Large-scale enhancements in NO/NO_y from subsonic aircraft emissions: Comparisons with observations

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/NO_y ratios from the stratospheric portions of this flight. The model is run with and without an additional in situ NO_x source from the 1992 Boeing-McDonnell Douglas (BMD) emissions climatology. During the northern section of this flight, the inclusion of this additional NO_x source resulted in a significant improvement with observed large-scale NO/NO_y ratios. This comparison suggests that air traffic over the eastern United States is sufficiently dense to enhance NO/NO_y 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/NO_y ratios agreed much better with the no-emissions scenario. This may be a reflection of the difficulty of using a climatological NO_x emissions database to infer instantaneous NO/NO_y ratios. It would be desirable to have a larger database of lower stratospheric NO and NO_y measurements downstream of the eastern United States. This would enable more stringent statistical comparisons of observed NO/NO_y ratios with the model-predicted enhancements of this ratio arising from aircraft emissions.