R R Friedl, California Institute of Technology, Pasadena, CA, USA
Copyright 2003 Elsevier Science Ltd. All Rights Reserved.
Introduction
Human society is becoming increasingly dependent on aircraft for long-distance travel and shipping. Among transportation modes, aviation is the fastest-growing;
the current passenger growth rate is approximately 4% per year and the average growth rate since 1960 has been nearly 9% per year. The fraction of transport fuel use by aviation has risen steadily to about 13%
currently. Because of the robust growth rate, concern has been expressed over possible environmental impacts of future aircraft operation. Vigorous science and technology programs have been pursued over the last decade to define potential atmospheric impacts and identify technological strategies to reduce specific exhaust emissions. Environmental compatibility issues have also been central to efforts to develop future aircraft technologies such as high-speed (i.e., supersonic) civil transport.
Jet aircraft burning hydrocarbon-based fossil fuels transport the bulk of air passengers and freight.
Currently there are over 15 000 aircraft serving nearly 10 000 airports worldwide and burning nearly 140 Tg of fuel per year. By the year 2015, fuel burn by aviation is forecast to increase to approximately 300 Tg per year. As with other fossil fuel transportation technol-ogies, jet aircraft operation results in gaseous and particle combustion byproducts. Aircraft engines emit principally carbon dioxide (CO2) and water (H2O) with minor contributions from nitrogen oxides (NOx), sulfur oxides (SOx), unburned hydrocarbons (HC), and soot. All of these exhaust species are atmospheric pollutants. CO2 and H2O are greenhouse gases that affect the Earth’s climate directly. NOx and HC are reactive gases that affect atmospheric ozone and methane levels. Soot, SOx, HC, and H2O are aerosol and cloud precursors that affect ozone and climate.
A major difference between aviation and other transportation modes is in the atmospheric placement of the combustion exhaust products. Unlike autos and trucks, by far the greater part (485%) of aircraft exhaust is released above the planetary boundary layer 60 AIRCRAFT EMISSIONS
(42 km) and a large fraction (B70%) of it is released in the upper troposphere (UT) and lower stratosphere (LS) between 9 and 13 km. Consequently, the major polluting effects of aircraft are expected to occur in the UT/LS region of the atmosphere.
The dynamics of the UT/LS region differ from those of the boundary layer in that there is less vertical mixing and less diurnal variation in wind direction.
Because of these differences, pollutants emitted into the UT/LS reside there longer and can spread over considerable longitudinal and, in some cases, latitudi-nal distances. Although aircraft exhaust is released in geographically narrow flight routes and corridors, its injection into the UT/LS means that the polluting effects of aircraft will be felt on regional and, perhaps, global scales. The longer residence times also enable some pollutants, such as NOx, to spend extended times cycling through catalytic chemical reaction sets that create or destroy ozone. Because of such enhanced catalytic chemical cycling in the UT/LS, the impact of a given amount of aircraft emissions on atmospheric ozone and climate may be much greater than the same amount of emissions from ground transportation sources.
There are also important dynamical and chemical differences between the UT and LS regions that com-plicate any analysis of aircraft effects. For example, the lifetime of ozone and the chemical mechanisms controlling its concentration are sensitive functions of altitude in the vicinity of the tropopause. Because of this altitude dependence, the sign of the ozone response to injections of NOx shifts from positive (net ozone formation) to negative (net ozone destruc-tion) at altitudes slightly above the tropopause (i.e., the transition between the stratosphere and tropo-sphere). The partitioning of aircraft exhaust between the UT and LS is difficult to define (estimates differ by factors of two) because of the high variability and latitudinal dependence of the tropopause height.
Aircraft Exhaust Products
Jet engines on modern aircraft are composed of three essential elements: compressors that increase the pressure and temperature of the entering air, combus-tors that mix and burn fuel with incoming air, and turbines that convert the hot gas energy, through compressor activity, to bypass airflow that propels the aircraft. The fuel-to-air ratio in modern combustors is approximately 1 : 9, hence large quantities of ambient air are processed in aircraft engines. Jet fuel is composed predominately of high-weight (C12–C15) alkanes, with substantially smaller quantities of alkenes and aromatics present. An important trace
species in the fuel is sulfur, which can represent up to 0.3% (by weight) of the fuel content.
Combustion of the fuel hydrocarbons to produce CO2 and H2O is nearly complete (499.5%) in commercial aircraft engines. In addition, the fuel sulfur is converted to sulfur dioxide and sulfuric acid, although the precise mechanism for this process remains uncertain. The small fraction of incompletely combusted fuel hydrocarbons give rise to CO and various smaller gaseous hydrocarbons (HC) such as ethene, ethine, and formaldehyde. Under fuel-rich combustor conditions, breakdown of the fuel hydro-carbons leads to formation of soot particulates com-posed primarily of carbonaceous material. The rate-limiting process in soot formation appears to involve the oxidation of C2species such as acetylene (C2H2).
Decomposition of ambient nitrogen and oxygen also occurs in the high-temperature portions of the com-bustor, giving rise to the important atmospheric pollutants nitric oxide (NO) and nitrogen dioxide (NO2) (i.e., NOx) (Table 1).
Aircraft Technology Considerations
Aircraft engine and airframe technologies have under-gone dramatic improvements over the last 30 years.
One result of these improvements has been a 70%
reduction in fuel burned per passenger seat from early to current jets. Gains in fuel efficiency are of benefit both economically and in environmental terms by reducing fuel costs and uniformly lowering CO2, H2O, and SOx emissions. These gains have derived primarily from increasing gas temperatures and pres-sures inside the engines. Without concomitant changes in engine design, increasing engine temperature leads to increasing NOxemissions.
Concern over urban pollution has led to increas-ingly stringent standards being adopted by the Inter-national Civil Aviation Organization (ICAO) regarding emissions of smoke, CO, HC, and NOx. Aircraft smoke refers to visible particulates in the aircraft plume and presumably includes the large diameter (41mm) part of the soot population. The
Table 1 Approximate emission index levels for cruise level operation of current commercial jet aircraft
Species Emission index (g kg!1)
CO2 3160
H2O 1240
CO 2
HC 1
NOx(as NO2) 12
SOx(as SO2) 0.8
Soot 0.04
AIRCRAFT EMISSIONS 61
ICAO standards have both reflected and motivated improvements in engine design and manufacture. How-ever, because the service lifetime of an individual aircraft is between 25 and 40 years, the current fleet consists of a combination of older and newer technologies.
Measurement of aircraft cruise emissions is an important facet of assessing impacts and documenting technological advances. These difficult measurements are made either in altitude simulation test cells or by in-flight measurements utilizing target and chase aircraft.
Aircraft Operational Considerations
Airline traffic patterns are highly inhomogeneous, with the bulk of current traffic located inside well-defined ‘flight corridors’ in the Northern Hemisphere (Figure 1). The chemical lifetimes of aircraft exhaust products such as NOx, soot, and sulfate injected in the UT/LS are comparable to atmospheric mixing times.
Consequently, a number of the aircraft chemical
perturbations are expected to be localized in regions around the flight corridors. A great deal of work has been done to compile accurate inventories of aircraft emissions. These efforts have involved development of aircraft movement databases based on simplifying assumptions about the airframe–engine combinations used and the paths flown between various city pairs.
Combining these movement databases with informa-tion on individual aircraft emission rates enables construction of global emissions inventories. For atmospheric modeling purposes, the aircraft emission databases are divided into spatial bins that are 11 longitude"11latitude"1 km altitude.
Impacts on Carbon Dioxide and Water
Although they are the most prevalent exhaust prod-ucts, emissions of CO2and H2O from aircraft repre-sent relatively small sources of these species compared
25 20 15 10 5 0
−90 −80−70−60−50−40−30−20−10 0 10 20 30 40 50 60 70 80 90 Latitude
Altitude (km)Latitude
0 20 40
NOx (thousand kg per day)
NOx (thousand kg per day) 90
60 30
−30
−60
−90
−180 −150 −120 −90 −60 −30 0
0 30 60 90 120 150 180
Longitude
0.00 0.25 0.50
(A)
(B)
Figure 1 Calculated NOxemissions for all aircraft traffic in May 1992 as a function of altitude and latitude, summed over longitude (A), and as a function of latitude and longitude summed over altitude (B). Values greater than the range maximum are plotted as black. (From NASA reference publication 1400.)
62 AIRCRAFT EMISSIONS
with the many other large natural and anthropogenic sources. Given past and current emission rates, aircraft are responsible for increasing atmospheric CO2levels by approximately 1 ppmv or 2% over the last 50 years.
Because CO2is very long-lived in the atmosphere and is well mixed, it is impossible to distinguish the CO2 emitted from aircraft from any other source.
Perturbations due to aircraft H2O emissions are far less than 1% globally. These small perturbations are impossible to detect on the large scale because water vapor has a short (days to weeks) tropospheric residence time and its ambient concentrations are highly variable. At very small spatial scales, H2O perturbations from aircraft are substantial and can lead to contrail and cirrus cloud formation. These effects have important climate consequences that will be discussed below.
Impacts on Ozone and Methane
Ozone chemistry throughout the stratosphere and troposphere is driven by solar-initiated free radical reactions. Aircraft emit a number of species (i.e., NOx, SOx, H2O, CO, and soot) that participate in ozone-controlling reactions of free radicals and free radical precursors. The relationship between aircraft exhaust products and ozone is complex and depends on the balance between a number of ozone-forming and -depleting chemical processes. These processes are summarized in the next two sections along with observational evidence that addresses the magnitude of the aircraft effect on ozone.
Atmospheric Chemistry
Nitrogen oxides in the UT/LS participate in both ozone-forming and ozone-depleting reaction cycles.
The balance between these processes, and their response to changes in ambient NOx levels, are sensitive functions of altitude. In the UT region, the primary influence of NOx is on the production of ozone from CO and CH4 oxidation. The CO cycle involves the following reactions:
OHþCO!HþCO2 ½I$
HþO2þM!HO2þM ½II$
HO2þNO!NO2þOH ½III$
NO2þSunlight!NOþO ½IV$
OþO2þM!O3þM ½V$
Net: COþ2O2!CO2þO3
(where M represents a gaseous third body such as N2 or O2). An analogous mechanism, which includes the reaction between NO and CH3O2, exists for CH4 oxidation.
The overall rate of ozone production from CO and CH4 oxidation decreases generally with height be-cause of decreasing ambient concentrations of CO and CH4. However, as can be seen from the above reaction sequence, the production rate depends also on the ambient concentrations of NO and OH. For example, increasing OH and NO will increase the rates of reactions [I] and [III], respectively, thereby increasing the ozone production rate. At high enough concentra-tions of NOx(4500 pptv) the ozone production rate begins to decrease owing to the increasing importance of NO2reactions that remove HOxspecies, i.e.,
OHþNO2þM!HONO2þM ½VI$
HO2þNO2þM!HO2NO2þM ½VII$
In the LS region, the primary influence of NOx is on destruction of ozone by the following radical-catalyzed processes:
OþXO!XþO2 ½VIII$
XþO3!XOþO2 ½IX$
Net:OþO3!2O2
(where X5NO, Cl and OH). Increases in ambient NOx due to aircraft emissions will enhance ozone destruction for the case where X5NO but will decrease ozone destruction for the case X5Cl by removing ClO through reaction [X]:
ClOþNO2þM!ClONO2þM ½X$
The exact balance between these contrary effects depends on the background concentrations of NOx
and ClOx. Throughout most of the year, the net effect of increasing LS NOxlevels through aircraft emissions will be to increase ozone concentrations. An exception is at high latitudes in summer, when NOx levels are high. In that case, addition of NOx will decrease ozone.
Increasing NOx levels due to aircraft exert an indirect effect on atmospheric CH4 concentrations.
This effect is initiated by the formation of OH radicals in reaction [III]. The increased levels of OH in the air traffic corridors lead to decreases in carbon monoxide (CO) through reaction [I]. Because CO lifetimes are longer than NOxlifetimes, the region of decreased CO concentrations spreads out from air traffic corridors to a much greater extent than do the aircraft NOx
emissions. The CO perturbation spreads all the way to AIRCRAFT EMISSIONS 63
tropical and subtropical regions where much of the global oxidation of CH4 takes place through its reaction with OH:
OHþCH4!CH3þH2O ½XI$
As CO levels are lowered in the tropics, OH levels are raised correspondingly. The higher levels of OH serve to lower CH4concentrations, which, in turn, lead to a further increase in OH. As a result of the complex interplay (i.e., atmospheric feedback cycle) between NOx, OH, CO, and CH4, an increase in NOxwill lead to an amplified decrease in CH4. The amplification factor is approximately 1.5.
Aircraft emissions of SOx, H2O and soot also effect atmospheric ozone concentrations by serving as aerosol precursors. In the UT, sulfate- and water-ice-containing aerosols promote ozone decreases by acting as surfaces for heterogeneous removal of the ozone precursors NOxand HOx. A major identified heterogeneous reaction involves conversion of the temporary NOx reservoir species nitrogen pentoxide (N2O5) into the longer-term reservoir nitric acid (HNO3).
N2O5þH2SO4=H2O!2 HNO3 ½XII$
HNO3, along with a number of other nitrogen and hydrogen acids and peroxides (e.g., HNO4and H2O2) are absorbed onto sulfate and water-ice. The absorbed species can be removed from the UT by sedimentation.
In the LS, sulfate- and water-ice-containing aerosol particles not only remove HOxand NOxspecies but also liberate ozone-destroying ClOxby heterogeneous reactions such as
ClONO2þH2O!HOClþHNO3 ½XIII$
The net effect of the heterogeneous processes is to decrease ozone in the LS and UT. However, the effect of the aircraft-derived aerosols on LS/UTozone offsets only partially the effect of the NOxemissions.
Much less is known about the effect of soot particulates on ozone. Ozone is observed to react directly on laboratory soot surfaces, but the reaction slows as the surface is modified. Heterogeneous reactions of NOx and nitrogen reservoir species also occur on soot surfaces – in some cases the reactions lead to more reactive species, in others to less reactive ones. Consequently, the effect of aircraft soot on atmospheric ozone concentrations is poorly deter-mined at present.
According to the current scientific understanding, the overall effect of aircraft emissions in the UT/LS is to increase ozone levels. Model calculations indicate that aircraft have increased ozone by about 6% in heavy traffic areas, with an associated 0.4% increase
in the total ozone column. In terms of climate effects, the radiative forcing changes due to increased ozone appear to be largely offset by the predicted decreases in methane. Considerable uncertainty is attached to these calculations, however (seeFigure 2).
Observing Ozone Impacts
Dense air traffic in Northern Hemisphere flight corri-dors will give rise to distinct geographical perturba-tions of NOx, aerosols and ozone under two condi-tions. First, large-scale dispersion of the exhaust must be slower than the chemistry that removes and/or links these emissions to ozone. Second, the strength of the aircraft emissions must be significant relative to other natural and anthropogenic sources of NOx and aerosols. The total NOxemission from current global aviation is approximately 0.5 Tg per year, of which roughly 60% is released into the upper troposphere and 15% is released into the lower stratosphere.
The major source of NOxin the lower stratosphere is chemical oxidationin situof nitrous oxide (N2O):
O3þsunlight!Oð1DÞ þO2 ½XIV$
Oð1DÞ þN2O!2 NO ½XV$
The global production rate of NO from N2O (B12 Tg per year) far exceeds that from current subsonic aircraft emissions in the lower stratosphere. Hence there is no expectation, nor observational evidence, that current aircraft are significantly perturbing stratospheric NOxlevels.
In the upper troposphere, the major non-aircraft sources of NOx include fossil fuel combustion
O3 NOx
CO CH4
Aircraft NOx
Aircraft soot
Aircraft H2O
Aircraft SOx and H2O
Figure 2 Influence of aircraft emissions on chemical balance in the UT/LS region. Atmospheric chemical reactions couple together O3, CO, NOx, and CH4. Among aircraft emissions, NOx is calculated to have the greatest effect on the coupled species, acting to increase ambient NOxand O3levels and decrease CO and CH4.
64 AIRCRAFT EMISSIONS
(autos, trucks, etc.), biomass burning, soil emissions, lightning, and N2O oxidation. Of these, only lightning deposits NOx directly into the UT. The fractions of NOx transported into the UT from sources at the Earth’s surface or in the stratosphere are small, occurring only during convective events, such as frontal activity or thunderstorms or during strato-sphere–troposphere exchange events triggered by meteorological features such as extratropical cy-clones. Source strength estimates for the various NOx sources are listed in Table 2. As shown in the table, aircraft emissions into the UT are of comparable strength to other sources and contribute a significant fraction of UT NOx.
Chemical sampling of the UT in and around traffic corridors has revealed each individual aircraft per-turbs ambient NOxlevels substantially for distances of several kilometers behind it. At larger spatial scales, aircraft signatures have not been discerned, owing to the high variability of background NOx. Likewise, there have been no identifiable spatial patterns in ozone concentrations that unambiguously point to production by aircraft NOx. Long-term ozone trend observations at specific measuring stations (e.g., Hohenpeissenburg, Germany, and Wallops Island, USA), do not correlate with the growth rate of air traffic from 1970 to the present, indicating that aircraft emissions are not a major factor in the observed upper-tropospheric trends.
Impacts on Clouds
Trails of ice particles – contrails – are the most readily identifiable exhaust signatures of aircraft (see Con-trails). Contrails often form, even under clear-sky conditions, because aircraft H2O emissions raise the relative humidity of the air near the exhaust plume above 100%. Water vapor in the supersaturated air subsequently condenses on aircraft-derived soot and sulfate nuclei and freezes to form ice. If the
surround-ing air is very dry and/or warm, contrails may be short-lived or may not form at all. In either case, the emitted soot and sulfate nuclei will remain in the atmosphere for days and weeks and possibly promote natural ice (cirrus) cloud formation in locations far from the initial aircraft plume. These same nuclei, upon contact with cirrus clouds, may change properties of the cloud particles such as size distribution, number density, and chemical composition.
Like other naturally occurring clouds, contrails and aircraft-induced (or modified) clouds impact the Earth’s climate by affecting the radiation balance.
For typical particle properties, cirrus clouds trap surface outgoing long-wave radiation more effectively than they reflect solar incoming short-wave radiation.
As a result, cirrus clouds tend to warm the climate.
However, the magnitude and even the sign of a cloud’s radiative effect on climate is a sensitive function of cloud particle size and shape as well as altitude and geographical location.
Cloud Formation Processes
Clouds or contrails can form when air moisture becomes supersaturated with respect to ice. The ice formation process takes place by one of several mechanisms. At higher supersaturations, low-volatil-ity gas phase species will cluster together to form liquid particles. These liquids can subsequently freeze in a process known as homogeneous freezing if the air is cooled by upward dynamical motion.
At lower supersaturations, the freezing process may be aided by the presence of a solid particle surface in a process termed heterogeneous freezing. The liquid and solid particles that readily promote freezing and ice crystal growth are typically in the 0.05–1mm diameter size range; they are referred to as cloud condensation nuclei (CCN) and ice nuclei (IN), respectively. Aircraft emissions may enhance the frequency of these freezing events by increasing the abundances of CCN and IN.
Aircraft soot emissions have attracted attention as a possible source of IN in the UT. The median size of a fresh aircraft soot particulate is approxi-mately 0.02mm. In order for a soot particulate to become an IN it must be activated (i.e., become more hydrophilic) by reaction with suitable species.
Lab studies have shown that acids such as H2SO4
induce this activation but that others such as HNO3do not. Hydrated samples of soot have been obtained from non-sulfur-containing flames, indicating the presence of other, as yet unidentified, activating species.
Aircraft emissions of condensable gases such as sulfur oxides and oxygenated hydrocarbons can
Table 2 Present-day sources of NOxin the troposphere and their approximate strengths
Source Emission rate
(Tg yr!1)
Emission rate (Tg yr!1)
Total 9–13 km altitude band
Aviation 0.5 0.3
Fossil fuel combustion 22 0.7
Biomass burning 8 0.2
Soil emissions 7 0.2
Lightning 5 1
N2O oxidation 12 0.6
AIRCRAFT EMISSIONS 65
contribute to CCN formation. In an aircraft plume, large numbers (B1016particles per kilogram of fuel) of small particles (o0.01mm radius) are formed from nucleation of sulfuric acid and water. The formation and subsequent growth of these particles may be accelerated by chemi-ions that are emitted into the plume following their production in high-temperature reactions occurring in the combustor. As the plume expands and is diluted by entrainment of ambient air, the small plume particles may continue to grow by uptake of additional gaseous species or they may be scavenged by larger ambient particles. The competi-tion between these two processes depends on a number of environmental variables such as air temperature, relative humidity, and background aerosol concentra-tion. Under low background aerosol conditions, such as exist during wintertime, a significant number of plume particles are expected to survive long enough to grow to CCN size (seeFigure 3).
Addition of aircraft-derived CCN and IN to the UT will increase cirrus cloud occurrence in areas where the air is supersaturated with respect to ice but crystal growth is limited by a lack of sufficient numbers of nuclei. Relative humidity measurements taken in the UT reveal that ice supersaturation occurs in more than 10% of the clear-sky cases examined. Cloud growth in these regions should be particularly susceptible to aircraft IN and CCN. In areas of developing cirrus
clouds, aircraft-derived CCN and IN may influence the properties of the cloud particles in one of several ways. If the aircraft particles are larger and function as more active growth nuclei than ambient particles, they may compete effectively for the avail-able water vapor and induce growth of larger ice crystals at the expense of crystal number density. If, on the other hand, the aircraft particles increase the number of CCN and IN, but do not change the overall rate of crystal growth, then increases in the crystal number density are expected, with concomitant decreases in average crystal size. The radiative prop-erties of the resulting clouds will be altered, but the magnitudes and characteristics of these modifications are uncertain.
Observing Cloud Impacts
Cirrus cloud coverage, as documented by surface and satellite observations, has been increasing over a number of regions in the last two decades, with the largest increases observed over regions of heavy air traffic in the United States and the North Atlantic.
Growth of cloud cover in air corridor regions has been approximately 1–2% per decade greater than in other areas; attributing this growth rate to aircraft impacts implies that there has been an overall 5% increase in traffic route cloud cover during the last 30 years of air
1017
1016
1015
1014
1013
100 101 102
Apparent emission index (particles per kg fuel)
Plume age (h) d>5 nm
d>25 nm
d>50 nm
Figure 3 Calculated time evolution of the ‘apparent’ emission indices of aircraft-generated particles for various size thresholds. Solid and dashed lines are for low and high ambient aerosol conditions, respectively. Appreciable concentrations of CCN size particles (450 nm) are predicted only for low ambient aerosol conditions. (Adapted with permission from Yu F and Turco RP (1999)Geophysical Research Letters26: 1703–1706. Washington, DC: American Geophysical Union.)
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