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.
