Formation and temperature dependence of Highly Oxygenated Organic Molecules from ∆3-carene ozonolysis

Monoterpenes, comprising 15% of global biogenic volatile organic compound emissions, play a pivotal role in atmospheric chemistry. ∆³-carene, the second most prevalent monoterpene, has been identified as a significant source of secondary organic aerosol (SOA) upon oxidation, potentially surpassing α-pinene under similar conditions. Despite its importance, research has predominantly focused on α-pinene , leaving gaps in our understanding of ∆³-carene's oxidation pathways, particularly its capacity to form highly oxygenated organic molecules (HOM).

To address this knowledge gap, we conducted an investigation into HOM formation during the ozonolysis of ∆³-carene using atmospheric simulation chambers. Employing a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer with nitrate as the reagent ion (NO3-CIMS), we measured HOM resulting from ∆³-carene ozonolysis. Additionally, we explored the impact of temperature and relative humidity on HOM composition and distribution across various conditions (0, 10, and 20 ºC, and humidity levels below 15% and around 80%).

Our analysis revealed diverse HOM monomers and dimers from ∆³-carene ozonolysis. Predominant HOM monomers included C₁₀H_14,16O₉ and C₉H_12,14O₉, while the largest dimers comprised C₁₉H₃₀O_6,10,11 and C₂₀H₃₂O_7,9,11. Significantly, HOM monomers with 9 or more oxygen atoms and all dimers irreversibly condensed onto particles, while those with 6-8 oxygen atoms behaved as semi-volatile organic species, maintaining notable gas-phase concentrations. Intriguingly, ∆³-carene ozonolysis produced higher HOM concentrations than α-pinene, suggesting distinct formation pathways for these two monoterpenes. Furthermore, we observed a substantial decrease in HOM concentrations at lower temperatures, consistent with previous studies on α-pinene ozonolysis. Despite similar main HOM species at temperatures of 20, 10, and 0 ℃, the ratio of HOM dimers to monomers increased from 0.78 to 1.51 as temperatures decreased. This temperature-dependent variation underscores the complexity of ∆³-carene's atmospheric processing, revealing nuanced behaviors of HOM under different environmental conditions.

In conclusion, this study provides valuable insights into the HOM formation pathways of ∆³-carene, shedding light on its unique atmospheric chemistry. The observed differences in HOM concentrations and temperature-dependent behaviors highlight the need for a more comprehensive understanding of various monoterpenes, moving beyond the well-studied α-pinene. These findings contribute to the broader knowledge of biogenic volatile organic compounds and their impact on atmospheric processes.