH3-buffered deliquesced aerosol particles in an environmental chamber. Malonic acid (MA) and ammonium nitrate (AN) seed aerosols were investigated, with aerosol pH buffered with gas-phase NH3 at values of 3.8–5.2 as calculated using the E-AIM model. Experiments 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. However, the ionic strength has a negligible effect on SO2 oxidation by NO2 in aerosol particles. An interfacial reaction at the surface of aerosol particles probably drives the very fast kinetics. The average aerosol pH in China has been increasing due to rising NH3 and declining SO2. NO2-induced sulfate formation may become increasingly important to the air quality and climate change in the future.
Associate Professor Tengyu Liu is the first and corresponding author of this paper. Professor Jonathan Abbatt from University of Toronto, Ontario, Canada is the co-corresponding author. The study is financial supported by the National Natural Science Foundation of China projects (92044301, 21806108), the National Key R&D Program of China (2016YFC0202000), Dengfeng Project of Nanjing University for funding and the Natural Sciences and Engineering Research Council (RPGIN-05972, BCPIR-537926).
Fig. 1: Reactions between SO2 and NO2 in bulk solutions and relationship between kexp and hydrogen ion activity.
a, Reactions between SO2 and NO2 in bulk solutions. b, Dependence of kexp on the hydrogen ion activity.
Fig. 2: Sulfate formation rates of NO2 and H2O2 pathways in aerosol particles under winter haze conditions.
Article link: https://www.nature.com/articles/s41557-021-00777-0