The heterogeneous reactions of gaseous NO2 and HNO3 [(2-29) X 10(14) molecules cm(-3)] with particles of NaCl in the 1-5-mu m size range at 298 K have been followed in real time using diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) to Obtain kinetic and mechanistic data. Both the NO2 and HNO3 reactions gave an identical sequence of absorption bands attributed to nitrate ions, the expected product of these reactions : (1) HNO3(g) + NaCl(s) --> NaNO3(s) + HCl(g) and (2) 2NO(2(g)) + NaCl(s) --> NaNO3(s) + ClNO(g). The reaction of DNO3 or NO2 with NaCl pretreated with D2O showed no new infrared absorptions, indicating that none of the bands were due to adsorbed HNO3. Bands observed in the earliest stages of the reaction at 1333 and 1460 cm(-1) in the nitrate upsilon(3) antisymmetric stretch region, and at 1042 cm(-1) in the upsilon(1) symmetric stretch region, are attributed to isolated nitrate ions in different localized environments on the NaCl surface. When this surface is exposed to gaseous water at pressures below the deliquescence points of NaCl and NaNO3 and then heated and pumped, bands due to the isolated NO3- disappear, and broad absorptions characteristic of bulk NaNO3 are formed. This suggests that the water exposure leads to the formation of a quasi-liquid layer and drying to recrystallization into separate regions of NaNO3 and NaCl. Finally, the kinetics of the NO2-NaCl reaction are shown to be, within experimental error, second order in NO2. Assuming that N2O4 is the reactant, the gas-solid reaction probability for the N2O4-NaCl reaction was found to be (6 +/- 2) x 10(-5) (1 sigma). Under typical tropospheric conditions, the NO2/N2O4 reaction will be too slow to compete with others, for example the N2O5-NaCl reaction.