Nitrogen Oxide (NO/NO2/HONO) Emissions Measurements in Aircraft Exhausts

Abstract

We present measurements of the emissions of the nitrogen oxide species nitric oxide, nitrogen dioxide, and nitrous acid in aircraft engine exhausts. In the Aircraft Particle Emissions Experiment test series, NO x (sum of nitric oxide and nitrogen dioxide) emissions were consistent with International Civil Aviation Organization certification measurements for a CFM56-2C1 engine, and the NO, emission index increases with increasing engine thrust At low powers, nitrogen dioxide represents up to 80% of the total NO x emissions for that engine, but this fraction decreases to 7% at the highest thrust setting. Nitrous acid, though a minor constituent (up to 7%) compared with NO,, is in part formed in the turbine through reaction with hydroxyl radical. It is therefore a sensitive monitor of exhaust chemistry through which chemical kinetic models capable of predicting concentrations of other important species, such as oxides of sulfur, can be validated. The CFM56 nitrous acid measurements are compared with previous observations, and a strong dependence on engine type is observed for both the nitrous acid emission index and its contribution to the emission index of the sum of all reactive gaseous nitrogen oxides. The nitrous acid emission index increases with increasing engine thrust, but the ratio of emission indexes of nitrous acid to nitrogen dioxide decreases with increasing thrust for the CFM56.