Large Discrepancy in the Formation of Secondary Organic Aerosols from Structurally Similar Monoterpenes

The two monoterpenes, Δ3-carene and α-pinene, have very similar chemical structures, which leads to the assumption that the formation of secondary organic aerosol (SOA) is also similar. This study aims to investigate if the formation of SOA from ozonolysis of Δ3-carene and α-pinene is actually analogous. This is addressed by conducting smog chamber studies of Δ3-carene ozonolysis and comparing the results to similar studies of α-pinene. Detailed offline analysis using ultra-high-performance liquid chromatography coupled to mass spectrometry seeks to elucidate differences in the product distribution between Δ3-carene and α-pinene. The experimental findings are supported by quantum chemical calculations of formation free energies of the first-generation oxidation products, cis-3-caric acid and cis-pinic acid. The smog chamber studies and detailed offline analysis show a considerable difference in the SOA formation from Δ3-carene and α-pinene. Δ3-carene produces higher SOA mass and larger particles, whereas α-pinene produces a larger number of smaller particles. This indicates that Δ3-carene oxidation products tend to condense on already existing particles, whereas α-pinene oxidation products to a larger degree contribute to new particle formation. The detailed offline analysis shows a much less diverse product distribution in Δ3-carene SOA compared to α-pinene SOA. In addition, three dimers from Δ3-carene are identified for the first time. The quantum chemical calculations indicate that cis-3-caric acid is expected to be more efficient in condensing on already existing particles compared to cis-pinic acid, which is in good agreement with the experimental results.