Relative Humidity Changes the Role of SO2 in Biogenic Secondary Organic Aerosol Formation

SO2 influences secondary organic aerosol (SOA) and organosulfates (OSs) formation but mechanisms remain elusive. This study focuses on this topic by investigating biogenic gamma-terpinene ozonolysis under various SO2 and relative humidity (RH) conditions. With a constant SO2 concentration (similar to 110 ppb), the increase in RH transformed SO2 sinks from stabilized Criegee intermediates (sCIs) to peroxides in aerosol particles. The associated changes in particle acidity and liquid water content may collectively first lead to decreased and then increased SOA yield with increasing RH, with the turning point appearing at similar to 30% RH. The abundance of most OSs formed under 45% RH was more than 5 times higher than that of OSs formed under 10% RH, possibly due to interactions of dissolved SO2 with hydroperoxides (ROOH) in SOA. ROOHs formed from the autoxidation processes of alkylperoxy radicals were proposed to be precursors for highly oxidized OSs (HOOSs) that decreased SOA volatility and showed a certain abundance in ambient aerosols. This study highlights that high RH potentially enhances the contribution of SO2 to OSs formation, and particularly, HOOSs formation during monoterpene ozonolysis in the atmosphere.

Automated summary

This study reveals that high relative humidity enhances the contribution of SO2 to the formation of OSs, particularly HOOSs, during monoterpene ozonolysis in the atmosphere. The interaction between SO2 and ROOHs under high humidity conditions promotes the formation of HOOSs, leading to changes in SOA yield. Additionally, variations in particle acidity and liquid water content also impact SOA formation.