Instituto de Astrofísica de Andalucía, CSIC, Granada 18008, Spain.
Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Serrano 119, Madrid 28006, Spain.
J Am Chem Soc. 2022 Jun 1;144(21):9240-9253. doi: 10.1021/jacs.1c12957. Epub 2022 May 23.
Iodine chemistry is an important driver of new particle formation in the marine and polar boundary layers. There are, however, conflicting views about how iodine gas-to-particle conversion proceeds. Laboratory studies indicate that the photooxidation of iodine produces iodine oxides (IO), which are well-known particle precursors. By contrast, nitrate anion chemical ionization mass spectrometry (CIMS) observations in field and environmental chamber studies have been interpreted as evidence of a dominant role of iodic acid (HIO) in iodine-driven particle formation. Here, we report flow tube laboratory experiments that solve these discrepancies by showing that both IO and HIO are involved in atmospheric new particle formation. IO molecules ( = 2, 3, and 4) react with nitrate core ions to generate mass spectra similar to those obtained by CIMS, including the iodate anion. Iodine pentoxide (IO) produced by photolysis of higher-order IO is hydrolyzed, likely by the water dimer, to yield HIO, which also contributes to the iodate anion signal. We estimate that ∼50% of the iodate anion signals observed by nitrate CIMS under atmospheric water vapor concentrations originate from IO. Under such conditions, iodine-containing clusters and particles are formed by aggregation of IO and HIO, while under dry laboratory conditions, particle formation is driven exclusively by IO. An updated mechanism for iodine gas-to-particle conversion is provided. Furthermore, we propose that a key iodine reservoir species such as iodine nitrate, which we observe as a product of the reaction between iodine oxides and the nitrate anion, can also be detected by CIMS in the atmosphere.
碘化学是海洋和极地边界层中新颗粒形成的重要驱动因素。然而，关于碘气体到颗粒的转化过程如何进行，存在相互矛盾的观点。实验室研究表明，碘的光氧化产生碘氧化物（IO），这是众所周知的颗粒前体。相比之下，在野外和环境室研究中，硝酸盐阴离子化学电离质谱（CIMS）的观测结果被解释为碘酸（HIO）在碘驱动的颗粒形成中起主要作用的证据。在这里，我们报告了流动管实验室实验，通过显示 IO 和 HIO 都参与了大气中新颗粒的形成，解决了这些差异。IO 分子（=2、3 和 4）与硝酸盐核心离子反应，生成与 CIMS 获得的相似的质谱，包括碘酸盐阴离子。通过高阶 IO 的光解产生的五氧化碘（IO）被水解，可能是通过水分子二聚体，生成 HIO，它也有助于碘酸盐阴离子信号。我们估计，在大气水蒸气浓度下，通过硝酸盐 CIMS 观察到的碘酸盐阴离子信号中约有 50%源自 IO。在这种条件下，通过 IO 和 HIO 的聚集形成含碘的团簇和颗粒，而在干燥的实验室条件下，颗粒的形成完全由 IO 驱动。提供了一个更新的碘气体到颗粒的转化机制。此外，我们提出，一种关键的碘储存物种，如我们观察到的碘氧化物和硝酸盐阴离子之间反应的产物碘硝酸盐，也可以在大气中通过 CIMS 检测到。
