Impact of temperature on the role of Criegee intermediates and peroxy radicals in dimer formation from β -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 a SCI 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 to 298 K, particularly for dimer formation in β-pinene ozonolysis. The SOA yield increased by 21 % from 298 to 273 K, while it decreased by 40 % from 273 to 248 K. Both changing [HO2]/[RO2] and scavenging SCIs significantly affect SOA yield and composition. SCI reactions accounted for more than 40 % of dimer and SOA mass formation for all temperatures. Increasing [HO2]/[RO2] inhibited dimer and SOA formation, and this inhibition became larger with decreasing temperature. Compared to low [HO2]/[RO2] (0.30–0.34), the dimer abundance at high [HO2]/[RO2] (1.53–1.88) decreased by about 31 % at 298 K and 70 % at 248 K. [HO2]/[RO2] has a specific impact on SCI-controlled dimers at lower temperatures by especially influencing the C9–SCI reactions with RO2. The dimers formed from C9–SCI 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 SCI reactions should be accounted for in describing colder regions of the atmosphere.