Role of stabilized Criegee Intermediate in secondary organic aerosol formation from the ozonolysis of α-cedrene

Atmospheric ozonolysis of sesquiterpenes is an important source of secondary organic aerosols (SOA). The mechanisms by which Criegee Intermediates (CIs) react to form SOA precursors and the influence of environmental conditions, however, remain unclear. On the basis of environmental chamber experiments coupled with detailed characterization of gas-phase and particle-phase products, we present evidence that a significant fraction of CIs from ozonolysis of α-cedrene are stabilized and bimolecular reactions of these stabilized CIs (SCIs) play a key role in the formation of SOA precursors. Ozonolysis experiments were conducted in a 4.5m³ collapsible fluoropolymer chamber under various conditions in the presence of the OH radical and SCI scavengers. The size and mass of SOA particles produced during ozonolysis were measured directly and used for calculation of particle effective density and mass yield. Gaseous and particulate products were analyzed by several mass spectrometry methods. A total of 14 compounds in gas phase and 17 compounds in particle phase were tentatively identified. The major gas-phase products are secondary ozonides (SOZ) from intramolecular reactions of SCIs. Multifunctional organic acids are dominant particle-phase products. The measured density of aerosol particles is 1.04±0.03 to 1.38±0.03g/cm³, and the aerosol mass yield is (23.7±0.4)% to (46.4±6.5)%, depending on reaction conditions. The presence of acetic acid, an SCI scavenger, inhibits new particle formation, but leads to increased aerosol mass yield. In contrast, the addition of SO₂ dramatically enhances new particle formation and total aerosol yield. The calculated OH formation yield decreases from (62.4±4.9)% to (9.0±1.6)% upon addition of SCI scavengers CH₃COOH and SO₂, indicating that a large fraction of excited CIs are collisionally stabilized and unimolecular decomposition of SCIs via the hydroperoxide channel can be suppressed by bimolecular reactions. The reaction of SCIs with SO₂ leads to the formation of sulfuric acid, an important nucleation precursor. From the consumption of SO₂ added as SCI scavenger, a lower-limit yield of SCIs from α-cedrene ozonolysis is estimated at ~88%. Our work underscores the key role of SCIs in SOA formation and observed composition of gas- and particle-phase products from α-cedrene ozonolysis. Bimolecular reactions of sesquiterpene CIs with atmospherically relevant species (e.g. SO₂, H₂O) need to be considered when assessing the atmospheric relevance of ozonolysis of sesquiterpenes.