Problems to be solved
Several factors are important when impact from aircraft emissions are studied and measures are taken to reduce aircraft impact. Emissions from aircraft flying at cruising altitudes (8 to 13 km) affect atmospheric composition in a height region where there might be significant climate impact through changes in the distribution of compounds like CO2, ozone, methane and the frequency and extent of contrails. Future emissions from aircraft are expected to increase much more rapidly than emission in general, therefore, not only will the overall impact of aircraft emissions increase, but also the importance relative to the total climate impact. The emissions occur in specific flight corridor where the atmospheric impact is significant. A large fraction of the emission, may be as much as one third, occur in the lower stratosphere, where atmospheric residence times are long and impact of emissions generally larger than in the upper troposphere. There are several options to reduce future impact from aircraft like fuel efficiency, pollution control, technology improvement and traffic routing, however, for the control measures to be efficient it has to be based on the estimates of the importance of the different climate compounds.

Scientific objectives and approach
The main objectives of TRADEOFF are: To calculate future changes in climate compounds in the atmosphere and the contributions from aircraft emissions to climate changes, to reduce the large uncertainties in the current calculations of the impact from future air traffic, and to provide industry and decision-makers with options to reduce future climate impact from aircraft emissions. Special objectives are: 1) To further develop current Chemical Transport Models (CTMs) and to test them against observations to improve their capability to estimate atmospheric impact, 2) to quantify the relative contributions of individual climate compounds emitted by future aircraft (e.g., CO2, O3, CH4, particles, contrails, H2O from supersonic aircraft) to radiative forcing and 3) to perform studies in the context of options for emission reductions. A co-ordinated 3 year research project will be performed, which includes the use of improved emission data base, the development of atmospheric model tools; estimates of changes in atmospheric composition and radiative forcing; analysis of options for reducing climate change. The studies will focus on changes in the lower stratosphere (LS) and the upper troposphere (UT), where perturbations of gaseous compounds and particles are likely to have a strong impact on climate. To quantify the impact of future aircraft emissions, several model runs will be performed. In these runs the participating groups will use different scenarios for future emissions in order to explore the consequences arising from uncertainties in emissions, and from different assumptions for atmospheric processes parameterisation (transport, chemistry, and physical processes) to narrow the uncertainties of the processes.

Expected impact
The project will contribute to improve European global scale modelling of processes in the upper troposphere and lower stratosphere, which are of general importance for ozone depletion and for climate studies. Results form the studies in the TRADEOFF project will contribute to a better scientific understanding of problems related atmospheric chemical processes through publishing in international journals and through presentations in workshops and in seminars. The TRADEOFF project is focusing on the impact form an industrial sub sector to the climate problem. This will promote a closer interaction between science, technology and policy makers. Another channel of dissemination of the results from the TRADEOFF project is through international assessments and reviews (IPCC, WMO, UNEP, EU). The results of the TRADEOFF project will thus extend far beyond the groups participating in the project.