Does Nitric Acid Dissociate at the Aqueous Solution Surface?

Nitric acid is a prevalent component of atmospheric aerosols, and the extent of nitric acid dissociation at aqueous interfaces is relevant to its role in heterogeneous atmospheric chemistry. Several experimental and theoretical studies have suggested that the extent of dissociation of nitric acid near aqueous interfaces is less than that in bulk solution. Here dissociation of HNO3 at the surface of aqueous solution is quantified using X-ray photoelectron spectroscopy of the nitrogen local electronic structure. The relative amounts of undissociated HNO3(aq) and dissociated NO3 –(aq) are identified by the distinguishable N1s core-level photoelectron spectra of the two species, and we determine the degree of dissociation, αint, in the interface (approximately the first three layers of solution) as a function of HNO3 concentration. Our measurements show that dissociation is decreased by ∼20% near the solution interface compared with bulk solution and furthermore that dissociation occurs in the topmost solution layer. The experimental results are supported by first-principles MD simulations, which show that hydrogen bonds between HNO3 and water molecules at the solution surface stabilize the molecular form even at low concentration by analogy to the stabilization of molecular HNO3 that occurs in bulk solution at high concentration.