Climate impact of aviation emissions in dependency of the emission location and weather situation – a basis for weather dependent, climate optimized flight planning

The climate impact of aviation emissions, in terms of aviation induced ozone or contrails, is highly dependent on ambient conditions, which vary considerably with emission location, altitude and with the actual weather situation. We present a methodological approach developed within the European project REACT4C (Reducing Emissions from Aviation by Changing Trajectories for the benefit of Climate) which determines climate cost functions, that reflect the climate impact of aviation emissions for defined emission locations, altitudes, times, and specific weather situations. These climate cost functions are calculated with an atmospheric chemistry-climate model and can be used in flight planning tools in order to perform route optimisation.

Within the project REACT4C the viability of changing flight trajectories and altitudes to reduce the climate impact of aviation emissions in dependency of the actual weather situation is investigated. One of the main objectives is to estimate the mitigation effect of such operational measures in terms of climate impact for a set of typical weather situations. Therefore, climate cost functions are determined which are utilized within current operational flight planning tools to calculate optimized flight trajectories considering the minimization of climate effects. The corresponding mitigation of climate impacts can then be evaluated by means of state of the art climate models. Finally, these findings will be exploited to derive recommendations for future flight concepts considering climate friendly flight planning.

To determine efficiently the climate change contribution of local aviation emissions in terms of ozone, methane, water vapour perturbations or contrail coverage and the respective radiative forcings, four-dimensional climate cost functions are computed. For this purpose the ECHAM5/MESSy Atmospheric Chemistry Model (EMAC) is employed and a new submodel (AIRTRAC) has been developed for the applications within this project. The emitted trace species (e.g. NO_x, H₂O) are placed on trajectories which are calculated on-line within EMAC by means of the Lagrangian tracer transport model ATTILA (Atmospheric Tracer Transport in a LAgrangian Model). The temporal development of chemical changes, such as decay processes affecting these emissions (e.g. conversion of NO_x to HNO₃, HNO₃ scavenging), the corresponding ozone production and methane destruction is calculated on the trajectories. Chemical processes are determined depending on the background chemistry proportionally to the actual concentration of the respective trace species. Also, the contrail formation, persistence, spreading and dissipation caused by a particular emission is computed directly on the trajectories. Such a comprehensive approach allows to study in detail the fate of aviation emissions and its radiative forcing, hence climate impact.

In order to derive an overall estimate of mitigation potential, weather situations over the North Atlantic flight corridor are classified and typical weather situations are considered to investigate differences in climate impact with respect to emission location, altitude and time. For each emission location, altitude, time and typical weather situation, the changes of radiatively active species (O₃, CH₄, H₂O, contrails) and the corresponding radiative forcing are computed, from which the climate cost functions are derived as outlined above. The climate cost functions incorporate the climate impact of emissions with respect to magnitude and lifetime of radiatively active species, e.g. in terms of GTP or GWP. These cost functions facilitate establishing an interface between the optimization of air traffic routes and corresponding climate impact, in order to investigate routes with minimum climate impact. Considerable differences (of the order of at least one magnitude) in radiative forcing and climate costs of ozone, methane and contrails were found for different emission locations, altitudes and for different weather situations. Such additional information on spatially and temporally resolved weather dependent climate impact provided by the climate-cost functions which represent the interface to operational flight planning tools, allows to perform flight route optimisation under various optimisation criteria, e.g. minimum costs or minimum climate impact.