Field observations of isoprene ozonolysis contributing to SOA formation

Isoprene (ISO) is a key atmospheric biogenic volatile organic compound (BVOC) due to its high emissions and reactivity. While OH radical-initiated oxidation is the main ISO degradation pathway, ISO ozonolysis is also a non-negligible pathway. Moreover, ISO ozonolysis produces OH radicals, further enhancing ISO oxidation under low actinic flux conditions. More importantly, the stabilized Criegee intermediate (sCIs) generated from the ozonolysis of ISO is an underappreciated yet crucial atmospheric oxidant. It can undergo oxidation reactions similar to those of OH radicals and produce low-volatility organic acids and other carbonyl compounds, which act as precursors to secondary organic aerosols (SOA). Recently, monomers of CIs have been detected in SOA from tropical rainforests, suggesting that CIs can directly participate in SOA formation through 1,2-insertion reactions. In this work, we developed a new technique based on atmospheric pressure interface chemical ionization mass spectrometry (CI-API-CIMS) to conduct in-situ measurements of sCIs after chemical derivatization. The CI-API-CIMS was field tested in summer 2025 at a forest site in Chengdu, China. VOCs and particulate organic matter were concurrently measured by a PTR-MS and an HR-ToF-AMS. Observations revealed that suspected sCIs fragments were highly correlated with gaseous sCIs and organic acids. These observations were consistent with our existing understanding of sCIs, providing supporting evidence for the mechanism by which sCIs directly participate in SOA formation. A 0-D box model was also developed to verify these findings.