The photochemical partitioning of stratospheric odd nitrogen and odd chlorine is examined using diurnal model calculations and data from the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment. This experiment, conducted during April--May 1985, used a solar occultation technique to obtain vertical profiles of temperature and various other trace constituents at 30¿N and 48¿S latitudes. The knowledge of total odd nitrogen and odd chlorine levels is a major advantage provided by this data set. We have conducted model calculations for different altitudes at 30¿N to evaluate photochemical parameters of interest. Calculated NO2 and NO profiles at sunset are compared with the ATMOS observations. The calculated ratio NO2/NO at sunset is smaller than the observed value by about 27% at 40 km. This difference could be explained by uncertainties in the data and model parameters. The ATMOS data set confirms the presence of N2O5 in the stratosphere; however, the observed mixing ratios at sunset are more than a factor of 2 smaller than the calculated values. We have used recent information on the sticking coefficient for N2O5 on sulfuric acid aerosol surfaces and background aerosol surface area density distribution to estimate the lifetime of N2O5 against removal by heterogeneous reaction.

It is shown that the conversion of N2O5 into HNO3 could take place in the presence of aerosol surface rapidly enough to affect the N2O5 mixing ratio below 30 km. The inclusion of the heterogeneous reaction brings the calculated N2O5 profile below 30 km into better agreement with the ATMOS data. The present calculations give lower HCl mixing ratios compared to the ATMOS data in the 30- to 43-km region. We have examined the effects of a secondary path for the reaction OH+ClO, leading to the production of HCl. With a branching ratio of 8% for this path we find that the discrepancy between the calculated and measured HCl is essentially removed. This modification reduces the chlorine-catalyzed destruction of odd oxygen and has the potential to significantly improve agreement between measured and modeled O3 in the middle stratosphere. ¿1991 American Geophysical Union