Concentrations of nitric oxide (NO), total reactive nitrogen (NOy), O3, and aerosol were measured between 14 and 32 km on balloon platforms launched from Aire sur l'Adour (44 ¿N, 0 ¿W), France, on October 20 and November 6, 1992. Aerosol surface area has been deduced from the observed concentrations of aerosols with radii ranging from 0.18 to 0.7 μm. On these days the surface area was enhanced due to the eruption of Mount Pinatubo, which occurred in June 1991. The data set is used to investigate the effect of heterogeneous reactions on sulfuric acid aerosol reducing NOx. A diurnal photochemical model, which takes into account heterogeneous chemistry, is used to calculate the changes in the partitioning of reactive nitrogen in the altitude region between 18 and 32 km. The observed values of NO and NOx/NOy ratio are significantly lower than those calculated by the model without taking into account heterogeneous chemistry. The degree of reduction of NOx by heterogeneous reaction was observed to vary sensitively with surface area for values lower than 2μm2cm-3. The net reduction of NOx by aerosol tended to saturate at surface area larger than 4μm2cm-3. Observations of NO and NOx/NOy are shown to be in generally good agreement with theoretical values found using the same model but including heterogeneous chemistry, indicating that NOx was reduced primarily by the hydrolysis of N2O5 on sulfuric acid aerosol.¿ 1997 American Geophysical Union