The Role of vPM Activation in Global Contrail Climate Assessments and Mitigation Implications

Global aviation contrail climate forcing could match or exceed the forcing from aviation’s cumulative CO₂ emissions. Aircraft engine exhaust contains various particles, including non-volatile particulate matter (nvPM), volatile particulate matter (vPM), and ambient aerosols. At cruise altitudes, these particles can activate to form water droplets if the relative humidity in the plume exceeds their activation threshold, where they subsequently freeze into contrail ice crystals. The initial number of contrail ice crystals is primarily driven by the nvPM in “soot-rich” conditions, where the nvPM number emissions index (EI_n) exceeds a threshold of around 10¹⁴ kg^-1; while vPM and ambient aerosols become more likely to activate to form contrail ice crystals under “soot-poor” conditions (nvPM EI_n < ~10¹⁴ kg^-1) (Yu et al., 2024). However, existing global contrail simulation workflows do not currently account for the potential activation of vPM, which may lead to an underestimation of the contrail climate forcing. This underestimation could likely be more significant for flights powered by: (i) cleaner lean-burn combustors, where their nvPM EI_n at cruise is typically below 10¹² kg^-1; or (ii) sustainable aviation fuel (SAF), which can reduce the nvPM EI_n by up to 70%.

An analytical model describing the microphysical pathway of contrail formation from nvPM and ambient aerosol particles was developed by Kärcher et al. (2015), which has since been extended to account for the potential activation of vPM in forming contrail ice crystals. Here, we aim to integrate this extended model into the contrail cirrus prediction model (CoCiP) to: (i) provide an updated estimate of the global annual mean contrail net radiative forcing (RF) for 2019; and (ii) quantify the simulated differences in contrail climate forcing between flights powered by conventional (rich-burn) and cleaner lean-burn combustors.

By accounting for vPM activation, our preliminary results estimates that the 2019 global contrail net RF could increase by up to 35%, depending on the assumed vPM properties (EI_n and particle size distribution). On average, the contrail climate forcing from lean-burn combustors could be around 50% to 90% lower than that from conventional rich burn combustors. When compared with the simulation without vPM activation, the increase in contrail warming effects due to vPM activation in the exhaust of lean-burn combustors becomes significant only when the ambient temperature is at least 10 K below the Schmidt-Appleman criterion threshold temperature. Further work is ongoing to quantify the contrail mitigation potential from a fleetwide adoption of SAF and cleaner lean-burn engines.

References

Kärcher, B., Burkhardt, U., Bier, A., Bock, L., and Ford, I. J.: The microphysical pathway to contrail formation, Journal of Geophysical Research: Atmospheres, 120, 7893–7927, https://doi.org/10.1002/2015JD023491, 2015.

Yu, F., Kärcher, B., and Anderson, B. E.: Revisiting Contrail Ice Formation: Impact of Primary Soot Particle Sizes and Contribution of Volatile Particles, Environ Sci Technol, 58, 17650–17660, https://doi.org/10.1021/ACS.EST.4C04340, 2024.