Box modelling of gas-phase atmospheric iodine chemical reactivity in case of a nuclear accident

A new atmospheric iodine gas-phase mechanism containing 248 reactions combined to the RACM air quality mechanism was implemented into a 0D box model in order to evaluate the behaviour of iodine fission products released in the atmosphere. Different simulation scenarios were performed unravelling the influence of several parameters on the iodine speciation (season, day/night, low/medium/high iodine concentration release, nature of the injected iodine species). The main outcomes of our work are the following: (i) the I2 reactivity is faster than that of CH3I, (ii) iodine nitroxides INOx are mainly formed during night with low and medium releases in winter, (iii) iodine oxides are promoted with a high release of I2 during summer, (iv) the formation of organic iodine compounds can be highlighted only in the case of a low I2 release. In this work, I2O, I2O5, and HOIO2 are the most abundant iodine oxides because their atmospheric removal pathways in the gas-phase are not yet known. The main iodinated organic species is CH3OOI arising from the reaction between I atom and the methyl peroxy radical. In case of a CH3I release, before the I atom formation, the reaction mechanism leads to either the formation or the destruction of several other organic iodinated compounds such as CH2IOH and CH2IOO. Iodine reaction scheme, 2013 August 8th, for an iodine release at 4 am 196 pptV of CH3I. [Display omitted] •New atmospheric iodine gas-phase mechanism containing 248 reactions.•Evaluate the behaviour of I2 and CH3I released in the atmosphere.•Iodine nitroxides mainly formed during nighttime.•Iodine oxides promoted in summer by high ozone levels.•Iodine organic reactivity should be improved to get firm conclusions.