Efficient Conversion of Nitrogen Dioxide into Nitrous Acid on Ice Surfaces

The hydrolysis of nitrogen dioxide (NO 2 ) on ice surfaces is studied by measuring the reaction products on the surface with the reactive ion scattering (RIS) technique, and the desorbing species is studied with temperature-programmed desorption (TPD) mass spectrometry. NO 2 adsorbs molecularly on an ice film surface at a temperature of 90 K. Upon heating the film to 140 K, NO 2 adsorbates are readily converted into nitrous acid (HONO), and nitrous acid products desorb intact from the surface at higher temperatures. The result indicates the efficient formation of nitrous acid gas on the ice surface with only a small activation energy. It is shown that the hydrolysis occurs from isolated NO 2 adsorbates rather than through NO 2 −NO 2 interactions. The implications of these findings for the atmospheric heterogeneous reaction of NO 2 are mentioned.