Soot in aircraft exhaust and the climate impact of air traffic
Aircraft emissions are associated with all sorts of environmental problems. Researchers have long been studying the different kinds of emissions in order to understand how air transport is altering the composition of the atmosphere, cloud cover and the climate. Aircraft contrails are anthropogenic clouds of ice. My work in the 1990s showed that the physical properties of the white wispy trails aircraft trace across the sky are best explained by their soot emissions – black carbon particles and condensable substances like sulphuric acid produced during the combustion of fossil fuels such as kerosene.##markend##
Ice crystals in contrails form around the tiny soot particles released by aircraft engines. They are smaller than a thousandth of the breadth of a human hair. In areas with high traffic and suitable weather conditions, contrails persist, spread out, and eventually form extensive contrail cirrus at cruising altitude, where the air is typically below minus 40 degrees Celsius and sometimes damp enough for ice clouds to last for several hours. While 'young' contrails are line-shaped, aged contrail cirrus clouds take on a variety of shapes and can be difficult to identify or distinguish from natural cirrus clouds.
We know that direct atmospheric warming caused by aircraft soot emissions plays a minor role, as the total mass of soot involved is too small. In recent years, climate models quantified the major impact of contrail cirrus on Earth's radiative balance for the first time and airborne observations confirmed the relationship between the amount of emitted soot particles and the number of ice crystals in contrails. However, there are questions about aircraft soot emissions and cloud formation that still remain unanswered – some field data have indicated correlations between soot and natural cirrus clouds. This suggests that soot particles not only form contrail cirrus clouds, but that they could also have an influence on the formation of natural ice clouds.
Although scientific understanding of contrail formation on emitted soot particles has advanced, the soot-to-contrail-to-cirrus cloud chain of events has remained on the list of pressing questions for research to this day due to a lack of understanding of the exact processes and any direct observational evidence. Some climate models have predicted a very large global radiative imbalance (radiative forcing) due to aircraft-soot induced changes in the cirrus clouds. Radiative forcing is a measure used to quantify anthropogenic climate effects. The higher the value, the greater the effect of a factor – in this case, soot emissions from aircraft – on the climate. Their radiative forcing would far surpass that of all the other aero engine components combined.
To quantify changes in cloud cover caused by air traffic – in other words, to express them in numbers – we must first understand how ice clouds form. After more than two decades of work to gain a better understanding of cirrus cloud and contrail formation and their representation in global climate models, my scientific research came full circle when I learned that laboratory measurements played a crucial role in solving the mystery of how and under what conditions soot particles form cloud ice crystals. I teamed up with Fabian Mahrt in Canada and Claudia Marcolli in Switzerland, who collected and analysed the laboratory data. The measurements show that the ice-forming ability of soot particles depends on the amount of time they remain in the atmosphere, and that ability strongly decreases with their size. I have integrated the new representation of soot-induced ice formation into a detailed, high-resolution cirrus model. In a deviation from previous approaches, we have worked together to address the elusive soot-to-cirrus problem from a radically different angle in our latest publication.
Predicting how soot emissions from aircraft affect the climate through interactions with cirrus clouds is only possible to a limited extent using cloud models. This is partly due to the huge variability in the atmospheric factors involved in cloud formation. Our strategy was therefore to deliberately overestimate the influence of aircraft soot in our model in order to determine the maximum conceivable effect on cirrus formation.
Turning conventional wisdom on its head, our research shows that aircraft-emitted soot particles previously frozen in contrails only exert a limited effect on cirrus clouds. This is mainly due to their small size. Less than one in 100 soot particles exhibit significant ice activity. It is surprising that a mere handful of ice-active soot particles in one litre of air at cruising altitude is enough to change the total number and mean size of the ice crystals in a cirrus cloud. We discovered that the soot particles do not significantly change the optical thickness of the cloud. This severely constrains the radiative forcing of aircraft–soot–cirrus interactions, and thus their effect on the climate.
The scientific community is now better prepared to devise dedicated field measurements that might confirm the limited effect of aircraft soot emissions on cirrus and to improve the cloud schemes programmed in global climate models. These are necessary to quantify the climate impact of indirect cirrus changes caused by soot particles, which we now expect to be much smaller than the direct impact from persistent contrails and contrail cirrus.
To sum up, soot particles produced by aircraft form contrail cirrus, and can also alter cirrus clouds. These are two different issues, as we know that the ice crystals created by soot from jet engines are formed differently to those occurring naturally in the atmosphere. The two topics are nevertheless related because only the soot residues released by dissipating contrails develop the ability to form ice under atmospheric conditions. Yet our study shows that this ability and the effect it has on the optical thickness of cirrus clouds is much weaker than previously thought, even under the most favourable conditions.
The future of air transport – alternative fuels reduce soot emissions and contrails
Recent measurements conducted by research aircraft have investigated how the addition of alternative aircraft fuels to kerosene reduces the number of ice crystals in contrails. This research is guided by a theoretical understanding of the ice formation process caused by soot. Large reductions in the number of soot particles are necessary in order to significantly reduce the amount of ice crystals in contrails. Low soot emissions, which are necessary to significantly reduce the influence of contrails on the climate, require the use of pure alternative fuels. Efforts to significantly reduce soot emissions from aircraft can be complemented by the introduction of new propulsion technologies based on a better understanding of the soot formation processes in the combustion chambers of aircraft engines.
This blog post is based on my behind-the-paper article published in the Nature Sustainability Community. The original can be found here.