Choice of Method

Emissions can be estimated from either the fuel consumed (represented by fuel sold) or the distance travelled by the vehicles. In general, the first approach (fuel sold) is appropriate for CO2 and the second (distance travelled by vehicle type and road type) is appropriate for CH4 and N2O.


Emissions of CO2 are best calculated on the basis of the amount and type of fuel combusted (taken to be equal to the fuel sold, see section and its carbon content. Figure 3.2.2 shows the decision tree for CO2 that guides the choice of either the Tier 1 or Tier 2 method. Each tier is defined below.

1 Urea consumption for catalytic converters in vehicles is directly related to the vehicle fuel consumption and technology.

Figure 3.2.1 Steps in estimating emissions from road transport


Figure 3.2.2 Decision tree for CO2 emissions from fuel combustion in road vehicles

Figure 3.2.2 Decision tree for CO2 emissions from fuel combustion in road vehicles

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees.

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees.

The Tier 1 approach calculates CO2 emissions by multiplying estimated fuel sold with a default CO2 emission factor. The approach is represented in Equation 3.2.1.



Emission = Emissions of CO2 (kg) Fuela = fuel sold (TJ)

EFa = emission factor (kg/TJ). This is equal to the carbon content of the fuel multiplied by 44/12.

a = type of fuel (e.g. petrol, diesel, natural gas, LPG etc)

The CO2 emission factor takes account of all the carbon in the fuel including that emitted as CO2, CH4, CO, NMVOC and particulate matter2. Any carbon in the fuel derived from biomass should be reported as an information item and not included in the sectoral or national totals to avoid double counting as the net emissions from biomass are already accounted for in the AFOLU sector (see section Completeness).

The Tier 2 approach is the same as Tier 1 except that country-specific carbon contents of the fuel sold in road transport are used. Equation 3.2.1 still applies but the emission factor is based on the actual carbon content of fuels consumed (as represented by fuel sold) in the country during the inventory year. At Tier 2, the CO2 emission factors may be adjusted to take account of un-oxidised carbon or carbon emitted as a non-CO2 gas.

There is no Tier 3 as it is not possible to produce significantly better results for CO2 than by using the existing Tier 2. In order to reduce the uncertainties, efforts should concentrate on the carbon content and on improving the data on fuel sold. Another major uncertainty component is the use of transport fuel for non-road purposes.


For estimating CO2 emissions from use of urea-based additives in catalytic converters (non-combustive emissions), it is good practice to use Equation 3.2.2:


Emissions = CO2 Emissions from urea-based additive in catalytic converters (Gg CO2)

Activity = amount of urea-based additive consumed for use in catalytic converters (Gg)

Purity = the mass fraction (= percentage divided by 100) of urea in the urea-based additive

The factor (12/60) captures the stochiometric conversion from urea (CO(NH2)2) to carbon, while factor (44/12) converts carbon to CO2. On the average, the activity level is 1 to 3 percent of diesel consumption by the vehicle. Thirty two and half percent can be taken as default purity in case country-specific values are not available (Peckham, 2003). As this is based on the properties of the materials used, there are no tiers for this source.


Emissions of CH4 and N2O are more difficult to estimate accurately than those for CO2 because emission factors depend on vehicle technology, fuel and operating characteristics. Both distance-based activity data (e.g. vehicle-kilometres travelled) and disaggregated fuel consumption may be considerably less certain than overall fuel sold.

CH4 and N2O emissions are significantly affected by the distribution of emission controls in the fleet. Thus higher tiers use an approach taking into account populations of different vehicle types and their different pollution control technologies.

Research on carbon mass balances for U.S. light-duty gasoline cars and trucks indicates that "the fraction of solid (unoxidized) carbon is negligible" USEPA (2004a). This did not address two-stroke engines or fuel types other than gasoline. Additional discussion of the 100 percent oxidation assumption is included in Section of the Energy Volume Introduction chapter.

Although CO2 emissions from biogenic carbon are not included in national totals, the combustion of biofuels in mobile sources generates anthropogenic CH4 and N2O that should be calculated and reported in emissions estimates.

The decision tree in Figure 3.2.3 outlines choice of method for calculating emissions of CH4 and N2O. The inventory compiler should choose the method on the basis of the existence and quality of data. The tiers are defined in the corresponding equations 3.2.3 to 3.2.5, below.

Three alternative approaches can be used to estimate CH4 and N2O emissions from road vehicles: one is based on vehicle kilometres travelled (VKT) and two are based on fuel sold. The Tier 3 approach requires detailed, country-specific data to generate activity-based emission factors for vehicle subcategories and may involve national models. Tier 3 calculates emissions by multiplying emission factors by vehicle activity levels (e.g., VKT) for each vehicle subcategory and possible road type. Vehicle subcategories are based on vehicle type, age, and emissions control technology. The Tier 2 approach uses fuel-based emission factors specific to vehicle subcategories. Tier 1, which uses fuel-based emission factors, may be used if it is not possible to estimate fuel consumption by vehicle type.

The equation for the Tier 1 method for estimating CH4 and N2O from road vehicles may be expressed as:

Equation 3.2.3 Tier 1 emissions of ch4 and n20


Emissions = emission in kg EFa = emission factor (kg/TJ)

Fuela = fuel consumed, (TJ) (as represented by fuel sold) a = fuel type a (e.g., diesel, gasoline, natural gas, LPG)

Equation 3.2.3 for the Tier 1 method implies the following steps:

• Step 1: Determine the amount of fuel consumed by fuel type for road transportation using national data or, as an alternative, IEA or UN international data sources (all values should be reported in terajoules).

• Step 2: For each fuel type, multiply the amount of fuel consumed by the appropriate CH4 and N2O default emission factors. Default emission factors may be found in the next Section (Emission Factors).

• Step 3: Emissions of each pollutant are summed across all fuel types. The emission equation for Tier 2 is:

Equation 3.2.4 Tier 2 emissions of ch4 and n20


Emission = emission in kg.

EFg,bc = emission factor (kg/TJ)

Fuela,b,c = fuel consumed (TJ) (as represented by fuel sold) for a given mobile source activity a = fuel type (e.g., diesel, gasoline, natural gas, LPG)

b = vehicle type c = emission control technology (such as uncontrolled, catalytic converter, etc)

Figure 3.2.3 Decision tree for CH4 and N2O emissions from road vehicles

Figure 3.2.3 Decision tree for CH4 and N2O emissions from road vehicles


1. See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees.

2.The decision tree and key category determination should be applied to methane and nitrous oxide emissions separately.

Vehicle type should follow the reporting classification 1.A.3.b (i to iv) (i.e., passenger, light-duty or heavy-duty for road vehicles, motorcycles) and preferably be further split by vehicle age (e.g., up to 3 years old, 3-8 years, older than 8 years) to enable categorization of vehicles by control technology (e.g., by inferring technology adoption as a function of policy implementation year). Where possible, fuel type should be split by sulphur content to allow for delineation of vehicle categories according to emission control system, because the emission control system operation is dependent upon the use of low sulphur fuel during the whole system lifespan3. Without considering this aspect, CH4 may be underestimated. This applies to Tiers 2 and 3.

The emission equation for Tier 3 is:

Equation 3.2.5 Tier 3 emissions of ch4 and n2o


Emission = emission or CH4 or N2O (kg) EFg,bcd = emission factor (kg/km)

Distanceabcd = distance travelled (VKT) during thermally stabilized engine operation phase for a given mobile source activity (km)

Ca,b,c,d = emissions during warm-up phase (cold start) (kg)

a = fuel type (e.g., diesel, gasoline, natural gas, LPG)

b = vehicle type c = emission control technology (such as uncontrolled, catalytic converter, etc.)

d = operating conditions (e.g., urban or rural road type, climate, or other environmental factors)

It may not be possible to split by road type in which case this can be ignored. Often emission models such as the USEPA MOVES or MOBILE models, or the EEA's COPERT model will be used (USEPA 2005a, USEPA 2005b, EEA 2005, respectively). These include detailed fleet models that enable a range of vehicle types and control technologies to be considered as well as fleet models to estimate VKT driven by these vehicle types. Emission models can help to ensure consistency and transparency because the calculation procedures may be fixed in software packages that may be used. It is good practice to clearly document any modifications to standardised models.

Additional emissions occur when the engines are cold, and this can be a significant contribution to total emissions from road vehicles. These should be included in Tier 3 models. Total emissions are calculated by summing emissions from the different phases, namely the thermally stabilized engine operation (hot) and the warming-up phase (cold start) - Eq 3.2.5 above. Cold starts are engine starts that occur when the engine temperature is below that at which the catalyst starts to operate (light-off threshold, roughly 300oC) or before the engine reaches its normal operation temperature for non-catalyst equipped vehicles. These have higher CH4 (and CO and HC) emissions. Research has shown that 180-240 seconds is the approximate average cold start mode duration. The cold start emission factors should therefore be applied only for this initial fraction of a vehicle's journey (up to around 3 km) and then the running emission factors should be applied. Please refer to USEPA (2004b) and EEA (2005a) for further details. The cold start emissions can be quantified in different ways. Table 3.2.3 (USEPA 2004b) gives additional emissions per start. This is added to the running emission and so requires knowledge of the number of starts per vehicle per year4. This can be derived through knowledge of the average trip length. The European model COPERT has more complex temperature dependant corrections for the cold start (EEA 2000) for methane.

This especially applies to countries where fuels with different sulphur contents are sold (e.g. "metropolitan" diesel). Some control systems (for example, diesel exhaust catalyst converters) require ultra low sulphur fuels (e.g. diesel with 50 ppm S or less) to be operational. Higher sulphur levels deteriorate such systems, increasing emissions of CH4 as well as nitrogen oxides, particulates and hydrocarbons. Deteriorated catalysts do not effectively convert nitrogen oxides to N2, which could result in changes in emission rates of N2O. This could also result from irregular misfuelling with high sulphur fuel.

This simple method of adding to the running emission the cold start (= number of starts • cold start factor) assumes individual trips are longer than 4 km.

Both Equation 3.2.4 and 3.2.5 for Tier 2 and 3 methods involves the following steps:

• Step 1: Obtain or estimate the amount of fuel consumed by fuel type for road transportation using national data (all values should be reported in terajoules; please also refer to Section

• Step 2: Ensure that fuel data or VKT is split into the vehicle and fuel categories required. It should be taken into consideration that, typically, emissions and distance travelled each year vary according to the age of the vehicle; the older vehicles tend to travel less but may emit more CH4 per unit of activity. Some vehicles may have been converted to operate on a different type of fuel than their original design.

• Step 3: Multiply the amount of fuel consumed (Tier 2), or the distance travelled (Tier 3) by each type of vehicle or vehicle/control technology, by the appropriate emission factor for that type. The emission factors presented in the EFDB or Tables 3.2.3 to 3.2.5 may be used as a starting point. However, the inventory compiler is encouraged to consult other data sources referenced in this chapter or locally available data before determining appropriate national emission factors for a particular subcategory. Established inspection and maintenance programmes may be a good local data source.

• Step 4: For Tier 3 approaches estimate cold start emissions.

• Step 5: Sum the emissions across all fuel and vehicle types, including for all levels of emission control, to determine total emissions from road transportation.

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