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

The two monoterpenes, Delta(3)-carene and alpha-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 Delta(3)-carene and alpha-pinene is actually analogous. This is addressed by conducting smog chamber studies of Delta(3)-carene ozonolysis and comparing the results to similar studies of alpha-pinene. Detailed offline analysis using ultra-high-performance liquid chromatography coupled to mass spectrometry seeks to elucidate differences in the product distribution between Delta(3)-carene and alpha-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 Delta(3)-carene and alpha-pinene. Delta(3)-carene produces higher SOA mass and larger particles, whereas alpha-pinene produces a larger number of smaller particles. This indicates that Delta(3)-carene oxidation products tend to condense on already existing particles, whereas apinene oxidation products to a larger degree contribute to new particle formation. The detailed offline analysis shows a much less diverse product distribution in Delta(3)-carene SOA compared to alpha-pinene SOA. In addition, three dimers from Delta(3)-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.

Automated summary

This study investigates the formation of secondary organic aerosol (SOA) from the ozonolysis of Delta(3)-carene and alpha-pinene. Through smog chamber studies and detailed offline analysis, it is found that Delta(3)-carene produces higher SOA mass and larger particles, while alpha-pinene produces a larger number of smaller particles. This suggests that the oxidation products of Delta(3)-carene tend to condense on existing particles, while those of alpha-pinene contribute more to new particle formation. The experimental results are in good agreement with quantum chemical calculations.