44717-AC6
Vibrational Overtone Initiated Bimolecular Reactions of Atmospheric Molecules
The OH radical plays an important role in controlling the oxidative capacity of the atmosphere. Recent work from our lab have suggested that the bimolecular reaction of electronically excited nitrogen dioxide, generated by absorption of visible light, with water molecules can lead to significant OH radical production that, apparently, are currently unaccounted for in tropospheric models:
NO2 + hn (l>420 nm) ą NO2* (1)
NO2* + M → NO2 + M (M≡ N2 or O2 or H2O) (2)
NO2* + H2O → OH + HONO (3)
Over the past year, using funds provided by the ACS-PRF grant, we have carried out laboratory studies directed towards establishing the rate for the NO2*+H2O reaction. By monitoring the OH reaction product as a function of the H2O concentration using laser induced fluorescence, we have been able to determine the rate of reation-3 to be ~1.7x 10-13 cm3/molecule sec. The magnitude of this reaction rate is sufficiently large to make the NO2*+ H2O reaction a significant source of tropospheric OH radicals. In addition to measuring the reaction rate we have also carried out several diagnostics studies to confirm that the observed OH products detected from the reaction mixture are not likely the result of multi-photon excitation of NO2. Through these diagnostics we have eliminated the possibilities that the OH is from the reaction of O(1D) atoms or translationally excited O(3P), formed from the multi-photon dissociation of NO2, reacting with water. We have also eliminated the possibility that the OH products result from two-photon excitation of NO2 to some bound, but forbidden, excited quartet state which then reacts with water. The results of our work have been published in the journal Science 319, 1657(2008). Finally, apart from providing the opportunity to pursue important scientific research, the ACS-PRF grant has also allowed the scientific training of student working on the project.
