Knudsen Cell Studies of the Reaction of Gaseous Nitric Acid with Synthetic Sea Salt at 298 K

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The uptake of nitric acid on synthetic sea salt (SSS) was studied at 298 K using a Knudsen cell with mass spectrometric detection of the gaseous reactant and products. HCl was the only product observed, with yields of 100% within experimental error. Nitric acid reaction probabilities decreased with reaction time by a factor of ∼2−3, ultimately reaching a constant value. Both the higher initial reaction probabilities (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 °C while pumping to decrease the amount of water on the salt surface prior to reaction. Several experiments using MgCl2·6H2O also gave very large nitric acid reaction probabilities, ≥0.14 in all cases, consistent with the important role of crystalline hydrates in the reactivity of SSS observed in earlier studies. The presence of large amounts of water on the SSS surface was illustrated by two phenomena:  (1) the uptake of D2O and liberation of large amounts of HDO and smaller amounts of H2O and (2) the formation of gaseous HCl, rather than DCl, as the major product of the DNO3 reaction with SSS. The results of these experiments confirm the critical role of surface-adsorbed water in the uptake and reaction of HNO3 with salt surfaces, as proposed earlier based on similar studies of the reaction of HNO3 with NaCl. They also suggest that the reaction probability for HNO3 with sea salt particles below the deliquescence point is approximately an order of magnitude larger than for reaction with pure NaCl, the major component of these particles. The atmospheric implications are discussed.