Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

EMISSIONS: EN ERGY, ROAD
TRANSPOR T
ACKNOWLEDGEMENTS
This paper was written by Simon Eggleston (AEA Technology Environment, UK) and Michael Walsh. Several
reviewers made valuable comments on draft versions of this paper, including Cindy Jacobs, and Tim Murrells.

ABSTRACT
Mobile sources account for a large fraction of fossil fuel combustion in most countries. Of this the largest sourceis road transport. In 1996, road transport accounted for 24% of CO2 emissions from fuel use in the USA while inEurope the figure was 22%.
Road transport emits mainly CO2, NOx, CO and NMVOCs, however it is also a small source of N2O, CH4 andNH3. Therefore the only major direct greenhouse gas emission is CO2. Emissions of CO2 are directly related tothe amount of fuel used. Emissions of the remaining gases depend on the amount of fuel used but are also
affected by the way the vehicle is driven (e.g. the speed, acceleration and load on the vehicle), the vehicle type,the fuel used and technology used to control emissions (e.g. catalysts). Thus the simplest way to estimate theemissions of the other gases is to use fuel based emission factors; this is only appropriate where there isinsufficient data to use the more complete methods available.
Methodology used depends on national legislation and availability of statistical data. The IPCC guidebook isbased on USA1 and European2 experience.
Across the entire globe, motor vehicle usage has increased tremendously. In 1950, there were about 50 million
vehicles on the world’s roads; this number now exceeds 700 million - almost 500 million light duty vehicles,
about 150 million commercial trucks and buses and another 100 million motorcycles. Over the past forty years,
each year on average, the fleet has grown by about 11 million automobiles, 3.6 million commercial vehicles and2.9 million motorcycles.1 While the growth rate has slowed in the highly industrialised countries, population
growth and increased urbanisation and industrialisation are accelerating the use of motor vehicles elsewhere.
Over the past several decades, motor vehicle production has gradually expanded from one region of the world toanother. Initially and through the 1950’s, it was dominated by North America. The first wave of competitioncame from Europe, and by the late 1960s European production had surpassed that of the United States. Over the
past two decades the car industry in Asia, led by Japan, has grown rapidly and now rivals both those in the
United States and Europe. Both Latin America and Eastern Europe appear poised to grow substantially in future
decades. For example, driven in large part by Brazil, motor vehicle production in South America now exceeds 2
million units per year.
One result is that areas of rapid industrialisation are now starting to experience similar air pollution problems tothose of the industrialised world. Cities such as Mexico, Delhi, Seoul, Singapore, Hong Kong, Sao Paulo,
Manila, Santiago, Bangkok, Taipei and Beijing, to cite just a few, already experience unacceptable air quality; insome cases, pollution levels are several times higher than acceptable health levels.
In terms of per capita motor vehicle registration for various regions, the United States, Japan, and Europe
currently account for the lion’s share of the ownership and use of motor vehicles. Indeed, the non OECD
countries of Africa, Asia (excluding Japan) and Latin America are home to more than four fifth’s of the world’s
population, yet account for only one fifth of world motor vehicle registrations! When one looks at the current percapita vehicle ownership rates in different parts of the world, it is clear where the future growth is likely to occur- Asia and Latin America. As a result, vehicle pollution in these regions is likely to increase.

1“World Motor Vehicle Data, 1998 Edition,” American Automobile Manufacturers Association, 1998.

2

CO2 emissions can be estimated from the mileage, however it is usually best to estimate the total emission fromthe fuel consumption (as this is the more reliable data) and allocate this emission to the vehicle types by
vehicle mileage data and relative fuel efficiencies.

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Background Paper

1

INTRODUCTION

It is important to make the estimates of CO2 as good as possible. Emissions of other gases only have a small
contribution to the total emissions of greenhouse gases. Emissions of CO2 depend on, and are estimated from,the amount of each fuel used. This is discussed in section 2.1. Effort spent on improving the estimates of CO2
emissions from road transport will result in overall improvements on the greenhouse gas inventories. Effort spent
on other pollutants may not result in significant improvements in the overall inventory, as they are only small
contributors to the greenhouse gas inventories. Thus greenhouse gas inventory experts should give CO2, fromroad transport, a higher priority when allocating resources to improving the inventories.
Road transport is a major source of CO2 and also emits several other gases including NOx, CO and NMVOCs
together with smaller quantities of N2O, CH4 and NH3. The carbon emission is estimated from the carbon
contained in the fuel3. Emissions of other gases are best estimated from the distance the vehicles are driven andemission factors, although in the absence of other information fuel based emission factors can also be used.
Emissions fall into three groups.
•

Exhaust (tail-pipe) emissions from the vehicle’s engine as it is driven;•

Cold Start emissions are additional emissions from the vehicle when started from cold;•

Evaporative emissions are evaporation of petroleum from the vehicles’ fuel system, engine, and fuel tanks.
Evaporation from vehicle refuelling and the supply of fuels is calculated elsewhere.

However, for inventories of greenhouse gases, the only large contribution is likely to be CO2. Evaporativeemissions are not likely to be significant, (they only relate to NMVOCs). A simple, fuel based, approach will
estimate emissions of CO2 accurately enough provided the fuel consumption is known. A similar approach canbe used for other gases where more detailed data cannot be found.
In general emissions are estimated using the following equation:
EQUATION 1
Emission = ∑ (EFabcd ● Activityabcd ) + ∑ Coldb + ∑ Evaporationb
abcd

b

b

Where:
Emission:

Total emissions from road transport

EF:

Emission factor, as mass per unit of activity rate

Activity:

activity rate (fuel consumed or distance travelled)

Cold:

Extra emissions due to cold starts

Evaporation:

extra emissions due to evaporation (NMVOCs)a:fuel type (petrol, diesel, LPG, etc)b:vehicle type (passenger car, light-duty truck, bus etc)c:emission controld:road type or vehicle speed

Estimation of emissions from road transport requires data for a range of parameters including:•

Fuel consumed, quality of each fuel type;•

Emission controls fitted to vehicle in the fleet;•

Operating characteristics (e.g. average vehicle speeds or types of roads;•

Maintenance;•

Fleet age distribution;

3

CO2 emissions can be estimated from the mileage, however, it is usually best to estimate the total emission
from the fuel consumption (as this is the more reliable data) and then allocate this emission to the vehicle types
by vehicle mileage data and relative fuel effiencies.

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Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories•

Distance driven, and•Climate.

Usually not all of these data are available. For example, total fuel consumption may be known but not how it isused by different types of vehicles (e.g. total petrol sales, but not petrol consumption by cars, light duty trucks
and motorcycles separately). Thus the simplest methodology is based on fuel consumption alone.
The Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC Guidelines) present two
sets of emission factors based on emission estimation research and emission estimation models from the UnitedStates and Europe. These are detailed models that take into account many factors that may not be available inmany other countries. In the past much of the effort in estimating emissions from road transport focused on themain local and regional pollutants and so there is not much reliable data for greenhouse gases such as N2O andCH4 although CO2 is better understood due to the interest in fuel consumption. It is important to remember that:•

The emission measurements and factors presented in the IPCC Guidelines are based on the vehicle fleets in
Europe and the USA. They reflect the vehicle types; emission controls and legislation; driving patterns androad types; vehicle maintenance; vehicle age distributions and mileage; and fuel types and quality in Europe
and the USA.

•The emission models proposed are also based on the statistical data available. Thus the vehicle types used
and the available statistics are related to the legislation (e.g. vehicle licensing) in these countries. Other
countries may well have different vehicle classifications and statistical data available and so this will needsome adjustment to fit with the emission factor information.

However, there is only limited guidance on how the information should be used outside these areas. Indeed thetables on the US data explicitly state that they themselves should not be used but that the underlying information
should be used to estimate emission rates and hence emissions for other countries. Data for vehicle types that do
not occur in Europe or the USA are not included, neither are the impacts of different fuel quality, maintenanceand age distributions in these countries.
This paper first considers the activity data and emissions data available and then the selection of good practicemethods. Following this reporting, issues about inventory quality are considered.

2
2.1

METHODOLOGICAL ISSUES
Fuel consumption

Fuel consumption is often better known than the distance travelled by vehicles. However, while the total of eachfuel used by road transport may be well known, the amounts used by each vehicle type are less well known. In
some countries, fuels other than petrol and diesel are significant. These can include LPG, CNG and methanol. It
is important to know how much of each of these fuels is used by transport. CO2 emissions can be estimated fromthe total amount of fuel consumed.
The approach proposed in the COPERT II manual is to use fuel consumption factors and distance travelled toestimated fuel use. This is then compared with the actual total fuel used and the vehicle kilometres adjusted tomatch the total fuel use.
Thus the total fuel use gives the total CO2 emission and the split between different vehicle types is determined by
the vehicle kilometre data for each vehicle category.
The CO2 emission factors proposed in the guidelines are:
TABLE 1CO2 EMISSION FACTORS FOR ROAD TRANSPORT IN THE IPCC GUIDELINES( g CO2/kg of fuel)PetrolUS
EuropeDiesel3172.31

3180

3140

In fact the emission factor can be estimated from the ratio of carbon to hydrogen in the fuel (Equation 2):

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Background Paper

EQUATION 2
1000
EF = 44.011 ●12.011 + 1.008 ⋅r H

C

Where:
EF:

mass emission of CO2 factor (g/kg)

rH/C: ratio of carbon to hydrogen atoms in the fuel (~1.8 for petrol and ~2.0 for diesel).
It should be noted that fuel consumption data may not be as clear as it appears at first sight. The following
questions should be addressed:•

Is all the fuel used sold for road transport? Is any sold for off-road use or for small machinery (such as
domestic appliances such as lawn mowers and garden equipment or small agricultural and forestry use suchas chain saws, or even small generators)?

•

Do some vehicles get their fuel from other sources (such as commercial or agricultural supplies or otherdirect supplies to consumers)?

•

How are vehicles that transit a country without refuelling dealt with in the statistics?

These are important issues. They may be particularly important with small countries, those with large throughtraffic from neighbouring countries or those with large fuel price differences with their neighbours. Off-roadsources should be estimated separately.

2.2

Activity data

In order to use the best method to estimate emissions, the types of information available need to be identified.
The activity data available in a country will determine the approach to be adopted.

2.2.1

Vehicle Types

Emission rates depend on the type of vehicle. The vehicle types defined in the IPCC Guidelines follow the
USEPA definitions. Other countries will need to match their vehicle data to these categories. Table 2 compares
categories in the IPCC Guidelines with those used for emission estimation in Europe (COPERT II).
TABLE 2ROAD VEHICLE TYPES: COMPARISON OF IPCC AND COPERT CLASSES
IPCC
Vehicle typeDefinition

COPERT
class

Definition

Light-duty
passenger
cars

Vehicle with gross weight less than 8,500 lbs.
(3,855kg) designed to carry 12 or fewer
passengers.

Passenger
Cars

Vehicles with four wheels( or threewhere the maximum weight exceeds
1,000kg) and being less than 2,500kg

Light-duty
trucks

Vehicle with a gross weight of 8,500lbs.
(3,855 kg) or less designed primarily for thetransportation of cargo or 12 or morepassengers, or are equipped for off-road
operations. This includes most pick-up trucks,
passenger and cargo vans four-wheel drive
vehicles and derivatives of these.

Light Duty
Vehicles

Vehicles used for the carriage of goods
having a maximum weight notexceeding 3,500kg.

Heavy duty
vehicles

Vehicles with a manufactures gross vehicle
weight greater than 8,500lbs (3,855kg).

Heavy Duty
Vehicles

Vehicles used for the carriage of goods
exceeding 3,500kg

Urban
Buses andCoaches

Vehicles used for the carriage ofpassengers and having more than 8
seats

Motorcycles

Two-wheeled vehicles and three-wheel
vehicles not exceeding 1,00kg andmotorcycles with sidecar.

Motorcycles

58

2 and 4 stroke motorcycles

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Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

While these definitions will give the same results for most vehicles there are some differences. In particular, the
IPCC passenger cars can be larger than under the COPERT definition and the IPCC light-duty vehicle classincludes some types (e.g. four-wheel drive vehicles) that may be passenger cars under the COPERT II
definitions. These differences arise for the local national legislation and the way it defines vehicles for licensing
purposes. Data in a country is usually collected in line with the national legislation on road transport.
In many cases, it is probably not worth trying to adjust the vehicle data to meet the IPCC definitions exactly: thisis only likely to add additional errors. However there may be cases where a national expert feels that someallocation of the national data to the IPCC classes is necessary and if this is done it should be clearly documented
and the assumptions underlying it explained.
In estimating the emissions, these vehicle types may be sub-divided. In particular the heavy-duty vehicle
category cover a wide range of vehicles from 3 500 kg to well over 30 000 kg. Sources such as COPERT II and
MOBILE5 give detailed emission rates for possible sub-classes by vehicle weights and engine capacity.
This vehicle fleet information is essential in order to estimate emissions (other than CO2) from road transport.
In the US, Gross Vehicle Weight Rating is the value specified by the manufacturer as the maximum design
loaded weight of a single vehicle. In Europe, this is the subject of plating and testing regulations but isessentially the same - the design (and legal) maximum laden weight. Emission factors for HGV may not assume
that the vehicles are fully laden (for example, COPERT II assumes that HGVs are 50% full, with conversion
factors provided for other loads, while the GVW is used to classify HGVs).

2.2.2

Emission controls

Different types of emission control technologies are fitted to vehicles; this must be taken into account. In Europe
and the US this can be done by the age of vehicle. Emission controls were introduced in response to legislationand so the age of a vehicle indicates the legislation for the vehicle was designed and built to meet and hence theemission controls fitted. (Care must be taken to account for the early introduction of new vehicles that meet afuture regulation and any ’grace’ periods allowing manufacturers to dispose of unsold stock.)In other countries, the link between vehicle age and emission control may not be as strong so that additional
effort may be needed to estimate the numbers of each vehicle type built with each emission control type.
The introduction of three-way catalysts has led to a significant change in the emissions of some pollutants. Whilethey are aimed primarily at gases such as NOx, CO and NMVOCs, they also have an impact on the direct
greenhouse gases. Fuel consumption generally increases a small amount with the fitting of a catalyst. However,other changes to vehicle design may counteract this. For example, cars with three-way catalysts also have fuelinjection and electronic engine management systems fitted. These are far less likely to drift out of tune, thus they
may have better fuel consumption over their lifetime. In any case, differences in driving patterns may swamp any
increases in fuel consumption due to catalysts.
Catalysts reduce hydrocarbon emissions. This is true for methane. However, the reduction in CH4 emissions isless than for other gases because of its relative inertness to catalytic reduction. On the other hand, N2O emissions
are increased by catalysts. Typically, road transport, in places where catalysts are fitted to petrol cars, is the only
significant source of N2O from fuel combustion. (The large sources are agriculture and process emissions.) Thus
the proportion of petrol cars fitted with catalysts is an important parameter in the estimation of N2O emissions.
Catalysts are poisoned by leaded fuel and fail to work. While cars with catalysts that have failed may emit morethan vehicles made without catalysts, there is little information on their emission rates. It is often assumed thatvehicle with failed catalysts emit at the same rate as vehicles built to the last pre-catalyst legislation. However, if
better information is available this should be used.
In the USA, the MOBILE model takes account of inspection and maintenance (I&M) programmes. Clearly theemission rate depends on the standard of vehicle maintenance and where specific programmes are in placeemissions are expected to be lower. Where a range of emission factors are given, where there is no I&M
programme, the high end of the emission factor range should be used.
In Europe the emission factors are based on in-service vehicles. Hence the standard of maintenance is includedin the measurements. This approach is fine for estimating current emissions where the measurement sample isrepresentative of the national fleet, but where forecasts are made, changes in I&M should be accounted for using
the expected improvements in the emission rates. In the UK, in-service testing is accounted for by assuming thatcatalysts fail and are mostly detected and repaired at an annual test.
Catalyst performance may degrade over time. Where information is available on the vehicle fleet age
distribution, this effect can be included by using emission rates dependent on age (or mileage).

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Background Paper

Where emission control efficiency is related to vehicle age it may be the case that older vehicles do lower annual
mileage. This needs to be taken into account when emissions are estimated. This vehicle fleet emission control
information is essential in order to estimate emissions (other than CO2) from road transport.
As well as emission control technology, the type of fuel the vehicle runs on is required. For example, petrol cars
emit very differently to diesel cars. The proportion of vehicle type (especially cars and vans) running on petroland diesel fuel needs to be known. Also the number of vehicles running on other fuels is also required (e.g. LPG,
CNG, ethanol or biofuels).

2.2.3

Vehicle kilometres

The distances driven by different vehicle types are needed for some approaches to estimate emissions. This data
is often of poor quality being derived from sample vehicle counts and modelling.
Usually only the main types of vehicle are counted and kilometres driven need to be allocated to vehicle sub-types
and by type of emission control according to their proportion in the fleet. Ideally, the distance driven annually byeach vehicle sub-type and emission control type should be estimated and taken into account in the calculations.
The different road types need to be defined. The COPERT II approach is based on the average speeds on
different road types. Emission factors are given as functions of speed for each vehicle type broken down byemission control and engine size. If this data is available then this approach can be used with estimates being
made for each road type. However this level of detail is unlikely to be available for every Party to the UNFCCC.
In this case vehicle kilometres need to be estimated for the different road types: typically urban, rural andhighway have been used as these three have very different driving patterns and average speeds.

2.3

Emission Factors

The tier I approach in the IPCC Guidelines gives fuel-based emission factors. This is a very simple approach thatcan be misleading if used directly, as major emission controls are not included. Thus a best practice approachshould at least be based on tables 1-25 to 1- 46, if not using more detailed approaches.
The simplest approach to estimating emissions from road transport is based on the amounts of each fuel consumed.
The approach for CO2 is indicated above in Section 2.1. This is based directly on the carbon content of the fuel.
For other gases (see section on N2O below), tables 1-25 to 1-46 give emission rates for US and European fleets,for vehicles at different stages of emission control. These emission factors are averaged over driving patterns andspeeds typical of the US or Europe and so the guidelines state they should not be used directly. However there
are situations where this may be appropriate where users have no information of vehicle kilometres driven on
different road types, of vehicle speeds and of driving patterns.
MOBILE 5 and COPERT II give detailed models that can be used to estimate emissions taking into account fleet
types, road types and driving patterns. There is also an intermediate approach where the models are too complicatedfor the national data available but where the emission factors can be taken from the models to take account of
national fleets and driving patterns. Table 3 shows the different approaches in order of increasing quality.
TABLE 3
EMISSION FACTORS FOR NON-CO2 EMISSIONS
Emission Factors

Notes

Quality

1

Fuel based factors in tables 1-1 and 1-7 to 11

Doe not take into account fleet
characteristics, emission controls etc

Not good practice- usefulfor preliminary
assessments only

2

Fuel Based Factors in tables 1-25 to 1-46

Takes into account fleet types andemission controls but NOT road types
and nationals differences

Simplest Acceptable
Practice3

Use detailed factors from COPERT II or
MOBILE

Estimate factors from these models totake account of road types and national
differences

Good Practice4

Use COPERT II or MOBILE or equivalent
national models

Takes into account fleet types emission
controls, road types, speeds and driving
patterns

Best Practice60

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Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

A country with information on the amount of fuel used by each vehicle type should instead use the more detailed
emission factors. Countries should use the energy specific emission factors for all of the pollutants (the g/kg fuelor g/MJ factors). The following things should be kept in mind: countries that use leaded fuel will have vehicles
that do not have operative catalyst control, as lead fouls the catalysts; and the ranges shown in the US emission
factor tables reflect the difference between the existence of inspection and maintenance programmes (I/M) andno I/M programmes (there are few I/M programmes for diesel vehicles in the US). Therefore, a country withleaded fuel and no I/M programme would use the “Uncontrolled” or “Non-Catalyst Control” factors and thehigher end of any range given. In most cases, the “Uncontrolled” category would be most applicable. In thisexample, the country would apply the US emission factors shown in Table 4.
TABLE 4
EMISSION FACTORS FROM THE USEPA FOR UNCONTROLLED PETROL ENGINED CARS
NOx

CH4

NMVOCs

CO

N2O

CO2

Average (g/kg fuel)9.76

0.82-0.90

39.88-42.09

181.14 -244.14

3172.31

Average (g/MJ)

0.222

0.019 -0.020

0.906-0.957

4.117-5.549

72.098Note that all values except the N2O values are from USEPA’s MOBILE 5 model. N2O values have been updated based on 1998 USEPA
data.

It is also possible to use the factors in Tables 1-27 through 1-33 to more accurately reflect the averagetemperature in a particular country. This should be done if the average temperature matches well with one of theseasonal temperature figures given in Table 1-26. In this case, the g/km emission factor will be the starting point.
This factor must be converted into an energy specific emission factor using the assumed fuel economy given in
the applicable table. Use one of the following equations below:
EQUATIONS 3 AND 4
Seasonal g/km ● X km/litre ● 1.33 = g/kg fuelSeasonal g/km ● X km/litre ● 0.0302 = g/MJ fuelThis same approach can be used for countries that do have vehicles with operational emissions control
equipment, if the vehicle kilometre travelled or the amount of fuel consumed in these vehicles is known and thecontrol technology can be matched to the descriptions given in the guidelines. However, countries with this levelof data may wish to use emissions factors that have been developed locally or the COPERT factors if those arelikely to be more applicable. Countries that do wish to use the MOBILE model, but run it to reflect their local
conditions (such as temperature and average speed, for example) should contact the USEPA for a copy of themodel and instructions on how to use it. In this case, the steps taken by the country in using MOBILE should beclearly documented in their inventory submission. Some cities and some countries have taken this approach
including China (for Beijing and Guangzhou), Mexico (for Mexico City and Monterrey), Bangkok, Thailand andKuala Lumpur, Malaysia.

2.3.1

N2O

Compared to regulated tailpipe emissions, there exist relatively few data that can be used to estimate nitrous
oxide emission factors for gasoline highway vehicles. Nitrous oxide is not a criteria pollutant, and measurementsof it in automobile exhaust are not routinely collected. Many of the recent measurements have been part ofresearch efforts attempting to understand why and under what conditions three-way catalysts produce nitrous
oxide, rather than trying to characterise the U.S. fleet.
The current IPCC Guidelines show markedly increased emission rates for vehicles with catalyst control. In
reviewing the literature data, and methods used to develop these emission factors, however, USEPA found they
are very limited, as described below:•

All the emission factors originate from testing done on five cars using European test cycles. Fuel sulphur
content for these tests was unspecified.

•

The new and aged three-way catalyst emission factors are based (90% of the data) on a single study using asingle car with eight non-production catalysts, new and bench-aged, with the catalysts located 1.4m from theengine. The other 10% of the data for the three-way catalyst emission factors came from two studies andthree more cars, all tested on European driving cycles only.

•

The non-catalyst emission factors were derived from four cars.

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Background Paper•

The emission factor for oxidation catalyst vehicles does not appear to be based on testing, but is instead thesame emission factor used for new three-way catalysts.

In order to refine the N2O emission factors, the USEPA Office of Mobile Sources in June and July 1998, carriedout a careful evaluation of available data supplemented by limited testing. They determined emission factors for
Early three-way Catalyst and previous vehicles primarily from the published literature. For (advanced) three-way
Catalyst vehicles and Low-Emission Vehicle Technology, data were used from the recent testing programme.
USEPA also assessed the limited data that exist for trucks. Based on all of the above, recommendations for N2Oemission factors by vehicle type and control technology have been made. The following sections will summarise
this effort.
As with the emission factors from the Mobile model, these newer factors developed by the USEPA for nitrous
oxide reflect the conditions in the United States. For example, in conducting the literature search, USEPA
reviewed only published values for the composite of the standard US federal test procedure, since it is thestandard driving cycle for the U.S. Still, the results of USEPA’s recent testing and literature review yields resultsthat improve the nitrous oxide emission factors now included in the IPCC Guidelines.
Emissions were always higher with commercial (high sulphur) fuel than with Indolene (low sulphur).
In 8 cases, tests were repeated with both high and low sulphur fuels. Six of the tests were with Low Emissions
Vehicles, and two with Advanced TWC vehicles. All showed higher emissions with commercial fuel (285 ppm
sulphur) than with Indolene (24 ppm sulphur). The ratio of nitrous oxide emissions using commercial fuel tothose using Indolene ranged from 1.2 to 4.4 and averaged 2.6. The mean of the ratio was significantly larger than
1 (p<.01). EPA believes that the basis for this difference is fuel sulphur content. The emission factors shown
below reflect the test results using the commercial, higher-sulphur fuel, as this is more representative of gasoline
sold today in the United States. Other countries may use gasoline with even higher sulphur levels. It is notpossible today to recommend that these countries scale up the N2O emission factor given below, as there are nodata to support this.
Emissions were usually higher with A/C On at 95°F than with A/C off at 75°F.
In 22 cases, tests were repeated under both A/C modes. In seventeen cases emissions were higher with A/C on,
in five cases with A/C off. The ratio of nitrous oxide emissions with A/C on to those with A/C off ranged from
0.9 to 3.4 and averaged 1.5. The mean of the ratio was significantly larger than 1 (p<.01). The emission factors
shown below reflect a weighted average of these results, based on U.S. activity data.
Nitrous Oxide was unrelated to the mileage of the vehicles.
A regression line of nitrous oxide emission factors against mileage for Advanced TWC passenger vehicles
yielded a slight positive slope not significantly different from zero (p<0.25). R2 was 0.06. Barton and Simpson(1994) similarly did not find a significant relationship between nitrous oxide emissions and mileage. Their slope
was negative.
Tables in Annex A contain replacement values for the N2O emission factors now shown in Tables 1-27 through1-33 of the IPCC Guidelines.

2.4

Good practice methods

The IPCC Guidelines identify two methods to estimate emissions from road transport. These are the USEPAMOBILE5 and the European COPERT II methods. The data presented in the guidelines themselves are
emissions per unit fuel used and distance driven in g/km and g/kg (and g/MJ) for each vehicle type. Theseemission factors assume a mix of driving on different road types (speeds and accelerations), an average cold start
per trip, average evaporative emissions across the fleet, typical climate and fuel quality, and a specific age
distribution for the vehicle fleet. As these assumptions are specific to Europe and North America these figures
should not, in principle, be used directly (See Box 4, page 1.67, Revised IPCC Guidelines for National
Greenhouse Gas Inventories: Reference Manual). If one or other of these methods cannot be used directly, andthere is no equivalent national methodology, then it is better to refer to the basic emission factors in these models
to estimate emissions.
Both these models have specific and extensive data requirements. However, the COPERT II model wasdeveloped to be applicable across Europe and so has to be useable in many countries. Therefore, the COPERT II
model’s data needs may be easier to satisfy.
To estimate emissions an appropriate emissions model should be used. The following approach is recommended:

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Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

Step 1 – Action data collection
Fuel consumption by road transport is REQUIRED. This can be total petrol, diesel and any other fuels used but
not broken down by vehicle type. (It is unlikely that fuel consumption by each type of vehicle separately is
known rather than being estimated in a similar manner to the emission estimates made below.) This information
is usually known as such fuels are often taxed. Estimate CO2 emissions from fuel use.
Notes: Data must be specific for fuel type used and then check the following:•

All fuel sold is for road transport use not off road vehicles and machinery, and•That all the fuel used is included. Tax exempt and agricultural vehicles may not appear in data depending on
where and how they obtain their fuel.

For some countries fuel purchased outside the country and used inside (or vice versa) may be significant. As thisfuel is in the tanks in ordinary vehicles, there will be records of it crossing a border. Luxembourg is an example.
For these countries it may be more appropriate to estimate fuel consumption from knowledge of the vehicle
mileage within the country. This should only be done where exports or imports are significant.
Step 2 - Action data collection
Vehicle kilometre data of the distance travelled by each vehicle type on different types of roads should becollected. Where vehicle kilometre data is not available, then it will be necessary to use fuel related emission
factors and the total amount of fuel consumed. If no other information is available then the total emission shouldbe allocated to the vehicle type by number of vehicles. However, this will result in an emission estimate with ahigher uncertainty and a lower quality.
Notes: Data should be specific for each fuel type used. Vehicle types should follow those used in the IPCC
Guidelines. However, national data may have to be mapped onto these IPCC classes using local expertise. In
some cases the IPCC classes may not be suitable and additional classes will have to be used. The road types usedwill depend on the local situation and traffic data that has been collected. The road type should reflect the driving
modes in the country, hence the widely used grouping of Urban, Rural and Highway (Motorway). Typically,
traffic data may not be available for all the possible vehicle and road type combinations. The local expert shouldattempt to allocate the vehicle kilometres. For example counts of total cars may be available but usually not the
number of a specific type of car (e.g. with catalyst).
Step 3 - Action data collection
Estimate the fuel consumption from the vehicle kilometre data. This requires the use of fuel economy data.
Compare this with the fuel consumption data collected in step 1. Except for countries where unreported imports
and exports are significant, the vehicle kilometre data should be adjusted to make the two sets of fuelconsumption data agree. Local experts should indicate the parts of the vehicle kilometre data that are least
reliable and the range of figures that are credible. If the two sets of fuel consumption data cannot be reconciledthen the original data be investigated to find the source of the problem - there is no point in continuing without
confidence in the data. If the problem cannot be solved, then the fuel related emission factors should be used toestimate emissions. For countries where unreported imports and exports are significant, the experts may prefer torely on the vehicle kilometre data rather the fuel use. In this case, they should estimate fuel use from the vehicle
kilometre data so that CO2 emissions can be estimated.
Notes: This is an important step as it gives some confidence that the vehicle data are realistic. Should the two
estimates agree exactly or should a margin of error be allowed? Where it cannot be done, due to imports andexports then some justification for the vehicle used should be chosen. The chosen vehicle kilometres will enable
an estimate to be made of the amount of fuel imported or exported. Where this is significant, it should bechecked that the other country involved is aware of the size of the imports/exports of their own estimates. Who
performs this check is unclear at present.
Step 4 - Action data collection
Does the car fleet most resemble that of the USA or Europe? Is there sufficient data available to use the models
MOBILE5 or COPERT II? If so then emission estimates can be made using the appropriate model and data. If
not then go to step 5.
Notes: The data requirements of these two models are fairly extensive and so they may not be suitable for many
countries. It should be noted that the COPERT II model includes data on some Eastern European types inparticular 2-stroke passenger cars.
Step 5 - Action data collection
Emission factors for each vehicle type and road type are needed. These can come from MOBILE5 and/or
COPERT II. However some countries will find that they have additional vehicle types that are not covered byEmissions: Energy, Road, Transport

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these models. In these cases, locally measured data can be used. It may be necessary to make some engineering
judgements to extrapolate from measured vehicle types to the vehicle types used in certain places.
Notes: If measure emission factors are not available, these alternative approaches to emission factors will result inincreased error and poorer quality inventories. The only way to improve the inventory is to make measurements.
The COPERT II model is based on combining emissions data from across Europe. This collaboration between many
countries has improved emission estimates and could be copied in other parts of the world. Experts should monitor
vehicle emission work in other countries as this may well provide data that will improve their own inventory.
Step 6 - Action data collection
The emissions factors from step 5 and the vehicle kilometres from step 3 are multiplied to estimate the emissions.
Notes: CO2 should be estimated from the vehicle kilometres data and, provided there are no mistakes, this will
be approximately equal to the estimate made in step 1 except in those countries where unreported exports andimports are significant. Some differences between these two estimates of CO2 are expected due to theuncertainties. HOW BIG ARE THESE?
Step 7 - Action data collection
An assessment of the emissions from cold starts and evaporative emissions is also needed.
Notes: This is only needed for some pollutants and is a minor contributor to greenhouse gas emissions.

2.4.1

Uncertainty

Assuming that the emission estimation process follows all the best practice the results will still be uncertain. This
results from:•

Uncertainties in emission factors. These arise from a number of areas. Measurements themselves are
uncertain. Most are laboratory-based dynamometer measurements that may not represent real road driving
conditions. Not all the conditions found on the road may be replicated by the measurement procedures.
Emissions vary from vehicle to vehicle and usually only relatively small samples of each vehicle type havebeen measured. Thus emission factors may differ from the real average emission rate for a particular vehicle
type. In addition, some very high emitting vehicles, due to age or malfunctions, may evade the measurement
programme altogether. (For example vehicles deemed unsafe or aggressive driving styles would not bemeasured but may be driven on the roads.)•

Uncertainties in activity rates. The activity rates are, like any statistical data, uncertain. In addition as they
are not collected specifically for use in emission inventories they may not be exactly the data required by theestimation methodology. Assumptions may be needed to use the activity data. Some of the data may be very
uncertain. Vehicle kilometres on different road types may be sound, based on detailed measurements, orthey may be extrapolations from a small dataset with a high degree of uncertainty.

•

Representativeness of the methodology. The methodology used to estimate emissions may not accuratelyreflect the true emission processes. This is difficult to assess theoretically but measurements may help.
Some roadside measurements indicate that the estimates of emissions of regulated pollutants (NOx, CO andNMVOCs) are approximately correct while some indicate systematic differences. In any case, thesecomparisons are difficult to assess as the methodology takes emission factors for cycles (drives including
starts, stops, acceleration and steady speeds) while the measurements are essentially spot measurements (orat least short periods with vehicles under one driving condition).

While experts may be able to assess the size and impact of the first two sources of uncertainty, the size of thethird is more difficult to address.
An early study looking at the first two sources of uncertainties (Eggleston, 1993), indicated that the uncertainty
in the estimated emission of NMVOCs was 50% (2 standard deviations as a percentage of the mean). While
uncertainties in fuel consumption, and hence CO2 emissions, are much lower, estimates of CH4 and N2O may
have similar or higher uncertainties than for NMVOCs.
The estimate of total CO2 from road transport may have a very small uncertainty. Fuel consumption statistics areoften quite accurate and so the total emission may have an accuracy of 3-5% in many cases. However the
allocation of this emission to specific vehicle types will be of much lower uncertainty. In other cases, the total
fuel consumption is not well known, either because the statistics are distorted by imports and exports, or because
the base data are not well known. Then the uncertainties could be much higher, relying on vehicle kilometre
estimates and fuel consumption rates.

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Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

Figure 1

Decision tree for choice of method to estimate
emissions from road transport

Start

Does fuelconsumption
data exist?

Collect fuel dataNo

Yes

Is there dataon vehicle
kilometres on
different roadtypes?

Can vehicle
kilometres on
different roadtypes beestimated?

No

Yes

Yes

Estimate fuelconsumption fromvehicle kilometres

Estimate vehicle
kilometres on
different road types

Does estimatedand actual total
fuel use agree?

No

No

Use average fuelbased factors from
IPCC guidelines

Adjust vehicle
kilometresaccording to local
experience

Yes
Calculate total CO2
emission from fueluse

Use MOBILE 5

Yes,
MOBILE 5

Is there enoughdata to use
MOBILE 5 or
COPERT II?

Yes

Does
estimated
and actual
total fuel use
agree now?

Yes,
COPERT II

No

Use average fuelbased factors from
IPCC guidelines

Use COPERT II

No

Use g/kg
emissionfactors

Yes

Emissions: Energy, Road, Transport

Are there
emission factors
for each vehicle
type/road typecombination?

No

Use average fuelbased factors from
IPCC guidelines formissing emission
factors and g/kg
where possible

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2.4.2

Completeness

It is very important that every effort is made for the estimate to be as complete as possible. There may be
specific national issues that need to be considered in addition to the issues discussed below:•

Tax exempt vehicles. Some classes of vehicles may be exempt from tax for a wide range of reasons.
However, they should be included in the estimates. Thus the numbers of these vehicles and the distances
they travel need to be estimated;•

Agricultural vehicles. It is possible that in some countries agricultural vehicles are not included in the data.
Some countries tax agricultural fuels differently and so it is important to be clear which vehicles are used on
the public roads and how much fuel is consumed by them;•

Military Vehicles. It can be difficult to collect information on military vehicles. Particular issues concern
where the fuel consumed is accounted for and how much such vehicles are used on the public roads;•

Import and export of fuel. This has been discussed above in section 2.1;•

Vehicle types. Are all the vehicle types that exist in a country included in the inventory?

•

Fuel types. Are all the fuel types in a country included? Are all the different fuel qualities covered by theinventory?

3

REPORTING AND DOCUMENTATION

It is important not only to estimate the emissions in the best way but also to report and document the approach
used. As was described above, the detail of the method used for the estimates depends on the statistical data
available. Information about the inventory can be split into two parts:•

Reporting. This is information needed by users of the emission inventories to compare and contrast theemission estimates with those of other countries, and•Documentation. This is more detailed information which will show an interested outsider exactly how theestimates have been made and any assumptions that have had to be made by the inventory compilers.

Therefore reporting should include:
1.

Emissions according to the categories described in the IPCC reporting instructions and guidelines. As has been
noted above, the national statistics and legislation may result in classes slightly different from those in the IPCC
Guidelines. In this case, an attempt should be made to report in the IPCC categories as closely as possible;2.

Any differences between reported vehicle classes and the requested IPCC classes should be reported (e.g.
differences in the treatment of four-wheel drive vehicles or in the maximum number of passengers in a car
or light-duty vehicle.);3.

Any adjustments made to the estimated vehicle emissions in order to fit the results into the IPCC classes, and4.

The estimation method used. This will include the availability of data describing road transport.

Documentation should include all of the above and:1.

The statistical data used and its sources. Has any data been estimated or assumed?

2.

The emission factors used and their sources and any assumptions that have had to be made to provide
emission factors to each national vehicle type;3.

Vehicle fleet by type and emission control, legislation controlling vehicle emissions, technologies used foremission control, fuel types and any alternative fuels used;4.

A comparison of reported fuel use and estimated fuel use from vehicle mileages, and5.

Checks and reviews that have been performed.

4
4.1

QUALITY ASSURANCE AND QUALITY CONTROL
Introduction

To ensure the best possible inventory quality a clear quality assurance/ quality control (QA/QC) system isneeded. This will need to cover a clear documented approach as follows:

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Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories•

Checks and internal review of the data used for the calculations and the results;•

Comprehensive documentation;•

External audit of the entire inventory, and•Inventory reviews including expert (peer review, stakeholder review and public review.

4.2

Internal inventory QA/QC systems

Internal checks should be made as part of the inventory QA/QC system. These should include checks that theprocedures have been followed correctly as well as checks that the results are reasonable. The comparison of thereported fuel use and fuel use estimated from vehicle mileage is a check on the reasonableness of the estimates.
All data used should be documented. Its source, data and use should be recorded. All assumptions about statistics
should be recorded. Differences between these estimates and other emission inventories should be consideredand explained.

4.3
4.3.1

External inventory QA/QC systems
Third party audit

This is a potential review mechanism that would give increased confidence in the emission estimates. An
independent third party would review the estimates and their documentation to see if the results are reasonable.
The first step in this would be the identification of widely accepted expert third parties to perform the audit. Such
parties should be:•

Independent;•

Experts, and•Accredited

4.3.2

Review

Publishing the inventory and inviting comments could perform peer review of the inventory. The three groups
who should be addressed are:•Experts (Peer Review): Those with the scientific and technical expertise to consider the data and emission
factors used and criticise the assumptions and approach used from a scientific background. Expert (peer)
review is needed to give the emission estimates scientific credibility;•

Stakeholders: Several groups have a direct interest in the results. These could include vehicle
manufacturers and traders, government departments, motorist organisations and the petroleum industry.
Giving these groups the chance to review and comment on the estimates will enable them to contribute thebest information they have as well as increasing their confidence in the emission estimates, and•Public: Review should attempt to include to widest range of input. Members of the public and groups such
as NGOs will comment from different viewpoints that may reveal problems in the data or in the way it isreported and documented.

REFERENCES
Eggleston HS (1993) “Uncertainties in the estimates of Emissions of VOCs from Motor cars” Proceedings of the
TNO/EURASAP working on the reliability of VOC Emission databases June 1993. TNO Netherlands
OPERT II(1997) “COPERT II Computer Programme to Calculate Emissions from Road Transport.
Methodology and Emission factors” Ahlvik P, Eggleston P, Gorissen N, Hassel D, Hickman A-J, Jourmard R,
Ntiziachristos, Rijkeboer R, Samaras Z, Zierock K-H, European Environment Agency, Copenhagen,November 1997.

Emissions: Energy, Road, Transport

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ANNEX 1N2O EMISSION FACTORS

Note that Table 1-29 of the IPCC Guidelines seems to be mis-labeled – more work is needed to determine if thisis true for all the emission factors in Table 1-29.
REPLACEMENT FOR N2O EMISSION FACTORS – TABLE 1-27
ESTIMATED EMISSION FACTORS FOR US GASOLINE PASSENGER CARS

Low-Emission Vehicle Technology; Assumed Fuel Economy: 8.5 km/litre (11.8 l/100 km)
Average (g/km)

0.0176

Average(g/kg fuel)0.199

Average(g/MJ)

0.00453

Three-way Catalyst Control; Assumed Fuel Economy: 8.3km/litre (12.01/100km)
Average (g/km)

0.0288Average (g/kg fuel)0.320

Average (g/MJ)

0.00728

Three-way Catalyst Control; Assumed Fuel Economy: 8.0 km/litre (12.51/100km)
Average (g/km)

0.057

Average (g/kg/fuel)0.539

Average(g/MJ)

0.01225

Oxidation Catalyst Control; Assumed Fuel Economy: 6.2 km/litre (16.11/100km)
Average (g/km)

0.0322Average (g/kg/fuel)0.266

Average(g/MJ)

0.00605

Non-Catalyst Control; Assumed Fuel Economy: 4.5 km/litre (22.21/100km)
Average (g/km)

0.0103

Average (g/kg/fuel)0.062

Average(g/MJ)

0.00140

Uncontrolled; Assumed Fuel Economy: 4.7 km/litre (21.31/100km)
Average (g/km)

0.0103

Average (g/kg/fuel)0.065

Average(g/MJ)

0.0047

Source: USEPA,1998.

REPLACEMENT FOR N2O EMISSION FACTORS – TABLE 1-28
ESTIMATED EMISSION FACTORS FOR US LIFHT DUTY GASOLINE TRUCKS

Low-Emission Vehicle Technology; Assumed Fuel Economy: 6.0 km/litre (16.7 l/100 km)
Average (g/km)

0.0249Average(g/kg fuel)0.199

Average(g/MJ)

0.0049Three-way Catalyst Control; Assumed Fuel Economy: 6.0 km/litre (16.71/100km)
Average (g/km)

0.0400

Average (g/kg fuel)0.320

Average (g/MJ)

0.00728

Three-way Catalyst Control; Assumed Fuel Economy: 4.8 km/litre (20.81/100km)

68

Average (g/km)

0.0846

Average (g/kg/fuel)0.539

Average(g/MJ)

0.01225

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Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

REPLACEMENT FOR N2O EMISSION FACTORS – TABLE 1-28 (CONTINUED)
ESTIMATED EMISSION FACTORS FOR US LIFHT DUTY GASOLINE TRUCKS
Oxidation Catalyst Control; Assumed Fuel Economy: 4.8 km/litre (20.81/100km)
Average (g/km)

0.0418Average (g/kg/fuel)0.266

Average(g/MJ)

0.00605

Non-Catalyst Control; Assumed Fuel Economy: 4.0 km/litre (25.01/100km)
Average (g/km)

0.0117

Average (g/kg/fuel)0.062

Average(g/MJ)

0.00140

Uncontrolled; Assumed Fuel Economy: 4.7 km/litre (21.31/100km)
Average (g/km)

0.01018

Average (g/kg/fuel)0.065

Average(g/MJ)

0.0047

Source: USEPA,1998.

REPLACEMENT FOR N2O EMISSION FACTORS – TABLE 1-29
ESTIMATED EMISSION FACTORS FOR US HEAVY-DUTY GASOLINE VEHICLES
Early Three-Way Catalyst; Assumed Fuel Economy: 2.3 km/litre (43.5 l/100 km)
Average (g/km)

0.1729Average (g/kg fuel)0.539

Average (g/MJ)

0.01225

Oxidation Catalyst; Assumed Fuel Economy: 2.3 km/litre(43.5l/100km)
Average (g/km)

0.0870

Average (g/kg fuel)0.266

Average (g/MJ)

0.00605

Uncontrolled; Assumed Fuel Economy: 1.8 km/litre(55.6l/100km)
Average (g/km)

0.0262Average (g/kg fuel)0.064

Average (g/MJ)

0.00144** The current Table 1-29 seems to be mis-labeled,at least as far as the N2O factors are concerned. The headings below are labeled asthey should be for N2O. The other factors need to be checked. Source: USEPA, 1998.

REPLACEMENT FOR N2O EMISSION FACTORS – TABLE 1-30
ESTIMATED EMISSION FACTORS FOR US DIESEL PASSENGER CARSAdvanced Control; Assumed Fuel Economy: 10.0 km/litre (10 l/100 km)
Average (g/km)

0.01

Average (g/kg fuel)0.134

Average (g/MJ)

00304

Moderate Control; Assumed Fuel Economy: 9.6 km/litre(10.4l/100km)
Average (g/km)

0.01

Average (g/kg fuel)0.134

Average (g/MJ)

0.00304

Uncontrolled; Assumed Fuel Economy: 7.5 km/litre(13.3l/100km)
Average (g/km)

0.01

Average (g/kg fuel)0.134

Average (g/MJ)

0.00304

Source: USEPA, 1998.

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REPLACEMENT FOR N2O EMISSION FACTORS – TABLE 1-31
ESTIMATED EMISSION FACTORS FOR US LIGHT DUTY DIESEL TRUCKS

Advanced Control; Assumed Fuel Economy: 7.2 km/litre (13.9 l/100 km)
Average (g/km)

0.02

Average (g/kg fuel)0.192

Average (g/MJ)

00437

Moderate Control; Assumed Fuel Economy: 7.2 km/litre(13.9l/100km)
Average (g/km)

0.02

Average (g/kg fuel)0.192

Average (g/MJ)

0.00437

Uncontrolled; Assumed Fuel Economy: 5.7 km/litre(17.5l/100km)
Average (g/km)

0.02

Average (g/kg fuel)0.192

Average (g/MJ)

0.00437

Source: USEPA, 1998.

REPLACEMENT FOR N2O EMISSION FACTORS – TABLE 1-32
ESTIMATED EMISSION FACTORS FOR US HEAVY DUTY VEHICLESAdvanced Control; Assumed Fuel Economy: 2.4 km/litre (41.7 l/100 km)
Average (g/km)

0.03

Average (g/kg fuel)0.096

Average (g/MJ)

0.00219

Moderate Control; Assumed Fuel Economy: 2.4 km/litre(41.7l/100km)
Average (g/km)

0.03

Average (g/kg fuel)0.096

Average (g/MJ)

0.00219

Uncontrolled; Assumed Fuel Economy: 2.2 km/litre(45.5l/100km)
Average (g/km)

0.03

Average (g/kg fuel)0.096

Average (g/MJ)

0.00219

Source: USEPA, 1998.

REPLACEMENT FOR N2O EMISSION FACTORS – TABLE 1-33
ESTIMATED EMISSION FACTORS FOR US MOTOR CYCLES
Advanced Control; Assumed Fuel Economy: 10.8 km/litre (9.3 l/100 km)
Average (g/km)

0.042

Average (g/kg fuel)0.062

Average (g/MJ)

0.00140

Moderate Control; Assumed Fuel Economy: 8.9 km/litre(11.2l/100km)
Average (g/km)

0.00054

Average (g/kg fuel)0.065

Average (g/MJ)

0.00147

Source: USEPA, 1998

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