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 a sulfuric acid concentration of 57.8 wt % (in the BS experiments), while hydration occurred under conditions of 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,K-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,K-rxn (similar to 5 x 10(-6) m(3)/mu g for limonene and similar to 2 x 10(-5) m(3)/mu g for terpineol) substantially exceed the physical partitioning coefficients (K-p = 6.5 x 10(-8) m(3)/mu g for limonene and =2.3 x 10(-6) m(3)/mu g for terpineol) derived from the absorptive partitioning theory. At RH higher than 5%, the apparent partitioning coefficients (K-p,K-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 (K-p,K-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.