Fewer soot particles in the exhaust plumes of airplanes reduces ice crystal formation and the climate impact of the resulting contrail cirrus. A new study, published in the Nature Partner Journal Climate and Atmospheric Science, shows that cutting the number of ice crystals formed in contrails by half leads to a reduction in the climate impact of contrail cirrus by 20 percent. When ice crystal numbers are cut by 80% the impact is reduced by a factor of two.
Aircraft engines emit soot particles. These act as condensation nuclei forming small supercooled water droplets which freeze immediately and form visible contrails in the sky. In cold and moist conditions contrail cirrus can persist for many hours and form so-called contrail cirrus, which have been shown to warm the climate. In the current model study it was found that a reduction in the number of ice crystals formed in contrails leads to a decrease in contrail cirrus optical depth and life time. This in turn causes a decrease in contrail cirrus cover and climate impact.
The occurrence of contrail cirrus is highly variable in time and space. The study shows that the climate impact of contrail cirrus is mainly due to a small number of contrail cirrus outbreaks. Reducing soot particle emissions in weather situations supporting those outbreaks is particularly effective.
The results are an important contribution towards the discussion on the mitigation of the climate impact of contrail cirrus and on the use of alternative fuels and new combustor technology, which reduce soot emissions. The design and testing of alternative fuels is currently a very active area of research.
Reduction of the net warming effect of contrail cirrus due to 80 % reduced ice crystal formation. Shaded areas indicate statistically significant changes at the 95 % significance level. (Graphics: ©DLR)