We report here the first measurement of the reaction probability (ϕ) at 298 K for the reaction (1) HNO3(g) +NaCl(s)→HCl(g)+NaNO3(s), ϕ=(4¿2)¿10-4, which was determined using X-ray photoelectron spectroscopy to follow the formation of nitrate on the surface of single crystals of NaCl. Along with recent kinetic data for the reaction N2O5(g)+NaCl(s)→ClNO2(g)+NaNO3(s), we show that in the dark with typical relative concentrations of N2O5 and HNO3, the N2O5 reaction should be competitive with, and may exceed, that of HNO3. The reactions of NO2 and ClONO2 with NaCl are expected to be much less important. Thus, the tropospheric production of chlorine atoms via photolysis of ClNO2 from the N2O5 reaction is anticipated, which has the potential to impact the ozone balance in the troposphere. ¿ American Geophysical Union 1994