NO2 vertical profiles observed from balloon borne UV-visible spectrometers during EASOE were simulated in order to quantify the NOx depletion by volcanic aerosol. Calculations were performed with a box photochemical model integrated along isentropic trajectories at 6 potential temperature levels (350, 380, 400, 475, 550, and 700 K), and initialized as far as possible from experimental data and from 2 D model zonal averages, when not available, NO2 vertical profiles are well captured by the model including current gas phase and heterogeneous chemistry. The very low NO2 concentrations reported during the coldest and darkest part of the winter might be reproduced by gas phase chemistry alone.

They are largely due to the lack of sunlight. Otherwise, in fall and spring at warmer temperature, NO2 concentrations are significantly lower than that simulated in pure gas phase chemistry.

The depletion is due to the presence of volcanic aerosols. The responsible reaction is the heterogeneous conversion of N2O5 into nitric acid on sulfuric acid droplets. Sunlight attenuation by the aerosol layer contributes for a minor part only. The role of the conversion of ClONO2 is negligible. However, NO2 cannot be totally removed by the heterogenous conversion of N2O5 on aerosol. There is a saturation effect which originates in the slow gas phase conversion of NO2 into N2O5. The NO2 concentration at saturation is then mostly controlled by temperature. ¿American Geophysical Union 1994