An updated global contrail forcing assessment accounting for volatile particulate matter

Contrails are ice clouds that form behind aircraft. Collectively, they are estimated to have a warming impact that is comparable to aviation’s accumulated CO₂ emissions [1]. Individually, the warming impact of a contrail depends, inter alia, on the apparent emission index of ice crystals (AEI_ice), which is governed by its formation pathway. Accordingly, contrails form when hot exhaust gases mix with cooler ambient air and the plume exceeds water saturation. Under these conditions, water vapour condenses upon particles that are either exhausted by the aircraft or entrained from the environment. These water (or solution) droplets subsequently freeze to generate (contrail) ice crystals.

For aircraft powered by conventional rich-quench-lean (RQL) combustors, contrails predominantly form via non-volatile particulate matter (nvPM) in the “soot-rich” regime. However, lean-burn combustors reduce nvPM emissions by up to several orders of magnitude relative to RQL combustors, driving engine emissions into the “soot-poor” regime. Here, contrails are thought to form via ambient particles and volatile particulate matter (vPM), the latter generated from condensable gaseous emissions [2], including sulphuric acid and lubrication oil. Moreover, sulphuric acid has also been reported as a source of contrail ice crystals behind RQL combustors, for fuel sulphur content of ~500 ppm [3]. Therefore, global simulations that do not incorporate the role of vPM in contrail formation may underpredict AEI_ice and hence the warming potential of contrails formed under these conditions.

Recently, a model was developed to estimate AEI_ice across both the “soot-poor” and “soot-rich” regimes by including the role of vPM [4]. We previously integrated this framework into the contrail cirrus prediction model (CoCiP) and showed that incorporating vPM raises the 2019 global contrail net radiative forcing (RF) by up to 30% [5]. Since then, this work has been extended to better constrain the assumed vPM properties, leveraging outputs from two recent flight campaigns that measured AEI_ice in the “soot-poor” regime [6]. Here, we provide an updated estimate for the 2019 global contrail net RF and characterize the effects of lubrication oil and sulphuric acid emissions. Additionally, we investigate the contrail mitigation potential via fleetwide adoption of 100% sustainable aviation fuel and low-sulphur Jet A-1.                                                                                                                                   

References

[1] D. S. Lee et al., Atmos. Environ., 2021, DOI: 10.1016/j.atmosenv.2020.117834.

[2] F. Yu, B. Kärcher, and B. E. Anderson, Environ. Sci. Technol., 2024, DOI: 10.1021/acs.est.4c04340.

[3] R. Dischl et al., Commun Earth Environ, 2025, DOI: 10.1038/s43247-025-02951-5.

[4] J. Ponsonby et al., Atmos. Chem. Phys., 2025, DOI: 10.5194/acp-25-18617-2025.

[5] R. Teoh, et al., EGU General Assembly, 2025. DOI: 10.5194/egusphere-egu25-17393.

[6] C. Voigt et al., In Review. DOI: 10.21203/rs.3.rs-6559440/v1.