KNUDSEN CELL STUDIES OF THE REACTION OF GASEOUS NITRIC-ACID WITH SYNTHETIC SEA-SALT AT 298 K

The uptake of nitric acid on synthetic sea salt (SSS) was studied at 2 98 K using a Knudsen cell with mass spectrometric detection of the gas eous reactant and products. HCl was the only product observed, with yi elds of 100% within experimental error, Nitric acid reaction probabili ties decreased with reaction time by a factor of similar to 2-3, ultim ately reaching a constant value. Both the higher initial reaction prob abilities (which ranged from 0.07 to 0.75) and the final steady-state values (which ranged from 0.03 to 0.25) decreased if the salt had been heated at 75 degrees C while pumping to decrease the amount of water on the salt surface prior to reaction. Several experiments using MgCl2 . 6H(2)O also gave very large nitric acid reaction probabilities, gre ater than or equal to 0.14 in all cases, consistent with the important role of crystalline hydrates in the reactivity of SSS observed in ear lier studies. The presence of large amounts of water on the SSS surfac e was illustrated by two phenomena: (1) the uptake of D2O and liberati on of large amounts of HDO and smaller amounts of H2O and (2) the form ation of gaseous HCl, rather than DCl, as the major product of the DNO 3 reaction with SSS. The results of these experiments confirm the crit ical role of surface-adsorbed water in the uptake and reaction of HNO3 with salt surfaces, as proposed earlier based on similar studies of t he reaction of HNO3 with NaCl. They also suggest that the reaction pro bability for HNO3 with sea salt particles below the deliquescence poin t is approximately an order of magnitude larger than for reaction with pure NaCl, the major component of these particles, The atmospheric im plications are discussed.