NOy partitioning and aerosol influences in the stratosphere

[1] The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument measured a variety of trace gases globally with high spatial resolution during two Space Shuttle missions. This paper concentrates on members of the NOy family and highlights differences between CRISTA 1 (November 1994) and CRISTA 2 (August 1997). A sequential assimilation technique is used to combine the CRISTA measurements of total NOy fields with corresponding model forecasts based on the National Center for Atmospheric Research Research for Ozone in the Stratosphere and its Evolution (ROSE) model. For this study we use a model version driven by wind and temperature data provided by the UK Met Office. NO2 and N2O show large- and medium-scale structures caused by dynamical processes. N2O5 shows a strong dependence on the aerosol load and solar zenith angles. N2O5 and NO2 changes from CRISTA 1 to CRISTA 2 are consistent with a reduction of aerosol concentrations in the Southern Hemisphere and minor aerosol changes in the Northern Hemisphere. For both missions the model reproduces well the measured diurnal cycles of the NOy family members. Measured diurnal variations of N2O5 and NO2 are consistent with the nighttime production of N2O5 from NO2. Compared to the effect of heterogeneous chemistry, the influence of ozone and temperature changes on the NOy partitioning is rather small. A model run based on a three-dimensional aerosol field derived from CRISTA observations indicates that zonal asymmetries in the background aerosol have strong local effects on the N2O5 and NO2 distribution