Monoterpenes belong to an important family of volatile organic compounds (VOC). They are emitted both from biogenic sources and anthropogenic activities. Once released into the atmosphere, monoterpenes can readily react with atmospheric oxidants, such as ozone, OH and NO3 radicals, and form complex mixtures containing carbonyls, carboxylic acids and alcohols. These oxidation products may partition in the gas and particle phases, making them a major source of secondary organic aerosols (SOA). It is generally perceived that the ozonolysis of monoterpene, such as α-pinene, takes place in multiple steps and proceeds likely via the "Criegee mechanism". This involves the initial 1,3-dipolar cycloaddition of ozone to the double bond to form 1,2,3-trioxilane intermediates, the so-called "primary ozonides" (POZ). The energy-rich POZ then rapidly undergoes unimolecular rearrangement to form activated carbonyl oxides, commonly referred to as "Criegee intermediates" (CI). However, direct experimental observation of RIs, particularly the CIs formed during ozonolysis of alkenes, has proved to be extremely challenging. Since the first prediction of CI by Rudolf Criegee in 1949, it is not until the last decade that the studies on the short-lived CIs have begun to flourish.
Recently, the research team led by Prof. Chia C. Wang investigated the ozonolysis of α-pinene over a broad range of temperature between 180 K and 298 K, by employing a custom-built, long-path, temperature-variable aerosol cooling chamber. By combining the time-resolved infrared spectroscopy and density functional theory (DFT) calculation, they provided the first experimental evidence of the Criegee intermediates and hydroperoxides formed during the ozonolysis of α-pinene and their temporal evolution. The evolution of aerosol size was also addressed in this study. The formation of hyperoxides (HP) and secondary ozonides (SOZ) are two major dissipation channels for CIs. The large organic HP once formed via Criegee mechanism, may either dimerize or agglomerate further to form larger SOA, or decompose into smaller carbonyl fragments leading to new particle formation with tens of nanometer in size. This study suggests that large CIs and HPs are formed during ozonolysis of α-pinene, unraveling new mechanistic and energetic insights into this atmospherically important, but highly complex reaction that lead to the formation of SOA.
Reference: Bagchi, A., Yu, Y., J.-H., Tsai, C.-C., Hu, W.-P., Wang, C. C.* (2020): Evidence and evolution of Criegee intermediates, hydroperoxides and secondary organic aerosols formed via ozonolysis of α-pinene, Phys. Chem. Chem. Phys., 22, 6528-6537. (link)