Chapter 4 Environmental impact of aviation
D. Cariolle
Aircraft emissions can have an impact on atmospheric chemistry and on the radiative balance of the atmosphere. Emissions of nitrogen oxides and the formation of ice particles within contrails perturb the natural chemical cycles and lead to ozone production or destruction depending on local air mass composition and insolation. These ozone perturbations along with the emissions of CO2, water vapour and ice particles formation, soot particles, sulphuric aerosols from the burning kerosene give an additional contribution to the greenhouse forcing. The most recent evaluations of those effects show the existence of a amplification factor of about 3 for greenhouse potential factor from aircraft emission: a molecule of CO2 emitted from a jet airplane is a factor of 3 more efficient for greenhouse forcing than a similar molecule emitted at ground level.
Figure 4.1: Steps involved in the formation and the transformation of gases and particles emitted by the kerosene combustion.
Given the exponential increase of the air traffic it is anticipated that the aircraft emissions will double by year 2020 compared to present. The air traffic would then be a major player of the climate change. There is no doubt that in future negotiation processes for the limitation of greenhouse gaz emissions aviation sources will be a central issue. It is therefore important that the regulations that could be imposed on aviation be based on well-sound scientific studies.
As this thematic was recognised as an important issue for the CERFACS shareholders and for the ministry of transportation (which has the administrative supervision of Météo-France), it was decided to settle a new project "aviation and the environment" within the CERFACS and CNRM teams. Starting in September 2003, the project is aimed at coordinating the work between the two teams, whose expertise in the field is internationally recognise.
The main scientific objective of the project is to better quantify the chemical and radiative atmospheric impacts of aviation at the various scales from the aircraft near field to the global atmosphere. An integrated evaluation of the different steps that involve the emission transformations must be performed, from the species generation within the combustion chambers, their chemical transformation within the airplane generated vortices, their vertical and horizontal dilution during contrail and track formation to the formation of corridors by the fleets and their transport by the general circulation of the atmosphere. At each of those steps the chemical and radiative atmospheric perturbations must be assessed. This will be done using a hierarchy of numerical models, AVBP and NTMIX from CERFACS and Méso-NHC, Arpège (1D and 3D) and MOCAGE from CNRM.
(a)
(b)
Figure 4.2: Iso-surface of the vorticity magnitude during the jet/vortex interaction:
(a) case 1; (b) case 2.
As this project was launched in autumn 2003, it has not generated specific results yet. However, R. Paoli actually at Stanford/Ames, has recently made simulations of contrail formations with various initial size distribution for emitted particles by the jet with growing characteristics varying according to a simplified microphysical model for ice particle nucleation.
Two simulations of the jet/vortex interaction were analyzed, as discussed in detail in Paoli et al.
[Paoli, 2002;
Paoli, 2003].
In the first case (see Fig. 4.2a), typical conditions of cruise flight, the jet and the vortex were sufficiently well separated in order to study initially the jet dynamics before considering its interaction with the vortex. The results show that dynamics and mixing are both controlled by the jet diffusion and its entrainment around the vortex core. In the second case (see Fig. 4.2b), the jet partially blowed into the vortex core, making the flow similar to a Batchelor vortex. The strong perturbations injected into the core caused an instability of the system which was continuously fed by the jet elements wrapping around the core. This leads to a strong decay of angular momentum and diffusion of the core. Global mixing properties, such as plume area and global mixedness evolutions, were analyzed and two applications to environmental problems were finally discussed. As the distribution of ice particles are predicted in those simulations, they will be used for 1D radiative model (used within the Arpege model) calculations to be performed in 2004.
Several responses to various calls for proposals have been prepared, anticipating that such funding will give support to the project for the next four years.
Fig. 4.1 Chart showing the steps involved in the formation and the transformation of gases and particles emitted by the kerosene combustion. Those various paths of transformation must be taken into account in microphysic models that predict particle formation and must be coupled to dynamical and radiative models covering a large range of time and geographical scale.
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