Stratospheric composition measurements as tests of photochemical theory

Abstract

This paper considers the question of the accuracy which is required of measurements of stratospheric trace constituents if they are to be accurate enough to lead to significant tests of our understanding of stratospheric photochemistry. We take as an example the measurement of the ten or so parameters (constituent densities, reaction rates, photodissociation coefficients) which affect the [NO 2] [NO] ratio. Using this example, we study two aspects of the problem: first, the measurement of all parameters as a test of the validity of the steady-state photochemical theory; and second the derivation of [ClO] by measurement of all other parameters in the balance equation. The latter approach might have value, it is pointed out, for species which are difficult to measure directly. In both cases it is shown, in an example calculation in which each parameter is constrained to contribute equally to the total error, that meaningful results (i.e. errors < ± 10%, say) require that the key parameters need to be measured to accuracies of the order of ±2%. Such accuracies are in general not currently achievable.