One of the DC-8 flights from the 1992 AASE 2 campaign flew south from Maine over the Atlantic Ocean, sampling air downstream of areas in the eastern United States associated with heavy air traffic. We use a photochemical trajectory model to help interpret observed NO/NOy ratios from the stratospheric portions of this flight. The model is run with and without an additional in situ NOx source from the 1992 Boeing-McDonnell Douglas (BMD) emissions climatology. During the northern section of this flight, the inclusion of this additional NOx source resulted in a significant improvement with observed large-scale NO/NOy ratios. This comparison suggests that air traffic over the eastern United States is sufficiently dense to enhance NO/NOy ratios on a regional scale, even when the characteristic NO spikes from exhaust plumes are absent. During the southern portion of the flight, in which the DC-8 flew at a higher altitude, observed NO/NOy ratios agreed much better with the no-emissions scenario. This may be a reflection of the difficulty of using a climatological NOx emissions database to infer instantaneous NO/NOy ratios. It would be desirable to have a larger database of lower stratospheric NO and NOy measurements downstream of the eastern United States. This would enable more stringent statistical comparisons of observed NO/NOy ratios with the model-predicted enhancements of this ratio arising from aircraft emissions. ¿ 1997 American Geophysical Union