DIFFUSE-REFLECTANCE INFRARED STUDIES OF THE REACTION OF SYNTHETIC SEA-SALT MIXTURES WITH NO2 - A KEY ROLE FOR HYDRATES IN THE KINETICS AND MECHANISM

The heterogeneous reactions of oxides of nitrogen with NaCl as a model for sea salt particles have been the focus of many studies, due to th eir potential to act as precursors to atomic halogens in the troposphe re. While a great deal has been learned about the kinetics and mechani sms of NaCl reactions, it is not clear how well this extrapolates to t he complex mixture of inorganics found in sea salt. We report here dif fuse reflectance infrared Fourier transform spectrometry (DRIFTS) stud ies in which nitrate formation on the salt surface is followed with ti me during the reaction of gaseous NO2 with synthetic sea salt at 298 K in the presence of either He or air as the carrier gas. The infrared bands due to surface nitrate formed during the reaction of NO2 are sho wn to be similar to those from the reaction of MgCl2 . 6H(2)O, a major hydrate in the mixture which was used as a surrogate for all of the c rystalline hydrates. Significant amounts of surface-adsorbed water are generated in the reaction of synthetic sea salt with NO2 in air, whic h appears at least in part to be due to liberation of bound water of h ydration in the crystalline hydrates. The reaction order with respect to NO2 is (1.8 +/- 0.2) (2 sigma) when the reaction of the synthetic s ea salt is carried out in He but only (1.2 +/- 0.2) (2 sigma) when air is used as the carrier gas. For comparison, the reaction order for th e NO2-NaCl reaction was reexamined and found to be (1.8 +/- 0.3) (2 si gma) in He and (1.6 +/- 0.3) (2 sigma) in air, in agreement with previ ous work(19) using this technique. It is assumed for slopes greater th an or equal to 1.6 that N2O4 is the reacting species for the purpose o f expressing the kinetics in the usual form of reaction probabilities. For the N2O4-NaCl reactions in He and air, and for the N2O4-synthetic sea salt reaction in He, the reaction probabilities are similar (simi lar to 10(-4)). The reaction of synthetic sea salt with NO2 in the pre sence of air is treated in terms of a first-order reaction with NO2 be ing the reactive species, which gives a reaction probability for the N O2-synthetic sea salt reaction of similar to 10(-8). The atmospheric i mplications are discussed.