Does HNO₃ dissociate on gas-phase ice nanoparticles?

We investigated the dissociation of nitric acid on large water clusters (H2O)(N), N & AP; 30-500, i.e., ice nanoparticles with diameters of 1-3 nm, in a molecular beam. The (H2O)(N) 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-MIDLINE HORIZONTAL ELLIPSISH3O+ 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.