The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source

Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O3 surface concentrations. Although iodic acid (HIO3) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO3 via reactions (R1) IOIO + O3 → IOIO3, and (R2) IOIO3 + H2O → HIO3 + HOI + O2. The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO3 in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation.

Henning Finkenzeller, Siddharth Iyer, Xu-Cheng He, Mario Simon, Theodore K. Koenig, Christopher F. Lee, Rashid Valiev, Victoria Hofbauer, Antonio Amorim, Rima Baalbaki, Andrea Baccarini, Lisa Beck, David M. Bell, Lucía Caudillos, Dexian Chen, Randall Chiu, Biwu Chu, Lubna Dada, Jonathan Duplissy, Martin Heinritzi, Deniz Kemppainen, Changhyuk Kim, Jordan Krechmer, Andreas Kürten, Alexandr Kvashnin, Houssni Lamkaddam, Chuan Ping Lee, Katrianne Lehtipalo, Zijun Li, Vladimir Makhmutov, Hanna E. Manninen, Guillaume Marie, Ruby Marten, Roy L. Mauldin, Bernhard Mentler, Tatjana Müller, Tuukka Petäjä, Maxim Philippov, Ananth Ranjithkumar, Birte Rörup, Jiali Shen, Dominik Stolzenburg, Christian Tauber, Yee Jun Tham, António Tomé, Miguel Vazquez-Pufleau, Andrea C. Wagner, Dongyu S. Wang, Mingyi Wang, Yonghong Wang, Stefan K. Weber, Wei Nie, Yusheng Wu, Mao Xiao, Qing Ye, Marcel Zauner-Wieczorek, Armin Hansel, Urs Baltensperger, Jérome Brioude, Joachim Curtius, Neil M. Donahue, Imad El Haddad, Richard C. Flagan, Markku Kulmala, Jasper Kirkby, Mikko Sipilä, Douglas R. Worsnop, Theo Kurten, Matti Rissanen, Rainer Volkamer