A concept of the PTI is proposed to characterize the photochemical interactions of aircraft exhausts with the ambient air. The PTI is the ratio of NOx, CO, ozone, and NOy transformed in the plume during its lifetime period tp to NOx and CO released into the plume initially. An estimate of tp is calculated by prescribing the 10% excess of emitted (NOx)p in the plume over the (NOx)a of the ambient air at the end of plume life. Simple analytical formulas for PTI are obtained from the known plume box model. The tp, PTI of NOx, HNO3 and O3 are calculated and analyzed for different moments of releases during the daytime in and out of flight corridor. The in-plume NOx conversion to HNO3 and O3 enhancement are found to be the largest in daytime, in July and in the lower stratosphere. ¿ 2000 American Geophysical Union