Effect of Pinatubo aerosols on stratospheric NO

Concentrations of nitric oxide (NO), total reactive nitrogen (NO y ), O 3 , 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 NO x . 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 NO x /NO y , ratio are significantly lower than those calculated by the model without taking into account heterogeneous chemistry. The degree of reduction of NO x by heterogeneous reaction was observed to vary sensitively with surface area for values lower than 2 μm 2 cm -3 . The net reduction of NO x by aerosol tended to saturate at surface area larger than 4 μm 2 cm -3 . Observations of NO and NO x /NO y are shown to be in generally good agreement with theoretical values found using the same model but including heterogeneous chemistry, indicating that NO x was reduced primarily by the hydrolysis of N 2 O 5 on sulfuric acid aerosol.