Matrix effect on surface-catalyzed photolysis of nitric acid

Photolysis rate constant of HNO₃ on the surface (HNO_3(s)) has been found to be enhanced by 1-4 orders of magnitude from that of gaseous HNO₃, with HONO and NO₂ as the main products. Such Re-NOx-ification pathway extends the apparent lifetime of reactive nitrogen species and modifies the atmospheric oxidative capacity along its long-rang transport. Despite of its importance, the detailed kinetics and mechanisms of HNO_3(s) photolysis are still not clear. Surface film of HNO₃ and organic compounds is ubiquitous in the environment and imposes matrix effect on HNO_3(s) photolysis. Here we studied photolysis of HNO₃ on Pyrex glass in a photochemical flow reactor over a wide range of HNO₃ surface density (D_HNO3) with or without the presence of model organic compounds. The photolysis rate constant of HNO_3(s) varied with D_HNO3 and surface-catalysis mechanism was proposed. Organic compounds further enhance the photolysis rate constant by up to one order of magnitude via both photosensitization and H-donating reaction. The H-donating reaction enhances as well the secondary HONO yield from reaction between the primary product NO₂ and adjacent H-donor, and thus increases the HONO/NO₂ production ratio. Finally, detailed mechanisms involving surface-catalyisis, photosensitization and H-donating reactions was integrated.