Fig. 4: Cu-catalyzed SO2 oxidation by NO2 as a sulfate source in urban air pollution.
From: Sulfate formation through copper-catalyzed SO2 oxidation by NO2 at aerosol surfaces
A The availability of Cu in PM2.5 in North China Plain haze events. On the map, circles (size) represent the concentration of total Cu elements in PM2.5 during the 2014 haze in Beijing, Tianjin, Langfang, Baoding, and Wangdu, as well as the 2016 haze in Shijiazhuang (See Table S2 for details). Data were acquired from refs. 38,59,60. Surrounding the map are the pie charts for the major Cu sources. Data were acquired from ref. 39. B Illustration for the Cu-catalyzed SO2 oxidation by NO2 in urban air pollution. C Sulfate PM2.5 formation rate through various heterogeneous SO2 oxidation pathways under urban haze conditions. The red solid line represents sulfate formation via the Cu-catalyzed oxidation of SO2 by NO2. The rate was estimated by using the kinetics expression in Equation 7. The red-shaded area represents the uncertainty arising from the one standard deviation of Cu concentration in urban air pollution (see Table S3). Also compared here are sulfate formation rates through other reaction pathways reported in literature, including the O3 pathway (purple curve)8, H2O2 pathway (blue curve)15, aqueous TMI-catalyzed oxidation (green curve)15, Mn-catalyzed oxidation on aerosol surface (yellow curve)13,32, and the uncatalyzed NO2 pathways (\({{\rm{HSO}}}_{3}^{-}\)/NO2 reaction8, red dashed-dotted curve; \({{\rm{SO}}}_{3}^{2-}\)/NO2 reaction14, red dotted curve). The conditions for urban haze follow that reported in ref. 8, which include 40 ppb SO2, 66 ppb NO2, 0.01 ppb H2O2, 1ppb O3, mean aerosol radius 0.15 µm, and aerosol water content (AWC) 300 µg m−3air. The water-soluble Fe and Mn were 18 and 45 ng m-3air, respectively32. The water-soluble and reactive Cu was 34 ± 19 ng m−3air. The gray shaded area represents the pH 4-to-5 range typical to China’s urban haze85,86.
