The impact of soot emissions and fuel composition on contrail microphysics

Understanding the microphysics of contrail formation is crucial for predictive models to be developed and deployed. Reliable model predictions are essential to accurately assess the environmental impact of aviation and to develop strategies to mitigate the effects of aircraft-induced cloudiness. This research aims to enhance our knowledge of the nucleation processes involved in contrail particle formation, by investigating the effect of soot emission levels, as well as the more complex fuel effects. To investigate these effects, we developed an altitude chamber facility dedicated to the study of atmospheric nucleation from ground level up to the stratopause. This facility enables precise control on ambient pressure, temperature, and gas composition. Exhaust gases, including particulates from relevant aviation fuels, are being fed to the chamber, emulating the exhaust stream of a jet engine. Utilizing a suite of advanced laser and optical diagnostic techniques, we characterized water nucleation processes varying soot emission levels and fuel composition, while changing ambient temperature under realistic atmospheric conditions around commercial airliners’ cruise altitudes.

Our experimental results provide compelling evidence for the significant role of soot emissions in the water nucleation process. Specifically, we observe that contrail nucleation is both delayed and less intense at lower soot levels in the exhaust. These findings align with previous research, indicating that contrail nucleation intensity diminishes with soot levels up to a certain threshold, beyond which further reductions in soot do not influence contrail formation. Notably, we do not observe an increase in contrail formation intensity at low soot levels, at all tested temperatures, potentially due to the absence of other species or particles that could offer alternative nucleation pathways. Additionally, our measurements reveal variations in fuel effects that extend beyond the soot-contrail relationship. Differences in soot properties, as well as the levels and types of volatiles emitted during combustion, likely account for the observed behaviors when comparing fuels. Ongoing and upcoming tests aim to evaluate the role of ambient humidity on contrail formation and to consider longer residence times to assess contrail development further downstream.