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By:
Michiel van Weele, Dominik Brunner, Hennie Kelder, Ernst Meijer, Veerle Pultau, Peter van Velthoven, Wiel Wauben KNMI 1999 |
The use of aircraft in transporting people and goods is an important part of today's economy. But what is the environmental impact of all those airplanes, flying ceaselessly through the atmosphere? This contribution to the ‘Recent highlights’ aims to give a short overview of our present knowledge of the effects of aviation on the atmosphere and also of the work that has recently been done in this field in the Atmospheric Composition Division at KNMI.
Even though most aircraft fly in so-called 'flight corridors', the effects of their emissions on atmospheric composition and chemistry propagate far beyond these regions and may even affect climate. The CO2 emitted by aircraft contributes 2-3% to the total anthropogenic CO2 emissions. Studies with global chemistry transport models show that aircraft emissions of nitrogen oxides NOx (= NO + NO2) in the North Atlantic Flight Corridor (NAFC) change the concentrations of long-lived greenhouse gases such as ozone and methane throughout the Northern Hemisphere.
When the aircraft emissions are injected in the stratosphere, they may affect the ozone concentrations in the ozone layer. Paul Crutzen [1] explained in the early seventies that ‘an artificial increase of nitrogen oxides in the stratosphere ... may lead to observable changes in the atmospheric ozone level’. Some years later these results became significant in the discussion on the effects of a fleet of supersonic aircraft flying in the stratosphere.
Aircraft NOx emissions account for only about 2-3% of the total anthropogenic NOx emissions. However, aircraft emissions have a larger impact than surface emissions due to the longer residence time of the emitted species at high altitudes. Furthermore, it is known that climate is especially sensitive to changes in atmospheric composition near the tropopause, which unfortunately coincides with the main cruise altitudes at mid-latitudes (8-13 km).
A potentially large climate effect of air traffic is the formation of contrails. Contrails are ice clouds (cirrus) that form in the wake of an aircraft. It is suspected that persistent contrails may initiate the formation of longer-lived cirrus cloud fields. Even a small increase in the frequency of occurrence of cirrus clouds would exert a large climate forcing.
The CO2 emissions, the effects of NOx emissions on the abundance of other greenhouse gases by chemical transformations, and the effects of water vapour exhaust via cloud formation may result in climate changes by air traffic. Due to the fast growth of air traffic around the globe (currently about 3-4% increase of fuel use per year), it is anticipated that the magnitude of these climate effects will increase rapidly during the next century.
Research on aircraft effects in the Atmospheric Composition Division is focused on:
Meteorological and scientific support is given to experimental campaigns that investigate the effects of air traffic on atmospheric composition and chemistry. Trajectory calculations are made during the campaigns to optimise flight planning and, at a later stage, to help the interpretation of the observations. At KNMI the measurements are compared with calculations with a global chemistry transport model, named TM3 (Tracer Model, version 3). Highlights of recent campaigns are briefly summarised below and include the POLINAT-2 campaign (Pollution and Aircraft Emissions in the North Atlantic Flight Corridor), the NOXAR project (Nitrogen Oxides and Ozone along Air Routes), and the EULINOX project (European Lightning Nitrogen Oxides project). In the Netherlands aircraft measurements were also performed within the AIRFORCE project (Aircraft Influences and Radiative Forcing from Emissions) in cooperation with IMAU (Institute for Marine and Atmospheric Research Utrecht ) and RIVM (National Institute of Public Health and the Environment).
In September and October 1997 a measurement campaign took place in the North Atlantic Flight Corridor in the framework of the European POLINAT-2 project. A wide variety of cases was investigated. In cyclones over the ocean and in anti-cyclones, aircraft emissions are accumulated at flight altitudes and in some cases the aircraft emissions contribute up to 80% to the NOx concentrations (Figure 1). In cyclones over land, the surface NOx emissions can be lifted up to cruise altitudes. In these cases the surface contribution to the NOx concentrations can be as large as the aircraft contribution (both about 40%). Flights from Ireland to the Canary Islands indicated a large-scale north-south gradient in NOx concentrations with high concentrations in the north due to air traffic. The measurements also indicated that lightning is an important NOx source in the south.
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| Figure 1. NO measurements in a stagnant anti-cyclone (West of Ireland in the NAFC on 23 October 1997). The highest flight level reveals the effect of 5 days of accumulated aircraft emissions without significant influence of other NOx sources. The lowest level is below the flight corridor. The measurements compare well with TM3 model simulations. The partitioning between the source categories was modelled by using an innovative labelling technique in the TM3 global chemistry-transport model [2]. Flight altitude (pressure level) is indicated on the right y-axis. |
The impact of these NOx perturbations on the ozone levels can only be estimated by model calculations. The ozone perturbations are much smaller than the natural variability of the ozone concentrations at flight altitudes. Therefore, it is very difficult to validate the calculated effects on ozone with measurements. The calculations show that aircraft emissions contribute for about 4-8% to the ozone concentrations in the tropopause region, whereas surface and lightning emissions explain approximately 10-15% each. The remaining ozone is transported downward from the stratosphere.
In the NOXAR project it was intended to establish a representative data set of NOx and ozone measurements in the upper troposphere and lower stratosphere over large parts of the Northern Hemisphere. The project was mainly carried out by Dominik Brunner, who did his PhD at the time at ETH (Swiss Federal Institute of Technology) in Zürich, Switzerland. An automated measurement system was in operation between May 1995 and May 1996 on a Swissair B-747 airliner between Zürich and destinations in the USA and the Far East. The upper tropospheric NOx distribution was found to be strongly influenced by large-scale plumes extending about 100 to 1300 kilometres along the flight track [3]. The plumes were frequently observed downwind of thunderstorms and frontal systems and are most probably caused by upward transport of polluted air from the continental boundary layer or NOx production by lightning strokes, or both. The large-scale plumes contrast with small-scale peaks that were detected on nearly every flight, and are most likely caused by the sampling of exhaust plumes from other aircraft in the flight corridor.
The amount of NOx produced by lightning is very uncertain. However, it is generally believed that it may, globally, exceed the current NOx production by aircraft by a factor of 10. Better understanding of the lightning NOx production is therefore necessary for studying the effect of aircraft on the atmosphere. For this reason the EULINOX experiment was started. During a field campaign in the summer of 1998 measurements were performed over central Europe with a FALCON aircraft which was able to penetrate thunderstorm anvils. The data are combined with surface- based measurements and with satellite data. In the year 2000 the experimental data sets will be evaluated by searching for parameterisations of the lightning activity and the associated NOx production as depending on parameters that are available operationally on larger scales (e.g. cloud depth and structures from satellite data). In a final step a new inventory of European lightning NOx production and an assessment of the environmental implications will be provided.
Modelling of the effect of aircraft emissions on atmospheric composition and chemistry is performed from the scales of physical processes up to the global scale. In most of these studies the global chemistry-transport model TM3 is used. The transport in the TM3 model is described using the meteorological fields from the ECMWF (European Centre for Medium Range Weather Forecasts) re-analysis projects. The chemistry includes (at least) species such as ozone, nitrogen oxides (NOx), nitric acid, methane, carbon monoxide, peroxides, aldehydes and peroxy radicals (HOx). Most calculations are currently performed on a 3.75° × 5° latitude- longitude grid with 19 vertical layers. The model is regularly validated by observations, for example in experimental campaigns as described above. The model is maintained in close cooperation with IMAU.
The TM3 model played a role in the recent IPCC (Intergovernmental Panel on Climate Change) assessment of the effects of air traffic. Scenario calculations for 1992, 2015 and 2050 are presented in the IPCC Special Report ‘Aviation and the Global Atmosphere’ [4] which appears early 1999. In cooperation with RLD (Dutch Civil Aviation Authorities) a parameterised version of the TM3 model is incorporated in a software environment that is used to assess the environmental and economic impacts of air traffic policies.
The chemical mechanism of ozone formation due to aircraft NOx emissions is similar to the ozone formation in the polluted boundary layer due to surface NOx emissions (‘smog formation’). The nitrogen oxides catalyse the oxidation of CO and hydrocarbons in the atmosphere and positive feedbacks in the chemistry enhance the ozone production. Because the background levels of NOx at flight altitude are very low, aircraft NOx emissions have considerable impact. Current aircraft emissions enhance the NOx concentrations in the flight corridors by several tens of percent, and the ozone concentrations by a few percent. Increases in ozone concentrations at flight altitudes of more than 10% (relative to background levels) are calculated for the beginning of the 21-th century (Figure 2). The scenario calculations for 2015 and 2050 indicate that ozone production increases in good approximation linearly with the increasing aircraft NOx emissions.
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| Figure 2. The effect of aircraft on the monthly mean concentrations of NOx, O3, and OH (in %) as a function of latitude and height for July, based on the IPCC emissions scenarios for 2015. In 2015 it is estimated that the aircraft emissions are about 150% of the 1992 emissions. The figures show the difference between two model calculations: one calculation with aircraft emissions and one calculation leaving the aircraft emissions out. Contour levels for O3 are 2, 4, 6, 8, 10%, contour levels for NOx and OH are 1,2,5, 10, 25, 50, and 100%. |
In a process study on the chemistry in the wake of an aircraft it has been found that incomplete mixing of the nitrogen oxides hampers ozone production. The nitrogen oxides are rapidly transformed into reservoir species such as nitric acid. A parameterisation of the plume processes for global chemistry models has been developed which includes a chemical reactive Gaussian plume model. The parameterisation has been tested in the TM3 model. The study showed that neglect of the mixing processes in the plume leads to an overestimation of the ozone production by about 20%.
Calculation of the possible climate effects of aircraft emissions is still a relatively new research field and present results are still rather uncertain. Most important climate forcings are probably CO2 emissions, ozone formation, contrail formation, and water vapour exhaust in the stratosphere. Further, it has recently been hypothesised that the aircraft NOx emissions may significantly reduce the lifetime of methane, which is also an important greenhouse gas.
The estimated ozone increase due to current air traffic causes a global mean radiative forcing of 0.02 Wm-2, which is about 1% of the total forcing due to the increase in greenhouse gases since pre-industrial times and is comparable with the forcing by the aircraft CO2 emissions. A side effect of changes in the (total) ozone amount is a change in the damaging ultraviolet (UV) radiation that is incident at the Earth's surface. An increase in total ozone of 1 DU (Dobson Unit) due to aircraft NOx emissions would decrease the incident damaging UV radiation by about one percent.
Water vapour that is emitted in the dry stratosphere has a very long residence time and may enhance the prevalence of polar stratospheric clouds, which play an important role in the stratospheric ozone depletion at high latitudes. The direct radiative forcing by the water vapour increase is quite uncertain, but may be substantial. In the troposphere contrails may form. Contrails are an artificial form of cirrus clouds that can often not be distinguished from natural cirrus clouds. Average contrail cover is estimated to be 0.5% over Europe and 0.1% globally. The associated radiative forcing is again quite uncertain, but it is most likely positive, suggesting that longwave heating prevails over shortwave cooling. An annual and daily mean forcing of about 0.02 Wm-2 has been calculated for 0.1% global contrail cover. Regionally, maximum values up to 0.7 Wm-2 have been calculated.
The IPCC assessment on the effect of aircraft emissions has led to the hypothesis of an indirect climate effect of aircraft NOx emissions by a reduction of the lifetime of methane. The lifetime of methane is largely determined by the concentrations of the OH radicals. Figure 2 shows the modelled increase in the OH concentration in 2015 due to aircraft NOx emissions calculated with the TM3 model. The increase amounts globally to about 2-3% and is largest at cruise altitudes in mid-latitudes. A possible explanation for the large-scale increase in OH is the reduction of CO in the flight corridors. Due to the long lifetime of CO the effect of the reduction in CO is also observed outside of the corridors. The calculated increase in OH implies that there may be a negative forcing by aircraft due to a reduction in methane which is of the same order as the positive forcing by the increase in ozone.
For further reading the IPCC Special Report ‘Aviation and the Global Atmosphere’ [4] is recommended. The report gives a detailed overview of our present knowledge on the effects of aircraft on the atmosphere. Additional information can be found in the recent review paper [5] and the references [6]-[11].
It is expected that more observations will become available in the future, both in-situ and by remote sensing. These observations will improve our knowledge on the global distribution and variation of NOx and ozone and other species in the upper troposphere and lower stratosphere. Current discrepancies between model calculations and measurements of aircraft effects are partly ascribed to insufficient knowledge of the natural variability of NOX, HNO3 and ozone, and partly to the coarse resolution of the global models. The latter may be improved when nested models become available. A correct description of the transport in convective events is also very important. The EULINOX project aims to reduce the uncertainty in the NOx production by lightning. The estimation of climate forcing by aircraft is still very uncertain. Given the fast growth in air traffic it is anticipated that research on the effects of aircraft emissions on atmospheric composition, chemistry and climate will intensify during the next century.