Impact of temperature on the role of Criegee intermediates and peroxy radicals in dimers formation from ß-pinene ozonolysis

Stabilized Criegee intermediates (SCIs) and organic peroxy radicals (RO2) are critical in atmospheric oxidation processes and secondary organic aerosol (SOA) formation. However, the influence of temperature on their corresponding reaction mechanisms in SOA formation is unclear. Through utilizing formic acid as SCIs scavenger and regulating the ratio of hydroperoxyl radials (HO2) to RO2 ([HO2]/[RO2]) from ~0.3 to ~1.9 using different concentrations of CO, the roles of RO2 and SCIs in SOA formation were investigated from 248 K to 298 K, particularly for dimers formation in β-pinene ozonolysis. The SOA yield increased by 21% from 298 K to 273 K, while decreased by 40% from 273 K to 248 K. Both changing [HO2]/[RO2] and scavenging SCIs significantly affect SOA yield and composition. SCIs reactions accounted for more than 40% of dimers and SOA mass formation for all temperatures. Increasing [HO2]/[RO2] inhibited dimers and SOA formation, and this inhibition became larger with decreasing temperature. Compared to low [HO2]/[RO2] (0.30–0.34), the dimers abundance at high [HO2]/[RO2] (1.53–1.88) decreased by about 31% at 298 K and 70% at 248 K. ... mehr[HO2]/[RO2] has a specific impact on SCIs-controlled dimers at lower temperatures by influencing especially the C9-SCIs reactions with RO2. The dimers
formed from C9-SCIs reactions with RO2 were estimated to decrease by 61% at high [HO2]/[RO2] compared to low [HO2]/[RO2] at 248 K. The high reactivity and substantial contribution to SOA of β-pinene-derived SCIs at lower temperatures observed in this study suggest that monoterpene-derived SCIs reactions should be accounted for in describing colder regions of the atmosphere.