J. Phys. Chem. 1994,98, 10373
10373
COMMENTS Comment on the Atmospheric Chemistry of FNO Timothy J. Wallington” and William F. Schneider Ford Research Laboratory, SRL-E3083, Ford Motor Company, Dearbom, Michigan 48121-2053
Ole John Nielsen* and Jens Sehested Section for Chemical Reactivity, Environmental Science and Technology Department, Ris@National Laboratory, DK-4000 Roskilde, Denmark Received: June 30. I994
In a recent publication dealing with a theoretical study of atmospheric photooxidation of fluorocarbons, it was concluded that FNO is photolyzed slowly in the atmosphere and that this species may be a reservoir species for stratospheric fluorine.’ In speculating that FNO may be an important radical reservoir species and that the atmospheric chemistry of FNO merits intensive investigation, the authors have overlooked a substantial body of data that shows that FNO is photolyzed rapidly under atmospheric conditions. FNO displays siwicant UV absorption at wavelengths below 220 nm and in the region 280-350 nm.2-6 The solar flux in the atmosphere at wavelengths aound 300 nm is several orders of magnitude more intense than that at 200 nm. For the purposes of estimating the photodissocation rate of FNO photolysis, we need only to consider the absorption in the region 300-350 nm. FNO excited to the Sl(1’A’’) state by absorption of 300-350 nm photons undergoes rapid dissociation (on the time scale of a few molecular vibrations, i.e., approximately 100 fs).’,* Collisional deactivation is unimportant, as is fluorescence. The quantum yield for photodissociation is expected to be unity.’,* The rate of FNO photolysis in the atmosphere can be estimated by summing the product of the absorption cross sections and solar flux over the wavelength range 300-350 nm. Using literature data for the FNO absorption spectrum6 and the solar flux (zenith angle = Oo)? the rate of photolysis of FNO can be estimated to be 7 x s-l near the earth’s surface. The lifetime of FNO with respect to photolysis is then about 24 min. The atmosphere is relatively transparent in the region 300-350 nm, and the solar flux in this region does not increase dramatically with a l t i t ~ d e .At ~ an altitude of 20 km the lifetime of FNO with respect to photolysis will decrease by approximately a factor of 2.
0022-365419412098- 10373$04.5010
The atmospheric fate of FNO is photolysis to give F atoms and NO. Fluorine atoms released from FNO will either combine with 0 2 to give FO2 radicals or react with hydrogen-containing species ( C h or H20) to give HF. HF is unreactive in the stratosphere and is transported to the troposphere where it is removed by incorporation into cloud-rain-sea water. Recombination of F atoms with NO to re-form FNO will not occur to any appreciable extent in the atmosphere. FNO can be regenerated by the reaction of FO2 radicals with NO. This reaction occurs in competition with other reactions of FO2 radicals, including decomposition to release F atoms, reaction with NO2 to give either F02N02 or FNO2, and possibly reaction with C&.l0 The two requirements for a radical reservoir species in the stratosphere are that it must have a relatively long lifetime and that it must be regenerated efficiently from the radical species. FNO is photolyzed on a time scale comparable with the lifetime of FOz radicals in the stratosphere, and it is regenerated from F02 radicals at a rate no greater than other competing reactions of FOz radicals. Thus, neither requirement for a reservoir species is fulfilled by FNO. While it is an important intermediate in the photooxidation of HFCs, FNO is not likely to be a reservoir for F atoms or F02 radicals in the stratosphere.
Acknowledgment. We thank Hanna Reisler (University of Southern California) for helpful discussions regarding the photodissociation dynamics of FNO. References and Notes (1) Dibble, T.S.; Francisco, J. S. J. Phys. Chem. 1994,98, 5010. (2) Johnston, H. S.; Bertin, H. J. J. Mol. Spectrosc. 1959,3, 683. (3)Solgadi, D.;Flament, J. P. Photophys. Photochem. 6 eV 1985,497. (4) Suter, H. U.;Huber, J. R.; von Dirke, M.; Untch, A,; Schinke, R. J. Chem. Phys. 1992,96, 6727. (5) Ogai, A,; Brandon, J.; Reisler, H.; Suter, H. U.; Huber, J. R.; von Dirke, M.;Schinke, R. J. Chem. Phys. 1992,96, 6643. (6) Burley, J. D.; Miller, C. E.; Johnston, H. S. J. Mol. Spectrosc. 1993, 158, 317. (7) Brandon, J. T.;Reid, S. A.; Robie, D. C.; Reisler, H. J. Chem. Phys. 1992,97, 5246. (8) Reid, S. A.; Brandon, J. T.; Reisler, H. J. Phys. Chem. 1993,97, 540. (9)Finlayson-Pitts, B. J.; Pitts, Jr, J. N. Atmospheric Chemistry: Fundamentals and Experimental Techniques; John Wiley: New York, 1986, p 110. (IO) Sehested, J.; Sehested, K.; Nielsen, 0. J.; Wallington, T. J. J. Phys. Chem. 1994,98, 6731.
0 1994 American Chemical Society
