Laser-spectroscopic investigation of OH-radical concentrations in the exhaust plane of jet engines

Hydroxyl radical (OH) emissions are relevant for oxidation reactions in the post flame chemistry of exhaust gases emitted from jet engines. No direct measurements of OH concentrations are available to date due to the low abun dance and the short lifetime of this radical species. The first application of a combined technique based on Raman scattering and laser-induced fluore scence (LIF) spectrometry is presented here for measurements in the exhaust gases of a commercial jet engine operated in a test rig. From the measurem ents, upper limits for OH concentrations in the exit plane were determined in the range of 90 ppbv for take off and 80 ppbv for ap-idle. The values ar e significantly below the predictions of model calculations based on HONO a nd HNO3 inflight concentration measurements presented recently. Possibiliti es for further increase of the detection sensitivity for OH are discussed.