Photochemical Mechanisms in Atmospherically-Relevant Iodine Oxide Clusters

Atmospheric new particle formation (NPF) events can be driven by iodine oxides or oxoacids via both neutral and ionic mechanisms. Photolysis of new particles likely plays a significant role in their growth mechanisms, but their spectra and their photolysis mechanisms remain difficult to characterize. We recorded UV photodissociation spectra of (I2O5)0-3(IO3-) clusters, observing O atom, I2O4, and (I2O5)1,2 fragments in the atmospherically-relevant range 300-340 nm. With increasing cluster size, absorption red shifts and generally increases in intensity, suggesting particles photolyze more frequently as they grow. Estimates of the rates indicate that even relatively small clusters are likely to undergo photolysis in ambient conditions. Vibrational spectra identify the covalent moiety I3O8- as the likely chromophore, not IO3-. The (I2O5)-loss pathway competes with particle growth while the slower O loss pathway likely produces triplet O + triplet cluster products that could drive subsequent intra-particle chemistry, particularly with co-adsorbed organic or amine species.