Environmental protection meets air traffic

In the field of air traffic, even relatively simple measures can help reduce the harmful effects of aviation on the climate. A young DLR scientist is now searching for such solutions. Observations from space show a criss-cross of white lines extending over western Europe on some days. On an annual average, aircraft contrails cover about 0.5 per cent of the sky in northern mid-latitudes. Water vapour emitted by aircraft becomes visible in the cold atmosphere when it condenses and forms tiny ice crystals.

"How these ice crystals interact with other aircraft exhausts and thereby change the climate has not yet been studied," explains Dr. Christiane Voigt from the DLR Institute of Atmospheric Physics in Oberpfaffenhofen. The young physicist recently became leader of a Young Investigators Group that cooperates with the University of Mainz and the Max Planck Institute of Chemistry in a research project called AEROTROP (Impact of Aircraft Emissions on the heteROgeneous chemistry of the TROPopause region). The research group takes a closer look at the environmental effects of air traffic. A well-known greenhouse gas emitted by aircraft is carbon dioxide, produced by kerosene combustion in the aircraft engines. Aviation carbon dioxide emissions solely contribute one to two per cent to the global warming. "But the contribution of the sum of aircraft emissions is much higher," Christiane Voigt points out. Two to eight per cent of the total anthropogenic radiative forcing, which contributes to the so-called greenhouse effect, is currently caused by air traffic. And the trend is increasing, because aviation is a growth industry. Since 1990, the transport capacity by passenger air traffic has almost doubled.

Besides carbon dioxide, the researchers' main concerns are contrails and nitrogen oxides emitted by aircraft. Nitrogen oxides emitted at cruise altitudes between 8 km and 12 km produce the greenhouse gas ozone. Ozone interacts with thermal long-wave radiation from the earth surface leading to a warming of the atmosphere. The radiative forcing through ozone resulting from aircraft nitrogen oxide emissions is of similar magnitude to the carbon dioxide effect. Besides producing ozone, nitrogen oxides from aircraft also trigger other processes in the atmosphere. For example, they can lead to reductions in the lifetime of the greenhouse gas methane and they can be processed in clouds. Large uncertainties remain in the exact quantification of aircraft induced ozone changes and their climate impact. To reduce such uncertainties and to understand the underlying processes is the topic of Christiane Voigt's research. Recently, Voigt and her colleagues made a surprising discovery on research flights over northern Germany. "We were able to detect increased amounts of reactive nitrogen compounds in the ice crystals in young contrails," explains Christiane Voigt. The uptake of reactive nitrogen compounds in ice particles reduces nitrogen oxide concentration in the ambient air hence ozone production is slowed down. In the future, Christiane Voigt and her Young Investigators Group want to study these unknown processes in more detail. In collaboration with German and international partners, she plans to perform aircraft measurements in contrails, in natural cirrus clouds and in the cloud free atmosphere with the new German research aircraft HALO (High Altitude Long Range).

Aircraft exhaust will directly be probed by chasing other aircraft. These measurements will provide an extensive dataset on aircraft emissions and contrails for process studies and global modelling. "We want to calculate the climate impact of present and future air traffic at different cruise altitudes," explains Christiane Voigt. In three years, the AEROTROP scientists want to find out flight altitudes with a low effect on the environment It is possible that the research findings could then help to reduce the climate impact of air traffic through mitigation: simply by adjusting the flight altitudes.