PERTURBATION OF STRATOSPHERIC NITROGEN-DIOXIDE BY VOLCANIC AEROSOL INTHE ARCTIC

NO2 vertical profiles observed from balloon borne UV-visible spectrome ters during EASOE were simulated in order to quantify the NOx depletio n by volcanic aerosol. Calculations were performed with a box photoche mical model integrated along isentropic trajectories at 6 potential te mperature levels (350, 380, 400, 475, 550 and 700 K), and initialized as far as possible from experimental data and from 2D model zonal aver ages, when not available. NO2 vertical profiles are well captured by t he model including current gas phase and heterogeneous chemistry. The very low NO2 concentrations reported during the coldest and darkest pa rt of the winter might be reproduced by gas phase chemistry alone. The y are largely due to the lack of sunlight. Otherwise, in fall and spri ng at warmer temperature, NO2 concentrations are significantly lower t han that simulated in pure gas phase chemistry. The depletion is due t o the presence of volcanic aerosols. The responsible reaction is the h eterogeneous conversion of N2O5 into nitric acid on sulfuric acid drop lets. Sunlight attenuation by the aerosol layer contributes for a mino r part only. The role of the conversion of ClONO2 is negligible. Howev er, NO2 Cannot be totally removed by the heterogeneous conversion of N 2O5 on aerosol. There is a saturation effect which originates in the s low gas phase conversion of NO2 into N2O5. The NO2 concentration at sa turation is then mostly controlled by temperature.