Center for Environmental Research and Technology

An experimental and modeling study was carried out to assess the impacts of methyl iodide on ground-level ozone formation compared to other chemicals that are emitted into the atmosphere. The experiments consisted of environmental chamber irradiations of methyl iodide and NOx with and without added CO, methyl iodide and O3 with CO, and incremental reactivity environmental chamber experiments to determine the effect of adding methyl iodide to irradiations of reactive organic gas (ROG) surrogate - NOx mixtures representing ambient conditions. The results were modeled using the SAPRC-07 mechanism with the reactions of methyl iodide and iodine species added. The data were reasonably well simulated after adjusting uncertain parameters concerning photolysis rate of INO2 and the formation of IxOx oligomers from the reactions of IO radicals. This mechanism was then used to calculate the atmospheric impact of methyl iodide in the box model scenarios to derive the Maximum Incremental Reactivity (MIR) and other reactivity scales. Methyl iodide was calculated to inhibit ozone under all conditions except those with very high NOx levels, where its impact is comparable to that for ethane. It is concluded that methyl iodide should not be regulated as contributing to ground level ozone formation.