Testing of photochemical theory with solar occultation data

We have investigated the problem of testing photochemical theory using solar occultation data. We show, simply using a diurnal model, that for many steady-state relationships sunrise and sunset are not good times for performing quantitative tests. Departures should be expected between the true profile of a substance and that derived using a steady-state argument. Critical tests of theory can, however, be made by considering what we call the rate ratio of a species. A rate ratio, ø, is defined as the ratio of the rate of production to the rate of loss of the species under consideration. This is a simple way to quantify departures from steady-state. Using ATMOS data rate ratios for NO₂, ClONO₂. HNO₄ and N₂O₅ have been evaluated. These can then be compared with theoretical (model) predictions of the relevant rate ratio. The tests performed confirm our general understanding of the chemistry controlling these species. To make firm conclusions, the measurement of all the relevant parameters would be required. The usefulness in applying rate ratios to measurements made at occultation times is demonstrated.