Heterogeneous interactions between SO2 and organic peroxides in submicron aerosol

Abstract

Abstract. Atmospheric models often underestimate particulate sulfate, a major\ncomponent in ambient aerosol, suggesting missing sulfate formation\nmechanisms in the models. Heterogeneous reactions between SO 2 and\naerosol play an important role in particulate sulfate formation and its\nphysicochemical evolution. Here we study the reactive uptake kinetics of\nSO 2 onto aerosol containing organic peroxides. We present chamber\nstudies of SO 2 reactive uptake performed under different relative\nhumidity (RH), particulate peroxide contents, peroxide types, and aerosol\nacidities. Using different model organic peroxides mixed with ammonium\nsulfate particles, the SO 2 uptake coefficient ( γ SO 2 ) was\nfound to be exponentially dependent on RH. γ SO 2 increases\nfrom 10 −3 at RH 25\u2009% to 10 −2 at RH 71\u2009% as measured for an\norganic peroxide with multiple O–O groups. Under similar conditions, the\nkinetics in this study were found to be structurally dependent: organic\nperoxides with multiple peroxide groups have a higher γ SO 2 than\nthose with only one peroxide group, consistent with the reactivity trend\npreviously observed in the aqueous phase. In addition, γ SO 2 is linearly related to particle-phase peroxide content, which in turn\ndepends on gas–particle partitioning of organic peroxides. Aerosol acidity\nplays a complex role in determining SO 2 uptake rate, influenced by\nthe effective Henry s Law constant of SO 2 and the condensed-phase\nkinetics of the peroxide–SO 2 reaction in the highly concentrated\naerosol phase. These uptake coefficients are consistently higher than those\ncalculated from the reaction kinetics in the bulk aqueous phase, and we show\nexperimental evidence suggesting that other factors, such as particle-phase\nionic strength, can play an essential role in determining the uptake\nkinetics. γ SO 2 values for different types of secondary organic\naerosol (SOA) were measured to be on the order of 10 −4 . Overall, this\nstudy provides quantitative evidence of the multiphase reactions between\nSO 2 and organic peroxides, highlighting the important factors that\ngovern the uptake kinetics.