Comparison of Gaseous and Particulate Highly Oxygenated Organic Molecules from the Ozonolysis of Terpenes

Terpenes are important constituents of volatile organic compounds (VOCs) emitted from both natural and anthropogenic sources, reacting rapidly in the atmosphere to form secondary organic aerosol (SOA), with climate and health implications. In this study, we explore the gas- and particle-phase mass spectra of highly oxygenated organic molecules (HOMs) from the ozonolysis of four monoterpenes (α-pinene, 3-carene, β-pinene, and limonene), a sesquiterpene, and a boreal forest-mimicking terpene mixture in chamber experiments. Measurements were performed by using the new vaporization inlet for aerosols (VIA) coupled to a nitrate-based chemical ionization mass spectrometer (NO₃-CIMS). We found that higher gas-phase HOM yields correlated with increased SOA formation, while detected particle-phase HOM mass fractions also depended on the terpene being oxidized. Elevated particle-phase dimer-to-monomer ratios compared to the gas phase were observed for α-pinene and β-pinene (by a factor of 4–8), which can not be solely explained by the volatility-dependent condensation, suggesting the existence of particle-phase processes. Additionally, a linear reconstruction of the terpene mixture mass spectra from the individual terpene spectra agreed much better for the particle phase than the gas phase, indicating particle-phase reactions forming common HOM species originating from different monoterpenes. Finally, the first ambient VIA–NO₃-CIMS measurements were conducted and showed similar trends for organics compared with aerosol mass spectrometer (AMS) measurements, while the sulfate tracked almost perfectly between the two instruments. However, the organic mass concentrations obtained by the VIA–NO₃-CIMS were lower than from the AMS, with our best estimates suggesting that particle-phase HOMs comprised about 17% of the organics in the ambient air, while in our chamber studies it was 14–29%.

Terpenes form SOA effectively, with climate and health impacts. This study investigates gas- and particle-phase HOMs in both laboratory and ambient experiments, revealing particle-phase processes of HOMs.