In situ measurements of NO and NOy are used to derive the ratio NOx/NOy along the flight track of the NASA ER-2 aircraft. Data are presented for two flights at mid-latitudes in October 1991 during the Airborne Arctic Stratospheric Expedition-II (AASE-II). Aerosol particle surface area was concurrently measured. The observations are compared with a photochemical model integrated along back trajectories from the aircraft flight track. Comparison of observations with the model run along trajectories and at a fixed position clearly and quantitatively demonstrates the importance of an air parcel's dynamic history in interpretation of local chemical observations. Comparison of the data with model runs under different assumptions regarding heterogeneous chemistry further reinforces the case for occurrence of the reaction of N2O5+H2O on sulfate aerosol surfaces in the atmosphere. Finally, comparisons for which relative changes in the model and the data are not consistent caution that our ability to resolve all the observations is not yet complete. ¿ American Geophysical Union 1993