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We report the first measurements of CO, CH 4 , N 2 O, CO 2 , and H 2 O (g) in the exhaust trails of T‐39, B‐757, and DC‐8 aircraft at cruise conditions. Emission indices (EI) derived from these in‐situ measurements are presented. Results are in agreement with ground‐based tests indicating aircraft act as a net sink for CH 4 and recent airborne in‐situ measurements that N 2 O is not an important exhaust constituent. Condensation of H 2 O (g) on exhaust particles resulted in EI(H 2 O (g) ) values less than those expected from the combustion of fuel alone. Observed apparent negative EI(H 2 O (g) ) values suggest that aircraft aerosol emissions, under unique atmospheric conditions, seed cloud formation and lead to dehydration of the exhaust‐influenced air parcel. Such conditions may induce the formation of cirrus clouds from persistent contrails. Comparisons with the Boeing EMIT Code show measurement‐derived CO emission index values consistent with model evaluations.
Vay et al. (Fri,) studied this question.