Roles of peroxy radicals and Criegee intermediates in β-pinene ozonolysis at different temperatures

Organic peroxy radicals (RO₂) and stabilized Criegee intermediates (SCIs), as important reactive species in the atmosphere, are critical in oxidation processes and secondary organic aerosol (SOA) formation. However, the influence of temperature on these reactive intermediates and the corresponding reaction mechanisms in SOA formation is still not well defined. In this study, through utilizing SCIs scavengers and regulating [HO₂]/[RO₂] from ~0.3 to ~1.9, the roles of RO₂ and SCIs in SOA formation were investigated at 298 K, 273 K, and 248 K, respectively, particularly for dimers formation in β-pinene ozonolysis. It was found that the dependence of the SOA yields on temperature was not monotonic. With the temperature decreasing from 298 K to 273 K, the SOA formation was promoted by the gas-particle partitioning of semi- and low-volatility products. However, when the temperature further decreased to 248 K, the SOA yields were lower due to the temperature effect on chemical reactions. The addition of SCIs scavengers showed that SCIs reactions accounted for more than 40% of both dimers and aerosol formation under all temperature conditions. The SCIs reactions predominantly contributed to the formation of C18 and C19 dimers. Increasing the [HO₂]/[RO₂] ratio suppressed SOA and dimers formation at all temperatures, indicating that in β-pinene ozonolysis RO₂+RO₂ reactions generate products of lower volatility, while RO₂+HO₂ reactions tend to form products of higher volatility. The lower RO₂ concentrations and suppressed RO₂+RO₂ reactions could partly explain the reduced SOA yield at 248 K. Additionally, it was found that the sensitivity of dimers formation on [HO₂]/[RO₂] was higher at lower temperatures. Even though the impact of temperature on the reaction coefficients of peroxy radicals was considered, the dimers formation at different [HO₂]/[RO₂] ratios could not be explained well at 273 K and 248 K. This suggests that SCIs reactions with RO₂ radicals may become more important at lower temperatures due to slower isomerization and decomposition of SCIs.