Study of the temperature dependence of the reaction of NO3 with CH3I and the estimation of its impact on atmospheric iodine chemistry

The rate constants of the reaction of NO3 With CH3I, which can affect iodine chemistry in the atmosphere, were measured in the temperature range of 298-323 K with cavity ring-down spectroscopy. The reaction has a rate constant of k = (4.1 +/- 0.2) x 10(-13) cm(3) molecule(-1) s(-1) at 298 K. Uncertainties reported herein are one standard deviation. On the basis of an Arrhenius plot in the region of 298-323 K, the reaction has an activation energy (E-a = 13 +/- 3 kJ mol(-1)). Density functional and coupled-cluster calculations suggest that the reaction proceeds via a transition state in which the hydrogen atom is nearly located at the middle position between the carbon atom of CH3I and the oxygen atom of NO3 to give HNO3 + CH2I. On the atmospheric implications, box model simulations indicate that the present reaction has an important effect on the levels and diurnal patterns of the mixing ratios of NO3 and CH3I, and also on the activation of iodine chemistry in the atmosphere.