83rd Annual

Monday, 10 February 2003
Photoenhancement… or Photodepression?… of the Hydrolysis of NO2: Implications for the Chemistry of Polluted Atmospheres
Kevin A. Ramazan, University of California, Irvine, CA; and D. Syomin and B. J. Finlayson-Pitts
Poster PDF (232.3 kB)

Nitrogen dioxide plays a key role in the formation of ozone, acids, and particles in both polluted and remote areas.  It is known that while the reaction of NO2 with water is slow in the gas phase, it occurs at a significant rate with the presence of surfaces, generating surface-adsorbed HNO3 and gas phase HONO, a major early morning source of OH radicals in polluted urban air. Photoenhancement of HONO formation from the NO2 heterogeneous hydrolysis was reported by Akimoto et al. (1987).  Using long path FTIR spectroscopy, we investigated the heterogeneous NO2 hydrolysis with initial NO2 concentration in range of (10 – 100) ppm at 20 to 80% RH in the presence of UV (320 - 400 nm) or visible (l > 400 nm) light to probe the photoenhancement of HONO formation.  We have developed a kinetics model composed of known gas phase reactions, as well as three parameterized heterogeneous reactions which simulates the NO2 hydrolysis reaction in the dark. To model the system in the presence of light, NO2 and HONO photolysis rate constants were measured under our experimental conditions to be (1.57± 0.11) x10-3 (2s) s-1 and (3.5 ± 0.4) x10-4 (2s) s-1, respectively.  These rate constants were added to the model to simulate the effect of UV light on the system. The resulting HONO time profile was compared to experimental data, showing that experimentally HONO decays faster than our model predictions.  This corresponds to the occurrence of a photodepression effect, rather than the expected photoenhancement. The atmospheric implications are discussed.



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