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