Climate impact and nature of condensation trails
Aviation sustainability has been a relevant topic among scientists throughout the 21st century. Awareness of increasing air travel volume is key in embracing experts concerns about the great impact of aviation on climate. The International Civil Aviation Organization (ICAO) and Airbus forecasted revenue passenger kilometers (RPK) - a metric showing the actual demand for air transport, evaluated as the number of paying passengers multiplied by the traveled distance -growth rates of 4.5-6% per annum over the next 20 years, which leads to inevitable emission related consequences on the environment. Despite mitigation strategies and improvements of operation efficiency, fuel usage may have 2–3–4-fold increases by 2050, depending on economic projections and emission scenarios. Taking into account that aviation is currently responsible for about 3% of total annual fossil fuel emissions of CO2 and the aforementioned data, radical changes in the industry are essential. Recent studies show that condensation trails (contrails) are non-CO2 aviation effects contributing for more than half of aviation related radiative forcing (RF) - the imbalance of Earth-atmosphere energetic budget, resulting from differences in incident radiation from the sun and upwelling radiation from earth, that induces changes in the temperature structure of low atmosphere. Reduction of such effects could be carried out rapidly as contrails live for hours whereas greenhouse gases for centuries, it would buy time to more in-depth changes such as decarbonization. Therefore, focus is put on understanding microphysical and radiative properties of contrails, determining to their environmental impact and source of possible mitigation solutions.
First of all, contrails can be categorized in short or long lived, depending on whether they remain for more than 10 minutes or not and they are globally referred to as aircraft-induced clouds (AIC). Short-lived contrails contribution to RF is negligible and they will not be further investigated. Long-lived ones can be categorized in persistent contrails and contrail cirrus, the latter are more interesting as their contribution to RF is superior. Persistent contrails keep their linear shape whereas contrail cirrus assume irregular shapes so that at an advanced level of evolution they cannot be distinguished from natural cirrus. Formation of contrails and evolution in different types entirely depend on surrounding atmosphere. They are composed of ice crystals (similarly to natural cirrus) and form in upper troposphere ice-supersaturated regions, where ambient air is moist and cool enough. Extension and magnitude of ice supersaturation are upper limits to the spread of contrail cirrus and determine contrail types. Life of contrails can be divided into formation stage and spreading stage.
Formation starts with the mixing of engine plumes, made of gases with water vapor content and particulate matter such as soot or ultra fine aqueous particles (UAPs), with surrounding ambient air. Cooling of hot exhaust gases produces supersaturation over the liquid phase. Consequently, aerosol particles emitted from the engine, together with mixed ambient ones, are water-activated and form droplets. The high number of soot particles and their hygroscopic properties make them the main activation nuclei. Ice nucleation occurs, only after droplets form, in shapes of droxtals, hexagonal prisms and columns. Laboratory studies showed that exhaust water vapor by itself would never form droplets nor ice crystals. Aerosol particles relevance as ice nuclei is furthermore highlighted by experiments showing that the more soot particles emitted from engines, the higher the number of ice crystals in contrails. No direct observation of ice crystals is available and their shape is currently under investigation as it affects contrails optical properties, hence their RF effects. If formation constraints of below 223 K temperature and above 8 km altitude are met, ice crystals grow and visible long-lived contrails appear. In this case, formation continues with turbulent mixing of engines plumes with the aircraft wing tip vortices - spiral fluxes of air that form in aircrafts wakes due to their aerodynamics - causing contrails descent of about 100 m under flight level. Therefore ice crystals in the lower part of the wake sublimate, in larger quantities for higher number of ice crystals and lower ambient ice supersaturation. On the other hand, ice crystals in the upper part of the wake keep growing by entraining ambient ice-supersaturated water vapor. This regular flow then vanishes in the dissipation regime, which ends the formation stage.
During spreading stage persistent contrails evolve into contrail cirrus. Increasing coverage of AIC is due to vertical shear in horizontal wind and sedimentation of contrails. In ice-supersaturated regions, ice crystals grow by uptake of water vapor and when they exceed a maximum dimension of 30 μm they start sedimenting. Encountering warmer and drier air layers, ice crystals sublimate. This process leads to an increase of contrails vertical extent, that together witch sheared horizontal wind causes spreading and enhanced coverage of AIC. Over traffic congested areas they merge with other contrail and natural cirrus forming cloud layers of greater optical depth (OD) - a measure of the attenuation of radiation at a given wavelength - with greater effects on RF. Presence of contrail cirrus around the globe is also affected by their considerable transportability from source regions. Simulations show that their coverage is greater on Europe rather than on US eastern coasts, albeit air traffic is not. This is due to advection caused by the Gulf Stream and pointing towards the old continent. Finally, it must be highlighted that formation constraints for AIC are often met at cruise altitudes whereas ice-supersaturated regions in which spreading can occur are less frequent. Extent, shape and optical properties of AIC determine their RF effect, contributing in large amount to the aviation impact on climate.
AIC RADIATIVE FORCING
Accordingly to a study from the Intergovernmental Panel of Climate Change, air transport contribution to the total anthropogenic RF in 2011 adds up to 4%. In assessing aviation potential to affect climate change, it cannot be neglected that it is a unique sector as all emissions occur at cruise altitudes of 8-12 km. This characteristic is responsible for typical phenomena such as contrails. The evidence is that more than half of aviation RF is due to AIC, the remaining part is caused by carbon dioxide and nitrogen oxides emissions. Among all types of AIC, contrail cirrus produce the highest RF, accounting for 80% of total AIC contribution. Unfortunately, observational data on contrail cirrus are lacking because of the inability to distinguish them from natural cirrus, on the other hand persistent contrails have been extensively studied and their radiative response is well defined. Dramatic progresses on assessing contrail cirrus effects were made by Ulrike Burkhardt and Bernd Kärcher in 2011. They overtook the knowledge gaps in the spreading stage by means of a process-oriented contrail-cirrus module (CCMod) in a global climate model, ECHAM4, simulating random merging and overlapping of contrail and natural cirrus. The core idea of the simulation was to introduce contrail cirrus in the model as a different cloud class, which allowed to extract data on contrail cirrus RF and additional information such as their coverage and optical depth around the globe. Physics of long-lived contrails support CCMod results. In fact, radiative forcing due to high and cold clouds is positive, they let a large amount of energy into the atmosphere and hold back the outgoing one. AIC are optically thin, so that they are transparent to solar radiation at short wavelengths. On the contrary, their high ice crystal density causes absorption of thermal emission from earth and consequent re-emission into the atmosphere. This analysis omits indirect effects of air travel and AIC on pre-existing natural clouds in upper troposphere. Caused by aircraft emitted aerosols and changes in water budget of surrounding atmosphere, physics and contribution to RF of this phenomenon are not clearly defined yet.
Modernization in the aviation industry is challenging and time-consuming. First of all, the long life-time of modern aircraft makes it impossible to perform short-term fleet replacements. Furthermore, new technologies must withstand testing to ensure compliance with the stringent safety and reliability requirements of the sector. Nonetheless, current knowledge of AIC and alternative technologies make mitigation of aviation RF possible with both short-term and long-term solutions. Key factors in reducing AIC are emission of soot particles, that act as ice nuclei when water-activated in formation stage and ice supersaturation of ambient air, essential to the spreading stage. Among short-term actions, the introduction of synthetic fuels and biofuels aims to substantial reduction of ice crystals forming rates in AIC. Since these fuels contain no aromatic species, engines would not emit soot particles. Moreover, lean combustion technology would induce the same effect by means of higher air to fuel ratios. Long-term solutions include non-C fuels such as liquid hydrogen that has zero soot emission. Besides obvious advantages, this would bring thorny consequences as production of LH2 should be carbon-neutral and stocking in aircraft would require larger airframe storage capacity, adding weight and drag to conventional airframes. Furthermore, full electrification or blended wind body technology would severely reduce overall emissions addressing to both CO2 and non-CO2 aviation effects. Nowadays, high energy density batteries are the outer reach in research as they represent the main obstacle to full electrified aircrafts. Finally, air traffic management strategies could be carried out. An intuitive change concerns reducing holding patterns on arrival and delays. AIC effects may also be reduced by lowering cruise altitudes so that formation constraints are not met, yet there are tradeoffs to be considered such as worse thermal efficiency of engines. Increasing knowledge and means of meteorology can really be of use to rescheduling flight routes, avoiding large scale ice-supersaturated areas. In the end, rapid modernization is crucial as current emissions do not meet the 2015 Paris COP21 agreement that sets a challenging +2.5° C upper limit for global warming by the end of the century. It is a duty for young engineers to rethink sustainability to be the core of creation.
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