INTERPRETATION OF NOX NOY OBSERVATIONS FROM AASE-II USING A MODEL OF CHEMISTRY ALONG TRAJECTORIES/

In situ measurements of NO and NO(y) are used to derive the ratio NO(x )/NO(y) along the flight track of the NASA ER-2 aircraft. Data are pre sented for two flights at mid-latitudes in October 1991 during the Air borne Arctic Stratospheric Expedition-II (AASE-II). Aerosol particle s urface area was concurrently measured. The observations are compared w ith a photochemical model integrated along back trajectories from the aircraft flight track. Comparison of observations with the model run a long trajectories and at a fixed position clearly and quantitatively d emonstrates the importance of an air parcel's dynamic history in inter pretation of local chemical observations. Comparison of the data with model runs under different assumptions regarding heterogeneous chemist ry further reinforces the case for occurrence of the reaction of N2O5 + H2O on sulfate aerosol surfaces in the atmosphere. Finally, comparis ons for which relative changes in the model and the data are not consi stent caution that our ability to resolve all the observations is not yet complete.