Aerosol growth due to heterogeneous reaction

Single Droplet Tweezer Revealing the Reaction Mechanism of Mn(II)-Catalyzed SO2 Oxidation

“…Some mineral dust can produce sulfate from SO 2 at a rate that is comparable to the missing sulfate source (Figure A, dark cyan and brown). , However, another study using different Fe 2 O 3 crystals showed lower sulfate formation rates (Figure A, light cyan), which decreases significantly with larger aerosol size and can be lower than other aqueous bulk reaction pathways. The reaction rates of aqueous bulk reactions are in general lower than those of missing sulfate source, ,, except that for autoxidation the reaction rate can overlap with the missing sulfate at pH = 5, and for the TMI involved pathway reported by Liu et al, the reaction rate can further increase with lower pH value (<3.5) . The bulk phase reaction rate reported by Martin et al (Fe­(III)-catalysis), and Adkins et al (Mn­(II)-catalysis) were under low ionic strength conditions; the reaction rate can further decrease due to the high ionic strength in atmospheric aerosols.…”

“…Reaction mechanisms include: (i) transition metal ion catalyzed surface oxidation of SO 2 (maroon (this study)-Mn­(II), yellow-Mn­(II), pink-Mn­(II), and blue-Fe­(III) 47 ); (ii) sulfate formation on mineral dust surface (brown-MnO 2 , dark cyan-Fe 2 O 3 , light cyan-Fe 2 O 3 ). (iii) S­(IV) oxidation in aqueous bulk (green-autoxidation, orange-TMI, black line-Fe­(III), and red line-Mn­(II)). The Mn­(II) concentration was set to be 64 ng/m 3 for the pathways that require Mn­(II) concentration (yellow area and red dashed line).…”

“…Furthermore, the accurate determination of heterogeneous reaction kinetics can uncover the reaction mechanism. Many studies have illustrated that the kinetics of Mn­(II)-catalyzed sulfate formation in the aqueous phase (e.g., in cloud droplets) are not applicable for heterogeneous reaction pathways, ,− partly due to the small aerosol size in the Aitken and accumulation mode. Previous studies revealed that the Mn­(II)-catalyzed oxidation of SO 2 at the aerosol surface was 2 orders of magnitude quicker than in the bulk phase, and the formation of sulfate is a multistep process involving the formation of intermediate complexes in the vicinity of air–water interface. , Moreover, reaction kinetics can be substantially affected by reaction conditions, such as the ambient temperature, the relative humidity (RH) that dictates the ionic strengths, and the gas phase composition that affects aerosol pH. ,− Therefore, achieving precise quantification of heterogeneous reaction kinetics, along with parametrization of reaction conditions, demands a technique that can probe heterogeneous reactions on aerosol surfaces with high sensitivity and time resolution in a controlled environment.…”