Photochemical loss of nitric acid on organic films: a possible recycling mechanism for NO(x).

The films coating urban impervious surfaces have been found to be comprised of about 7% inorganic nitrate and approximately 10% organic compounds (by mass). A simple steady-state analysis of the lifetime of the nitrate in the film suggests the existence of a loss process(es) in addition to washout by rainfall. We show here that gas-phase nitric acid can be taken up in organic films and lower the film pH. Photolysis of nitrated films using actinic illumination causes loss both of protons and of nitrate anion. We argue that this is possibly due to a combination of direct and indirect (photosensitized) photochemistry involving nitrate ions, yielding gas-phase HONO and/or NO2.