On the mechanism of iodine oxide particle formation

Abstract

The formation of atmospherically relevant iodine oxides I_xO_y (x = 1,…,3, y = 1,…,7) has been studied experimentally using time-of-flight mass spectrometry combined with a soft ionisation source, complemented with ab initio electronic structure calculations of ionisation potentials and bond energies at a high level of theory presented in detail in the accompanying paper (Galvez et al., 2013). For the first time, direct experimental evidence of the I₂O_y (y = 1,…,5) molecules in the gas phase has been obtained. These chemical species are observed alongside their precursors (IO and OIO) in experiments where large amounts of aerosol are also generated. The measured relative concentrations of the I_xO_y molecules and their dependence on ozone concentration have been investigated by using chemical modelling and rate theory calculations. It is concluded that I₂O₄ is the most plausible candidate to initiate nucleation, while the contribution of I₂O₅ in the initial steps is likely to be marginal. The absence of large I₃O_y (y = 3,…,6) peaks in the mass spectra and the high stability of the I₂O₄–I₂O₄ dimer indicate that dimerisation of I₂O₄ is the key step in iodine oxide particle nucleation.