Experiments are in hand to determine the real impact aircraft are having on the atmosphere.

Martin Hindley/LONDON

SCIENTISTS STUDYING the effects of aircraft emissions on the Earth's atmosphere have produced results, which may dispel one of the most commonly held theories about air pollution. After more than 2,000 flights, amassing data at the uppermost edge of the troposphere, researchers have the first evidence to suggest that ozone is not being destroyed at airline cruising heights.

The findings, announced in January (Flight International, 17-23 January), are the most important milestone to date in Europe's search to understand the ecological impact of aircraft on the atmosphere. Airbus Industrie is heading the European Union-backed project, in which German and French climate-research agencies have agreed to co-ordinate measures to assess the cause and effect of ozone production at tropospheric level.

The programme, called the MOZAIC (Measurement of Ozone by Airbus In-service Aircraft), is intended to be Europe's most extensive investigation of the atmosphere ever carried out from the air.

"MOZAIC will provide unprecedented data on the troposphere, enabling us to build a very solid picture of the climate conditions in which aircraft fly," says Dr Alain Marenco, who heads the French side of the programme at the Centre National de la Recherche Scientific (CNRS), the Toulouse-based aerospace-research institute.

"The results will be used to validate numerical models of the atmosphere, which will allow us to forecast ozone distribution and the impact of aircraft on the troposphere and stratosphere," he says. The preliminary findings represent a step forward, for climate-researchers and serve to confirm certain theories about the impact of air transport, on ozone dynamics.

Airlines, already alerted to climate issues by the debate on the ozone layer, have also recognised that they have a role to play in helping scientists understand and predict the impact of trace-exhaust gases on ozone. Air France, Austrian Airlines and Lufthansa all agreed to install atmospheric-monitoring equipment on a total of five Airbus A340-300s, allowing the MOZAIC scientists to record ozone and water-vapour data in the air corridors over the North Atlantic.

Attempting to measure the atmosphere from the air is a lot easier than from the ground, Marenco says, and scientists hope to get a clearer picture of ozone dynamics this way.

If there is any environmental impact, other than noise, which threatens to undermine the commercial viability of air transport, aero-engine emissions are close to making it.

Sweeping changes to existing anti-pollution standards, requiring airlines to reduce the level of fossil-fuel engine emissions, threaten to cost the industry millions of dollars in engine upgrades and technical modifications.

One of the biggest challenges facing climatologists and regulators in addressing the issue of air pollution is understanding and quantifying the effects of these emissions, particularly in combination with other man-made pollutants and natural sources.

While most jet-engine emissions have been reduced to comparatively low levels over the past decade, very little is known about the behaviour of nitrogen oxides (NOx), other than their role in the formation of ozone and smog.

Given the increasing concern over the environment in general, it is only to be expected that governments and regulatory authorities in Europe and the USA would seek to police jet-engine emissions more strictly. Most agree, however, that this can only be done on the basis of informed environmental judgements.

To what extent, for example, will the predicted rise in air traffic de-rail the drive for a sustained, long-term reduction in aircraft emissions? Also, perhaps more importantly, on what evidence, specifically linking NOx with aircraft pollution, will environmental regulators base future aircraft controls?

Airbus hopes that the results of the MOZAIC programme will go a long way towards answering these questions, and that they will be adopted, by regulatory bodies as a basis for future environmental impact assessments, as well as helping manufacturers in designing, more environmentally friendly aircraft.


According to Airbus, airlines fly around 2,000 billion passenger-seat-kilometres a year, and, despite a recent lull in traffic, this figure is set to increase by around 5% a year, leading to a doubling over the next 10-15 years.

This growth, the company says, will obviously be accompanied by parallel increases in fuel consumption and emissions to the atmosphere. Aircraft already produce eight times more pollution than cars and around 22 times more than electric-powered trains, per passenger mile.

Gases such as carbon dioxide, methane, nitrous oxide and ozone in the troposphere have all increased dramatically in concentration over the last 50 years. According to Airbus, while the majority of studies have concentrated on the environmental effects of aircraft higher up in the stratosphere, ozone production by man-made pollutants such as aircraft NOx in the troposphere has received far less attention.

Over Europe alone, tropospheric ozone has increased by a factor of five since 1970, the company points out. Concentrations are increasing at a rate of between 1% and 2% per year in the northern hemisphere, and around half that in the southern hemisphere.

Measuring air composition at this level could help verify current theories about photochemical processes as a way of understanding and predicting the effects of aircraft emissions, the company says.

While ozone is one of the key elements in the troposphere, high concentrations at this level are hazardous to humans. In the stratosphere, ozone plays a key role in protecting the earth's surface from harmful radiation from the Sun. Few commercial aircraft fly that high, however, so their effects on the ozone layer are less pronounced, and are even reversed in some cases.

Ozone in the atmosphere comes from several sources, but chemical reactions involving NOx are a major concern. In the troposphere, high concentrations of nitrogen oxides result in high levels of ozone. In contrast, NOx emissions lead to ozone depletion in the stratosphere because a different set of chemical reactions predominates higher up in the atmosphere.

The task for the MOZAIC scientists is pinpointing the level of ozone production at the troposphere/stratosphere interface, which varies in height according to the time of year.

"It is well known that the tropopause altitude is lowest in winter," says Marenco. "Subsonic aircraft flying over the North Atlantic are therefore spending more time in the stratosphere during this period," he says.

"The question is, how could NOx releases in lower-stratospheric air effect the upper layers of the atmosphere, particularly if they are transported to higher altitudes by vertical mixing?"

The detection of ozone and other trace products arising from jet-exhaust emissions, such as water vapour, is difficult to measure directly. By collecting data from the troposphere, however, and then using them to validate models, researchers believe that they will be able to plot its formation.

Test instrumentation, developed jointly by CNRS and the German KFA nuclear-research centre in Julich, and installed at Aerospatiale's Toulouse plant, is designed to take measurements in the air every 4s. The equipment is in the A340 avionics bay just below the cockpit.

An onboard ozone analyser, samples pressurised air which flows through an external inlet below the flight deck. The air inlet and pumps are coated internally with Teflon to avoid destroying the ozone as it enters the aircraft.

About 200,000 measurements are taken on a typical 12h flight, recording speed and position data, as well as ozone, mainly during cruise flight. Air humidity and temperature is monitored using a water-vapour sensor, housed outside the aircraft close to the ozone probe.

The sensor consists of a tiny plastic chip, the electrical properties of which change when it absorbs humidity, says Prof Dieter Kley, the leader of the Julich team, which developed the device.

Ozone and water vapour are closely interrelated, says Kley. The lifetime of an ozone molecule depends greatly on the concentration of water vapour in the air, a correlation, which has been confirmed by the first results from the MOZAIC measurements, he adds.

Even at this early stage in the data analysis, it is clear to Kley that it is unlikely that water vapour from jet engines contributes to the greenhouse effect.

If additional ozone in the troposphere changed the total amount of trace gases at that level, however, it may have a negative effect on the troposphere's ability to clean itself, he says. "If aircraft make a significant contribution to the total amount of ozone, we should be able to determine this in the context of the MOZAIC programme."

The Cambridge researchers have developed three-dimensional general-circulation models, which include ozone as a variable. The model ozone distributions will be compared to the MOZAIC data in an attempt to validate seasonal and short-term variations.

The MOZAIC programme is expected to finish at the end of this year, and will possibly include two more A340s on transpacific routes.

Source: Flight International