Does HNO₃ dissociate on gas-phase ice nanoparticles?

We investigated the dissociation of nitric acid on large water clusters (H₂O), N̄ ≈ 30-500, i.e., ice nanoparticles with diameters of 1-3 nm, in a molecular beam. The (H₂O) clusters were doped with single HNO₃ molecules in a pickup cell and probed by mass spectrometry after a low-energy (1.5-15 eV) electron attachment. The negative ion mass spectra provided direct evidence for HNO₃ dissociation with the formation of NO₃^-⋯H₃O⁺ ion pairs, but over half of the observed cluster ions originated from non-dissociated HNO₃ molecules. This behavior is in contrast with the complete dissociation of nitric acid on amorphous ice surfaces above 100 K. Thus, the proton transfer is significantly suppressed on nanometer-sized particles compared to macroscopic ice surfaces. This can have considerable implications for heterogeneous processes on atmospheric ice particles.