Reactive Uptake of Gaseous Sesquiterpenes on Aqueous Surfaces

Sesquiterpenes emitted from biogenic sources play important roles in atmospheric HOx cycles and new particle formation. Current atmospheric models, however, fail to account for their fates, possibly due to missing heterogeneous sinks. Here we apply interface-specific mass spectrometry to detect carbocation products of the reactive uptake of gaseous sesquiterpenes C₁₅H₂₄ (β-caryophyllene (β-C), α-humulene (α-H), and alloaromadendrene (a-d)) on the surface of aqueous microjets as functions of water acidity and gas concentration. We find that these gases are effectively protonated to C₁₅H₂₅⁺ upon colliding with the surface of pH < 5 water microjets. We determine inflection points from plots of product yields vs bulk pH: pH_1/2 = 4.17 ± 0.05, 4.28 ± 0.06, and 4.36 ± 0.19, and kinetic isotope effects (KIEs) from H₂O/D₂O (1:1 = vol/vol) experiments: KIE = 2.31 ± 0.08, 1.95 ± 0.05, and 2.71 ± 0.11, for β-C, α-H, and a-d, respectively. These results are analyzed vis-a-vis previous reports on isoprene and monoterpenes experiments. We estimate 6.2 × 10^-5 ≤ γ ≤ 3.1 × 10^-4 for the reactive uptake of gaseous sesquiterpenes on acidic (1 < pH < 3) water surfaces. The atmospheric implications of present findings are discussed.