The potential impact of aircraft exhaust gas on entrained ambient air in the stratosphere has been studied by using several numerical models to manifest the importance of mixing processes occurring at different scales. These models include perfectly stirred reactor (PSR), transient well-mixed reactor (WMR), partially stirred reactor (PaSR), and joint scalar probability density function (PDF) simulation for the near-field jet exhaust. Sensitivities of major species to chemical reactions are presented to illustrate the chemical transformation in the near-field jet regime. An accurate reduced chemical mechanism that captures major reaction pathways has been systematically developed and tested. The predicted results with the reduced mechanism are found in good agreement with those from the detailed mechanism, but the computing time is decreased by a factor of 3. The effect of microscale turbulent mixing and chemistry interaction on species evolution is studied, and its impact is found most noticeable in the near field where the timescale of chemistry for highly reactive radicals, such as OH and HO2, is comparable or faster compared with that of microscale mixing. When microscale mixing is included, the predicted H2SO4 level increases at locations where sulfate aerosol formation may be important in the near field.¿ 1997 American Geophysical Union