Fig. 2: Reaction mechanism and kinetics.
From: Sulfate formation through copper-catalyzed SO2 oxidation by NO2 at aerosol surfaces
A A schematic diagram for the reaction mechanism of Cu-catalyzed SO2 oxidation by NO2. Reactions 2 and 3 are the rate-limiting steps. R2 stands for Reaction 2 and so forth. B–D Reaction rate measured with an aerosol optical tweezer (AOT). B Normalized reaction rate \(R\) increases linearly with the total Cu ions molarity (\(\left[{\rm{Cu}}\right]\), which is also the initial Cu(II) ions molarity). Reaction conditions were 500 ppb SO2 and 500 ppb NO2. Droplets initially comprised (NH4)2SO4/NH4HSO4/CuSO4 at mixing molar ratio of 1:1:0.0002, 1:1:0.0004, 1:1:0.001, and 1:1:0.002. The droplet was buffered at approximately pH 4 with 800 ppb NH3. The dotted line represents a linear fit. During reactions, sulfate (\({{\rm{SO}}}_{4}^{2-}\)) molar concentration increased linearly with time. The size of the circle indicates droplet radius, \(a\). The color indicates droplet pH. Conditions were 500 ppb SO2 and 500 ppb NO2. Droplets comprised a NH4Cl/HCl/CuCl2 mixture (1:0.005:0.001) and were buffered at pH approximately 3, 4, or 5 with 40, 400, or 4000 ppb NH3, respectively. C Normalized reaction rate \(R/[{\rm{Cu}}]\) is plotted as a function of SO2 mixing ratio, \({P}_{{{\rm{SO}}}_{2}}\) (5–200 ppb). Data are colored per NO2 mixing ratio, \({P}_{{{\rm{NO}}}_{2}}\) (50–500 ppb). Droplets initially comprised a mixture of (NH4)2SO4/NH4HSO4/CuSO4 at a mixing molar ratio of 1:1:0.002. The droplet was buffered at approximately pH 4 with 800 ppb NH3. D The same data in C is further normalized as \(R/([{\rm{Cu}}]{P}_{{{\rm{NO}}}_{2}})\) versus \({P}_{{{\rm{SO}}}_{2}}/{P}_{{{\rm{NO}}}_{2}}\). Also overlaid is the kinetic data determined from the Raman microspectrometry (micro-Raman). The dashed-dotted line illustrates the piecewise trends of the dataset. In B–D, errors in AOT data arise from the 95% confidence interval values of droplet growth rate determined by analyzing the stimulated Raman spectra (see Methods for details). In D, the error in micro-Raman data represents the one standard deviation value of R amongst the 10 measurements shown in Fig.1.
