Aviation soot particles emitted from aircraft have the potential to act as ice nuclei, competing with other ice formation processes in cirrus clouds and thus affecting their radiative properties. However, quantification of the resulting climate effect is still subject to significant uncertainties. There is great disagreement between different model studies, not only concerning the magnitude of this effect, but also its sign (warming or cooling effect). Reasons for this are the insufficient knowledge of the ice-forming properties of (aviation) soot particles, the complexity of ice-forming processes in the atmosphere, and their difficult representation in global climate models.
Here, a newly developed configuration of the global climate chemistry model EMAC-MADE3 (Righi et al., 2020) was applied to investigate the uncertainties of the impact of aviation soot on natural cirrus clouds. The model uses the aerosol microphysics submodel MADE3 coupled to a cloud microphysics scheme that includes a detailed parameterization of aerosol-induced ice formation in cirrus clouds.
A large number of numerical simulations were performed with the EMAC-MADE3 model to estimate the sensitivity of the aviation soot-cirrus effect related to the assumptions on the ice-forming properties of the soot particles and to the model representation of the updrafts that trigger cirrus cloud formation. Soot particle microphysics, as well as atmospheric dynamics that determine the occurrence and strength of updrafts, are considered key processes for the effect of soot-induced ice formation in cirrus clouds.
The model describes the ice-forming ability of aerosol particles with two parameters: the critical supersaturation at which ice formation is triggered (Scrit), and the activated fraction of the aerosol population that effectively forms ice (fact). Based on previous studies and the results of laboratory experiments, these two parameters were varied over a range of possible values. A total of nine numerical experiments were conducted to illuminate the corresponding parameter space. Using this method, radiative forcing caused by the airborne effect on cirrus was quantified in the range of -35 to +13 mW m-2, but with a confidence level below 95% in several cases. Compared to this, the effect of CO2 from aviation is about +34 mW m-2 according to latest estimates (referring to the year 2018).
To further investigate the sensitivity of this result to model dynamics, idealized experiments were performed with prescribed vertical velocities ranging from 2 to 50 cm s-1. These show that the uncertainty due to the model dynamics aspect has a crucial influence on the studied effects, contributing a factor of ±1.7 additional uncertainty to the model results of the radiative forcing of the aviation soot-cirrus effect.
A comparison with previous model studies of the effect of aviation soot on cirrus clouds shows a large range and a striking lack of agreement among the various model results. This indicates the need for further detailed analyses to explain the origin of these discrepancies. The international collaborations currently underway as part of the European ACACIA Horizon 2020 project will provide additional important insights into the ice-forming properties of aviation soot through laboratory measurements, helping to improve the assessment of this effect in future follow-up studies.
Righi, M., Hendricks, J., Lohmann, U., Beer, C. G., Hahn, V., Heinold, B., Heller, R., Krämer, M., Ponater, M., Rolf, C., Tegen, I., and Voigt, C.: Coupling aerosols to (cirrus) clouds in the global aerosol-climate model EMAC-MADE3, Geosci. Model Dev., 13, 1635-1661, https://doi.org/10.5194/gmd-13-1635-2020, 2020.
Righi, M., Hendricks, J., and Beer, C. G.: Exploring the uncertainties in the aviation soot–cirrus effect, Atmos. Chem. Phys., 21, 17267-17289, https://doi.org/10.5194/acp-21-17267-2021, 2021.