The Effect of Altitude Upon Jet Transport Emissions of Nitrogen Oxides: A Flight and Empirical Data Study

Consideration of aviation’s climatological impact encompasses both CO2 and non-CO2 emissions, notably contrails and nitrogen oxides (NOx). To quantify altitude-dependent NOx emissions, empirical adjustments are typically applied to sea-level (SL) engine test-cell emission indices (EINOx) to represent conditions encountered in upper troposphere–lower stratosphere (UTLS) operations. However, cruising flights involve extended durations, making altitude corrections derived from ground-based test-cell data highly extrapolative. This NRC study evaluates NOx emissions during aircraft takeoff, climb, and cruise phases (Mach 0.5 and 0.75) using corrected predictions derived from ground-based test cell measurements, applying two empirical models: the Boeing Fuel Flow Method 2 (FFM2) and the Aviation Environmental Design Tool (AEDT). Both methods predict a decreasing trend in EINOx from takeoff to climb and then cruise at Mach 0.5. However, as the cruise speed increases to Mach 0.75, the EINOx in AEDT exceed those observed during the climb phase, whereas in FFM2 they remain below climb-phase values. Comparisons between FFM2 and AEDT reveal differences of approximately 10–40%, primarily due to differing thermodynamic modeling capabilities not fully captured by FFM2. Additionally, detailed total reactive nitrogen content (NOy) emission measurements from Jet A-1, on an experimental contrail flight project conducted on 20th November 2017 were compared against AEDT and FFM2 predictions. Significant discrepancies in magnitude and trends between measured emissions and model predictions underscore the limitations inherent in current sea-level extrapolation techniques. These results emphasize the necessity for refined altitude-correction methods and targeted flight tests specifically designed to accurately measure NOx emissions across typical jet transport flight profiles, explicitly accounting for chemical transformations of NOx into reservoir species (NOy). Furthermore, the nonlinear partitioning of NOx across altitude underscores the need to explicitly account for altitude-dependent chemical transformations in future atmospheric assessments and modeling refinements.