First Direct Measurement of the Rate Constant for the Reaction Cl + CH3 and a Reexamination of its Role in Perturbing Laboratory Measurements of the Important Stratospheric Reaction Cl + CH4
Abstract
Two recent papers, one experimental (Wang and Keyser, 1999) and one theoretical (Michelsen and Simpson, 2001) have called attention to possible contributions from the reaction Cl + CH3 + M -> CH3Cl + M to the observed rate of loss of Cl in laboratory studies of the stratospheric reaction Cl + CH4 -> CH3 + HCl, especially at low temperatures ( T = 200-250 K) and low pressures (P = 1 Torr He). However, the only data available in the literature is from a very complex system, the photochlorination of methane (Timonen et al., 1986). The experiments were performed at P = 50-300 Torr CO2 and T = 298-423 K and yield k = 3.7 x 10-10 exp(-185/T) with k(298 K) ~ 2 x 10-10, both in units cm3 molecule-1 s-1. The purpose of the present Cl + CH3 experiments is to supply data under conditions appropriate for correction of laboratory experiments on Cl + CH4. In our discharge flow-mass spectrometric study of Cl + CH3, reaction of F with mixtures of HCl/CH4 rapidly and simultaneously generates Cl and CH3 in the ratio [Cl]/[CH3] = 13:1 to 70:1. With Cl in excess, [Cl]0 = (2 - 14) x 1012 molecule cm-3 and [CH3]0 ~ 2 x 1011 molecule cm-3, we monitor the decay of CH3 via low energy (12eV) electron impact mass spectrometry. Under these conditions, the Cl + CH3 reaction is well isolated and potential secondary reactions are negligible. The rate constant has been measured at P = 0.5, 1.0, 1.5 and 2.0 Torr He and at T = 298, 250 and 202 K. The rate constant is observed to increase with increasing pressure, i.e. the reaction is in the fall-off region. At each pressure k is observed to increase with decreasing temperature. At T = 202 K and P = 1 Torr He, k = 1.6 x 10-11 cm3 molecule-1 s-1. The range of values for k now measured for T = 202-298 K and P = 1.0 Torr He are at least 10 times smaller than those used to correct previous Cl + CH4 laboratory data at these temperatures and pressures. Since Wang and Keyser report that the value of k must be greater than about 5 x 10-11 cm3 molecule-1 s-1 to fit their observations at T = 298 K, P = 1 Torr He and we observe k = 6.6 x 10-12 cm3 molecule-1 s-1 under these same conditions, some other explanation besides contribution from Cl + CH3 must be sought to account for the observations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.A72C0195S
- Keywords:
-
- 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0317 Chemical kinetic and photochemical properties;
- 0340 Middle atmosphere: composition and chemistry