8976
J. Phys. Chem. 1993,97, 8976-8982
Rate Coefficients for Reactions of Several Hydrofluorocarbons with OH and O(lD) and Their Atmospheric Lifetimes A. M. Schmoitner, R. K. Talukdar, R. F. Warren? A. Mellouki,#L. Goidfarb,f T. Gierczak,l S. A. McKeen, and A. R. Ravishankara'9s National Oceanic and Atmospheric Administration, Aeronomy Laboratory, Boulder, Colorado 80303, and The Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309 Received: April 23, 1993; In Final Form: June 4, 1993
The rate coefficients for the reaction of O H with CHJF, CHF3, CzHsF, C ~ H ~ Fand S , CSHZFlO were measured at temperatures between 232 and 378 K using the pulsed laser photolysis-laser-induced fluorescence technique. The rate coefficients for the reaction of O(lD) with the above molecules and CHzFz were measured at room temperature using time-resolvedvacuum-UV atomic resonance fluorescencedetection of Ot3P). The atmospheric lifetimes needed for the evaluation of global warming potentials were calculated for all six molecules using a one-dimensional atmospheric model using the kinetic data obtained in the present study.
Introduction The decision to phase out chlorofluorocarbons (CFCs), which are believed to be resonsible for the dramatic reductions in stratospheric ozone observed in the polar regions, as well as the smaller depletions at lower latitudes, has prompted the search for alternatives. In recent years, fluorine-substitutedalkanes (hydrofluorocarbons, HFCs) have been suggested and used increasingly as substitutes for CFCs. HFCs do not lead to ozone destruction, since they do not contain C1 or Br atoms which participate in the catalyticozonedestructioncycles. Furthermore, the presence of hydrogen atoms enables OH to destroy these molecules by chemical reactions and can result in significantly lower atmospheric lifetimes compared to those of CFCs. However, HFCs are potent greenhouse gases due to the strong IR activity of the C-F bonds. They also absorb IR radiation in wavelength regions where CO, and H2O do not absorb. It is therefore important to determine the potential contribution to the greenhouse effect due to emission of these molecules. Even though many uncertainties remain in the details of these processes, the concept of global warming potentials (GWPs) provides a convenient tool for ranking the global warming effects of different molecules compared to a standard (C02 or CFCI,).' The longtime-horizon GWP of a molecule increases with increasing atmospheric lifetime, making this one of the central parameters necessary for determining a compound's environmental impact. Generally, three main pathways lead to the destruction of organicmolecules in the atmosphere: reaction with OH radicals, UV photolysis, and reaction with O(lD) in the stratosphere. Reactions with C1 atoms can lead to removal of the HFCs as well. However, these reactions are significant in the stratosphere only if their rate coefficients are much larger than those for reaction with OH. This is because the C1concentration in the stratosphere is about 2 orders of magnitude lower than that of OH. If the recently proposed24 C1 atom production in the troposphere is indeed taking place, Cl atoms could play a role in the troposphere, Current address: Centre for EnvironmentalTechnology,Imperial College of Science, Technology, and Medicine, 48 Prince's Gardens, London SW7 ZPE, UK. t Current address: C.N.R.S., L.C.S.R., lC, Avenue de la Recherche Scientifique, 45071 Orltans Ctdex 2, France. 4 Also affiliated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO. I Current address: Department of Chemistry, Warsaw University, ul. Zwirki i Wigury 101, 02-089, Warszawa, Poland. Address correspondence to this author at NOAA/ERL, R/E/AL2, 325 Broadway, Boulder, CO 80303.
especially in the marine boundary layer. In the case of HFCs, UV photolysisdoes not play a significant role since these molecules do not absorb at wavelengths greater than about 145 nm.5 This paper describes studies of the following OH reactions:
-
+ CH,F O H + CHF, O H + C,H,F O H + C,H,F, OH
OH
+ C,H,F,,
H,O H,O
H,O
-
H,O H,O
+ CH,F; + CF,;
k,
k,
(1)
(2)
+ C,H,F;
k,
(3)
+ C,HF,; + C,HF,,;
k,
(4)
k,
(5)
In addition, we present the rate coefficients for the reactions of O(1D) with these five HFCs and CH2F2. Lastly, the atmospheric lifetimes of all six molecules are calculated by including the rate coefficients of their reactions with OH, O(lD), and C1. The studies of the C1 atom reactions are described elsewhere.6 The compounds were chosen to continue our systematic efforts to obtain the lifetimes of small fluorinated hydrocarbons and to extend these studies to newly synthesized compounds of potential industrial importance.
Experiments and Results Samples. CH3F (HFC 41) and C2H5F (HFC 161) were obtained from PCR. GC/MS analysis and UV spectroscopy of CH3F and CZHSFperformed in this laboratory revealed the presence of a few impurities (summarized in Table I). In CH3F the levels of impurities are too small to contribute significantly to the measured rate coefficient. Only very small amounts of C2H50H were seen in CzHsF, which makes a negligible contribution to the measured rate coefficient. CH2F2 (HFC 32) and CHF3 (HFC 23) were obtained from Allied-Signal Corp. and du Pont de Nemours and Co., respectively. According to the manufacturer, GC/MS analysis indicated the absence of unsaturated hydrocarbons (whose reactions with OH are several ordersof magnitude faster than with alkanes) in theCHF3sample. C4H2Fs (lI1,2,2,3,3,4,4-octofluorobutane, CHF2CF2CF2CF2H, HFC 338 pcc) and C5H2F10 (1,1,1,2,3,4,4,5,5,5-decafluoropentane, CF3CHFCHFCF2CF3, HFC 43-10Mee) were obtained from du Pont de Nemours and Co. and were analyzed by the
0022-365419312097-8976$04.00/0 Q 1993 American Chemical Society
Reactions of HFCs with OH and O(lD)
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 8977
TABLE I: Impurities Detected in the Samples of HFCs Used in the Present Study "pd impurity amount, ppmv
