Past studies designed to test photochemical theory have had varying degrees of success in reconciling NOx (NO+NO2) measurements with model predictions. Because of the importance of NOx to ozone chemistry, it is important to be able to quantitatively account for deviations of observed NOx concentrations from predictions. The aircraft source for NOx can provide a plausible explanation for deviations from photochemical theory, in some regions, since multidimensional models have predicted aircraft emissions to be the dominant source of NOx within flight corridors. Here, NOx concentrations have been observed to be higher within the dense flight corridors over the North Atlantic Ocean, as compared to background concentrations. This thesis examines whether the origin of observed NOx deviations from model results can be more clearly identified by including an additional source term of NOx emissions from aircraft. This source is included in a photochemical trajectory model run along 10-day back-trajectories from aircraft flight tracks. Data are presented for three flights at mid- to high-latitudes, in winter, during the Second Airborne Arctic Stratospheric Expedition (AASE-II). In situ measurements of NO and NOy are used to derive the ratio NO/NOy along the flight tracks. The NO/NOy observations are then compared with model results for cases that either exclude or include an NOx emission source. These measurements have been taken in the lower stratosphere at altitudes of 9-12 km where most of the commercial aircraft emissions occur. The only available database of aircraft emissions of NOx is that of a yearly climatology for 1990, compiled by Boeing and McDonnell Douglas (BMD). Thus, these emission values are not expected to fully resolve the deviations between modeled and observed ratios of NO/NOy at any given time and location. Nonetheless, the inclusion of the aircraft NOx emission source in model simulations has shown an overall improvement in the level of agreement between modelled and observed NO/NOy, particularly downstream of dense aircraft traffic. With respect to the NOx emission database, the results emphasize the need to (1) take into account the addition source term of NOx from aircraft when identifying large enhancements in observed NOx sub-species, and (2) the need to increase the resolution of the database (i.e., reducing the timescale of the compiled data). Results also identify two chemical environments: (1) Outside flight corridors, the main source of NOx is HNO3 photolysis. These regions are characterized by well-processed air and small contributions from aircraft NOx emissions. (2) Conversely, within the North Atlantic flight corridor and downstream of the eastern U.S., high levels of aircraft NOx confirms the presence of a highly perturbed region. The trajectory model approach has demonstrated significant skill in reproducing the observed regional scale enhancements in NO/NOy where perturbations associated with aircraft emissions are large.