Abstract
Photolysis rate constant of HNO3 on the surface (HNO3(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 HNO3(s) photolysis are still not clear. Surface film of HNO3 and organic compounds is ubiquitous in the environment and imposes matrix effect on HNO3(s) photolysis. Here we studied photolysis of HNO3 on Pyrex glass in a photochemical flow reactor over a wide range of HNO3 surface density (DHNO3) with or without the presence of model organic compounds. The photolysis rate constant of HNO3(s) varied with DHNO3 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.