Determination of the absolute concentration of O atoms and OH radicals in laboratory studies

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Abstract
The fast reaction of O atoms with NO2 has been used in measurements of absolute concentrations of O atoms. Similarly, the reaction of H with NO2 can be used to generate OH radicals in known concentrations. Relative concentrations of both O atoms and OH radicals have frequently been measured by resonance fluorescence determinations in the ultra‐violet. It will be shown that the stoichiometry of these reactions is strongly dependent on the initial concentration of reactants and on the contact time (in the case of OH on secondary reactions as well), making it impossible to equate directly the loss of NO2 with the loss of O atoms or the production of OH radicals.In the first part of this work a simple analytical mathematical method for the determination of the concentration of atomic oxygen will be developed. The method is based on the integrated second order kinetic equation, and the effect of the experimental conditions on the results is discussed.In the second part, the production of OH as a function of contact time and of the initial concentrations of H and NO2 is examined using a five reaction mechanism. By careful choice of the initial concentrations of reactants it is possible to reproduce the experimental results using simplified analytical expressions for the concentration of OH and hence to calculate a calibration factor.The importance of carrying out the calibration measurements under the same experimental conditions as those employed in kinetic experiments is highlighted.

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