These observations provide evidence of autocatalytic iodine release from atmospheric aerosol

Atmospheric iodine, dominated by ocean emissions, is important in atmospheric chemistry for two main reasons, a) its remarkable capacity to form new particles that can grow to cloud condensation nuclei (CCN) sizes, thereby establishing a direct link between iodine emissions and CCN in remote regions; b) iodine is a very efficient ozone depleting substance in the troposphere. The changing evolution of these two processes is particularly relevant since global iodine emissions have tripled in the last 70 years.

Key to the abovementioned atmospheric implications is the recycling of reactive iodine from heterogeneous reactions on seasalt aerosols. This process was hypothesized over two decades ago to proceed via aerosol uptake of gaseous hypoiodous acid (HOI) and its subsequent reaction with seasalt halides to produce iodine chloride (ICl) and iodine bromine (IBr), which would then be released to the gas phase.  Although this mechanism has been studied in the laboratory, it had not been confirmed in the atmosphere until now. In this new study, scientists have measured HOI, ICl and IBr for the first time in the ambient atmosphere at the Mace Head Atmospheric Observatory, Ireland, picture courtesy of Colin O´Dowd. The work finding that iodine heterogeneous processing in marine aerosols is remarkably fast, indeed faster than anticipated. The work also reveals that the photolysis of the resulting iodine interhalogen species leads to a faster cycling of atmospheric reactive iodine than previously thought. This new fundamental information will now be included in a climate model to assess its relevance on iodine-mediated new particle formation and ozone loss.

Yee Jun Tham, Xu-Cheng He, Qinyi Li, Carlos A. Cuevas, Jiali Shen, Joni Kalliokoski, Chao Yan, Siddharth Iyer, Tuuli Lehmusjärvi, Sehyun Jang, Roseline C. Thakur, Lisa Beck, Deniz Kemppainen, Miska Olin, Nina Sarnela, Jyri Mikkilä, Jani Hakala, Marjan Marbouti, Lei  Yao, Haiyan Li, Wei Huang, Yonghong Wang, Daniela Wimmer, Qiaozhi Zha, Juhani Virkanen, Gerard Spain, Simon O'Doherty, Tuija Jokinen, Federico Bianchi, Tuukka Petäjä, Douglas R. Worsnop, Roy L. Mauldin III, Jurgita Ovadnevaite, Darius Ceburnis, Norbert M. Maier, Markku Kulmala, Colin O’Dowd, Miikka Dal Maso, Alfonso Saiz-Lopez and Mikko Sipilä. Direct field evidence of autocatalytic iodine release from atmospheric aerosol, PNAS.  DOI: 10.1073/pnas.2009951118