S. Langer et al., 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 journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(7), 1997, pp. 1277-1286
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.