CHAPTER 11

Subsonic and Supersonic Aircraft Emissions

Lead Authors: A. Wahner M.A. Geller

Co-authors: F. Arnold W.H. Brune D.A. Cariolle A.R. Douglass C. Johnson D.H. Lister J.A. Pyle R. Ramaroson D. Rind F. Rohrer U. Schumann A.M. Thompson CHAPTER 11

SUBSONIC AND SUPERSONIC AIRCRAFT EMISSIONS

Contents

SCIENTIFIC SUMMARY ...... 11.1

11.1 INTRODUCTION ...... 11.3

11.2 AIRCRAFT EMISSIONS ...... 11.4 11.2.1 Subsonic Aircraft ...... 11.5 11.2.2 Supersonic Aircraft ...... 11.6 11.2.3 Military Aircraft ...... 11.6 11.2.4 Emissions at Altitude ...... 11.6 11.2.5 Scenarios and Emissions Data Bases ...... 11.6 11.2.6 Emissions Above and Below the Tropopause ...... 11.7

11.3 PLUME PROCESSES ...... 11.10 11.3.1 Mixing ...... ll.lO 11.3.2 Homogeneous Processes ...... 11.10 11.3.3 Heterogeneous Processes ...... 11.12 11.3.4 Contrails ...... 11.13

11.4 NOx/H20/SULFUR IMPACTS ON ATMOSPHERIC CHEMISTRY ...... 11.13 11.4.1 Supersonic Aircraft ...... 11.13 11.4.2 Subsonic Aircraft ...... 11.14

11.5 MODEL PREDICTIONS OF AIRCRAFTEFFECTS ON ATMOSPHERIC CHEMISTRY ...... 11.15 11.5.1 Supersonic Aircraft ...... 11.15

11.5.2 Subsonic Aircraft ...... 11.20

11.6 CLIMATE EFFECTS ...... 11.22

11.6.1 Ozone ...... 11.23

11.6.2 Water Vapor ...... 11.24

11.6.3 Sulfuric Acid Aerosols ...... 11.24

11.6.4 Soot ...... 11.24

11.6.5 Cloud Condensation Nuclei ...... 11.24

11.6.6 Carbon Dioxide ...... 11.24

11.7 UNCERTAINTIES ...... 11.25

11.7 .1 Emissions Uncertainties ...... 11.25

11.7.2 Modeling Uncertainties ...... 11.25

11.7.3 Climate Uncertainties ...... 11.27

1 1.7.4 Surprises ...... 11.27

ACRONYMS ...... 11.27

REFERENCES ...... 11.28 AIRCRAFT EMISSIONS

SCIENTIFIC SUMMARY

Extensive research and evaluations are underway to assess the atmospheric effects of the present and future subsonic aircraft fleetand of a projected fleetof supersonic transports. Assessment of aircraft effects on the atmosphere involves the following: i) measuring the characteristics of aircraft engine emissions; ii) developing three-dimensional (3-D) inventories for emissions as a function of time; iii) developing plume models to assess the transformations of the aircraft engine emissions to the point where they are governed by ambient atmospheric conditions; iv) developing atmospheric models to assess aircraft influences on atmospheric composition and climate; and v) measuring atmospheric trace species and meteorology to test the understanding of photochemistry and transport as well as to test model behavior against that of the atmosphere.

Supersonic and subsonic aircraft fly in atmospheric regions that have quite different dynamical and chemical regimes. Subsonic aircraft fly in the upper troposphere and in the stratosphere near the tropopause, where stratospheric residence times due to exchange with the troposphere are measured in months. Proposed supersonic aircraft will fly in the stratosphere near 20 km, where stratospheric residence times due to exchange with the troposphere increase to years. In the upper troposphere, increases in NOx typically lead to increases in ozone. In the stratosphere, ozone changes depend on the complex coupling among HOx, NOx, and halogen reactions.

Emission inventories have been developed for the current subsonic and projected supersonic and subsonic aircraft fleets. These provide reasonable bases for inputs to models. Subsonic aircraft flying in the North Atlantic flight corridor emit 56% of their exhaust emissions into the upper troposphere and 44% into the lower stratosphere on an annual basis.

Plume processing models contain complex chemistry, microphysics, and turbulence parameterizations. Only a few measurements exist to compare to plume processing model results.

Estimates indicate that present subsonic aircraft operations may have increased NOx concentrations at upper tropospheric altitudes in the North Atlantic flight corridor by about 10-l 00%, water vapor concentrations by about 0.1% or less, SOx by about 10% or less, and soot by about 10% compared with the atmosphere in the absence of aircraft and assuming all aircraft are flying below the tropopause.

Preliminary model results indicate that the current subsonic fleet produces upper tropospheric ozone increases as much as several percent, maximizing at the latitudes of the North Atlantic flight corridor.

The results of these rather complex models depend critically on NOx chemistry. Since there are large uncertainties in the present knowledge of the tropospheric NOx budget (especially in the upper troposphere), little confidence should be put in these quantitative model results of subsonic aircraft effects on the atmosphere.

Atmospheric effects of supersonic aircraft depend on the number of aircraft,the altitude of operation, the exhaust emissions, and the background chlorine and aerosol loading. Rough estimates of the impact of future supersonic operations (assuming 500 aircraft flying at Mach 2.4 in the stratosphere and emitting 15 grams of nitrogen oxides per kilogram of fuel) indicate an increase of the North Atlantic flight corridor concentrations of NOx up to about 250%, water vapor up to about 40%, SOx up to about 40%, H2S04 up to about 200%, soot up to about 100%, and CO up to about 20%.

11.1 AIRCRAFT EMISSIONS

One result of two-dimensional model calculations of the impact of such a projected fleetin a stratosphere with a chlorine loading of 3.7 ppbv (corresponding to the year 2015) implies additional annually averaged ozone column decreases of 0.3-1.8% for the Northern Hemisphere.Although NOx aircraftem issions have the largest impact on ozone, the effects from H20 emissions contribute to the calculated ozone change (about 20%).

Net changes in the column ozone from supersonic aircraftmodeling result from ozone mixing ratio enhancements in the upper troposphere and lower stratosphere and depletion at higher stratospheric altitudes.

There are important uncertainties in supersonic assessments. In particular, these assessment models produce ozone changes that differ among each other, especially in the lower stratosphere below 25 km. When used to calculate ozone trends, these same models predict smaller changes than are observed in the stratosphere below 25 km between 1980 and 1990. Thus, these models may not be properly including mechanisms that are important in this crucial altitude range.

Increases in ozone at altitudes near the tropopause, such as are thought to result from aircraft emissions, enhance the atmosphere's greenhouse effect. Research to evaluate the climate effects of supersonic and subsonic aircraft operations is just beginning, so reliable quantitative results are not yet available, but some initial estimates indi­ cate that this effect is of the same order as that resulting from the aircraft C02 emissions.

11.2 AIRCRAFT EMISSIONS

11.1 INTRODUCTION Since supersonic aircraft engines may emit significant amounts of NOx, the fear is that large fleets of superson­ Tremendous growth occurred in the aircraft indus­ ic aircraft flying at stratospheric levels, where maximum try during the last several decades. Figure 11-1 shows ozone concentrations exist, might seriously deplete the the increasing use of aircraft fuel as a fu nction of time. stratospheric ozone layer, leading to increased ultravio­ Aircraft fuel consumption has increased by about 75% let radiation flux on the biosphere. Also, climate during the past 20 years and is projected to increase by sensitivity studies have shown that ozone changes in the 100 to 200% over the next 30 years. At the present time, upper troposphere and lower stratosphere will have approximately 3% of the worldwide usage of fossil fuels greater radiative effects on changing surface and lower is by aircraft. Ninety-nine percent of this aircraft fuel is tropospheric temperatures than would ozone changes at burned by subsonic aircraft, of which a large proportion other levels (see Chapter 8). occurs in the upper troposphere. Table 9.2 of the previ­ Al so, in the 1950s, "smog reactions" were discov­ ous assessment (WMO, 1992) demonstrates that ered that implied the depletion of tropospheric ozone subsonic aircraft emit a significant fraction of their ex­ when NOx concentrations are low and ozone production haust products into the lower stratosphere. This depends when NOx concentrations are high. Thus, there is a con­ on factors such as latitude and season. cern that new fleets of supersonic aircraft flying in the Despite the small percentage of the total fossil fuel stratosphere would lead to harmful stratospheric ozone usage for aviation, the environmental effects of aircraft depletion, while present and future subsonic aircraft op­ should be closely examined for several reasons. One rea­ erations will lead to undesired enhanced levels of ozone son is the rapid growth that has occurred and is projected in the upper troposphere. to occur in aircraft emissions, and another is that aircraft Development of any successful aircraft requires a emit their exhaust products at specific altitudes where period of about 25 years, and each aircraft will have a significant effects might be expected. For instance, an useful lifetime of about 25 years as well. Thus, even if an environmental concern of the 1970s was the effect that environmentally motivated decision is made to utilize large fleets of supersonic aircraft would have on the new aircraft technologies, it will take decades to fully stratospheric ozone layer. The main concern was then realize the benefits. and still is that catalytic cycles involving aircraft-emitted One can get some perspective on possible atmo­ NOx (NO plus N02) enhance the destruction of ozone. spheric effects of aircraft operations by noting the following. Current subsonic aircraft operations in the North Atlantic flight corridor are probably increasing

500 NOx concentrations at upper tropospheric altitudes by about 10-100%, water vapor concentrations by about 400 0.1% or less, and SOx by about 10% or less compared to � an atmosphere without aircraft. Future supersonic opera­ � 300 c tions in the stratosphere might increase the North 0

·;;·� 200 Atlantic flight corridor concentrations of NOx up to " · ·· . ·· about 250%, water vapor up to about 40%, SOx up to . . 100 . about 40%, H2S04 up to about 200%, soot up to about 100%, and CO up to about 20%. Thus, present subsonic

i 970 1980 1 990 2000 2010 2020 aircraft perturbations in atmospheric composition are Year now probably significant, and future large supersonic aircraft fleet operations will also be significant in affect­ Figure 11-1. Aviation fuel versus time. Data up to ing atmospheric trace gas concentrations. 1989 from the International Energy Agency (1990).� Extrapolations according Kavana�gh (19�8) wi h These and other concerns have led to an increasing 2.2% per year in a low-fuel scenano and w1th 3.6 Yo amount of research into the atmospheric effects of cur­ up to 2000 and 2.9% thereafter in a high-fuel sce­ rent and future aircraft operations. In the U.S., NASA's nario. (Based on Schumann, 1994.) Atmospheric Effects of Av iation Project is composed of

11.3 AIRCRAFT EMISSIONS two elements. The Atmospheric Effects of Stratospheric as it adjusts from the physical conditions of the aircraft Aircraft (AESA) element was initiated in 1990 to evalu­ exhaust leaving the engine tailpipe to those of the ambi­ ate the possible impact of a proposed fleet of high-speed ent atmosphere. Some of the atmospheric effects of the (i.e., supersonic) civil transport (HSCT) aircraft. A Sub­ different chemical families that are emitted by aircraft sonic Assessment (Wesoky et al., 1994) was begun in are then considered, and finally, modeling studies of the 1994 to study the impact of the current commercial air­ atmospheric effects of aircraft emissions on ozone are craft fleet. In Europe, the Commission of the European presented, along with a discussion of possible climate Communities (CEC) has initiated the Impact of NOx effects of aircraft operations. A discussion of the level of Emissions from Aircraft upon the Atmosphere uncertainty of these predictions, and some conclusions (AERONOX) and Measurement of Ozone on Airbus In­ are presented. service Aircraft (MOZAIC) programs (Aeronautics, Further details of the NASA effort to assess the at­ 1993) and Pollution from Aircraft Emissions in the mospheric effects of future supersonic aircraft North Atlantic Flight Corridor (POLINAT) to investigate operations can be found in Albritton eta!. ( 1993) and the effects of the emissions of the present subsonic aircraft references therein. An external evaluation of these ef­ fleet in flight traffic corridors. In addition, there are also forts can be found in NRC (1994). No similar documents several national programs in Europe and Japan looking exist at this time pertaining to the atmospheric effects of at various aspects of the atmospheric effects of aircraft subsonic aircraft operations. emissions. Atmospheric models play a particularly important 11.2 AIRCRAFT EMISSIONS role in these programs since there does not appear to be any purely experimental approach that can evaluate the The evaluation of the potential impact of emis­ global impact of aircraft operations on the atmosphere. sions from aircraft on atmospheric ozone levels requires The strategy is to construct models of the present atmo­ a comprehensive understanding of the nature of the sphere that compare well with atmospheric measurements emissions produced by all types of aircraft and a knowl­ and to use these models to try to predict the future atmo­ edge of the operations of the total global aircraft fleet in spheric effects of changed aircraft operations. At the order to generate a time-dependent, three-dimensional present time, the subsonic and supersonic assessment emissions data base for use in chemical/dynamical at­ programs are in quite different stages of maturity and are mospheric models. utilizing different approaches in both modeling and ob­ Emissions from the engines, rather than those as­ servations. Therefore, in this chapter the subsonic and sociated with the airframe, are considered to be supersonic evaluations will be considered separately dominant (Prather et al. , 1992). These are functions of since the chemical and dynamical regimes are quite dif­ engine technology and the operation of the aircraft on ferent. In this context the "lower stratosphere" refers to which the engines are installed. Primary engine exhaust the region above the local tropopause where there are products are C02 and H20, which are directly related to lines of constant potential temperature that connect the the burned fuel, with minor variations due to the precise stratosphere and troposphere. In this region, strato­ carbon-hydrogen ratio of the fu el. Secondary products sphere-troposphere exchange can occur by horizontal include NOx (=NO + N02), CO, unburned and partially advection with no need to expend energy in overcoming burnt fuel hydrocarbons (HC), soot particulates/smoke, the stable stratification. In the stratosphere near 20 km, and SOx. NOx is a consequence of the high temperature where Mach 2.4 HSCT operate, no lines of constant po­ in the engine combustor; the incomplete combustion tential temperature connecting the stratosphere and products (CO, HC, and sooUsmoke) are functions of the troposphere exist. Therefore residence times of tracers engine design and operation and may vary widely be­ are much larger (about 2 years) in the stratosphere at 20 tween engines. SOx is directly related to fuel km than in the lower stratosphere. composition. Currently, typical sulfur levels in aviation In this chapter, we will review what is known kerosene are about 0.05% sulfur by weight, compared about aircraft emissions into the atmosphere and discuss with an allowed specification limit of 0.3% (ICAO, the transformations that take place in the aircraft plume 1993).

Jl.4 AIRCRAFT EMISSIONS

Table 11-1. Emission Index (grams per kilograms of fuel used) of various materials for subsonic and supersonic aircraft for cruise condition. Values in parentheses are ranges for different engines and oper· ating conditions.

Species Subsonic Aircraft* Supersonic Aircraft# (gm MW) Short range Long range

C02 (44) 3160 3160 3160 H20 (18) 1230 1230 1230 co (28) 5.9 (0.2-14) 3.3 (0.2-14) 1.5 (1.2-3.Q) HC as methane (16) 0.9 (0.1 2-4.6) 0.56 (0.1 2-4.6) 0.2 (0.02-0.5) so2 (64) 1.1 1.1 1.0 NOx as N02 (46) 9.3 (6-19) 14.4 (6-19) depends on design (5-45)

* Mean (fuel-consumption weighted) emission indices for 1987 based on Boeing (1990). The values were calculated from a data base containing emission indices and fuel consumptions by aircraft types. The difference between short range (cruise altitude around 8 km) and long range (cruise altitude between 10 and 11 km) reflects different mixes of aircraft used for different flights. # Based on Boeing (1990) and McDonnell Douglas (1990).

The measure of aircraft emissions traditionally trate) should be considered, along with SOx and soot used in the aviation community is the Emissions Index particles as aerosol-active species. HC and CO may also (EI), with units of grams per kilogram of burntfu el. Ty p­ play an important role in high altitude HOx chemistry. ical El values for subsonic and anticipated values for supersonic aircraft engines are given in Table 11-1 for 11.2.1 Subsonic Aircraft cruise conditions. By convention, EI(NOx) is defined in Engine design is a compromise between many terms of N02 (similarly, hydrocarbons are referenced to conflicting requirements, among which are safety, econ­ methane). omy, and environmental impacts. For subsonic engines, Historically, the emissions emphasis has been on the various manufacturers have resolved these conflicts limiting NOx, CO, HC, and smoke, mainly for reasons with different compromises according to their own in­ relating to boundary layer pollution. Standards are in house styles. This has resulted in a spread of emission place for control of these over a Landing!fake-Off values for HC, CO, NOx, and smoke, all meeting the (LTO) cycle up to 915 m altitude at and around airports LTO cycle regulatory standards. (ICAO, 1993). Currently there are no regulations cover­ Historical trends ( 1970-1988) in aircraft engine ing other flight regimes, e.g. cruise, though ICAO ( 1991) emissions for the typical LTO cycle show that very sub­ is considering the need and feasibility of introducing stantial decreases in HC and CO emissions have been standards. realized over the past two decades due to improvements It is now recognized that the list of chemical spe­ in fuel-efficient engine design and emissions control cies (emitted from engines or possibly produced in the technology. A substantial increase in NOx emissions young plume, also by reactions with ambient trace spe­ would have been expected due to the much higher combus­ cies like hydrocarbons) that may be relevant to ozone tion temperatures associated with the more fuel-efficient and climate change extends well beyond the primary engine cycles. However, other improvements in engine combustion species and NOx. A more complete set of technology have kept NOx relatively constant. Combin­ "odd nitrogen" compounds, known as NO -including y ing the increased passenger miles in the period from NOx, N205, N03, HN03-and PAN (peroxyacetylni- 1970 to 1988 with that of the technology improvements

11.5 AIRCRAFT EMISSIONS would imply that the actual mass output should have de­ frequencies. Also, national authorities are reluctant to creased by about 77% for HC and 30% for CO, while disclose this information. Thus it is extremely difficultto NOx mass output should have increased by about 110%. make realistic assessments of the contribution of mili­ Considerable further reductions of HC and CO will tary aircraft in terms of fuel usage or emissions. Earlier come as older aircraft are phased out, but little change estimates (Wuebbles et al., 1993) were that the world's can be expected for NOx without the introduction of military aircraft used about 19% of the total aviation fuel low-NOx technology engines. and emitted 13% of the NOx, with an average EI(NOx) The firststeps to develop combustion systems pro­ of 7.5. With the changes following the breakup of the ducing significantlylo wer NOx levels relative to existing former Soviet Union, there has been considerable reduc­ technology were made in the mid-1970s (ClAP 2, 1975). tion in activity, and an estimate of about 10% fuel usage These systems achieve at least a 30% NOx reduction, may be more appropriate (ECAC/ANCAT, 1994). and are now being developed into airworthy systems for introduction in medium and high thrust engines. 11.2.4 Emissions at Altitude

As noted above, engines are currently only regulated 11.2.2 Supersonic Aircraft for some species over an LTO cycle. Internationally ac­ The first generation of civil supersonic aircraft credited emissions data on these are available (ICAO, (Concorde, Tu polev TU144) incorporated turbojet en­ 1994). However, experimental data for other flight con­ gines of a technology level typical of the early 1970s. ditions are sparse, since these can only realistically be The second generation, currently being considered by a obtained from tests in flightor in altitude simulation test number of countries and industrial consortia, will have facilities. Correlations, in particular for NOx, have been to incorporate technology capable of meeting environ­ developed from theoretical studies and combustor test mental requirements. A comprehensive study of the programs for prediction of emissions over a range of scientificissues associated with the Atmospheric Effects flight conditions. A review of these is given elsewhere of Stratospheric Aircraft (AESA) was initiated in 1990 (Prather et al., 1992; Albritton et al., 1993 ). Engine tests as part of NASA's High Speed Research Program under simulated altitude conditions are being carried out (HSRP; Prather et al., 1992). No engines or prototypes within the AERONOX program (Aeronautics, 1993) and exist and designs are only at the concept stage. A range should be useful to check this approach for subsonic en­ of cruise EI(NOx) levels (45, 15, and 5) has been set as gines. the basis for use in atmospheric model assessments and in developing engine technology. An EI(NOx) of 45 is 11.2.5 Scenarios and Emissions Data Bases approximately what would be obtained if HSCT engines Air traffic scenarios have been developed as a ba­ were to be built using today 's jet engine technology sis for evaluating global distributions of emissions from without putting any emphasis on obtaining lower aircraft (Mcinnes and Walker, 1992; Prather et al., 1992; EI(NOx) emissions. Jet engine experts have great confi­ Wuebbles et al., 1993; ECAC/ANCAT, 1994). The first dence in their ability to achieve an HSCT engine design two based their traffic assessment on scheduled com­ with EI(NOx) no greater than 15 and have set a goal of mercial flight information from timetables and designing an HSCT engine with EI(NOx) no greater than supplemented these data with information from other 5. Laboratory-scale studies of new engine concepts, sources for non-scheduled charter, general aviation, and which appear to offer the potential of at least 70-80% military flights. The third is based on worldwide Air reduction in NOx compared with current technology, are Traffic Controldata supplemented by timetable informa­ being pursued. Early results indicate that these systems tion and other data as appropriate. seem able to achieve the low target levels of EI(NOx) = 5 Mcinnes and Walker (1992) generated 2-D and (Albritton et al., 1993). 3-D inventories of NOx emissions from subsonic air­ craft, using relatively broad assumptions for numbers of 11.2.3 Military Aircraft aircraft types, flightprofiles/ distance bands, and cell siz­ In contrast to the majority of civil aviation, mili­ es. However, the evaluation did not include tary aircraft do not operate to set flight profiles or non-scheduled, military, cargo, or general aviation, and

11.6 AIRCRAFT EMISSIONS both inventories accounted for only 51% of the total esti­ El(NOx) than those of the other inventories and are like­ mated fuel consumption of 166.5 x 109 kg for the year ly to represent upper bounds on the aircraft NOx 1989 (lEA, 1990). The fuel consumption was simply emission burden. The current grid scale is larger than scaled to match the total estimated fuel consumption in that of the HSRP/ AESA inventory, but this may give a order to estimate the total NOx mass. Their average more realistic representation of the NOx distribution EI(NOx) value of 11.6 is within the range quoted else­ within the heavily traveled air traffic routes, such as the where (NuBer and Schmitt [1990] 6- 16.4; Egli [1990] North Atlantic, where there is known to be a significant ll-30; and Becket al. [1992] 17.9). divergence of actual flight paths from the ideal great cir­ Wuebbles et al. (1993) generated for the HSRP/ cle routes currently assumed by all inventories. Further AESA (Prather et al., 1992; Stolarski and We soky, work is being carried out to produce forecast inventories 1993a) a comprehensive assessment of all aircraft types for the years 2003 and 2015. to determine fuel, NOx, CO, and HC for general scenar­ Considerable comparative analysis is being under­ ios comprising the 1990 fleet and projected fleets of taken between the ECAC/ ANCA T and the HSRP/ AESA subsonic and supersonic aircraft (HSCTs) for the year inventories in order to understand the reasons underlying 2015. A much better match (7 6%) of the calculated fuel the differences (EI(NOx) 10.9 to 16.8 ; NOx mass 1.92 to use with the total estimated fuel consumption for 1990 2.8 Tg) and to refine the inventories. For example, it is was achieved. The remainder is likely to be mainly at­ already known that there is some double counting of tributable to factors such as the non-idealized flight traffic in some geographically important areas of the routings and altitudes actually flown by aircraft due to ECAC/ AN CAT inventory. Another significant factor is a factors such as air traffic control, adverse weather, etc., large difference in the contribution from military air­ as well as low-level unplanned delays and ground opera­ craft. A comparison summary of the inventories is given tions. However, scaling to match the total estimated fuel in the table at the bottom of the page. consumption gave a total annual NOx mass (1.92 Tg) similar to that of Mcinnes and Walker. Illustrations of 11.2.6 Emissions Above and Below the the global NOx inventories as functions of latitude/longi­ Tropopause tude, or altitude/latitude for both 1990 and 2015 are In a global perspective, the North Atlantic, apart given in Figures 11-2 and 11-3. from North America and Europe, contains the largest The European Civil Aviation Conference (ECAC) specific subsonic traffic load. In 1990 the average daily Abatement of Nuisance Caused by Air Traffic (AN CAT) movements across the Atlantic (both directions) between work, carried out to complement the AERONOX pro­ 45° and 60°N amounted to 595 flights in July and 462 gram, has also considered NOx emissions from subsonic flights in November. One recent study (Hoinka et al., and supersonic fleetsfor the year 1992. Unlike the other 1993) has assessed the aircraft fleet mix and the resulting inventories, the traffic data have been compiled for four emissions for this flight corridor. By correlation of the equally spaced months throughout the year to provide traffic data with the tropopause height from the Euro­ information on the seasonal variation. Preliminary re­ pean Centre for Medium-Range Weather Forecasts sults indicate a higher fuel bum, NOx annual mass, and

Mcinnes and Walker, Wu ebbles et al., ECACIANCAT, 1992 1993 1994

Year 1989 1990 1992

Grid size 7.5° X 7.5° X 0.5km 1° X 1° X lkm 2.8° X 2.8° X lkm Fuel match 51% 76% 99% EI (NOx) global 11.6 10.9 16.8 NOx mass (Tg)# 1.91# 1.92# 2.8#

#Note: all data for NOx mass have been scaled to 100% fuel match.

11.7 AIRCRAFT EMISSIONS

90

60

-30

-60

-90 L=��------=� -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 Longitude

0.01 50.01 100.01 150.01 29 Molecules/Year of NOx (xl0 )

90

60

30 C) "'d ...... ;::l ...... 0 ell .....:l -30

-60

-90 -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 Longitude

"" -- �$;: Ji,::.:..., 0.01 10.01 20.01 30.01 40.01 50.01 29 Molecules/Year of NOx (xl0 )

Figure 11-2. Annual NOx emissions for proposed 2015 subsonic and Mach 2.4 (EI(NOx)=15) HSCT fleets as function of latitude and longitude. Top panel shows emissions below 13 km (primarily subsonic traffic) while bottom panel shows emissions above 13 km (primarily HSCT traffic) . (Albritton et at., 1993)

11.8 -�· -�· -�· -�·

AIRCRAFT EMISSIONS

30

25

� � 20 '-" il) "'d 15 II I I ::::: ......

5

OL______u�wU��.u�-. -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Latitude

;�To tt,i� J:.; 0.01 500.01 1000.01 1500.01 2000.01 2500.01 3000.01 3500.01

29 Molecules/Year of NOx (x10 )

30

25

� 15 3 ...... � 10

5

0 �------� -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Latitude

0.01 500.01 1000.01 1500.01 2000.01 2500.01 3000.01 3500.01 29 Molecules/Year of NOx (xl0 )

Figure 11-3. Annual NOx emissions as a function of altitude and latitude for 1990 subsonic fleet (Scenario A, top panel) and for proposed 2015 subsonic and Mach 2.4 (EI( NOx)=15) HSCT fleets (bottom panel). (Albrit­ ton eta/., 1993)

11.9 AIRCRAFT EMISSIONS

(ECMWF) data, it is estimated that 44% of the NOx cross-section of the trailing vortex pair represents an up­ emissions are injected in the lower stratosphere and 56% per bound for the mixed area of the plumes. However, are injected in the upper troposphere. measurements of water vapor concentration and temper­ ature in the jet and vortex regime (>2 kmbehind a DC-9 at cruising altitude) exhibit a spiky concentration field 11.3 PLUME PROCESSES within the double vortex system, indicating that the indi­ Plume processing involves the dispersion and con­ vidual jet plumes may not yet be homogeneously mixed version of aircraft exhausts on their way from the scales over the vortex cross-section at such distances (Bau­ of the jet engines to the grid scales of global models. The mann et al., 1993). details of plume mixingand processing can be important The lift of the aircraft induces downward motion for conversion processes that depend nonlinearly on the of the double vortex structure at about 2.4 ± 0.2 m s-1 for concentration levels, such as the formation of contrails, a Boeing-747, which decreases when the vortices mix the formation of soot, sulfur and nitric acid particles, and with the environment at altitudes that may be typically nonlinear photochemistry. Also, the vertical motion of 100 m lower than flight level. During this descent, parts the plumes relative to ambient air and sedimentation of of the exhaust gases are found to escape the vortex cores. particles may change the effectivedistribution of emitted In the supersonic case, the vortex pair has more species at large scales. Contrails may impact the mixing, vertical momentum (descent velocity of about 5 m/s), sedimentation, heterogeneous chemistry, and the forma­ and its vertical motion will continue (possibly in the tion of cirrus clouds, with climatic consequences. form of vortex rings) well after the vortex system has broken up. This will lead to exhaust species deposition a 11.3.1 Mixing few hundred meters below flight altitude (Miake-Lye et al. , 1993). Radiation cooling of the exhaust gases may The aircraft wake can be conveniently subdivided contribute to additional sinking (Rodriguez et al., 1994), into three regimes (Hoshizaki et al., 1975): the jet, the in particular when contrails are forming. vortex, and the dispersion regimes. The vortex regime Very little is known about the rate of mixing in the persists until the vortices become unstable and break up dispersion range, and it is this rate of mixing that plays a into a less ordered configuration. Thereafter, the disper­ large role in determining the time evolution of the gas sion regime follows, in which further mixing is composition of the plume (Karol et al., 1994). In fact, it influenced by atmospheric shear motions and turbulence is yet unknown at what time scales the emissions be­ depending on shear, stratification, and other parameters come indistinguishable from the ambient atmosphere. (Schumann and Gerz, 1993). With respect to mixing Table 1 1-2 shows estimates of the concentration increas­ models, the jet and vortex regime, including the very ear­ es due to aircraft emissions in a young exhaust plume ly dispersion regime, can be computed with models as (vortex regime) and at the scales of the North Atlantic described by Miake-Lye et al. (1993). The engine flight corridor (Schumann, 1994). These are the scales in plumes grow by turbulent mixing to fill the vortex pair between which global models will be able to resolve the cell. Due to rotation, centripetal acceleration causes in­ concentration fields. The background concentration esti­ ward motions of the relatively warm plumes so that mates are taken from Penner et al. (1991) for NOx, the exhaust gases get trapped near the narrow well­ Mohler and Arnold (1992) for S02, and Pueschel et al. mixed core of the vortices. The radial pressure gradient (1992) for soot. With respect to background, the concen­ also causes adiabatic cooling and hence increases the tration increases in young plumes are of importance for formation of contrails. These centripetal forces are much all aircraft emissions included in Table 11-2. A strong larger for supersonic aircraft than for subsonic aircraft. It corridor effect is expected for NOx and, at least in the should be noted, however, that these model results re­ lower stratosphere, also for SOx and soot particles. main largely untested, observationally. Details of the plume fluid dynamics depend criti­ 11.3.2 Homogeneous Processes cally on the aircraft scales. For a Boeing-747, one may estimate that the jet regime lasts for about 10 s and the Several models have been developed to describe following vortex regime for about 1 to 3 minutes. The the finite-rate chemical kinetics in the exhaust plumes

11.10 AIRCRAFT EMISSIONS

Table 11-2. Mean concentration increases in vortex regime (5000 m2 cross-section) of a B-747 plume, and mean concentration increase in the North Atlantic flight corridor due to traffic exhaust emissions from 500 aircraft. (Table adopted from Schumann, 1994.)

Species EI (g/kg) Background Mean Mean concentration concentration concentration at 8 km increase in increase in vortex regime North Atlantic flightcorridor

C02 3150 358 ppmv 14 ppmv 0.02 ppmv HzO 1260 20-400 ppmv 14 ppmv 0.02 ppmv NOx(N02) 18 0.01-0.05 ppbv 78 ppbv 0. 1 ppbv so2 1 50-300 pptv 3100 pptv 4 pptv soot 0.1 3 ngfm3 240 ngfm3 0.3 ngfm3

(Danilin et a!., 1992; Miake-Lye et al., 1993; Pleijel et 1% of the emitted odd-nitrogen underwent chemical al., 1993; Weibrink and Zellner, 1993). Most models fol­ conversion to longer living HN03. Hence, most of the low a well-mixed air parcel as a function of plume age or emitted odd nitrogen initially remains in a reactive form, distance behind the aircraft. The models are initialized which can catalytically influence ozone. either with an estimate of emissions from the jet exit or a separate model describing the kinetics after the combus­ 2 tion chamber within the engine. Considerable deviations -45-92 MPI H from local equilibrium are predicted at the jet exit, in F91-12 DC-9 TRAIL ALTITUDE 9.5km particular for CO, NO, N02, HN03, OH, 0, and H. In 5 DEC 1991 DISTANCE 2.0km NO 0.01 X 0 -9 N0 xQ01 + the models, the air parcel grows in size as a prescribed 0 0 2 0 0 0 1-Q <>: function of mixing with the environment, and the con­ 0:: >­ f- (0 centrations in the plume change according to mixing z Ui x with the ambient air and due to internal reactions in the g_ -10� 0{1S w homogeneous mixture. The models differ in the treat­ 0:: ::;:: w :::> m __J ment of mixing, in the reaction set used to simulate the ::;:: 0 :::> > z exhaust plume finite-rate chemical kinetics, photolysis (0 8 8- -11 rates, treatment of heterogeneous processes, and in the __J s prescription of the effective plume cross-section as a function of time or distance. Since most of the NOx 12.27 12.28 12.29 12.30 12.31 12.32 12.33 emissions are in the form of NO, a rapid but local de­ UNIVERSAL TIME struction of ozone is to be expected. Besides some incidental measurements in flight Figure 11-4. Time plot of nitrous acid (HN02) and corridors or contrails (Hofmann and Rosen 1978; Doug­ nitric acid abundance measured during chase of a lass et al., 1991), very few data exist at this time on the DC-9 airliner at 9.5 km altitude and a distance of 2 km. Periods when the research aircraft was inside gaseous emissions in aircraft plumes in the atmosphere. the exhaust-trail of the DC-9 are marked by bars. Measurements of the gases HN02, HN03, NO, N02, For these periods NO and N02 abundance are also et a!. , and S02 were recently made (Arnold 1992, 1994a) given. (Arnold et a/., 1992, 1994b; recalibration in the young plume of an airliner at cruising altitude (see changed conversion factors shown in figure to: NO x Figure 11-4). The data imply that not more than about 0.006 and N02 x 0.003.)

ll.ll AIRCRAFT EMISSIONS

1978; Pitchford et al. , 1991; Hagen et al., 1992; White­ > .0 May 18,1993 62690 GMT(s) 0. 0.6 field et al. , 1993) in plumes under flight conditions . 0� The molecular physics details of nucleation are z not well known and the theory of bimolecular nucleation

> .0 is only in a rudimentary state. For a jet engine exhaust 0. scenario, nucleation takes place in a non-equilibrium 5� 0 z mechanism, which further complicates a theoretical de­ scription. It seems, however, that jet aircraft may form long-lived contrails composed of H2S04·H20 aerosols

OM and soot particles covered with H2S04·H20. Under u conditions of low ambient temperatures around 10 km > altitude, particularly in winter at high latitudes, contrails 8 0. composed of HN03·H20 aerosols may also form (Ar­ 5 0 ::t: nold et al., 1992). Even if HN03·H20 nucleation does M � '4W' Ar c:.-- not occur, some HN03 may become incorporated into -.H 160 condensed-phase H2S04·H20 by dissolution at low tem­ as � :::- 80 peratures. " " 8"g" 0 There are several potential effects of newly formed o�--L-�1o��-2�o�l-�3�o--L-�4o���so���6o· CN and activated soot. Such CN may trigger water con­ Elapsed time (s) trail formation, induce heterogeneous chemical reactions, and serve as cloud condensation nuclei Figure 11-5. Time series for NO, NOy. C02, H20, (CCN). Thereby, jet aircraft-produced CN may have an and CN during the plume encounters on May 18, impact on trace gas cycles and climate. However, at 1993 . The approximate Greenwich Mean Time present this is highly speculative. (GMT) is noted in the top panel. The scale on the Numerical calculations with chemical plume mod­ left side indicates the absolute value of each spe­ els show that the impact of aircraft emissions on the cies. The zero in the right scale is set to the approximate background values of each species. atmosphere in the wake regime critically depends on het­ At the ER-2 airspeed of 200 m s-1, the panel width erogeneous processes where considerable uncertainties of 60 seconds corresponds to 12 km. (Based on still exist (Danilin et al., 1992, 1994). Danilin et al. Fahey et at., 1994.) (1992) have considered the heterogeneous reaction

N20s + H20 --7 2HN03 on ambient aerosol particles only. They have found that this reaction does not play an In situ measurements of NOy, NO, C02, H20, condensation nuclei, and meteorological parameters important role at time scales of up to one hour in the (Figure 11-5) have been used to observe the engine ex­ wake, but may get important at larger time scales. Taking haust plume of the NASA ER-2 aircraft approximately contrail ice (or/and nitric acid trihydrate [NAT]) particle 10 minutes after emission operating in the lower strato­ formation into account, Danilin et al. (1994) estimate sphere (Fahey et al. , 1994). The obtained EI(NOx) of 4 is that heterogeneous processes are more important at in good agreement with values scaled from limited lower temperatures, but their impact on heterogeneous ground-based tests of the ER-2 engine. Non-NOx nitro­ conversion is small during the first day after emission. In gen species comprise less than about 20% of emitted contrast, Karol et al. (1994) found noticeable "heteroge­ reactive nitrogen, consistent with model evaluations. neous impact" on the chemistry in the plume taking into account the growth of ice particles. 11.3.3 Heterogeneous Processes Around 10 km altitude, there seems to exist a strong CN source, which is not due to aircraft but to New particles form in young exhaust plumes of jet H2S04 resulting from sulfur sources at the Earth's sur­ aircraft. This is documented by in situ condensation nu­ face (Arnold et al. , 1994a). Hence, the relative clem (CN) measurements made (Hofmann and Rosen, contribution of aircraft to CN production around 10 km

11.12 AIRCRAFT EMISSIONS

Table 11-3. Estimates of stratospheric perturbations due to aircraft effluents of a fleet of approxi­ mately 500 Mach 2.4 HSCTs (NOx El=15) relative to background concentrations. (Perturbations are estimated for a broad corridor at northern midlatitudes.) (Expanded from Stolarski and Wesoky, 1993b.)

Species Perturbation Background

NOx 3-5 ppbv 2-16 ppbv H20 0.2-0.8 ppmv 2-6 ppmv SOx 10-20 pptv 50-100 pptv H2S04 350-700 pptm 350-700 pptm Soot -7 pptm -7 pptm Hydrocarbons 2 ppbv (NMHC) 1600 ppbv (CH4) co -2ppbv 10-50 ppbv C02 -I ppbv 350 ppmv

altitude needs to be determined. It is uncertain whether contrails sediment quickly at approximately 10 km alti­ CN production around 10 km actually has a significant tude (Schumann, 1994). impact on trace gas cycles and CCN.

11.4 NOx/H 0/SULFUR IMPACTS ON 11.3.4 Contrails 2 ATMOSPHERIC CHEMISTRY Miake-Lye et al. (1993) have applied the analysis of Appleman (1953) to the standard atmosphere as a 11.4.1 Supersonic Aircraft function of altitude and latitude. Their result shows that The impacts of HSCT emissions on chemistry are much of the current high-flying air traffic takes place at discussed in detail in Stolarski and Wesoky (1993b). altitudes where the formation of contrails is very likely, Here we give a short summary. Effects of emissions in particular in the northern winter hemisphere. A small from HSCTs (see Table 11-3) on ozone are generally reduction of global mean temperature near and above the predicted to be manifested through gas phase catalytic tropopause, by say 2 K, would strongly increase the re­ cycles involving NOx, HOx, ClOx, and BrOx. The gion in which contrails have to be expected. Also, a amounts of these radicals are changed by two pathways. slight change in the threshold temperature below which First, they are changed by chemistry, either addition of contrails form has a strong effect on the area of coverage or repartitioning within nitrogen, hydrogen, and halogen with contrails. chemical families. Predicted changes in ozone from this Except for in situ measurements by Knollenberg pathway are initiated primarily by NOx chemistry. Sec­ (1972), little is known about the spatial structure and ond, they are changed when HSCT emissions affect the microphysical parameters of contrails. Recent measure­ properties of the aerosols and the probability of polar ments (Gayet et al., 1993) show that contrails contain stratospheric cloud (PSC) formation. Changes in ozone more and smaller ice particles than natural cirrus, lead­ from this pathway are determined primarily by ClOx and ing to about double the optical thickness in spite of their BrOx chemistry, with a contribution from HOx chemis­ smaller ice content. Contrail observations from satellite try (see Chapter 6 for more detail). data, Lidar measurements, and climatological observa­ Heterogeneous chemistry on sulfate aerosols also tions of cloud cover changes have been described by has a large impact on the potential ozone loss. Most im­ Schumann and Wendling (1990). Large (1 to 10 km wide portant is the hydrolysis of N20s: N20s H20 and more than 100 km long) contrails are observed re­ + ---7 2 HN03. Several observations are consistent with this gionally on about a quarter of all days within one year, reaction occurring in the lower stratosphere (e.g., Fahey but the average contrail coverage is only about 0.4% in et al., 1993; Solomon and Keys, 1992). Its most direct mid-Europe. Lidar observations show that particles from

11.13 AIRCRAFT EMISSIONS

effect is to reduce the amount of NOx. Indirectly, it in­ rates of heterogeneous reactions on sulfate and the prob­ creases the amounts of CIO and H02 by shifting the ability of PSC formation. balance of CIO and ClON02 more toward CIO during The addition of sulfur to the stratosphere from the day and by reducing the loss of HOx into HN03. As a HSCTs will increase the surface area of the sulfate aero­ result, the HOx catalytic cycle is the largest chemical sol layer. This change in aerosol surface area is expected loss of ozone in the lower stratosphere, with NOx sec­ to be small compared to changes from volcanic erup­ ond, and both the ClOx and BrOx catalytic cycles have tions, with a possible exception being the immediate increased importance compared to gas phase conditions. vicinity of the aircraft wake. Model calculations by Bek­ The addition of the emissions from HSCTs will ki and Pyle ( 1993) predict regional increases of the mass affect the partitioning of radicals in the NOy, HOy, and of lower stratospheric HzS04·H20 aerosols, due to air ClOy chemical families, and thus will affect ozone. The traffic, by up to about 100%. The importance of sulfur NOx emitted from the HSCTs will be chemically con­ emissions from HSCTs in the presence of this large and vertedto other forms, so that the NOx!NOy ratio of these variable background needs to be assessed. emissions will be almost the same as for the background atmosphere. As a result, the NOx emissions will tend to 11.4.2 Subsonic Aircraft decrease ozone, but less than would occur in the absence The emissions from subsonic aircraft take place of sulfate aerosols. both in the lower stratosphere and troposphere. The pri­ The increase in H20 will lead to an increase in mary chemical effects of aircraft in the troposphere seem OH, because the reaction between O(ID) that comes to be related to their NOx emissions. The concentration from ozone photolysis and H20 is the major source of of ozone in the upper troposphere depends on transport OH; however, increases in NO will act to reduce HOx y of ozone mainly from the stratosphere and on upper tro­ through the reactions of OH with HN03 and HN04. On posphere ozone production or destruction. The impact of the other hand, HN03, formed in the reaction of OH with subsonic aircraft occurs through the influence of NOx on N02, can be photolyzed in some seasons and latitudes to the tropospheric HOx cycle (see Chapter 5 for a fuller regenerate OH. When all of these effects are considered, discussion of tropospheric ozone chemistry). the amount of HOx is calculated to decrease-H02 by The HOx cycle is initialized by the photolysis of up to 30% and OH by up to 10%. Thus, the catalytic de­ ozone itself, which results in the production of OH radi­ struction of ozone by HOx, the largest of the catalytic cals and destruction of ozone. OH radicals have two cycles, is decreased. possible reaction pathways: reaction with CO, CH4, and Finally, ClOx concentrations decrease with the ad­ non-methane hydrocarbons (NMHC) resulting in H02 dition of HSCT emissions for two reasons. First and and R02 radicals; or reaction with N02, removing OH most important, with the addition of more N02, the day­ and NOx from the cycle. The H02 radicals that are pro­ time balance between ClO and ClON02 is shifted more duced also have two possible pathways: reaction with toward ClON02. Second, with OH reduced, the conver­ ozone or reaction with NO. The first one removes ozone sion of HCl to Cl by reaction with OH is reduced, so that from the cycle; the second one (also valid for R02 radi­ more chlorine stays in the form of HCl. Thus, the catalyt­ cals) produces ozone and regains NO. Additionally, both ic destruction of ozone by ClOx is decreased. pathways regain OH radicals. The addition of HSCT emissions results in in­ As a consequence, ozone is destroyed photochem­ creases in the catalytic destruction of ozone by the NOx ically in the absence of NOx· Only in the presence of cycle that are compensated by decreases in the catalytic NOx can ozone be produced. The net production/de­ destruction by ClOx and HOx. Because the magnitudes struction depends on the combination of these processes. of the changes in catalytic destruction of ozone are simi­ Their relative importance is controlled mainly by the lar for the NOx, HOx, and ClOx cycles, compensation NOx concentration. In a regime of low NOx, the ozone results in a small increase or decrease in ozone. Model concentration will be reduced photochemically. At high­ calculations indicate a small decrease. The decreases in er NOx concentrations (on the order of 10 pptv NOx) the catalytic destruction of 03 by CIOx and HOx involve NOx will lead to a net ozone production. In both re­ the effects of increased water vapor and HN03 on the gimes, additional NOx will result in higher ozone

11.14 AIRCRAFT EMISSIONS

concentrations. Only when the concentration of NOx is N03) on aerosol surfaces will reduce the concentration so high (over a few hundred pptv NOx) that the OH con­ of photochemically active NOx during the day, giving centration starts to decline, will additional NOx result in rise to lower ozone and OH concentrations in the upper a lower ozone production. troposphere (Dentener and Crutzen, 1993). The impact of NOx emitted by aircraft depends, therefore, on the background NOx concentration and on 11.5 MODEL PREDICTIONS OF AIRCRAFT the increase in NOx concentration. Measurements show EFFECTS ON ATMOSPHERIC CHEMISTRY that background NOx concentrations (including NOx emitted from subsonic aircraft) are in the range of 10- The first investigations concerning the potential 200 pptv NOx. Therefore, airplane emissions take place effects of supersonic aircraft on the ozone layer were in the regime of increasing ozone production most of the conducted in the 1970s. Early assessments were ob­ time, where increasing NOx results in increased local tained using one-dimensional (1-D) photochemical ozone concentrations. models; more recent assessments rely on 2-D models In this regime, the concentration of OH radicals is (e.g., Stolarski and Wesoky, 1993b). In addition, the enhanced also by additional NOx. First, enhanced ozone transport in 2-D models has been compared to 3-D mod­ means higher production of OH by photolysis of ozone. el transport by examining the evolution of the Second, the partitioning in the HOx family is shifted to­ distribution of passive tracers. wards OH by the reaction of H02 with NO. The loss process of OH by reaction with N02 is not yet important. 11.5.1 Supersonic Aircraft This enhancement of the OH concentration reduces the Evaluations of the effects of the emissions of the tropospheric lifetime of many trace species like CH4, HSCT on the lower stratosphere have used two-dimen­ NOx, etc. sional (2-D) models. These are zonally averaged (lati­ The emission of sulfur from aviation is much tude-height) models and are discussed in detail in smaller than from surface emissions and negligible in Chapter 6. For use in such 2-D models, both the source terms of the resultant acid rain, but may be important if of exhaust and the emission transport (both horizontal emitted at high altitudes. Hofmann (1991) reported ob­ and vertical) are zonally averaged. In fact, the source of servations that show an increase of non-volcanic emissions is not zonally symmetric, as HSCT flight is stratospheric sulfate aerosol of about 5% per year. He expected to be restricted to oceanic corridors. Further­ suggests that if about 1/6 of the Northern Hemisphere air more, the transport processes through which trace spe­ traffic takes place directly in the stratosphere and if a cies are removed from the stratosphere are not well small fraction of other emissions above 9 km would en­ represented by a zonally averaged model. Stratosphere­ ter the stratosphere through dynamical processes, then troposphere exchange processes (STE) occur preferen­ the jet fleetappears to represent a large enough source to tially near jet-systems, above frontal perturbations, and explain the observed increase. On the other hand, Bekki during strong convection in tropical regions. The two and Pyle (1992) conclude from a model study that al­ former processes may transport effluents released by though aircraft may represent a substantial source of HSCTs irreversibly to lower levels and lead to tropo­ sulfate below 20 km, the rise in air traffic is insufficient spheric sinks. Effluents may be rapidly advected also to to account for the observed 60% increase in large strato­ lower latitudes by large-scale motions. Such processes spheric aerosol particles over the 1979-1 990 period. are poorly represented in 2-D models. The horizontal Sulfate particles generated from SOx may also contrib­ scale for STE is small and can only be represented using ute to nucleation particles (Arnold et al., 1994a). 3-D models with high resolution. These small scales are Whitefield et al. (1993) find a positive correlation be­ not explicitly resolved in most global 3-D models. Thus, tween sulfur content and CCN efficiency of particles any use of a 3-D model to evaluate the use of a 2-D mod­ formed in jet engine combustion. el for these assessments must include a critical evalua­ The possible enhancement of aerosol surface area tion of the 3-D model STE. 2-D models do have the may affect the nighttime chemistry of the nitrogen ox­ practical advantage that it is possible to complete many ides. The heterogeneous reaction of N205 (and possibly

ll.l5 AIRCRAFT EMISSIONS

assessment calculations, using a reasonably complete vations (Jackman et al. , l989b; Prather and Remsberg, representation of stratospheric chemistry, and also by 1993). Model results for a "best" simulation, as well as considering the sensitivity of the results to model param­ for various applications and constrained calculations, eters one can take some aspects of feedbacks among at­ were compared with each other and with observations. mospheric processes into account. There are significant differences in the models that lead Current 3-D models, though impractical for full to differences in the model assessments as discussed be­ chemical assessments, are practical for calculations that low. In addition, there are some features, such as the very consider the transport of aircraft exhaust, which is treat­ low observed values of N20 and CH4 in the upper tropi­ ed as a passive tracer. Such calculations have been cal stratosphere, and the NOyf03 ratio at tropical compared directly with 2-D models (Douglass et al. , latitudes, that are not well represented by all 2-D models. 1993; Rasch et al., 1993). Their results show that for There are also many areas of agreement between seasonal simulations, provided that the residual circula­ models and observations that suggest that an evaluation tion derived from the 3-D fieldsis the same as used in the of the effects of the HSCT may be an appropriate use of 2-D calculation, the tracer is dispersed fa ster vertically these models. For example, the models' total ozone and has similar horizontal spread for 3-D compared with fields show general consistency when compared with 2-D calculations. Although the tracer is also transported observed fields such as Total Ozone Mapping Spectro­ upward more rapidly in 3-D than in 2-D (where vertical meter (TOMS) data, the overall vertical and latitudinal upward transport is minimal), the more rapid downward distributions of such species as N20, CH4, and HN03, transport is the more pronounced effect. Accumulation and the ozone climatology that is based on Stratospheric of aircraft exhaust in flightcorridors is found in regions Aerosol and Gas Experiment (SAGE) and Solar Back­ of low wind speed, but only a small number of typical scatter Ultraviolet (SBUV) observations. If the SAGE corridors (North Atlantic, North Pacific, and tropical) results for 03 loss over the past decade at altitudes just have been considered. The effect of such local accumu­ above the tropopause are correct (see Chapter 1 ), howev­ lation would be largest if a threshold chemical process er, then the inability of present models to reproduce this such as particle formation is triggered at high concentra­ 03 decrease (see Chapter 6) casts doubt on their ability tion of aircraft exhaust constituents. In 2-D models that to correctly model aircraft effects in this important re­ use residual mean formulation, transport to the tropo­ gion. sphere takes place principally through two mechanisms: At the beginning of the NASA HSRP/ AESA pro­ advective transport by the residual mean circulation gram, the assessment models contained only gas phase (mostly at middle to high latitudes) and diffusive trans­ photochemical reactions. The importance of the hetero­ port across the tropopause (all latitudes). The latter is geneous reaction (temperature independent) N20s + largest where the 2-D model's tropopause height is dis­ H20 --7 2 HN03 on the surface of stratospheric aerosols continuous (to represent the downward slope of the was noted by Weisenstein et al. (1991) and Bekki et al. tropopause from the tropics to middle and polar lati­ (1991) and has been further explored by Ramaroson and tudes) (Shia et al. , 1993). The difference in the character Louisnard (1994). This process changes the balance be­ of STE in 2-D and 3-D models leads to different sensitiv­ tween the reactive nitrogen species, NO and N02 (NOx), ities to the latitude at which exhaust is injected in the and the reservoir species, HN03. For gas phase evalua­ models. For the 3-D model, the atmospheric lifetime of a tions, lower stratospheric ozone was most sensitive to tracer species is relatively insensitive to the latitude of the amount of NOx from aircraft exhaust injected into injection. For the 2-D model, the tracer species lifetime the lower stratosphere. For evaluations including this is much longer for injection at lower latitudes than at heterogeneous process, the NOx levels in both the base higher latitudes, since transport to higher latitudes must atmosphere and in the perturbed atmosphere are much take place before most of the pollutant is removed from lower than in the gas phase evaluations, and the calculat­ the stratosphere. ed ozone change is greatly reduced (Ko and Douglass, Treatments of the transport and photochemistry 1993). used in 2-D models have been examined through a series 2-D models have also been used to examine other of model intercomparisons and comparisons with obser- processes that are of potential significance. For example,

11.16 AIRCRAFT EMISSIONS

Ta ble 11-4. Calculated percent change in the averaged column content of ozone between 40°N and 50°N.

Scenarios AER GSFC LLNL OSLO CAMED NCAR

I: Mach 1.6, NOx EI=5* -0.04 -0. 11 -0.22 +0.04 +0.69 -0.01 II: Mach 1.6, NOx El=l5* -0.02 -0.07 -0.57 +0. 15 +0.48 -0.60 III: Mach 2.4, NOx EI=5* -0.47 -0.29 -0.58 -0.47 +0.38 -0.26 IV: Mach 2.4, NOx EI=l5* -1.2 -0.86 -2.1 -1.3 -0.45 -1.8 V: Mach 2.4, NOx El=l5** -2.0 -1.3 -2.7 -0.42 -1.1 -2.3 VI: Mach 2.4, NOx EI=45* -5.5 -4. 1 -8.3 -3.5 -2.8 -6.9

Ta ble 11-5. Calculated percent change in the averaged column content of ozone in the Northern Hemisphere.

Scenarios AER GSFC LLNL OSLO CAMED NCAR

I: Mach 1.6, NOx EI=5* -0.04 -0. 12 -0.18 +0.02 +0.63 -0.04 II: Mach 1.6, NOx EI=l5* -0.02 -0. 14 -0.48 +0. 10 +0.63 -0.54 III: Mach 2.4, NOx El=5* -0.42 -0.27 -0.50 -0.39 +0.25 -0.25 IV: Mach 2.4, NOx El=l5* -1.0 -0.80 -1.8 -1.0 -0.26 -1.5 V: Mach 2.4, NOx EI=l5** -1.7 -1.2 -2.3 -0.43 -0.80 -1.9 VI: Mach 2.4, NOx EI=45* -4.6 -3.6 -7.0 -3.1 -2.1 -5.1 * 3 Relative to a background atmosphere with chlorine loading of .7 ppbv, corresponding to the year 2015 ** Relative to a background atmosphere with chlorine loading of 2.0 ppbv, corresponding to the year 2060 if HSCT planes are flown, the lower stratospheric levels bation. This paper also showed that the annual cycle of of total odd nitrogen and water vapor are expected to the zonally averaged total ozone is sensitive to the annu­ rise. In addition to a general increase over background al cycle in the residual circulation. A similar sensitivity levels throughout the lower stratosphere, there is a possi­ to the residual circulation has been demonstrated for a bility for large enhancements in areas of high traffic (air 3-D calculation using winds from a data assimilation "corridors"). Peter et al. (1991) and Considine et al. procedure for transport (Weaver et al., 1993). (1993) have considered the possibility that the increases The supersonic aircraft assessment scenarios dis­ in H 0 and in HN03 (a consequence of the heteroge­ cussed here are for Mach numbers 1.6 and 2.4, which 2 neous conversion of NOx) will lead to an increase in the correspond to the two aircraft cruise altitudes 16 km and amount of nitric acid trihydrate (NAT) cloud formation. 20 km, respectively, and for three values for EI(NOx) They indeed find this to be so. (see Stolarski and Wesoky [1993b] for specific details). The evaluation of the effects of a fu ture fleet of The emission indices are given in Table 11-1. The calcu­ supersonic aircraft on stratospheric ozone was made by lated total ozone changes are given for each participating Johnston et al. (1989) and by Ramaroson (1993) using model in Table 1 1-4 for the calculated annually averaged gas phase models. The ozone loss for an injection at a column ozone change in the latitude band where the air­ fixed level was found to increase nearly linearly as the craft emissions are largest (40°-50°N), and in Table 1 1-5 amount of NOx injected was increased. The ozone loss for the Northern Hemisphere average. The model calcu­ was found to be larger for injection at higher levels be­ lations use an aerosol background similar to that cause the ozone response time decreases with altitude, observed in 1979 (e.g., before the Mt. Pinatubo erup­ and because the pollutant has a longer stratospheric life­ tion). Some similarities and differences are seen among time when injected farther from the model tropopause. the model results. For all of the models, the ozone Jackman et al. (1989a) used a 2-D model to test change for Mach 2.4 is more negative than that for Mach the dependence of the supersonic aircraft assessments on 1.6. The ozone change at Mach 2.4 is more negative as model dynamical inputs. As anticipated, the calculated the EI is increased, but the change is more rapid than a change in ozone is larger (smaller) for a slower (faster) linear change. The complexity of the assessment is cap­ residual circulation because the circulation controls the sulated by the change in ozone calculated at Mach 1.6 magnitude of the steady-state stratospheric NOx pertur- for the two different Eis in Table 11-4. For all models,

11.17 AIRCRAFT EMISSIONS

NOy (Scenario IV) · AER NOy (Scenano IV) · CAMEO 60 60

50 - 50

40

2 � � 30 " N 20 20

10 10

0 0 - - 60 90 90 -60 LAT30 ITUDE0 (DEG)30 60 -90 -60 ·30LAT ITUDE0 (DEG)30

NOy (Scen arioIV ) - GSFC NOy (Scenario IV)- LLNL 60

40 0.5 :::E � 30 :.. N 20

10 10

0 0 - 9 -90 -60 -30 0 60 90 0 ·60 -30LATITU DE0 (DEG) 30 60 90 LATITUDE (DEG)30

(ScenarioIV ) - NCAR (Scenario IV) - OSLO 60

50 50

40

� � a.s � 30 30 N N 20 20

10 10

oi 0 0 30 60 90 90 -90 -60 -30LATITU DE (DEG) ·90 -60 -30LATITU DE0 (DEG)30 60

Figure 11-6. Calculated changes in the local concentration of NOy (ppbv) in June for Mach 2.4 (EI(N0x)= 15) case. The contour intervals are 1 ppbv, 2 ppbv, 3 ppbv, 4 ppbv, and 5 ppbv (Stolarski and Wesoky, 1993b).

11.18 AIRCRAFT EMISSIONS

(Scenano IV) - June 03 AER ------�------� 5o r- � �� 60

50 50 oo 40 40 r1 :::! � 30 N

20 20

10

0 0.0

-60 -30 0 30 60 90 ·90 -60 -30 0 30 60 90 LATITUDE (DEG) LATITUDE (DEG)

03 GSFC (Scenario IV) - June 03 LLNL (Scenano IV) -June . ' ,. .. .·.· - 60 60 . . . . . '.·' . 0 • • . 50 ...... 50 ...... -1. 0

4 40 : .• 0 . . . -1 .0 ...... � � 0 " '· 3 ... . () . :X:: . . . � 30 ".·3. . . . N N 20 20

0 .0 10 10

o �--�--��--��--� 0 - 90 -60 -30 0 30 60 90 - 90 ·60 -30 0 30 60 90 LATITUDE (DEG) LATITUDE (DEG)

03 NCAR (Scenario IV) - June 03 OSLO (ScenarioIV) - June �--�--��--�--��--�--� ------60 5o .- � �� �--��------.

50 50

. . . . · ...... 40

. '.0 ..... • :::! ' ,,,.,, � 30 •••• •••• • • '•• • ': :I N 20

1 0 10

o �Mr�--��--���----�--� 0 �--�--��--�--�----�--� -90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90 LATITUDE (DEG) LATITUDE (DEG)

Figure 11-7. Model-calculated percent change in local ozone for June for Mach 2.4 (EI(NOx)=15) fleet in the 2015 atmosphere. The contour intervals are -4%, -3%, -2%, -1%, -0.5%, 0%, 0.5%, 1%, 2%, 3%, 4% (Albrit­ ton eta/., 1993).

11.19 AIRCRAFT EMISSIONS

and for both cases at ClOx mixing ratios of 3.7 ppbv, the 11.5.2 Subsonic Aircraft changes are less than 1%. For three of the models (At­ The Chapter 7 discussions indicate that tropo­ mospheric and Environmental Research, Inc., AER; spheric photochemical-dynamic modeling is much less Goddard Space Flight Center, GSFC; and the University developed than is this type of stratospheric modeling; of Oslo, OSLO), the ozone change is less negative (more however, several types of models have been used to as­ positive) for EI = 15 than for EI = 5. For the other three sess the impact of subsonic aircraft emissions. These models (Lawrence Livermore National Laboratory, include global photochemistry and transport models in LLNL; the University of Cambridge and the University latitude-height dimensions ignoring the longitudinal of Edingburgh, CAMED; and the National Center for variation of emissions. This is an important drawback for Atmospheric Research, NCAR), the ozone change is species with short lifetimes. Another type of model used more negative (less positive) for the larger emission in­ is the longitude-height model that addresses a restricted dex. range of latitudes. They neglect the effect of latitudinal The assessment initiated by the "Comite Avian­ transport. Three-dimensional global dynamical models Ozone" shows similar results. A 2-D model including are being developed to study the impact of aircraft emis­ heterogeneous reactions on aerosol and PSC surfaces sions, but the results from these models are as yet and a similar emission scenario to that for the HSRP as­ restricted to NOx and NO species. The published results sessments shows a global mean decrease of total ozone y from two-dimensional models have used a range of esti­ of 0.3% (Ramaroson and Louisnard, 1994). The results mates to represent present and future aircraft emissions, depend upon the prescribed background atmosphere and consequently, the results are not easily comparable. (e.g. , aerosol loading) used (see also: Tie et al. , 1994; There have been no organized efforts to intercompare Considine et al., 1994). models for subsonic aircraft as there have been for the The change in NO is given in Figure 1 1-6 for each y supersonic aircraftproblem. of the models for a scenario in which the HSCT fleet is The sensitivity of modeled ozone concentrations assumed to fly at Mach 2.4 with an EI(NOx) = 15 and a to changes in aircraft NOx emissions has been found to backgr�un� chlorine mixing ratio of 3.7 ppbv. This NO y be much higher than for surface emissions, with around change md1cates the sensitivity to the different transport. twenty times more ozone being created per unit NOx LLNL has the largest change in NO , and also the largest y emission for aircraft compared to surface sources global ozone changes in Tables 11-4 and 11-5. However, (Johnson et al., 1992). Several authors have investigated the calculated global changes are clearly not ordered by the role of hydrocarbon and carbon monoxide emissions the magnitude of the NO change. The latitude height y from aircraft on ozone concentrations, but have found change in ozone for each of the models is given in Figure small effects (Beck et al. , 1992; Johnson and Henshaw, 11-7. There are remarkably large differences in the local 1991; Wuebbles and Kinnison, 1990). The increase in ozone changes, particularly in the upper troposphere/ net ozone production with increasing NOx is steeper at lower stratosphere region where the aircraft issiem ons lower concentrations of NOx (Liu et al. , 1987), and produce an increase in the ozone production as well as therefore larger ozone sensitivities are expected for an increase in the ozone loss. Although changes in NOx emissions to the Southern Hemisphere, where NOx con­ have the largest impact on 03, the effects from H20 centrations are lower (Johnson and Henshaw, 1991). emissions contribute to the calculated 03 changes (about Beck et al. (1992) note the influence of lightning pro­ 20%). duction of NOx in controlling the sensitivity of ozone to The assessment models' representation of upper aircraft NOx emissions. These studies indicate the im­ tropospheric chemistry was not considered as a part of portance of predicting a realistic background NOx the Models and Measurements Wo rkshop (Prather and concentration, and underline the importance of measure­ Remsberg, 1993). Further attention must be paid to the ments in model testing. upper tropospheric chemistry to understand the spread in Several recent publications (Johnson and Hen­ the results for these assessments. This subject is dis­ shaw, 1991; Wuebbles and Kinnison, 1990; Fuglestvedt cussed in the fo llowing section on the evaluation of the et al., 1993; Beck et al. , 1992; Rohrer et al., 1993) esti- impact of the subsonic fleet.

11.20 AIRCRAFTEMISSIONS mate the percentage increases in ozone concentrations due to the impact of aircraft emissions. The results show maximum increases at around 10 km of between 12% and 4% between 30° and 50°N. NOx concentrations in the upper troposphere are ""E controlled by the transport of NOx downwards from the stratosphere, by aircraft and lighting emissions, and by the convection of NOx from surface sources (Ehhalt et al., 1992). The available measurements of NOx in the 3 \ free troposphere are discussed in Chapter 5. There are a \ number of observations where the vertical NO profile is 70°W soow 50°W 40°W 30°W 20°W 10°W oo strongly and unequivocally influenced by one or the oth­ er of these sources, e.g., lightning (Chameides et al., Longitude 1987; Murphy et al., 1993), aircraft emissions (Arnoldet al., 1992), fast vertical transport (Ehhalt et al. , 1993), Figure 11-8. Daytime NO mixing ratio distribution which makes it clear that all these sources can and do (altitude vs. longitude) across the North Atlantic 4-6, 1984, make a contribution to the NOx in the upper troposphere. during the period June of the STRATOZ Ill campaign. (Based on Ehhalt eta/., 1993.) An example is given in Figure 11-8, which presents the daytime NO distribution across the North Atlantic dur­ ing the period June 4-6, 1984, of the Stratospheric Ozone Figure 1 1-10), Ehhalt et al. (1992) could reproduce quite (STRATOZ III) campaign (Ehhalt et al., 1993). Large reasonably the measured vertical profiles shown in Fig­ longitudinal gradients of NO mixing ratio up to a factor ure 11-9. The transport of polluted air masses from the of 5 were observed at all altitudes in the free troposphere planetary boundary layer to the upper troposphere by in which the effectsof an outflow of polluted air from the fast vertical convection is considered an important pro­ European continent are seen. This tongue of high NO cess for NOx by these authors. However, Kasibhatla over the Eastern Atlantic was accompanied by elevated (1993) suggests that the stratospheric source is a more CO and CH4 mixing ratios and therefore was probably important source than that arising from rapid vertical due to surface sources. Figure 11-8 also illustrates the convection, but the calculations did not consider light­ variance superimposed by longitudinal gradients on av­ ning, biomass burning, and soil emissions, and the erage meridional cross sections. However, at present heterogeneous removal of N205. there are not enough data to derive the respective global Despite considerable differences in model trans­ contributions from atmospheric measurements alone. In­ port characteristics and emission rates, all the studies dependent estimates of the various source strengths are suggest that aircraft are important contributors to upper needed. Our lack of knowledge about the NOx budget in tropospheric NOx and NOy concentrations. For example the troposphere, especially in the upper troposphere, Ehhalt et al. (1992) suggest that aircraft emissions ( esti­ makes model predictions for this region questionable. mated for 1984) contribute around 30% to upper Thus, at present, we can have little confidence in our tropospheric NOx (Figure 11- 10). Kasibhatla (1993) es­ ability to correctly model subsonic aircraft effectson the timates that about 30% of the NOx in the upper atmosphere. troposphere between 30° and 60°N are from aircraft. It is Figure 11-9 shows published comparisons of clear from the results of Beck et al. (1992) and Kasibhat­ available NO measurements (Wahner et al. , 1994) with la (1993) that despite large latitudinal variations in the predictions from two-dimensional models (Berntsen and rate of aircraft emissions, the impacts become manifest Isaksen, 1992). Using a quasi-two dimensional longi­ over the entire zonal band, though not evenly. This be­ tude-height model and considering estimates of all havior is in contrast to the behavior in the lower important tropospheric sources of NOx (input from the troposphere, and is due to the slower conversion of NOx stratosphere, lightning, fossil fuel combustion, soil emis­ to formHN0 3, and the slower removal rates for HN03, sions and aircraft) for the latitude band of 40° -50°N (see which allow for reconversion back to NOx·

11.21 AIRCRAFT EMISSIONS

- STRATOZIII June 1984 (Drummondet al. 1988)

E' 1o Model calculation June 1984 � ...... Ehhalt et al I 1992 I Model calculation August I Berntsen et al 1992 \ 5 ----a- TROPOZ II January 1991

Model calculation January 1991 Wahner et al 1993

0 0 100 200 300 400 500 NO MIXING RATIO (ppt)

Figure 11-9. Comparisons of measured vertical profiles of NO (June 1984 and January 1991) with calcula­ tions from two-dimensional models. (Based on data from: Wahner eta/., 1994; Berntsen and Isaksen, 1992; Drummond eta/., 1988.)

Several authors discuss the changes to OH concen­ composition changes, as well as uncertainties associated tration consequent to the growth in ozone, and the with the climate effects themselves. Therefore, the fol­ consequences to methane destruction. Beck et al. (1992) lowing discussion will be on possible mechanisms by

predicts OH changes of + 10% at around 10 krn for the which aircraft operations might affect climate, along region 30°-60°N. Similar values are suggested by Fug­ with some estimates of their relative importance. lestvedt and Isaksen (1992) (+20%) and Rohrer et al. Increases of C02 and water vapor, and alterations (1993) ( + 12% ). These subsonic aircraft results should be of ozone and cirrus clouds have the potential to alter in considered as being preliminary given the complexity of situ and global climate by changing the infrared (green­ the models, the lack of model intercomparison exercises, house) opacity of the atmosphere and solar forcing. as well as the paucity of measurements to test against Sulfuric acid, which results from SOx emissions, may model results. cool the climate through producing aerosols that give in­ creased scattering of incoming solar radiation, while soot has both longwave and shortwave radiation im­ 11.6 CLIMATE EFFECTS pacts. The direct radiative impact for the troposphere as Both subsonic and supersonic aircraft emissions a whole is largest for concentration changes in the upper include constituents with the potential to alter the local troposphere and lower stratosphere, where the effective­ and global climate. Species important in this respect in­ ness is amplified by the colder radiating temperatures. clude water vapor, NOx (through its impact on 03), However, the impact (including feedbacks) on surface sulfur, soot, cloud condensation nuclei, and C02. How­ air temperature may be limited if changes at the tropo­ ever, quantitative assessments of the climate effects of pause are not effectively transmitted to the surface (see aircraft operations are difficult to make at this time, giv­ Chapter 8). en the uncertainty in the resulting atmospheric

11.22 AIRCRAFT EMISSIONS

65" 55" 45" 35° 25" 1 5" 5° w

E 10 � - +-' a .c 5 0> ·a; I 0.1 0.2 0.3 0 OJ 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3

NO I ppb June 1984

0 stratosphere • aircraft �lightning � surface

° ° ° sso 55 45" 35° 25 1 5 5° w

E 10 � - b +-' .c 5 0> ·a; I 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3 0 0.1 0.2 0.3

NO I ppb January 1991

Figure 11-10. Calculations of vertical profiles of NO during summer (June, top panel) and winter (January, bottom panel) using a quasi-two dimensional longitude-height model for the latitude band of 40°-50°N. The different shadings relate to the different sources: stratosphere, lightning, surface (fossil fuel combustion and soil emissions), and aircraft (Ehhalt et at., 1992).

11.6.1 Ozone atmosphere model (Rind et al., 1988), the resulting change in global average surface air temperature was As has been discussed in Chapter 8, the impact of approximately -0.03°C. The net result is a consequence ozone changes on the radiation balance of the surface­ of the net effect of varying influences: ozone reduction troposphere system depends on the vertical distribution in the stratosphere at 20 km, and ozone increases in the of the ozone changes. Reduction in tropospheric and upper troposphere produce surface warming, while lower stratospheric ozone tends to cool the climate, by ozone reduction in the lower stratosphere produces sur­ reducing the atmospheric greenhouse effect. Reduction face cooling. The net result provides the small in middle and upper stratospheric ozone tends to warm temperature changes found in this experiment. the climate, by allowing more shortwave radiation to Assuming a local ozone increase (8 to 12 km, 30° reach the surface (Lacis et al. , 1990). to 50°N) of 4 - 7% due to doubling of the subsonic air­ The preliminary assessments of the HSRP/AES A craft NOx emission and incorporating these changes into program are that supersonic aircraft operations could de­ the Wang et al. ( 1991) model, the inference can be drawn crease ozone in the lower stratosphere by less than 2 that a radiative forcing of 0.04 to 0.07 W m-2 will result percent for an EI(NOx) of 15, while increasing it in the (Mohnen et al. , 1993; Fortuin et al. , 1994). This radia­ upper troposphere by a similar percentage. When these tive forcing is of the same order as that resulting from the ozone changes were put into the NASA Goddard Insti­ aircraft C02 emissions (see Chapter 8.2.1). The estimat­ tute for Space Studies (GISS) 3-D climate/middle ed feedback on radiative forcing from methane

11.23 AIRCRAFTEM ISSIONS

decreases (due to the OH increase from increasing NOJ 11.6.4 Soot has been estimated to be small using two-dimensional Particles containing elemental carbon are the re­ models (Johnson, 1994; Fuglestvedt et al., 1994). sult of incomplete combustion of carbonaceous fuel.

11.6.2 Water Vapor Such particles have greater shortwave absorbing charac­ teristics than do sulfuric acid aerosols, and thus a Water vapor is the primary atmospheric green­ different shortwavellongwave impact on net radiation. house gas. Increases in water vapor associated with Upper tropospheric aircraft emissions of soot presently aircraftemissions have the potential to warm the climate account for about 0.3% of the background aerosol at low tropospheric levels, while cooling at altitudes of (Pueschel et al. , 1992). release, due to greater thermal emission. The effects are The total soot source for the stratosphere is cur­ largest when water vapor perturbations occur near the rently estimated as 0.00 1 teragrams/year (Stolarski and tropopause (GraBl, 1990; Rind and Lacis, 1993), as is Wesoky, 1993b), most likely coming primarily from likely to be the case. commercial air traffic.This accounts for about 0.01% of High-speed aircraft may increase stratospheric the total stratospheric (background) aerosol loading water vapor by up to 0.8 ppmv for a corridor at Northern (Pueschel et al. , 1992). It is estimated that the proposed Hemisphere midlatitudes, with a Northern Hemispheric HSCT fleet would double stratospheric soot concentra­ effect perhaps 114 as large (Albritton et al. , 1993). When tions for the hemisphere as a whole, while increases of changes of this magnitude were used as input to the up to a factor of ten could occur in flight corridors (Tur­ stratosphere, the GISS climate/middle atmosphere mod­ co, 1992). el failed to show any appreciable surface warming, as the radiative effect of the negative feedbacks (primarily 11.6.5 Cloud Condensation Nuclei cloud cover changes) were as important as the strato­ Contrails in the upper atmosphere act in a manner spheric water forcing. In general, the stratosphere cooled somewhat similar to cirrus clouds, with the capability of by a few tenths of a degree, associated with the increased warming the climate by increasing longwave energy ab­ thermal emission. sorption in addition to the shortwave cooling effect. Subsonic tropospheric emissions of water vapor Aircraft sulfur emissions in addition to frozen droplets could possibly result in increases on the order of 0.02 are the most likely contributor to this "indirect" effectof ppmv. Shine and Sinha (1991) estimate that a global in­ aerosols, but soot might also be important. crease of 1 ppmv for a 50 mbar slab between 400 and The impact of aircraft particle emissions on upper 100 mbar would increase surface air temperature by tropospheric cloud amounts and optical processes is not 0.02°C. Therefore the climate effects from subsonic wa­ yet known, though it is likely to grow with increased air ter vapor emission by aircraft seem to be very small. traffic.Chang es in cloud cover and cloud optical thick­

11.6.3 Sulfuric Acid Aerosols ness resulting from aircraft operations might be the most significant aircraft/climate effect, but quantitative evalu­ Subsonic aircraft, flying both in the troposphere ations of this are very uncertain. In a 2-D analysis, and stratosphere, are presently adding significant increases in cirrus clouds of 5% between 20-70°N pro­ amounts of sulfur to the atmosphere. Hofmann (1991) duced a warming of 1 °C, due to increased thermal has estimated that the current fleet may be contributing absorption (Liou et al. , 1990). For 0.4% additional cloud about 65% of the background non-volcanic stratospheric coverage by contrails and mid-European conditions, an aerosol amount, whose optical thickness is approximate­ increase in surface temperature of about 0.05°C is esti­ ly 1 - 2 x lQ-3; note however, that this view is a mated (Schumann, 1994). controversial one as can be seen in Section 3.2.1 of Chapter 6. This added optical thickness would imply a 11.6.6 Carbon Dioxide contribution to the equilibrium surface air temperature While aircraft C02 emissions are at a different al­ cooling on the order of 0.03°C due to aircraft sulfur titude from other anthropogenic emissions, the climate emissions (Pollack et al., 1993).

11.24 AIRCRAFT EMISSIONS

impact should be qualitatively similar, as C02 is a rela­ 11.7.2 Modeling Uncertainties tively well-mixed gas. Therefore the climate impact There are two types of modeling uncertainties in from subsonic C02 emissions can be estimated to be ap­ the aircraft assessment process. One is related to model­ proximately 3% of the total anthropogenic C02 impact, ing of small-scale plume processes, while the other since subsonic aircraft fuel consumption is about 3% of relates to the global atmospheric modeling. the global fo ssil fuel consumption.

PLuME MoDELING 11.7 UNCERTAINTIES As was indicated earlier in this chapter, consider­ This chapter deals with the atmospheric effects of able modeling is required to characterize the evolution of both the present subsonic aircraft fleetand an envisioned the aircraft exhaust leaving the engines' tailpipes to future supersonic aircraft fleet. The uncertainties in as­ flight corridor spatial scales and then to the scales that sessing these two atmospheric effects are of a different are treated in the atmospheric models of aircraft effects. nature. For instance, there is a real uncertainty in the These plume models must treat turbulent dynamics and present emissions data base that results from uncertain­ both gas phase and heterogeneous chemistry. Only one ties in the aircraft engine characteristics, engine such model presently exists that treats the full problem operations, and air traffic data. There are also uncertain­ and there exists no measurement program that is aimed ties relating to the models being used to examine the at the validation of this model (Miake-Lye et al. , 1993). atmospheric effects of these subsonic emissions. In the There have been very few actual measurements in air­ supersonic case, assessments are being made for a hypo­ plane exhaust wakes. There are the chemical thetical aircraft fleet, so modeling uncertainties are the measurements at altitudes of about 10 km by Arnold et main concern. The modeling uncertainties are probably al. (1992), and there were turbulence and humidity data much greater than the emission uncertainties at the taken by Baumann et al. (1993) at the same time. Also, present time. there are the SPADE (Stratospheric Photochemistry, Aerosols, and Dynamics Expedition) measurements tak­ 11.7.1 Emissions Uncertainties en during crossings of the ER-2 exhaust plume (Fahey et al., 1994). These measurements, while valuable, are not As was indicated previously, the evaluation of a sufficient to validate the plume processing model. time-dependent emissions data base for use in atmo­ spheric chemical-transport models requires a rather ATMOSPHERIC MODELING complete knowledge of the specific emissions produced by all types of aircraft, as well as a knowledge of the The upper troposphere and lower stratosphere, the operations and routing of the aircraft fleet. regions of major interest in this chapter, are particularly There has been very limited aircraft engine testing difficult regions to model. In 2-D models of supersonic under realistic cruise conditions for the present subsonic aircraft effects, the meridional transport circulation is aircraft fleet. At the present time, some engine tests are difficult to obtain since the radiative heating is com­ being carried out under simulated altitude conditions to prised of a number of small terms of differentsign. Thus, see if the present method of determining NOx, for exam­ small changes in any radiation term can have important ple, from a combination of theoretical studies and consequences for transport. Similarly, the time scales for laboratory combustor testing can be validated. both transport and chemistry to modify the ozone distri­ A disagreement exists between the quantity of fuel bution are generally long and comparable. The complete produced and predicted fu el usage by the data bases. problem must be solved. The NOx, HOx, and ClOx This discrepancy probably results from uncertainties in chemical processes are highly coupled in the strato­ emissions for the non-OECD (Organization for Eco­ sphere. Modeling the chemical balance correctly, in nomic Cooperation and Development) countries and for regions where few measurements are available, presents military traffic, and from the uncertain estimates ofload­ formidable difficulties. This situation is even worse in ing and power settings of the aircraft fleet. the upper troposphere than in the stratosphere, given that

11.25 AIRCRAFTEMISSIONS the chemistry of the upper troposphere is more complex It is also necessary to model stratospheric-tropospheric and there are fewer existing observations of this region. transport processes carefully so that NOx fluxesand con­ Supersonic aircraft have their cruising altitudes in centrations in the region near the tropopause are the middle stratosphere (near 20 km) while subsonic air­ realistic. This requires a substantial effortto improve our crafthave cruise altitudes that lie both in the troposphere understanding of stratosphere-troposphere exchange and lower stratosphere. Supersonic assessment calcula­ processes. tions have been done using 2-D models up to the present time, while it is generally appreciated that 3-D models CHEMICAL CHANGES will be necessary for credible subsonic assessments. The effectof NOx emitted by subsonic aircraft de­ Thus, separate discussions of modeling uncertainties pends on the amount ofNOx in the free troposphere. The follow for aircraft perturbations in the stratosphere and ambient NOx concentrations are not very well known, in the troposphere. and depend on several factors such as surface emission from anthropogenic and natural biogenic sources, the TRANSPORT strength of the lightning source for NOx, and the trans­ Two particular problems relating to atmospheric port of stratospheric NOx into the troposphere (see transport are extremely important for the supersonic air­ Chapter 2, Table 2-5). The inclusion of wet and dry dep­ craft problem. First, stratosphere-troposphere exchange, osition processes and entrainment in clouds in which cannot be modeled in detail with great confidence assessment models is at a very preliminary stage. in global (2-D or 3-D) models, is clearly of special sig­ Heterogeneous chemistry is another important nificanceto the chemical distribution in these regions, to area of uncertainty for models of the troposphere and the lifetime of emitted species, etc. More work on this lower stratosphere. For example, the hydrolysis of N20s topic is essential. Second, the present 2-D assessment is important in both the troposphere and stratosphere, but models do not model well the details of the polar vortex, the precise rate for this reaction is not known. Observa­ although improvements are anticipated when these mod­ tional studies are needed to elucidate the exact nature els include the Garcia (1991) parameterization for and area of the reactive surfaces. Furthermore, at the breaking planetary waves. If the ideas of the polar vortex present time, heterogeneous chemistry is being crudely as a "flowingpr ocessor" are correct (see Chapter 3), then modeled. Although there do exist models describing the the correct modeling of polar vortex dynamics will have size distribution and composition of stratospheric aero­ a crucial impact on the distribution of species in the low­ sols, no aircraft assessment model presently exists that er stratosphere, and present 2-D models would clearly be incorporates and calculates aerosol chemistry. performing poorly there. There is also the larger issue In supersonic assessment models, it is important to that the uncertainty connected with the use of 2-D mod­ properly model the switch over (at some altitude) from els to assess the inherently 3-D aircraft emission NOx-induced net ozone production to net ozone destruc­ problem needs to be evaluated further. Even when 3-D tion. The precise altitude at which this switch over models are available to model this problem, however, the occurs differs from model to model, and this can lead to question will remain as to how well these 3-D models very different ozone changes in different models of su­ simulate the actual atmosphere until adequate measure­ personic aircraft effects. The different responses of the ment-model comparisons are done. various models used in the HSCT/AESA assessment of For modeling aimed at assessing the atmospheric the impact of changed EI (see Tables 11-4 and 11-5, for effects of both subsonic and supersonic aircraft, it is cru­ example) point to important, unresolved differences in cial to properly model ambient NOx distributions in the these models that must be addressed before a satisfac­ upper troposphere, and these, in tum, depend on proper­ tory assessment of the atmospheric effects of supersonic ly modeling transport between the boundary layer and aircraft can be made with confidence. Also, it is clear the free troposphere, on proper modeling of the fast up­ from examining the modeled 03 changes in Chapter 6 ward vertical transport accompanying convection, and that the model results at altitudes below about 30 km dif­ on modeling the lightning source for NOx. Considerable fer significantlyfrom one another. They also do not give effort is needed to improve our capability in these areas. as large 03 losses as are observed (see Chapter 1). This

11.26 AIRCRAFT EMISSIONS problem is particularly acute if one accepts the SAGE ACRONYMS results indicating large decreases in ozone concentra­ AER Atmospheric and Environmental tions just above the tropopause (see Chapter I) as being Research, Inc. correct. Then, the fact that present stratospheric models AERONOX The Impact of NOx Emissions from do not correctly give this effect casts doubt on present Aircraft upon the Atmosphere assessment models to correctly simulate that atmospher­ AESA Atmospheric Effects of Stratospheric ic region. Since it is in this region where effects from Aircraft aircraft operations are particularly significant, there is AN CAT Abatement of Nuisance Caused by Air the question of how well we can correctly predict atmo­ Traffic spheric effects in this altitude region. It may be that the CAMED University of Cambridge and University SAGE ozone trends in this region are in error, or it may of Edingburgh be that important effects in this region are not properly CEC Commission of the European included in present models. Communities

11.7.3 Climate Uncertainties ClAP Climatic Impact Assessment Program ECAC European Civil Aviation Conference The study of the possible impact of aircraft on cli­ ECMWF European Centre for Medium-Range mate is now just beginning. One can make some Weather Forecasts preliminary extrapolations based on existing climate re­ EI Emission Index search, but one should appreciate that the complexity of GISS NASA Goddard Institute for Space climate research, in general, implies that it will be some Studies time before great confidencecan exist in estimates of air­ GSFC NASA Goddard Space Flight Center craft impacts on climate. HSCT High-Speed Civil Transport HSRP High Speed Research Program 11.7.4 Surprises ICAO International Civil Aviation Organization Early assessments of the impact of aircraft on the lEA InternationalEnergy Agency stratosphere varied enormously with time as understand­ LLNL Lawrence Livermore National Laboratory ing slowly improved. Our understanding of the lower LTO Landing/Take-Offcycle stratosphere/upper troposphere region is still far from MOZAIC Measurement of Ozone on Airbus complete and surprises can still be anticipated, which In-service Aircraft may either result in greater or lesser aircraft effects on NASA National Aeronautics and Space the atmosphere. Administration NCAR National Center for Atmospheric Research NRC National Research Council OECD Organization for Economic Cooperation and Development OSLO University of Oslo POLINAT Pollution from Aircraft Emissions in the North Atlantic Flight Corridor SAGE Stratospheric Aerosol and Gas Experiment SBUV Solar Backscatter Ultraviolet spectrometer SPADE Stratospheric Photochemistry, Aerosols, and Dynamics Expedition WMO World Meteorological Organization

11.27 AIRCRAFT EMISSIONS

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