Radical reactions of atmospheric importance
Abstract
This thesis describes experiments to measure the rate coefficients for some reactions of atmospherically important radicals. The discharge-flow technique coupled with resonance fluorescence, optical absorption and mass spectrometry was used to measure the rate constants for reactions of OH, NO3 and ClO. The rate constant for the reaction of OH with ketene was measured over the temperature range 193-423 K. The reaction displays a negative temperature dependence (E(a) = 4.2 plus or minus 3.5 kJ mol(exp -1)) and an independence of pressure. An explanation for these observations is presented which is consistent with previous work on the reaction, and the implications are discussed. The temperature dependence of the rate constants for the reaction of OH with some hydrochlorofluorocarbons (HCFC's) have been measured together with the infra-red spectra of the HCFC's in the 800-1600 cm(-1) wavenumber range. Ozone depletion potentials and greenhouse warming potentials are calculated. The measured rate constants are compared with a recent evaluation of all previous laboratory data determining the rate of reaction of OH with HCFC's. The rate constants for the reaction of NO3 with a series of alkanes have been studied between the temperatures 289 K and 553 K. Analysis of the rate constants for the reaction of NO3 with enthane, n-butane and i-butane allowed expressions for the temperature dependence for the rate of abstraction of an H atom at primary, secondary and tertiary sites to be determined. The validity of the use of these expressions was tested by comparing the measured rate of reaction of NO3 with i-pentane with that predicted using these expressions. The agreement was found to be good. The reaction between NO3 and ClO has been found to display little or no temperature dependence over the temperature range 210-353 K. An average over all the data gives a temperature-independent rate constant of (5.0 plus or minus 1.4) x 10(exp -13) cm(exp 3) molecule(exp -1) s(exp -1) in agreement with previous room temperature results. Product branching ratios have been investigated and a minor channel producing OClO has been directly observed. The overall rate constant is small for a radical-radical reaction, and possible explanations are discussed. Modulated photolysis has been used to measure for the first time the rate constant for the recombination of ClO with OClO. The low-pressure limiting rate constant at 226 K was found to be (3.0 plus or minus 0.6) x 10(exp -31) cm(exp 3) molecule(exp -1) s(exp -1). The implications for polar stratospheres are discussed.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT........22P
- Keywords:
-
- Air Pollution;
- Atmospheric Chemistry;
- Hydroxyl Radicals;
- Nitrates;
- Radicals;
- Reaction Kinetics;
- Alkanes;
- Atmospheric Heating;
- Chlorofluorocarbons;
- Greenhouse Effect;
- Ketenes;
- Mass Spectroscopy;
- Ozone Depletion;
- Rates (Per Time);
- Resonance Fluorescence;
- Temperature Dependence;
- Geophysics