Acid-Catalyzed Condensed-Phase Reactions of Limonene and Terpineol and Their Impacts on Gas-to-Particle Partitioning in the Formation of Organic Aerosols

We investigated the condensed-phase reactions of biogenic VOCs with CC bonds (limonene, C10H16, and terpineol, C10H18O) catalyzed by sulfuric acid by both bulk solution (BS) experiments and gas-particle (GP) experiments using a flow cell reactor. Product analysis by gas chromatography−mass spectrometry (GC−MS) showed that cationic polymerization led to dimeric and trimeric product formation under conditions of relative humidity (RH) 20% (in the GP experiments) and sulfuric acid concentrations of 46.3 wt % or lower (in the BS experiments). Apparent partitioning coefficients (K p,rxn) were estimated from the GP experiments by including the reaction products. Only under extremely low RH conditions (RH < 5%) did the values of K p,rxn (∼5 × 10−6 m3/μg for limonene and ∼2 × 10−5 m3/μg for terpineol) substantially exceed the physical partitioning coefficients (K p = 6.5 × 10−8 m3/μg for limonene and =2.3 × 10−6 m3/μg for terpineol) derived from the absorptive partitioning theory. At RH higher than 5%, the apparent partitioning coefficients (K p,rxn) of both limonene and terpineol were in the same order of magnitude as the K p values derived from the absorptive partitioning theory. Compared with other conditions including VOC concentration and degree of neutralization (by ammonium) of acidic particles, RH is a critical parameter that influences both the reaction mechanisms and the uptake ability (Kp,rxn values) of these processes. The finding suggests that RH needs to be considered when taking the effects of acid-catalyzed reactions into account in estimating organic aerosol formation from CC containing VOCs.