Secondary organic aerosol (SOA) has received lots of attention in the research field due to its significant impact on human health, climate forcing and regional visibility. Acid catalyzed reactions of VOCs in the gas-particle phase have been suggested as an important pathway of SOA formation. Sesquiterpenes (SQT) are known as important SOA precursors due to their low volatility and the high potential of aerosol yield. β-caryophyllene is a sesquiterpene commonly found emitted by vegetation and was used as the model compound in this study....[ Read more ]

Secondary organic aerosol (SOA) has received lots of attention in the research field due to its significant impact on human health, climate forcing and regional visibility. Acid catalyzed reactions of VOCs in the gas-particle phase have been suggested as an important pathway of SOA formation. Sesquiterpenes (SQT) are known as important SOA precursors due to their low volatility and the high potential of aerosol yield. β-caryophyllene is a sesquiterpene commonly found emitted by vegetation and was used as the model compound in this study.

Two experimental sessions were conducted to investigate the heterogeneous acid-catalyzed reactions of β-caryophyllene with sulfuric acid particles: the bulk solution (BS) experiments and the gas particle (GP) experiments. A custom made flow cell reaction system was used to perform the reactions in GP experiments. Significant reactive uptake was observed and semi-quantified by gas chromatography mass spectrometry (GC-MS) in both the BS and GP experiments. The reaction mechanisms were proposed including isomerization, hydration, cationic dimerization and dehydrogenization. They were strongly affected by the aerosol acidity or relative humidity (RH). The isomerization and hydration processes occurred under wide range of conditions (sulfuric acid concentrations of 17.6wt%–57.8wt% in BS experiments and RH conditions of 3.4%–30.3% in GP experiments), while the dehydrogenation and dimerization were only observed in very acidic environment (around 3% RH in GP experiments). Apparent partitioning coefficients (Kp,rxn) were determined from the GP experiments ranged from 7×10^-9 m³/μg to 3×10^-6 m³/μg for 60% to 3% RH. Reactive uptake coefficients (γ) for relatively larger particles (about 100um in diameter) were also estimated ranged from 2.72×10^-5 to 1.80×10^-2 for 60% to 3% RH. In assessing the significance of such acid catalyzed reactions in the atmosphere based on the reported ambient SQT concentration (100 ppt), 0.024% to 12% mass of SQT could be consumed by the studied reaction corresponding to a RH of 60% to 3%.