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Advancing the Science for Aviation and ClimAte

Periodic Reporting for period 3 - ACACIA (Advancing the Science for Aviation and ClimAte)

Berichtszeitraum: 2022-07-01 bis 2024-02-29

Global aviation significantly contributes to global warming. The combustion of kerosene in aircraft jet engines leads to emission of gases and particles that alter the chemical composition of the atmosphere, lead to formation of condensation trails and that may perturb natural cloud formation processes. Some of these effects contribute to global warming, others to cooling, but overall the warming contribution predominates. Due to the strong growth in demand, aviation's contribution to climate change will potentially also grow.

In order to identify sustainable pathways for future aviation, comprehensive knowledge on CO2 and non-CO2 climate effects is required.
Here, the ACACIA project aimed at improving scientific understanding of those impacts that have the largest uncertainty. It formulated concepts for international measurement campaigns with the goal to constrain numerical models and theories with data.
Implementation work has been performed on putting all aviation effects on a common scale which will eventually allow providing an updated climate impact assessment. Uncertainties are treated in a transparent way, such that trade-offs between different mitigation strategies can be evaluated explicitly. Finally, the project strives for the knowledge basis necessary to allow strategic guidance for future implementation of mitigation options, that is, to give robust recommendations of no-regret strategies for achieving reduced climate impact of aviation.
ACACIA has been further exploring how changes to aviation might help to bring emissions and impacts in line with the goals of the Paris Agreement. Due to a better understanding of aviation's non-CO2 climate effects, the project delivered necessary input for an eco-efficient planning of flight trajectories.
Aircraft engine soot was collected in the test facility of Zurich airport and then used in the laboratory in contrail pre-processing and ice formation experiments. The result is that aircraft soot, both unprocessed and pre-processed is an inefficient ice nucleating particle. The aerosol-cirrus interaction via aviation soot has probably a minor influence on climate. This result has also been obtained from simulating the soot activation (i.e. ice nucleation) process with a global circulation model, which only gave significant cooling when it was unrealistically assumed that the soot is an efficient ice nucleating particle. Moreover, simulations in cloud-resolving spatial resolution demonstrate that it is not sufficient that soot from aviation is available for the aerosol-cirrus interaction, it must also be at the right place at the time when the cirrus is forming to have an effect. If the cirrus has already formed (via natural ambient aerosol), the advent of additional soot particles has little effect anymore.
Aircraft flying through already existing cirrus lead to higher crystal numbers in the flight track behind the aircraft. The effect is largest not at flight altitude but rather a couple of 100 meters below to where the downward travelling vortex pair transports the engine emissions during the vortex phase, that is, during a couple of minutes directly behind the aircraft.
There are long-lasting effects of the initial plume processing on aerosol number concentrations. A highly sensitive parameter is the initial size of sulphate particles upon emission. Plume models can be used to derive factors to correct the emissions used in global models.
The impacts of aircraft NOx-emissions on radiative forcing occur often far downstream from the emission location. There is a considerable local variation of these effects relative to the global mean, which implies that future shifts of the major air routes will have an effect on the global mean forcing even if the total amount of emissions would not change.
The relative humidity field on cruise levels, simulated with current numerical weather prediction models, is currently not of sufficient quality for contrail-avoiding flight routing.
Analysis of contrail data from an earlier measurement campaign (ML-Cirrus, 2014) showed that contrails can survive in a slightly subsaturated environment for a couple of hours. This finding has a consequence on our understanding of “persistence” and on strategies to avoid persistent contrails.
An extensive evaluation of the global models with IAGOS has been performed at flight altitude based on a newly developed evaluation tool. The models have been updated and improved based on this evaluation. The chemical response of the atmosphere to NOx emissions from aviation has been studied with five models. These resulted in similar response patterns, but considerably variable amplitudes. Critical components of the chemistry schemes have been identified which will be investigated further. Background NOx from lightning is critical due to the non-linear character of atmospheric NOx chemistry. The five global model perturbations have been used to calculate the radiative forcings associated with NOx changes (ozone and methane) for both present-day and future (2050) conditions under different scenarios for aircraft and background atmospheric conditions.
Interhemispheric contrasts of ice crystal number concentrations in cirrus clouds have been determined from a pole to pole airborne measurement campaign (ATom). It turns out that ice number densities are very variable (more than one to two orders of magnitude variation), and that they tend to be larger in the northern than in the southern extratropics.
A White Paper on concepts for observation strategies for aviation non-CO2 emissions has been written, covering the three aviation effects with the highest uncertainties.
A novel approach to uncertainty assessment for the three main aviation non-CO2 effects has been developed and applied. The approach is to build conceptual models (equations) of the type: “aviation-driven perturbation × radiative efficiency × fractional coverage” and then use the published literature to estimate uncertainties in the different terms.
Four scenarios developed by ICAO-CAEP assuming various types of fuel and covering both tailpipe emissions plus emissions from manufacturing the fuel have been investigated. The temperature response of the atmosphere for these scenarios has been calculated with the CICERO simplified climate model.
Various options for a potential inclusion of aviation in the Paris Agreement have been discussed.
ACACIA aimed at a breakthrough in the understanding and quantification of the overall impact of aviation aerosol on both ice and liquid clouds. This will be achieved by assessing newly available modelling capabilities across scales, from the aircraft -plume and cloud-resolving to the global scale. This takes advantage of high-quality observational data from in situ measurements and dedicated laboratory experiments. ACACIA takes a game-changing step towards the use of climatological results from long-term observations for the design of proof-of-concept studies for aviation climate impact mitigation. This requires the application of both long-term and global-scale atmospheric airborne databases from research infrastructures combined with data from field campaigns. This will lead to the design of novel large-scale field experiments. ACACIA has explored mechanisms for how international aviation can align with the temperature goal as well as the greenhouse gas balance goal of the Paris agreement.
Work-package structure of the ACACIA project