This objective of this project was to determine emission indices (EIs) for several important atmospheric pollutants including oxides of nitrogen (NOX), particulate matter (PM), and a number of toxic air contaminants from military aircraft by conducting extractive sampling of the aircraft exhaust downstream of the engine exit plane (EEP). Until recently nearly all aircraft engine exhaust measurements were made by extractive sampling at the exhaust exit.

This project employed a strategy of outdoor sampling at ground level, downstream behind operational military aircraft. This permitted rapid change-out of the aircraft so that different types of engines could be tested quickly, and multiple engines of the same type could be sampled in a short period of time to assess engine to engine variability. This approach allowed for extended sampling intervals at known engine power settings so that emissions could be related to engine power. It also eliminated many of the difficulties of sampling just behind the exhaust nozzle by making the measurements far enough downstream of the engine that chemical reactions and particle formation are largely complete. The downstream distance at which processes are essentially complete is 20-25 nozzle exit diameters behind the engine. Sampling at this location greatly simplified probe requirements due to reduced exhaust temperature, and data analysis was simplified because a carbon mass balance approach could be used to determine emission factors for trace chemicals without the need for detailed mapping of the exhaust plume area. This downstream approach permitted use of less complex sampling probes compared to exit plane sampling, and yields results more representative of true atmospheric emissions.

This approach was used to develop emission factors for carbon dioxide (CO₂), carbon monoxide (CO), nitric oxide (NO), and NO_X, and more than 60 volatile organic compounds (VOCs), for C-130H transport aircraft engines, as well as for F-15 and F-22 fighter aircraft engines. For each aircraft type, emissions were measured at multiple throttle settings, including afterburner for the fighter aircraft. Testing was performed on multiple engines of each type to assess inter-engine variability, and multiple test runs were performed on each engine to assess intra-engine variability.

In addition to extractive sampling of the engine exhaust, this study also employed optical remote sensing (ORS) techniques, which involve no sample handling, to unobtrusively interrogate the exhaust. The ORS methods do not have the sensitivity to measure many of the trace chemicals in the exhaust that can be measured by extractive techniques, but ORS can measure some of the main exhaust constituents without risk of perturbing the sample, so comparison of downstream extractive and ORS measurements should be of interest.

A preliminary measurement campaign was performed in conjunction with SERDP Project WP-1401 that took emissions measurements from a C-130H aircraft. During this campaign, extractive measurements were made at the EEP and approximately 15 m downstream. Optical remote sensing measurements were made at two downstream locations, approximately 9 and 14 m downstream of the exit plane. The results of that measurement campaign demonstrated that EIs generated using the downstream sampling approach used in this project agreed very well with the traditional exit plane measurements.

Two additional measurement campaigns were also performed that focused on characterization of emissions from fighter aircraft. The first of these campaigns involved emissions measurements from multiple F-15 aircraft. The second campaign included emissions measurements from multiple F-22 aircraft as well as from a single F-15 for comparison to the previous results. During each campaign, extractive measurements were made approximately 23 m downstream of the exit plane for several days to characterize non-afterburner conditions, and then 38 m downstream for a single day to characterize emissions from afterburner conditions. Optical remote sensing measurements were made 23 m downstream of the exit plane for all test conditions during the F-15 campaign. A comparison of the results from the F-15 aircraft shows good agreement between the two measurement campaigns with agreement between EIs from the two campaigns typically within 20% for the major species. For most organic gases the agreement between the two campaigns was within a factor of two.

A comparison of the results from all three aircraft types indicates comparable EIs for CO and NOX and several organic species, but differences of a factor of 10 or greater for some other organic species.  In general, F-15 and F-22 EIs for the air toxics are lower than those for C-130, especially at idle conditions.  At higher power conditions, similar trends are observed although the measurements were frequently near instrumental detection limits and the observed differences are not statistically significant. This emissions data will help to maintain military training schedules and to permit planners to consider movements of airborne units from one facility to another.