Kinetics of the Heterogeneous Reaction of HNO₃ with NaCl:  Effect of Water Vapor

The kinetics of uptake of gaseous HNO 3 on thin films and crystallite grains of NaCl has been studied in a coated wall tubular flow reactor coupled to a quadrupole mass spectrometer. A rapid gas−solid reaction occurs with release of gaseous HCl. Uptake coefficients were measured at room temperature with a range of partial pressures of HNO 3 and H 2 O in the flow tube. γ values decreased with increasing HNO 3 concentration (4 × 10 11 to 700 × 10 11 molecule cm -3 ) and increased with H 2 O over the range 2 × 10 -4 to 10 mbar. The results were interpreted in terms of a mechanism of the reaction HNO 3 (g) + NaCl(s) → HCl(g) + NaNO 3 (s), which involves ionization of adsorbed HNO 3 molecules aided by surface-adsorbed water followed by reaction of H 3 O + with Cl - and desorption of product HCl molecules. The results allow estimation of the reactive uptake coefficient for HNO 3 on sea salt aerosol under atmospheric conditions.