Dry Deposition of Biogenic Terpenes via Cationic Oligomerization on Environmental Aqueous Surfaces

Unraveling the complex interactions between the atmosphere and the biosphere is critical for predicting climate changes. Although it is well-recognized that the large amounts of biogenic volatile organic compounds (BVOCs) emitted by plants must play important roles in this regard, current atmospheric models fail to account for their fate due to missing chemical sinks. Here, we applied online electrospray mass spectrometry to monitor aqueous microjets exposed to gaseous monoterpenes (α-pinene, β-pinene, and d-limonene) and found that these BVOCs are readily protonated (to C_(10)H_(17)^+) and undergo oligomerization (to C_(20)H_(33)^+ and C_(30)H_(49)^+) upon colliding with the surface of pH < 4 microjets. By considering that the yields of all products show inflection points at pH ≈ 3.5 and display solvent kinetic hydrogen isotope effects larger than 2, we conclude that the oligomerization process is initiated by weakly hydrated hydronium ions, H_(3)O^+, present at the gas–water interface. Present results provide a universal mechanism for the dry deposition of unsaturated BVOCs and may account for recent observations on the uptake of terpenes in forest canopies and over grassland.