Iodide Accelerates the Processing of Biogenic Monoterpene Emissions on Marine Aerosols

Abstract

Marine photosynthetic organisms emit organic gases, including the polyolefins isoprene (C_5H_8) and monoterpenes (MTPs, C_(10)H_(16)), 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 O_3(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 O_3(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 O_3(g) under present conditions. In the presence of ≥ 0.01 mM I–, in contrast, new signals appear at m/z = 169 (C_9H_(13)O_3–), m/z = 183 (C_(10)H_(15)O_3–), m/z = 199 (C_(10)H_(15)O_4–), m/z = 311 (C_(10)H_(16)IO_3–), and m/z = 461 (C_(20)H_(30)IO_4–), plus m/z = 175 (IO_3–), and m/z = 381 (I_3–). Collisional fragmentation splits CO_2 from C_9H_(13)O_3–, C_(10)H_(15)O_3– and C_(10)H_(15)O_4–, and I– plus IO– from C_(10)H_(16)IO_3– as expected from a trioxide IOOO•C_(10)H_(16)– 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 O_3. A mechanism in which IOOO– reacts with α-pinene (and likely with other unsaturated species) in competition with its isomerization to IO_3–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.

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