Global warming impacts of transport

The Science of the Total Environment, 134 (1993) 117-124 Elsevier Science Publishers B.V., Amsterdam

117

Global warming impacts of transport* Laurie Michaelis Strategic Studies Department, Energy Technology Support Unit, ltarwell Laboratory, O~ford~hire, OXII, ORA, UK

ABSTRACT Global warming is expected to occur due to carbon dioxide and other gases emitted in the course of human activities. Due to uncertainties about atmospheric chemistry, few attempts have been made to quantify the impact of emissions of NO~, non-methane hydrocarbons and carbon monoxide. Atmospheric chemistry modelling at Harwell Laboratory has resulted in preliminary estimates of the impact of trace gases including NO, many hydrocarbon species, carbon monoxide and water vapour on global warming. These estimates have been applied to emissions from transport, with particular attention to cars. It is found that the use of three. way catalysts on cars could reduce their global warming impact. IDI diesel engines have still less global warming impact. High N O , CO and hydrocarbon emissions from DI diesels could result in a higher impact from these engines than from IDI diesels. Emissions from aircraft are injected into the troposphere at the height where they have a maximal effect on global warming. Although modelling of aircraft impact is at a very early stage, the first results in. dicate that air travel could become an important cause of global warming. Key words: global warming: impact: transport; carbon dioxide; nitrogen oxides RESUME

On s'attend h ce qu'un rdcha~fement gdndral ,re produise grt}ce au dioxyde de carbone el aux autres gaz emis lots des activitds humaines. Etant donnd les incertitudes ~ propos de la chimie atmosphdrique, pcu de tentattves ont dtd fattes pour dvaluer quantitativement l'impact des emissions de NO~, des hydrocarbures (exptd h, mdthane) et du monoryde de carbone. Le mod~lisation de la chimie atmosphdrique effeetude au Harwell Laboratory a permis de montrer en premibr¢ estimation, I'¢ffet sur le r~chauffement gdndra/ de traces de gaz incluant du NO~° de nombreuses espbces d'hydrocarbures, du monoxyde de carbone et de ia vapeur d'eau. Les estimations prennent en compte les dmissions provenant des modes de transport, avec un attention particulibre pour les voitures. !1 apparait que I'utilisation de convertisseurs catalytiques trois voles sur les voitures pourrait rdduire leur impact sur le Nchauffement g~n~ral. Les moteurs diesels IDi ont encore moins d'effet sur le rechauffement g~n~rai. Les fortes dmissions de NOlo de CO et d'hyrocarbures des diesels DI pourraient induire un plus fort impact de ces moteurs que celui des diesels IDI. Les dmissions des avions sont inject~es dans la troposphi're iz l'altitude oi~ ils ont *This paper describes results of work carried out under the Corporate Research and Application Dfvelopment Programme of AEA Environment and Energy.

0048.9697/93/$06.00

© 1993 Elsevier Science Publishers B.V. All rights reserved

L. MICHAEl.IS

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un effet maximum sur le rdchauffement gdndral, Bien que ia modelisation de l'impact des nylons n'en soil qu'it ses d~bu,, les premiers r6sultats indiquent clue le transport adrien dans la Communauld europdenne pourrait devenir une cause importante du rdchauffement general. Mots ¢1~: richouffement gdndral; impact; transport; dmissions INTRODUCTION

Over the past 5 years, there has been growing acknowledgement of the possibility that carbon dioxide emissions from industrial society may lead to global wakening. Concern has spread to other so called 'greenhouse gases', especially methane, CFCs and nitrous oxide. Some attempts have been made to quantify the emissions of these gases In the case of carbon dioxide and CFCs measures are already being taken to halt the rise in emissions. There has been somewhat less awareness of the importance of ozone for global warming. Ozone bas a lifetime of only 0. I-0.3 years in the troposphere, but it can have a significant warming effect. Using results supplied by Harwell atmospheric chemistry modeilers, an attempt has been made at quantifying the warming impact of pollutants from specific transport technologies. The results will be presented in this paper although the large uncertainties involved in the calculations must be em. phasised. The paper will discuss: (i) the effects of transport emissions on greenhouse gases: (ii) the effects of ozone and other gases on global warming and the sources of uncertainty: (iii) the global warming impact of various car technologies, and (iv) the warming impact of different transport modes. ROLE OF VEHICLE EMISSIONS

Emissions from vehicles can have direct warming impacts (as in the case of carbot~ dioxide) or indirect impacts (as in the case of nitrogen dioxide). Most of the emissions have both direct and indirect effects. Table 1 gives an indication of the role played by some internal combustion engine emissions. The greatest uncertainty surrounds the atmospheric chemistry involving CO, hydrocarbons and NOx, These gases all affect the tropospheric balance of ozone and methane, both strong greenhouse gases. The magnitude and overall direction of these effects is still subject to uncertainty. Part of this uncertainty is due to the interaction between methane and ozone. Methane is removed from the troposphere by OH and the level of OH is raised when the ozone concentration is increased, O:on¢ production Tropospheric ozone is produced as a result of a large number of reactions

| 19

GLOBAL W A R M I N G IMPACTS OF TRANSPORT

TABLE 1 Effect of emissions on greenhouse gases Emissions

CO: CO CH4 Other HCs NO, NO., N.,O H~O

Effect on greenhouse gases CO2

CH4

N20

Direct l l l -. .

. . ! Direct ! 1 . .

. l? Direct

O~

H20

!" l" :" ! ==

! ! Direct

.

"The direction of these effects d e ~ n d s on the NO~ concentration. At low NO, levels ozone is removed by these gases, but at high NOs levels the ozone level is increased.

involving oxygen, hydrocarbons and nitrogen oxides, ~ith the help of sunlight. Ozone is one of the most damaging chemicals in 'photochemical smog' produced in this way. Although photochemical smogs are relatively rare events associated with high concentrations of pollutants, some ozone is produced in the troposphere from background concentrations of chemicals. A number of research groups have used computer models to calculate the effects of pollution on ozone concentration. Although the models are reasonably good at predicting ground level ozone levels, there are inaccuracies in prediction of the height distribution. Perhaps more importantly, the models do not predict water vapour levels we!!. It is the interaction between ozone and water vapour that generates OH which scavenges methane and other pollutants. G L O B A L W A R M I N G IMPACT

Table 2 shows initial estimates of the global warming potentials (GWPs) of some gases relative to that ofcarbon dioxide. The GWP of a gas as defined here is the number of kilograms of CO2 that would have to be emitted to have the same radiative forcing as one kilogram of that gas. These estimates are based on computer models of the type described above. Account has been taken in the Intergovernmental Panel on Chmate Change (IPCC) data of the formation of tropospheric ozone, stratospheric water vapour and carbon dioxide. The IPCC data is largely influenced by the work of Derwent [1] which was based on the Harwell model. The data was included in the IPCC report as an example of calculation of GWPs. There is continuing debate about, arid research into, the values of the GWPs.

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L, MICHAELIS

TABLE 2

Global warming potentials of selected gases Gas

Time horizon (years) Provisional revised estimates a

IPCC (3) I00

:SO0

CO~

I

I

I

I

I

I

CO

? 31 63 1:9) -

3 II 21 40

~ 6 9 14

4

2 4 6 0

-

-

,42b 0 660

2,5 $ 14 0

270

290

190

270

NMHCs Ci~ NO2 (ground level emissions) NO: (aircraft tqrnissions} N20

II

166 290

66 190

'This data includes revised estimates of impacts through r a i d ozone levels and an allowance for the impact of d ~ ~ methane levels. bData for methane needs further revision,

The revised estimates in Table 2 are based on a re-examination of the modelling output reported in 1990 [2], They take some account of the height distribution of ozone in the troposphere, and the effects of ozone on the rate of methane removal. The uncertainty relating to the GWP of NO~ in particular is very great, It is possible that NO~ emissions may have a negative greenhouse impact, because they reduce the level of methane in the troposphere, Examination of the model results has also shown that use of a simple lifetime approach to calculation of the warming impact of methane may be inappropriate, There seems to be some buffering effect which compensates for increases in methane emissions, Further runs of the Harwell model are planned, to investigate the effects on the model results of recently collected data on reaction rates and species concentrations in the atmosphere. Model runs co Vermg longer timespans are ' also planned to improve understanding of the chemistry involving methane. GLOBAL WARMING DUE TO CAR EMISSIONS

I ~ ¢ t of CO~and regulatedpollutants Several surveys have been carried out in Europe of emissions from cars. Most of the samples used are quite small and the cars are tested on specific

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GLOBAL WARMING IMPACTS OF TRANSPORT

duty cycles, so it is not possible to extrapolate the results to give total European emissions. Dunne [4] gives an account of measurements of emissions from a number of alternative vehicle technologies. Many of these tests involved only one vehicle of a given type although samples of six or more were used for the R15-04, three-way catalyst and IDI diesel cars. A number of driving cycles were used including the ECE R I 5 test cycle, the new Extra-Urban Driving Cycle and various real driving cycles. In order to give a representative view of overall emissions by various vehicle types, a weighted mean has been calculated here for each gas and these are shown in Table 3. The weights are chosen to reflect the breakdown of traffic in the UK between urban roads, rural roads and motorways [5]. No firm conclusion, can be drawn from testing of single vehicles and very small samples regarding the relative merits of the technologies. However some interesting points emerge. Firstly, it is generally accepted that three-way catalysts increase CO2 emissions by 5-10%. Table 3 suggests that the greenhouse impact of this is more than offset by the reduction in other emissions. This is true for each set of GWPs considered here. Secondly, the warming impact of the leanburn vehicle (only one was tested) is less than that of the catalyst vehicle if the revised GWPs are used. If the IPCC GWPs are used the catalyst vehicle has less impact. Thirdly, of the vehicles tested, indirect injection diesel vehicles appear to be the most attractive in greenhouse terms. This is true for all the sets of GWPs considered. TABLE 3 Global warming impacts of altemativ~ technology vehicles Vehicle

RI5-04 Three-way catalyst Leanburn Leanburn/Oxidising catalyst IDI diesel DI diesel LPG

Mean emissions (g/kin)

CO2 equivs. (g/kin)

CO2

CO

THC

NO~

20 year n

I00 year a

20 year b

151 180 132 144

6.1 0.87 8.2 4.5

1.27 0. ! 3 I. 1 0.50

2.7 0.13 1.5 0.65

635 210 452 289

291 189 230 190

190 185 177 168

131 126 192

0.43 ~.53 37

0.23 0.73 0.86

0.46 I.I 1.4

211 324 686

153 182 367

135 137 348

abased on IPCC GWPs from Table 2. bBased on vrovisional revised estimates from Table 2,

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I.. MICHAELlS

The further benefit of reduced CO2 emissions of DI diesel may be outweighed by the impact of higher NO~ emissions.

Impact of ~regulated emissions In addition to the gases mentioned in Table 3, cars emit nitrous oxide (N20) and some methane. Neither of these unregulated gases is normally measured in car exhaust but some data is available. Difficulties have been encountered in the measurement of t~itrous oxide emissions, because concentrations in car exhaust are very close to atmospheric background levels and emissions are strongly dependent on the vehicle and driving conditions. In certain circumstances exhaust contains less nitrous oxide than background 0

air.

N20 emissions from three-way catalyst vehicles have been measured and are three to five times higher (J. Bailey, pers. commun.) than those from noncatalyst cars. This has been an additional source of concern regarding the greenhouse impact of catalysts. With emissions in the region of 30-50 mg/km, the greenhouse impact of N20 from catalyst vehicles would amount to 8-15 g/km CO~ equivalent or about 5% of total catalyst car emissions. Methane emissions from non-catalyst cars are about 4-5% (J. Bailey, pers. commun.) of the total hydrocarbon emissions, with a warming impact of 1.$-4.$ g/kin CO2 equivalent (depending on time horizon). With three-way catalysts methane emissions can be raised. IMPACT IN EUROPE OF EMISSIONS FROM VARIOUS TRANSPORT MODES

The analysis of global warming impact has been repeated in relation to emissions from trains, road vehicles and aircraft, using aggregate data for Europe [6]. Emissions from aircraft require particular attention. The ratio of NO~ emiuions to CO~ emissions from aircraft engines is lower than that from car engines. However the impact of NO~ emissions at aircraft cruising heights is thought to be much greater [7]. No measurements are currently available of NO, emissions from cruising aircraft. Data is available [8] for emissions from aircraft in take-off and landing cycles. It is found that the NO, emission rate from a given engine is roughly proportional to the square of the fuel consumption rate. On this basis NO~ emissions during cruise can be estimated from known fuel consumption rates. Surprisingly the values found (around 10-12 g NO~/kg fuel) agree with the results of detailed modelling of engine conditions during cruise (M. Williams, pers. commun.).

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GLOBAL WARMING IMPACTS OF TRANSPORT

TABLE 4 Global warming impact of European transport Transport mode

Emissions a (Mr/year) C02

Road (petrol) Road (diesel) Rail (diesel) Rail (electric) Air

CO

353.72 14.3 260.37 1.5 8.48 0.13 35.78 0.01 85.37 0.35

Impact (Mr/year) CO2 equivs,

THCs NOx

2.97 2. I 0,18 0.01 0.06

6.32 3.5 0.2"/ 0.14 0,25

20 year b

100 year b

20 yearc

1493 861 55 57 127

796 423 22 42 97

394 272 9 36 251

~These emissions have been calculated on the basis of energy consumption data for each ~ctor and pollutant emission factors estimated from a number of sources. bBased on IPCC data from Table 2. eBased on provisional revised estimates from Table 2,

Using an emission factor of |1 g NO~/kg fuel the greenhouse impact of aircraft emissions has been calculated for the European Community. It can be seen from Table 4 that the aircraft impact is over one-third that of road transport on a 20-year time horizon. CONCLUSIONS

At this stage the global warming impact of emissions of NOs, hydrocarbons and carbon monoxide is not quantified well enough to give firm guidance on priorities for greenhouse gas abatement. It does appear that NOx emissions from aircraft have a particularly large impact. If, as widely predicted, air travel is to continue to increase, significant changes in aircraft technology will be required. NOx, THC and CO emissions from surface transport may also be important for the greenhouse effect but the growing use of three-way catalysts and diesel cars will reduce emissions significantly. The main challenge for the transport sector will therefore be abatement of carbon dioxide emissions, through improved vehicle energy efficiency and changing patterns of demand. Further modelling work is planned at Harwell to incorporate recently produced data on atmospheric chemistry and to improve understanding of the interactions between ozone and methane.

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L. MIC,^ELIS

REFERENCES 1 2 3 4 5 6 7 8

R.G. Derwent, Trace Gases and Their Relative Contribution to the Greenhouse Effect. Elarwell Laboratory, Report R 13716, January 1990. A.M. Hough and C.E~ Johnson, Modelling the Role of Nitrogen Oxides. Hydrocarbons and Carbon Monoxide in the Global Formation of Tropospheric Oxidants, Harwell Laboratory, Report R 13545, May I ~ . Intergovemmental Panel on Climate Change, Climate Change: The lPCC Scientihc Assessment, Cambridge University Press, 1990. J.M. Dunne, A Comparison of Various Emission Control Technology Cars and Their Influence on Exhaust Emissions and Fuel Economy, Report LR 770 (AP), Warren Spring Laboratory, UK, June I ~ . Department of Transport, UK, Transport Statistics Great Britain 1977-1987, HMSO, 1988. Eurostat, Energy Balance Sheets 1987--1988,Office des publications officielles des Coral munaut~ Europ6ennes, Luxembourg, 1990. C.E. Johnson and J. Henshaw, The Impact of NO~ Emissions from Tropospileric Aircraft, CEC Study Contract CEC EV4V-020$.UK(A), Harwell Laboratory, January 1991. United States Department of Commerce, Compilation of Air Pollutant Emission Factors, Vol. 2, Mobile Sources, Document No. I)(387-205266, National Technical Information Service, 4th edn., 1985.