A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides

Emitted from the oceans, iodine-bearing molecules are ubiquitous in the atmosphere and a source of new atmospheric aerosol particles of potentially global significance. However, its inclusion in atmospheric models is hindered by a lack of understanding of the first steps of the photochemical gas-to-particle conversion mechanism. Our laboratory results show that under a high humidity and low HO(x) regime, the recently proposed nucleating molecule (iodic acid, HOIO(2)) does not form rapidly enough, and gas-to-particle conversion proceeds by clustering of iodine oxides (I(x)O(y)), albeit at slower rates than under dryer conditions. Moreover, we show experimentally that gas-phase HOIO(2) is not necessary for the formation of HOIO(2)-containing particles. These insights help to explain new particle formation in the relatively dry polar regions and, more generally, provide for the first time a thermochemically feasible molecular mechanism from ocean iodine emissions to atmospheric particles that is currently missing in model calculations of aerosol radiative forcing.