Simulation of the oxidation of biogenic volatile organic compounds in an atmospheric simulation chamber
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Date
2025-10-08
Authors
Βουτσινά, Ελένη
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Abstract
Atmospheric simulation chambers (ASCs) have been traditionally used for the experimental investigation of the oxidation of volatile organic compounds (VOCs) and the subsequent secondary organic aerosol (SOA) formation. β-Caryophyllene (C₁₅H₂₄) is one of the most reactive biogenic VOCs. The ozonolysis and photo-oxidation of β-caryophyllene result in the formation of several gas-phase products and a considerable amount of SOA. β-Caryophyllene ozonolysis and subsequent photo-oxidation experiments were recently conducted in the atmospheric simulation chamber of the Foundation for Research and Technology-Hellas (FORTH-ASC) under different conditions. In this study, a model based on the Master Chemical Mechanism (MCM) and an SOA formation module were used to simulate the corresponding gas-phase oxidation processes and partitioning of semi-volatile compounds between the gas and particulate phases in order to evaluate the mechanism and explain the experimental results. Additional simulations were performed to investigate the effect of relative humidity (RH) and reactant levels on the formation of first-ozonolysis gas-phase
products including C₁₅H₂₄O₃ and C₁₅H₂₄O₂ as well as on the predicted SOA formation.
First-ozonolysis gas-phase oxidation products C₁₅H₂₄O₃ (secondary ozonide and β-caryophyllonic acid) and C₁₅H₂₄O₂ (β-caryophyllon aldehyde), which were formed by the cleavage of the endocyclic double bond of β-caryophyllene, were identified during the measurements and their relatively high contributions to the product mixture were reproduced by the model. Under stoichiometric conditions, under which most of the ozone reacts with only the endocyclic bond, their predicted molar yields were 58% for C₁₅H₂₄O₃ (secondary ozonide), 12% for C₁₅H₂₄O₂ (β-caryophyllon aldehyde), and 8% for C₁₅H₂₄O₃ (β-caryophyllonic acid). These predicted values changed to 45.5%, 21.1%, and 15.3% when the RH increased from 20% to 50%, indicating RH dependence. In contrast, under excess ozone, these first-ozonolysis products were not observed experimentally, whereas they were predicted by the model in substantial quantities. Later-generation compounds, with five or less carbons, formed via cleavage of both endocyclic and exocyclic double bonds were predicted and measured in both stoichiometric and excess ozone conditions in the gas phase.
Overall, SOA formation was significantly enhanced when the ozonolysis products reacted with the OH radical. In experiments with stoichiometric concentrations, the model closely matched the SOA observations, with measured aerosol mass yields of 10% for ozonolysis and 69% during aging, compared to predicted values of 6% and 73%, respectively. In contrast, under conditions with excess ozone, larger discrepancies were observed, with measured yields of 25% and 90%, while the model underpredicted these values at 8% and 64%. The model predicted a similar degree of oxidation for both experiments in both the gas and particulate phases, while it slightly underpredicted the O:C of the SOA in the excess ozone experiment, with values of 0.34 versus 0.41 during ozonolysis and 0.45 versus 0.52 during aging. Moreover, based on the model RH has a small effect on SOA formation, whereas ozone levels and β-caryophyllene concentration played an essential role. Higher ozone levels nearly doubled the ozonolysis yield (8% versus 17%), while photo-oxidation yields remained similar (64% versus 61%), indicating greater sensitivity of dark ozonolysis to ozone concentration. Under the same excess ozone, higher initial β-caryophyllene concentrations enhanced SOA formation as expected.
The model captured quite well first-ozonolysis gas-phase products and SOA formation, whereas under excess ozone notable discrepancies were observed. These differences highlight the need for further research of the mechanism to improve model performance. Additionally, further experiments under excess ozone conditions could be conducted at varying β-caryophyllene to ozone ratios. These experiments would enable a more detailed investigation of the factors responsible for the absence of first-ozonolysis products, as well as provide experimental insight into the SOA formation behavior across different ratios and the extent to which these results align with model predictions.
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Keywords
Atmospheric pollution, Atmospheric chemistry, Volatile organic compounds (VOCs), Secondary organic aerosol (SOA), β-Caryophyllene