Photochemical Renoxification of Nitric Acid on Real Urban Grime

The fate of NO_x (=NO + NO₂) is important to understand because NO_x is a significant player in air quality determination through its role in O₃ formation. Here we show that renoxification of the urban atmosphere may occur through the photolysis of HNO₃ deposited onto urban grime. The photolysis occurs 4 orders of magnitude faster than in water with J values at noon on July 1 in Toronto of 1.2 × 10^–3 s^–1 for nitrate on urban grime and 1.0 × 10^–7 s^–1 for aqueous nitrate. Photolysis of nitrate present on urban grime probably follows the same mechanism as aqueous nitrate photolysis, involving the formation of NO₂, OH, and possibly HONO. Thus NO_x may be rapidly returned to the atmosphere rather than being ultimately removed from the atmosphere through film wash off.