Trapping intermediates of the NO₂ hydrolysis reaction on ice

Using molecular beam methods, a mixture of stable NO₂, O₂NNO₂, and up to 30% relative abundance of metastable t-ONONO₂, a potential heterogeneous hydrolysis reaction intermediate, was prepared by heating the quasi-effusive molecular beam nozzle to 600 K. The chemical speciation of hot nitrogen dioxide vapours was established using reflection-absorption IR spectroscopy (RAIRS) at very high (i.e., 1 : 1000) dilution by exploiting selective enhancement in absorbance features due to electric field standing waves (EFSW). Mode-selective shifts in the NO stretching vibrational frequencies of these species are observed upon their adsorption to the surface of amorphous solid water (ASW) at 40 K compared to their value in a crystalline solid argon matrix. Their sensitivities to hydration were assessed by computational chemistry methods using clusters of up to ten water molecules. This revealed that the shifts in the vibrational frequency of the terminal NO stretching mode and of the asymmetric ONO stretching mode of the terminal -NO₂ group of t-ONONO₂ upon its adsorption onto the surface of ASW signal that its ON-ONO₂ bond is significantly polarized. Upon thermal annealing of the sample to 130 K, spectral signatures attributed to adsorbed nitrate anions can be observed suggesting that the activation barrier to heterogenous hydrolysis of the ON⁺·^-ONO₂ zwitterionic reaction intermediate is sufficiently small to be overcome at cryogenic temperatures. Facile NO₂ hydrolysis on aqueous interfaces could contribute to their acidification and to elevated nitrous acid emission fluxes to the lower troposphere.