Iodide Accelerates the Processing of Biogenic Monoterpene Emissions on Marine Aerosols

Marine photosynthetic organisms emit organic gases, including the polyolefins isoprene (C₅H₈) and monoterpenes (MTPs, C₁₀H₁₆), into the boundary layer. Their atmospheric processing produces particles that influence cloud formation and growth and, as a result, the Earth's radiation balance. Here, we report that the heterogeneous ozonolysis of dissolved α-pinene by O₃(g) on aqueous surfaces is dramatically accelerated by I^-, an anion enriched in the ocean upper microlayer and sea spray aerosols (SSAs). In our experiments, liquid microjets of α-pinene solutions, with and without added I^-, are dosed with O₃(g) for τ < 10 μs and analyzed online by pneumatic ionization mass spectrometry. In the absence of I^-, α-pinene does not detectably react with O₃(g) under present conditions. In the presence of ≥ 0.01 mM I^-, in contrast, new signals appear at m/z = 169 (C₉H₁₃O₃ ^-), m/z = 183 (C₁₀H₁₅O₃ ^-), m/z = 199 (C₁₀H₁₅O₄ ^-), m/z = 311 (C₁₀H₁₆IO₃ ^-), and m/z = 461 (C₂₀H₃₀IO₄ ^-), plus m/z = 175 (IO₃ ^-), and m/z = 381 (I₃ ^-). Collisional fragmentation splits CO₂ from C₉H₁₃O₃ ^-, C₁₀H₁₅O₃ ^- and C₁₀H₁₅O₄ ^-, and I^- plus IO^- from C₁₀H₁₆IO₃ ^- as expected from a trioxide IOOO^•C₁₀H₁₆ ^- structure. We infer that the oxidative processing of α-pinene on aqueous surfaces is significantly accelerated by I^- via the formation of IOOO^- intermediates that are more reactive than O₃. A mechanism in which IOOO^- reacts with α-pinene (and likely with other unsaturated species) in competition with its isomerization to IO₃ ^- accounts for present results and the fact that soluble iodine in SSA is mostly present as iodine-containing organic species rather than the thermodynamically more stable iodate. By this process, a significant fraction of biogenic MTPs and other unsaturated gases may be converted to water-soluble species rather than emitted to the atmosphere.