Vertical distributions of NO2 and O3 mixing ratios and the aerosol extinction coefficient were measured by night between 18 and 32 km altitude (9--80 hPa) at mid latitudes using the balloon-borne instrument AMON on March 24, 1994. The NO2 profile is compared with results of simulations involving the REPROBUS 3D Chemical-Transport Model and a Lagrangian model in which the ozone mixing ratio and the aerosol surface area are initialized using AMON measurements (other mixing ratios are initialized with REPROBUS). As confirmed by Lidar observations, the surface areas were larger than the monthly and zonally averaged SAGE 2 data available for March 1994 at 45 ¿N. The Lagrangian model shows relatively good agreement with AMON results for pressure higher than 40 hPa and smaller than 15 hPa, but the computed NO2 value is too high between 15 and 40 hPa. This seems to indicate that heterogeneous reactions involving the NOy species in the aerosol layer are still incompletely understood.¿ 1997 American Geophysical Union