Quantification of SO₂ Oxidation on Interfacial Surfaces of Acidic Micro-Droplets: Implication for Ambient Sulfate Formation

Abstract

Sulfate formation on the surface of aqueous microdroplets was investigated using a spray-chamber reactor coupled to an electrospray ionization mass spectrometer that was calibrated using NaSO(aq) as a function of pH. The observed formation of SO, SO, and HSO at pH < 3.5 without the addition of other oxidants indicates that an efficient oxidation pathway takes place involving direct interfacial electron transfer from SO to O on the surface of aqueous microdroplets. Compared to the well-studied sulfate formation kinetics via oxidation by HO(aq), the interfacial SO formation rate on the surface of microdroplets was estimated to be proportional to the collision frequency of SO with a pH-dependent efficiency factor of 5.6 × 10[H]/([H]+10). The rate via the acidic surface reactions is approximately 1-2 orders of magnitude higher than that by HO(aq) for a 1.0 ppbv concentration of HO( g) interacting with 50 μg/m of aerosols. This finding highlights the relative importance of the interfacial SO oxidation in the atmosphere. Chemical reactions on the aquated aerosol surfaces are overlooked in most atmospheric chemistry models. This interfacial reaction pathway may help to explain the observed rapid conversion of SO to sulfate in mega-cities and nearby regions with high PM2.5 haze aerosol loadings.

Possible heterogeneous chemistry of hydroxymethanesulfonate (HMS) in northern China winter haze

“…Several heterogeneous reaction pathways have been proposed (He et al, 2014;Cheng et al, 2016;Wang et al, 2016;Li et al, 2017a;Hung et al, 2018;Qin et al, 2018;Yu et al, 2018), including the oxidation of SO 2 in aerosol water (by NO 2 , transition-metal-catalyzed O 2 , or H 2 O 2 ) and on aerosol surfaces (by NO 2 and/or O 2 ). Their relative importance for sulfate production in winter haze, however, is unknown due to uncertainties in relevant reaction rates and estimates for aerosol water pH values (most reaction pathways are pH-dependent) (Guo et al, 2017b;Liu et al, 2017a;Li et al, 2018b;Wang et al, 2018;Zhao et al, 2018).…”

Enhanced heterogenous hydration of SO 2 through immobilization of pyridinic-N on carbon materials

“…Here, we found the SO 2 can be hydrated feasibly to form bisulfite, which might be readily oxidized in further steps to form sulfuric acid. The oxidation pathway has been suggested recently based on experiment results on water microdroplets [ 47 , 48 ]. To generalize our findings, the effects of H 2 O for SO 2 hydration should be discussed in two points: (i) what are the effects if more H 2 O molecules present for SO 2 hydration on PyN-doped substrate, and (ii) can the hydrogenated substrates be dehydrogenated to recycle as a catalyst?…”

China's Clean Air Action has suppressed unfavorable influences of climate on wintertime PM&lt;sub&gt;2.5&lt;/sub&gt; concentrations in Beijing since 2002

“…Meteorological initial and boundary conditions were obtained from the National Centers for in Beijing have been well documented in previous studies (Cheng et al, 206;Gao et al, 2016aGao et al, , 2018aSong et al, 2018), attributed to missing reaction pathways and aging/deposition 195 treatments in models (Song et al, 2019). Several heterogeneous reaction pathways for sulfate formation have been proposed, including the oxidation of SO2 by NO2, transition-metacatalyzed O2, or H2O2 in aerosol water, and by NO2 or O2 on aerosol surfaces (Cheng et al, 2016;He et al, 2014;Hung et al, 2018;Li et al, 2018). More recently, Song et al (2019) proposed that the heterogeneous production of hydroxymethanesulfonate (HMS) from the 200 reaction of SO2 and formaldehyde could be an important chemical mechanism for wintertime haze in China.…”