Field evidence of autocatalytic iodine release from atmospheric aerosol

Reactive iodine plays a key role in determining the oxidation capacity of the atmosphere in addition to being implicated in the formation of new particles in the marine environment. Recycling of reactive iodine from heterogeneous processes on sea-salt aerosol was hypothesized over two decades ago but the understanding of this mechanism has been limited to laboratory studies and has not been confirmed in the atmosphere until now. Here, we report the first direct ambient observations of hypoiodous acid (HOI) and heterogeneous recycling of iodine monochloride (ICl) and iodine monobromide (IBr) at Mace Head Observatory in Ireland (53°19’ N, 9°54’ W) during the summer of 2018. A newly developed bromide based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (Br-CI-APi-TOF) was deployed to measure I₂, HOI, ICl, and IBr. Significant levels of ICl and IBr, with mean daily maxima of 4.3 and 3.0 pptv (1 min-average), respectively, have been observed throughout the campaign. We show that the heterogeneous reaction of HOI on marine aerosol and subsequent production of iodine interhalogens (ICl and IBr) are much faster than previously thought. These results indicate that the fast formation of iodine interhalogens, together with their rapid photolysis, results in more efficient recycling of atomic iodine than currently estimated by the models. The photolysis of the observed ICl and IBr leads to 32% increase in the daytime average of atomic iodine production rate, thereby enhancing the average daytime iodine-catalyzed ozone loss rate by 10-20%. Our findings provide the first direct field evidence that the autocatalytic mechanism of iodine release from marine aerosol is important in the atmosphere and can have significant impacts on atmospheric oxidation capacity and new particle formation in the troposphere.