Interaction process between gaseous CH3I and NaCl particles: implication for iodine dispersion in the atmosphere

Abstract

Gaseous iodomethane (CH₃I) is naturally emitted into the atmosphere by biological activity in oceans and during severe accidents (SAs) in nuclear power plants. In this latter case, a part of radioactive iodine such as ¹³¹I may be released. Improving the knowledge of CH₃I transport and reactivity in the atmosphere is important since they are strongly linked to first the cycle of ozone and second to the dispersion of radioactive CH₃I with potential radiological consequences on both the environment and human health. Here, the interaction process of CH₃I with NaCl as a surrogate of atmospheric aerosols was investigated under ambient air conditions by using Diffuse Reflectance Fourier Transform Spectroscopy (DRIFTS). The DRIFTS spectra of NaCl clearly evidenced CH₃I adsorption on the NaCl particle surface. A part of CH₃I ((1.68 ± 0.85) × 10¹⁴ molecule per mg_NaCl) was found to be strongly bonded to NaCl since no desorption was observed. The CH₃I adsorption on the NaCl surface presented a 1^st order kinetics relative to its gas phase concentration. The uptake coefficient was determined to be in the order of 10⁻¹¹. These results show a low probability of CH₃I to be taken up by halide-containing aerosols. These data are crucial for completing the iodine atmospheric chemical scheme.