Heterogeneous interactions between SO₂ and organic peroxides in submicron aerosol

Abstract

Atmospheric models often underestimate particulate sulfate, a major component in ambient aerosol, suggesting missing sulfate formation mechanisms in the models. Heterogeneous reactions between SO2 and aerosol play an important role in particulate sulfate formation and its physicochemical evolution. Here we study the reactive uptake kinetics of SO2 onto aerosol containing organic peroxides. We present chamber studies of SO2 reactive uptake performed under different relative humidity (RH), particulate peroxide contents, peroxide types, and aerosol acidities. Using different model organic peroxides mixed with ammonium sulfate particles, the SO2 uptake coefficient (γSO2) was found to be exponentially dependent on RH. γSO2 increases from 10−3 at RH 25 % to 10−2 at RH 71 % as measured for an organic peroxide with multiple O–O groups. Under similar conditions, the kinetics in this study were found to be structurally dependent: organic peroxides with multiple peroxide groups have a higher γSO2 than those with only one peroxide group, consistent with the reactivity trend previously observed in the aqueous phase. In addition, γSO2 is linearly related to particle-phase peroxide content, which in turn depends on gas–particle partitioning of organic peroxides. Aerosol acidity plays a complex role in determining SO2 uptake rate, influenced by the effective Henry's Law constant of SO2 and the condensed-phase kinetics of the peroxide–SO2 reaction in the highly concentrated aerosol phase. These uptake coefficients are consistently higher than those calculated from the reaction kinetics in the bulk aqueous phase, and we show experimental evidence suggesting that other factors, such as particle-phase ionic strength, can play an essential role in determining the uptake kinetics. γSO2 values for different types of secondary organic aerosol (SOA) were measured to be on the order of 10−4. Overall, this study provides quantitative evidence of the multiphase reactions between SO2 and organic peroxides, highlighting the important factors that govern the uptake kinetics.