Heterogeneous Reactions of HNO_3(g) + NaCl(s) → HCl(g) + NaNO_3(s) and N_2O_5(g) + NaCl(s) → ClNO_2(g) + NaNO_3(s)

The heterogeneous reactions of HNO_3(g) + NaCl(s) → HCl(g) + NaNO_3(s) (eq 1) and N_2O_5(g) + NaCl(s) → ClNO_2(g) + NaNO_3(s) (eq 2) were investigated over the temperature range 223-296 K in a flow-tube reactor coupled to a quadrupole mass spectrometer. Either a chemical ionization mass spectrometer (CIMS) or an electron-impact ionization mass spectrometer (EIMS) was used to provide suitable detection sensitivity and selectivity. In order to mimic atmospheric conditions, partial pressures of HNO_3 and N_2O_5 in the range 6 x 10^(-8) ~ 2 x 10^(-6) Torr were used. Granule sizes and surface roughness of the solid NaCl substrates were determined by using a scanning electron microscope. For dry NaCl substrates, decay rates of HNO_3 were used to obtain y(1) = 0.013 ± 0.004 (lo) at 296 K and >0.008 at 223 K, respectively. The error quoted is the statistical error. After all corrections were made, he overall error, including systematic error, was estimated to be about a factor of 2. HC1 was found to be the sole gas-phase product of reaction 1. The mechanism changed from heterogeneous reaction to predominantly physical adsorption when the reactor was cooled from 296 to 223 K. For reaction 2 using dry salts, y(2) was found to be less than 1.0 x 10^(-4) at both 223 and 296 K. The gas-phase reaction product was identified as ClNO_2 in previous studies using an infrared spectrometer. An enhancement in reaction probability was observed if water was not completely removed from salt surfaces, probably due to the reaction of N_2O_5(g) + H_2O(s) → 2HNO_3(g). Our results are compared with previous literature values obtained using different experimental techniques and conditions. The implications of the present results for the enhancement of the hydrogen chloride column density in the lower stratosphere after the El Chichon volcanic eruption and for the chemistry of HCl and HNO_3 in the marine troposphere are discussed