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Report on the European Workshop“Aviation, Aerosols, Contrails and Cirrus Clouds” (A2C3),

Seeheim near Frankfurt/Main, July 10-12, 2000

Aircraft engine emissions cause the formation of small aerosol particles and contrails in theupper troposphere and lower stratosphere, which - together with other aerosols - mayinfluence cirrus cloud formation (Figure 1, 2). The special report by the IntergovernmentalPanel on Climate Change (IPCC) on "Aviation and the Global Atmosphere" (IPCC, 1999)concluded that the largest uncertainties limiting our ability to project aviation impacts onclimate and ozone comes from the influence of contrails and aerosols on cloud changes(Figure 3). Under guidance of the World Meteorological Organisation (WMO), the ”ScientificAssessment of Ozone Depletion: 1998” was prepared (WMO, 1999) which notes incompleteunderstanding of the influence of aerosol and cloud particles on ozone in the lowerstratosphere and the upper troposphere. The Second (IPCC, 1996) and the forthcoming ThirdAssessment Report of IPCC on ”Climate Change” note large uncertainties in quantifying thedirect and indirect effects of aerosols and changes in cloudiness on climate. At present, less isknown about the impact of aerosols and contrail-induced cirrus changes than about the impactof gaseous emissions. Many of the emissions of aircraft engines may be reduced with moreadvanced technology. But what can be done against contrails and cirrus changes? Thereforethis workshop concentrate on the aerosol and contrail issues.

The three-days European workshop “Aviation, Aerosols, Contrails and Cirrus Clouds” (A2C3)took place in Seeheim, near Frankfurt/Main, Germany, July 10-12, 2000. The workshopstarted with two days with four scientific sessions followed by one day with working groupmeetings and discussions. 26 invited oral papers and 55 poster contributions were presentedand discussed by the 96 participants from Europe, Russia, and the USA, with representativesfrom atmospheric sciences, combustion sciences, industry, and various agencies. The firstsession provided the frame of reference for the following discussions by reviewing results ofprevious assessments and by presenting results from scientific work in related fields. Thefollowing three sessions were devoted to the specific workshop topics: 1) particle formationand properties; 2) contrails and cloud changes; and 3) parameters effecting aviation impact.The third day was dedicated to discussions at the posters, working groups and a closing panelsession. The proceedings (Schumann and Amanatidis, 2000) are in press and will appear inthe series of Air Pollution Research Reports of the European Commission. Main results of thepapers presented in the proceedings and in posters during the workshop are summarisedbriefly below.

Black-Carbon and Nonvolatile Soot Particles

Aircraft soot particles (composed of black carbon and other non-volatile materilas) may act ascondensation and freezing nucleus for ice particle formation.

Soot emissions have been measured in altitude test facilities (A. Döpelheuer and C. Wahl, D.E. Hagen et al.) and in exhaust plumes and contrails behind cruising aircraft (A. Petzold etal.). Such measurements have been used to validate the soot calculation procedures which arenecessary to set up a 3d global aircraft-generated soot inventory (Figure 4). Aircraft emit onaverage 0.04 g black carbon per kg of burnt fuel; 40% of all soot gets emitted above 10 kmaltitude, mainly over North America and Europe (A. Döpelheuer and C. Wahl). The numberand mass of soot particles emitted per kg of burnt fuel by modern engines is lower (typically0.01 g/kg, 3×1014 particles/kg fuel) than in older ones (typically 0.1 g/kg and 2×1015/kg). The

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soot emission mass and number is independent of fuel sulphur content. Primary soot particlesare typically 25 to 60 nm large in diameter, soot agglomerates reach larger than 150 nm insize (A. Petzold et al.).

Aircraft engine soot may not be as hydrophobic as assumed in the past. High initial wateruptake has been measured on aviation soot. Such soot particles acquire a substantial fractionof a water monolayer already in the very young exhaust plume, which would explain contrailformation independent of fuel sulphur content (O.Popovitcheva et al.).

The activation of soot particles from a graphite spark gap generator with mobility equivalentdiameters of 50 to 200 nm has been investigated in a new large aerosol chamber (AIDA inKarlsruhe) at 255 and 239 K. For 255 K, the soot was observed to become activated to formdroplets when reaching liquid saturation; for 239 K, the droplets freeze and evaporate quicklywhen the humidity drops below ice saturation (A. Nink, O. Möhler et al.).

The main processes leading to soot particles are now known and can be reasonablyapproximated in models at least for laminar flame situations. However, a reliable techniquefor quantitative calculation of soot emissions from engine combustors has still to bedeveloped. The most important contribution to soot comes from surface growth. Particleinterception and coagulation dominate the particle dynamics. Large amounts of soot forms inthe early combustion process but most of it gets oxidised before leaving the combustionchamber. Soot prediction within aero-gas turbine combustion chambers require soot modelscoupled to three-dimensional (3d) fluid dynamics and radiation transfer calculations (H.Bockhorn, H. Brocklehurst, and D. Hu, P. Frank et al.).

Chemiions (CI) and Volatile particles

Large numbers (about 2.4×1017 per kg of burnt fuel) of volatile particles form behind aircraftat cruise. The volatile particles may contribute to contrail and cirrus particle formation (Figure5). The size spectrum of particles from 3 nm to 100 nm has been measured with a battery ofcondensation nucleus counters of different size sensitivities; size spectra of larger particles aremeasured with optical probes (A. Petzold et al.; F. Schröder). Smaller particles of differentmasses are detectable as chemiions (CI).

Engines emit large numbers of positive and negative chemiions. Chemiions are produced bychemistry in the combustors. They may enhance the growth of volatile particles, and henceinfluence the formation of ice particles. Chemiions may have large sizes, possibly more than8500 atomic mass units, and hence may reach up to the smallest condensation nucleimeasurable today (about 3 nm) (F. Arnold et al., K.-H. Wohlfrom et al.). Very massive CIwere observed even for nearly sulphur free fuels. However the number of negative CI growswith fuel sulphur content (FSC). Negative CI are larger in mass than positive CI (K.-H.Wohlfrom et al.). Negative CI contain HSO4

-H2SO4 cluster ions. Positive CI have beenidentified as at least partly oxygenated hydrocarbons (A. Kiendler et al.). Models explain themeasured number of chemiions in hot aircraft exhaust plumes by ion-ion-recombination andinitial ion concentrations of the order of 108 cm-3 (A. Sorokin et al.; F. Arnold et al.).

The concentration of volatile particles strongly increases as diameter decreases toward thesizes of the large molecular clusters. The number of volatile particles larger than 3 nmdetectable in a young (order 0.5 to 10 s) plume increases with FSC for FSC in the range from60 to 1000 µg/g (Figure 6). Particle formation does not change significantly when the FSCgets reduced below 60 µg/g, apparently because of condensable hydrocarbons contributing to

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the formation of volatile aerosols in the exhaust plume (Petzold et al.). The variation in thenumber of measured volatile particles behind several aircraft can be explained with a model interms of variations in plume age, size detection limit of the particle counters, FSC, the amountof condensable organic material, and, most important, with the number of CI emitted (B.Kärcher et al.).

Sulphuric acid

Part of the sulphur contained in the aircraft fuel may get converted to sulphuric acid whichaffects the formation of volatile particles and the hydration properties of soot. The conversionfraction of fuel sulphur to S(VI) (SO3, H2SO4) depends on pressure and temperature atcombustor exit and on the residence times. The conversion is largest at a temperature of about1000 K and high pressures. Cooling of turbine blades in the high pressure turbine enhancesthe conversion (R. Miake-Lye et al.). In flight measurements indicate that around 3 % of FSCgets converted to sulphuric acid, less than believed a few years ago (F. Arnold et al., K.-H.Wohlfrom et al.).

Nitric acid and other aerosol consitutents

H2O-H2SO4 aerosols take up HNO3 only at small mole fractions (<1%) and only fortemperatures below 222 K and high ambient humidity (G. Gleitsmann and R. Zellner). Theuptake coefficient of HNO3 (order 10-3 mg-1) and H2O (5×10-4 mg-1) on soot have beenmeasured. Uptake of HNO3 seems to be of little importance for volatile aerosols, sootactivation, and contrail formation in aircraft exhaust plumes (T. Keil et al.). The uptake ofNOy on cirrus cloud particles was inferred from measurements of gas- and particle phase NOy,H2O, HNO3, and aerosols for temperatures near 200 K. About 2 to 10% of the NOy getsremoved by a coverage of ice particles with about 0.1 monolayers of NOy (H. Schlager et al.).A thermodynamic model of hydrogen-ammonium-sulphate-nitrate-water aerosols has beenprovided for upper tropospheric temperatures (S. Clegg and P. Brimblecombe). The non-reactive uptake of HCl on frozen film ice surfaces was determined for temperatures from 205– 235 K. The uptake coefficient decreases substantially when T increases from 205 to 215 K(R. G. Hynes et al.).

Global aerosol distribution

A global stratospheric and tropospheric sulphate aerosol model has been developed, comparedto observations from the SAGE satellite instrument in terms of extinction, optical depth, massand surface area density, and applied to study future changes of stratospheric aerosols inrelation to projected changes of anthropogenic sulphur emissions. Subsonic aircraft werefound to contribute about 3% to the total aerosols mass in the stratosphere and 20 % of thesurface area density at 14 km altitude north of 45°N (G. Pitari and E. Mancini).

Contrails

Contrails trigger cirrus cloud formation. In air masses with ice-supersaturation of a fewpercent no cirrus cloud would have formed otherwise, see Figure 2 (U. Schumann). Aerosolsfrom aircraft may influence cirrus formation, possibly long after the time of emission. Thecrucial contrail parameters are their cover and optical properties. Contrails may warm theEarth surface in particular over warm and bright surfaces. During day, the enhanced Earth’salbedo caused by contrails may cause a cooling. Around 17% of the Earth is covered with airmasses which are ice-supersaturated and cold enough so that persistent contrails form when

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aircraft fly in these region. The area coverage by such regions susceptible to contrails is 21%when aircraft would burn liquid hydrogen instead of kerosene (K. Gierens and S. Marquart).Several recent studies show that supersaturation with respect to ice is very common in theupper troposphere (K. Gierens, E. Jensen, J. Ovarlez et al.), see Figure 7. In the Arctic winter,supersaturation occurs even in the lowermost stratosphere due to the extremely lowtemperatures in this region. Also the tropical upper troposphere is often ice supersaturated.Cirrus clouds rarely extend more than 1 km above the tropopause (E. Jensen et al.). Large icesupersaturation also occurs within cirrus clouds (J. Ovarlez et al.).

Short lived contrails of 2-engined aircraft were observed to evaporate earlier than contrailsfrom 4-engines aircraft, consistent with model calulations (K. Gierens and R. Sussmann).

The microphysical properties of contrails and cirrus clouds have been measured during morethan 20 airborne missions over Central Europe at temperatures below –50°C. Contrailscontain large concentrations of nearly spherical ice crystals, initially up to 105 cm-3, laterdiluted to typically a few 100 cm-3, while the mean diameter range from 1-10 µm. Youngcirrus clouds contain mostly regularly shaped ice crystals in the size range of 10-20 µm attypical concentrations of 2-5 cm-3. Contrail growth is affected only weakly by pre-existingcirrus clouds (F. Schröder).

In contrails and cirrus clouds, the scattering phase function has been measured in situ with aPolar Nephelometer. The scattering by cold ice clouds differs strongly from that of sphericalparticles, implying 40% higher albedo, i.e. a stronger shortwave cooling by contrails thancomputed for spherical particles (F. Auriol et al.).

Particle measurement systems may be strongly disturbed by electromagnetic interferences.Older probes with slower electronics may have difficulties in registering crystal size andnumber density accurately (E. Raschke et al.).

Contrails have been estimated to cover currently about 0.1% of the global sky (K. Gierens andS. Marquart). Over the USA, the mean contrail coverage has been determined from 4 monthsof satellite data to be 1.8%. For 55 contrails over different backgrounds the mean opticaldepth and the effective particle diameter have been determined and found to be 0.46 and 36µm, respectively, see Figure 10 (P. Minnis et al.). Over central Europe, the annual daytimeaverage contrail has been determined from 2 years of NOAA-14 satellite data with anautomated contrail detection algorithm. The annual day time average cover is 0.7% (Figure 9)with a strong annual cycle (1% in winter, 0.4% in summer). The mean visible optical depth inthis region is determined from the same data to be 0.11. During night the cover is 3 timesmaller. The resultant radiative forcing is found to be one order of magnitude below theresults of previous assessments. (R. Meyer et al.). Data from traffic management systemsindicate strong diurnal variations in air traffic, which is important in assessing the meanradiative forcing by contrails; contrails warm during night but may cool during day time (S.Baughcum et al.).

A case study of a contrail has been reported which has been measured from ground with aLidar, and in situ with particle size spectrometers. Pyranometers and pyrgeometers were usedto measure the radiative shortwave and longwave vertical fluxes near the contrail. The solaroptical depth of the contrail varies from 0.05 to 0.23. The measured radiation fluxes indicatesmaller radiative forcing than what is computed for the measured size spectrum assumingspherical ice particles (P. Wendling et al.).

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Gas phase chemistry and heterogeneous reactions on contrail and cirrus particles

Aerosols and ice particles contribute to changes in ozone and other air constituents. NOx

emissions by subsonic aircraft lead to net ozone production in the tropopause region.However, within contrails, halogen components may get activated and NOx and hydroxylradicals may get reduced, causing less ozone production, and ozone depletion is possible atleast locally within contrails. In the lowermost stratosphere, ozone destruction is caused byheterogeneous chlorine activation on contrail ice particles, whereas in the upper troposphereozone production is compensated by heterogeneous reduction of OH radicals (S. Meilinger etal.). A chemistry box model study shows a large sensitivity of upper tropospheric chemistryto heterogeneous processes in cirrus clouds. Ice particle sedimentation may reduce uppertropospheric HNO3, NOx, and OH, resulting in significant ozone loss (A. Meier and J.Hendricks). A 3d chemical transport model (CTM) has been used to investigate the role ofsubvisible ice clouds in chemical ozone loss near the tropopause. The calculated zonal meanozone loss reaches a maximum in the mid- and high latitude tropopause region of about 1% inwinter and 3% in summer (B. Bregman et al.).

Seven years of ozone, humidity and temperature profile observations at 21°S, 55°E reveal asystematic diminution of ozone abundance in tropical cirrus clouds, possibly due to ice cloudinduced denoxification (S. Roumeau et al).

Only a few attempts have been made to model the formation and microphysics of cirrusclouds (W. Wobrock and A. Flossmann; T. Chourlaton et al.). From measurements andmodels, it was found that aerosols which passed through cirrus behave differently fromaerosol that was not yet contained in frozen particles (T. Choularton). Large aerosol may actas ice nuclei by homogeneous freezing, small aerosols may enhance freezing by acting asheterogeneous ice nucleus (W. Wobrock). Bulk and size-resolved cirrus process-models showlarge differences even when applied to idealised cases with prescribed ambient conditions;particle sedimentation rate is one of the crucial model parameters (GEWEX-cirrus modellinginitiative, poster of D. O’C. Starr).

Volatile aerosol freezes for temperatures, T, in the range 233.5 K < T < 273.15 K, when theambient air reaches liquid saturation. Below 233 K, homogeneous ice nucleation occurs inparticles of 0.2 µm radius within one minute when the relative humidity over ice (RHI)exceeds RHI = 238.7 – 0.398 T (RHI in %, T in K; Koop et al.).

The ECHAM4 global circulation model (GCM) has been used to study the sensitivity of iceclouds to different assumptions about ice nuclei. Ice nuclei are assumed to be either just afunction of temperature and altitude, insoluble carbonaceous aerosols or dust aerosols. Thedifferent models cause variations in the longwave cloud forcing by 4 W m-2 (U. Lohmann). Afirst GCM study with parameterised contrails was presented, showing variations in the coverand optical depth of contrails with latitude, altitude, and season; the optical depth of contrailsappears to be larger over the USA than over Europe because of different temperature andhumidity conditions (M. Ponater et al.).

Lidar observations in Southern France (44°N, 6°E) have been used to determine theoccurrence frequency and altitude range of thin cirrus at various optical depths. Subvisiblecirrus (532 nm optical depth < 0.03) constitute a substantial portion (40%) of the calculatedmean optical depth. Fewer than 5% of the cirrus occurrences are (at most <1 km) above thetropopause (L. Goldfarb et al.).

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For the first time, in situ observations of cirrus clouds have been performed in the Southernhemisphere during the Interhemispheric Differences in Cirrus Properties From AnthropogenicEmissions (INCA) experiment in March/April 2000. An extensive scientific payload tocharacterise aerosol-cloud interactions, cloud microphysical properties, and trace gases wasdeployed on the DLR research aircraft Falcon. A second experiment in the NorthernHemisphere midlatitudes has been performed in Sept./Oct. 2000 to compare clean andpolluted air masses in order to study whether anthropogenic emissions have a measurableeffect on cirrus cloud properties. First results were presented showing clear interhemisphericdifferences in various aerosol components (volatile and non-volatile particles of various sizes;crystal residuals and interstitial aerosols; see Figure 8) and trace gases (CO, O3, NO, andNOy) (J. Ström et al.). The observations are accompanied by ground based Lidar (Immler etal.) and satellite observations (P. Minnis et al.). At Punta Arenas the mean cirrus cover was45%. On the way from Punta Arenas to Bremerhaven, the Lidar was mounted on thePOLARSTERN and observed extended cirrus clouds in the tropics (F. Immler et al.).

Climate Impact

Little progress has been made beyond that described in the IPCC report in understandingwhich aviation parameter is most important in controlling the climate impact by contrails andcirrus. The total aviation climate impact (in terms of radiative forcing, and without cirruschanges) is about three times as large as the radiative forcing from aircraft-induced carbondioxide alone (R. Sausen; U. Schumann), see Figure 3 (IPCC,1999).

The climate response is more sensitive to aircraft induced ozone than to a CO2 perturbationwith the same radiative forcing. Global warming leads to a smaller increase in contrailcoverage than would be expected from growing air traffic in an unchanged atmosphere (R.Sausen). Some observations indicate long-term cirrus cover trends (P. Minnis et al.). Ingeneral, long term observations are needed to identify any systematic cloud or climate changedue to aviation, if present, because of the large variability of cloud systems (V. Ramaswamy).

Mitigation Potentials

Contrails form for thermodynamic reasons and, hence, can be avoided only by flying inwarmer air either below or above the altitude range (around the tropopause at midlatitudes)which is cool enough to let contrails form. Recent experiments have provided evidence thatcontrails form at lower altitude and hence more frequently when using more efficient engines.However, more efficient engines also emit less carbon dioxides. The trade-off between botheffects depends on the relative sensitivity of climate to contrails and carbon dioxide changesand on the scenario of future aviation (U. Schumann).

Fuel properties have some impact on the formation of soot, volatile particles, and the carbondioxide and water vapour emissions. The impact of variations within various jet fuels for sootformation is small compared to the variability between various combustor concepts (poster byP. Madden). Elimination of fuel sulphur would reduce the size, and to minor degree also thenumber of volatile aerosol particles formed. However, volatile particles would form even for avery low fuel sulphur content (B. Kärcher et al.).

Hydrogen fuelled aircraft have been proposed in order to reduce carbon dioxide emissions byaircraft. Very low NOx emissions from such aircraft have been demonstrated. The radiativeforcing by the emitted water vapour is small, but the impact on contrails and on cirrus clouds

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has still to be determined (H. Klug and M. Ponater). Replacing the fleet of kerosene drivenaircraft by a fleet of liquid hydrogen driven aircraft would pay off in terms of reducedradiative forcing by carbon dioxides only in the long term, after several decades (S.Marquartet al.). Contrails behind liquid hydrogen driven engines may have smaller climatic impactbecause of different particle properties (larger in size but less in number) compared tocontrails behind kerosene burning engines.

It is not yet known whether the climatic impact depends just on fuel consumption of theaircraft flying in sufficiently cold and humid air masses, or whether other parameters are moreor equally important, such as the number of aircraft in operation, the number and size of sootparticles emitted, the sulphur content in the fuel used, or just the flight route. In the absence ofa reliable estimate of the climatic impact of contrails, it is not yet possible to assess variousflight routing and operational options (U. Schumann).

Ongoing and future research programmes

Several research programmes were described: Atmospheric research in Germany supportedby the Bundesministerium für Bildung und Forschung (G. Hahn); laboratory studies ofheterogeneous processes on ice (CUT-ICE; J. N. Crowley et al.); aerosol/particulate researchactivities supported by the NASA ultra-efficient engine technology (UEET) program (C. C.Wey); measurements of aged aircraft exhaust in the ACCENT mission (R. Friedl et al.);environmental considerations in aircraft design (C. Hume); the CRYOPLANE projectconsidering the potential of liquid hydrogen as aircraft fuel (H. Klug); perspective of the U.S.federal aviation administration (H. Wesoky); overview on NASA’s plans to address theconcerns identified in the IPCC special report (D. Anderson and R. Friedl); programmaticperspectives of the European research on aviation’s atmospheric effects (G. Amanatidis); andindustry and agencies perspectives were presented in opening remarks and panel discussions(J. Raps, F. Beulé, P. Newton, F. Walle). A series of SULFUR experiments (1-7) wasperformed by the Deutsches Zentrum für Luft- und Raumfahrt (DLR) in Germany; DLR andpartners of other major national research centres (within the Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren, HGF) perform a 3-years project Particles fromAircraft Engines and Their Impact on Contrails, Cirrus Clouds, and Climate (PAZI).

Important open questions addressed in the working groups include: The prediction of thenumber of soot particles emitted from the fleet of aircraft; the hydration and ice nucleationproperties of soot and other aerosol components; the global cover and optical properties ofcontrails and aircraft induced cirrus, and their long-term trends; the heterogeneous chemistryon cirrus and contrail particles and their effects on air composition; the statistics of verticalwind, humidity, and cirrus properties in the tropopause region; the identification of the mostrelevant aerosols component which should get reduced for an environmentally sustainable airtraffic development.

Aerosols are key issue for understanding of aircraft impact, ozone chemistry and climatechanges. Aviation emissions are contributing a small but growing fraction of allanthropogenic disturbances to the climate system. In view of the rather well defineddisturbance, understanding of aviation effects helps in understanding the more generalaerosol-cloud-climate-chemistry interactions from all sources.

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Figures Captions:

Figure 1. Mean altitude of the tropopause in June and December, distribution of NOx

emission source rate from aircraft versus altitude and latitude (the emission rate is the largerthe darker the shaded area), and indications of mean circulation and some relevant processes.

Figure 2. Spiral contrail observed in NOAA-14 AVHRR satellite data west of Denmarkinduced by a military aircraft circulating over the North Sea at 1236 UT May 22, 1998,indicating that long-lasting cirrus clouds are triggered by contrails which would not formotherwise (U. Schumann)

Figure 3. Radiative forcing (W m-2) due to aircraft emissions in 1992 (IPCC, 1999). Thecolumns indicate the best estimate of forcing. The error bars denote the range within whichthe best-estimate value is expected with a two-third probability. The available information oncirrus clouds is insufficient to determine either a best estimate or an uncertainty range; thedashed line indicates a range of possible best estimates. The estimate for total forcing does notinclude the effect of changes in cirrus cloudiness. The evaluations below the graph (“good,”“fair,” “poor,” “very poor”) are a relative appraisal associated with each component andindicates the level of scientific understanding.

Figure 4. Global aircraft-generated soot inventory (vertical integral of emissions per month in1992) (A. Döpelheuer et al.).

Figure 5. Formation processes of aerosols in an aircraft exhaust plume (K.-H. Wohlfrom).

Figure 6. Apparent particle emission indices of particles larger diameter d (PEId, d = 3, 5, and14 nm), measured in aircraft exhaust plumes that do not cause a contrail as a function of fuelsulphur content (FSC) behind the ATTAS jet aircraft (squares), a Boeing 737-300 (circles),and an Airbus 310-300 (diamonds) (A. Petzold et al.).

Figure 7. Relative humidity with respect to liquid water versus ambient temperature. Thehorizontal dashed line denotes liquid saturation, the full curve denotes the relative humidityfor ice saturation, and the thin curves with various line codings are lines of constant watermixing ratio. The symbols denote the measured data as derived from frostpoint hygrometerand temperature sensors during the POLINAT 2 project (J. Ovarlez and U. Schumann).

Figure 8. Number densities of particles versus latitude during the outbound transfer flight toPunta Arenas, Chile. a) Particles larger than 6 nm diameter b) Particles larger than 120nmdiameter. (Note difference in scale between Figures a and b) (J. Ström et al.).

Figure 9. Contrail cover over Europe from NOAA-14 noon scenes for March 1995-February1997. The lines indicate main traffic routes over Europe (R. Meyer et al.).

Figure 10. Contrails, 0.65 µm optical depth τ, effective diameter De, and longwave (LW) andshortwave (SW) cloud radiative forcing (CRF) in a NOAA-12 infrared image, 1214 UTC 26September 1996, with letter-identified contrails (left) and contrails selected by automatedalgorithm (right, in green). Properties of identified contrails are given in the table (P. Minniset al.).

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References

IPCC, Climate Change 1995 – The Science of Climate Change, edited by J. T. Houghton etal., Intergovernmental Panel on Climate Change, Cambridge University Press, 878 pp.,1996.

IPCC, Aviation and the Global Atmosphere, edited by J. E. Penner et al., IntergovernmentalPanel on Climate Change, Cambridge Univ. Press, New York, 373 pp., 1999.

Schumann, U. and G. T. Amanatidis (Editors), Aviation, Aerosols, Contrails and CirrusClouds (A2C3), Proceedings of a European Workshop, Seeheim (near Frankfurt/Main),Germany, July 10-12, 2000, Air Pollution Research Report 74, Report EUR 19428 EN,European Commission, Brussels, pp. 314, in press, 2000.

WMO, Scientific Assessment of Ozone Depletion: 1998, World Meteorological Organisation,Geneva, 1999.

Ulrich Schumann, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik derAtmosphäre, Oberpfaffenhofen, 82230 Wessling, Germany, (ulrich.schumann@dlr.de)

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Figures:

Fig. 1. Mean altitude of the tropopause in June and December, distribution of NOx emissionsource rate from aircraft versus altitude and latitude (the emission rate is the larger the darkerthe shaded area), and indications of mean circulation and some relevant processes.

Fig. 2. Spiral contrail observed in NOAA-14 AVHRR satellite data west of Denmark inducedby a military aircraft circulating over the North Sea at 1236 UT May 22, 1998, indicating thatlong-lasting cirrus clouds are triggered by contrails which would not form otherwise (U.Schumann)

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Fig. 3. Radiative forcing (W m-2) due to aircraft emissions in 1992 (IPCC, 1999). Thecolumns indicate the best estimate of forcing. The error bars denote the range within whichthe best-estimate value is expected with a two-third probability. The available information oncirrus clouds is insufficient to determine either a best estimate or an uncertainty range; thedashed line indicates a range of possible best estimates. The estimate for total forcing does notinclude the effect of changes in cirrus cloudiness. The evaluations below the graph (“good,”“fair,” “poor,” “very poor”) are a relative appraisal associated with each component andindicates the level of scientific understanding.

Fig: 4. Global aircraft-generated soot inventory (vertical integral of emissions per month in1992) (A. Döpelheuer).

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Fig. 5. Formation processes of aerosols in an aircraft exhaust plume. (K.-H. Wohlfrom).

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Fig. 6. Apparent particle emission indices of particles larger diameter d (PEId, d = 3, 5, and 14nm), measured in aircraft exhaust plumes that do not cause a contrail as a function of fuelsulphur content (FSC) behind the ATTAS jet aircraft (squares), a Boeing 737-300 (circles),and an Airbus 310-300 (diamonds) (A. Petzold et al.).

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Fig. 7. Relative humidity with respect to liquid water versus ambient temperature. The horizontaldashed line denotes liquid saturation, the full curve denotes the relative humidity for ice saturation,and the thin curves with various line codings are lines of constant water mixing ratio. The symbolsdenote the measured data as derived from frostpoint hygrometer and temperature sensors during thePOLINAT 2 project (J. Ovarlez and U. Schumann).

Fig. 8 Number densities of particles versus latitude during the outbound transfer flight toPunta Arenas, Chile. a) Particles larger than 6 nm diameter b) Particles larger than 120nmdiameter. (Note difference in scale between Figures a and b) (J. Ström et al.).

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Ferry to Punta Arenas: Latitudional distribution of aerosol > 6nmFlights performed in upper troposphere (ozone < 100 ppbv) and devided in bins by 1 deg of Latitude

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Fig. 9. Contrail cover over Europe from NOAA-14 noon scenes for March 1995-February1997. The lines indicate main traffic routes over Europe (R. Meyer et al.).

Fig. 10. Contrails, 0.65 µm optical depth τ, effective diameter De, longwave (LW) andshortwave (SW) cloud radiative forcing (CRF) in NOAA-12 IR images, 1214 UTC, 26September 1996, with letter-identified contrails (left) and contrails selected by automated

A B C D E F G H I τ τ τ τ 0.37 0.25 0.42 0.28 0.22 0.49 0.14 0.21 0.20De (µm) 18 19 25 14 30 21 45 30 21LWCRF (Wm-2) 24.2 13.0 21.7 7.0 5.2 16.8 6.7 6.9 4.7SWCRF (Wm-2) -10.1 -2.6 -6.8 -2.2 -1.1 -7.1 -0.8 -5.2 -2.2

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algorithm (right, in green). Properties of identified contrails are given in the table. (P. Minniset al.).