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The Tier 1 method is fuel-based, since emissions from all sources of combustion can be estimated on the basis of the quantities of fuel combusted (usually from national energy statistics) and average emission factors. Tier 1 emission factors are available for all relevant direct greenhouse gases.

The quality of these emission factors differs between gases. For CO2, emission factors mainly depend upon the carbon content of the fuel. Combustion conditions (combustion efficiency, carbon retained in slag and ashes etc.) are relatively unimportant. Therefore, CO2 emissions can be estimated fairly accurately based on the total amount of fuels combusted and the averaged carbon content of the fuels.

However, emission factors for methane and nitrous oxide depend on the combustion technology and operating conditions and vary significantly, both between individual combustion installations and over time. Due to this variability, use of averaged emission factors for these gases, that must account for a large variability in technological conditions, will introduce relatively large uncertainties.


In the Tier 2 method for energy, emissions from combustion are estimated from similar fuel statistics, as used in the Tier 1 method, but country-specific emission factors are used in place of the Tier 1 defaults. Since available country-specific emission factors might differ for different specific fuels, combustion technologies or even individual plants, activity data could be further disaggregated to properly reflect such disaggregated sources. If these country-specific emission factors indeed are derived from detailed data on carbon contents in different batches of fuels used or from more detailed information on the combustion technologies applied in the country, the uncertainties of the estimate should decrease, and the trends over time can be better estimated.

If an inventory compiler has well documented measurements of the amount of carbon emitted in non-CO2 gases or otherwise not oxidised, it can be taken into account in this tier in the country-specific emission factors. It is good practice to document how this has been done.

Figure 1.1 Activity and source structure in the Energy Sector

1A1a Main Activity Electricity and Heat Production

1A1 Energy Industries

1A1a1 Electricity Generation f 1 A1aii Combined Heat and Poi

1A1b Petroleum Refining 1A1c Manufacture of Solid Fuels andOtherEn ergy I ndu s tri es_

Manufacture of Solid Fuels :r Energy Industries iflZ Manufacturing Industries and Construction

Manufacture of Solid Fuels :r Energy Industries iflZ Manufacturing Industries and Construction

1 A2a

Iron and Steel


1 A2b

Non-Ferrous Metals

1 A2c



1 A2d

Pulp, Paper and Print

1 A2e

Food Processing, Beverages and Tobacco

1 A2f

Non-Metallic Minerals

1 A2g

Transport Equipment

1 A2h


1 A2i

Mining (excluding fuels) and Quarrying


Wood and wood products


1 A2k



Textile and Leather


1 A2m

Non-specified Industry:

1A3a Civil Aviation

1A3ai International Aviation (International Bunkers) 1A3aii Domestic Aviation

's with 3-wey catalysts

1 A3bi Cars

1 A3bi1 Pass

1 A3bi2 Pass

1A Fuel Combustion Activities

National Greenhouse Gas Inventory

1 A3b Road Transportation

1 A3bii Light-duty trucks

>ut 3-way catalysts 1A3bii1 Light-duty trucks with

3-v\ay catalysts_

1A3bii2 Light-duty trucks without

3-way catalysts_

1A3biii Heavy-duty trucks and buses , 1 A3biv Motorcycles

1A3bv Evaporative emissions from vehicles

I 1A3bvi Urea-based catalysts

International v\rater-bor itional bunkers)_

1 A3d Water-borne Navigation /lA3dii Domestic Water-bor V Navigation_

I 1 A3 e Qth er Tran sp ort ati on

Commercial/Institutional 1A4b Residential

Pipeline Transport

1A4 Other Sectors J

1A4c Agrie ul tu re./ Fores try J1 Fishing/Fish Farms_

1 A4ci Stationary

1 A4cii Off-road Vehicles and Other Machinery

Fishing (mobile combustion)

1A5a Stationary

\ 1A5 Hon-Specified

1A5bi Mobile (aviation component)_

' Mobile / 1A5bii Mobile (water-borne component) \ 1A5biii Mobile (Othei^" Multilateral Operations

1B1 a Coal rr and handling iB1 ai3 Abandoned underground mines

1B1ai4 Flaring of drained methane or n of methane to C02

1B1aii Surface mines

, 1B1b Spontaneous combustion and burning coal dumps 1B2a Oil ai Venting aii Flaring

1B Fugitive from fuels

1B2 Oil and Natural Gas

1 B2a

2 Production and Upgrading

1 B2a

3 Transport

1 B2a


1 B2a

5 Distribution of oil products

1 B2a

6 Other

V. 1B2a




/ 1B2


MC Carbon dioxide I Transport and Storage

1B2b Natural Gas i

I 1B3 Other emissions from Energy Production Pipelines

1C1 Transport of CQ2 / 1C1b Ships

> (please s 1C2a Injection

1B2biii All Other I








Transmission and Storage





1C2 Injectior 1C3 Other ind Storage




In the Tier 3 methods for energy, either detailed emission models or measurements and data at individual plant level are used where appropriate. Properly applied, these models and measurements should provide better estimates primarily for non-CO2 greenhouse gases, though at the cost of more detailed information and effort.

Continuous emissions monitoring (CEM) of flue gases is generally not justified for accurate measurement of CO2 emissions only (because of the comparatively high cost) but could be undertaken particularly when monitors are installed for measurement of other pollutants such as SO2 or NOx. Continuous emissions monitoring is particularly useful for combustion of solid fuels where it is more difficult to measure fuel flow rates, or when fuels are highly variable, or fuel analysis is otherwise expensive. Direct measurement of fuel flow, especially for gaseous or liquid fuels, using quality assured fuel flow meters may improve the accuracy of CO2 emission calculations for sectors using these fuel flow meters. When considering using measurement data, it is good practice to assess the representativeness of the sample and suitability of measurement method. The best measurement methods are those that have been developed by official standards organisations and field-tested to determine their operational characteristics. For further information on the usage of measured data, check Chapter 2, Approaches to Data Collection in Volume 1.

It should be noted that additional types of uncertainties are introduced through the use of such models and measurements should therefore be well validated, including a comparison of calculated fuel consumption with energy statistics, thorough assessments of their uncertainties and systematic errors, as described in Volume 1, Chapter 6.

If an inventory compiler has well documented measurements of the amount of carbon emitted in non-CO2 gases or otherwise not oxidised, it can be taken into account in this tier in the country-specific emission factors. It is good practice to document how this has been done. If emission estimates are based on measurements then they will already include the direct emissions of CO2 only. Selecting tiers: a general decision tree

For each source category and greenhouse gas, the inventory compiler has a choice of applying different methods, as described in the Tiers for the source category and gas. The inventory compiler could use different tiers for different source categories, depending on the importance of the source category within the national total (cf. key categories Chapter 4 of Volume 1) and the availability of resources in terms of time, work force, sophisticated models, and budget. To perform a key category analysis, data on the relative importance of each source category already calculated is required. This knowledge could be derived from an earlier inventory, and updated if necessary.

Figure 1.2 presents a generalized decision tree for selecting Tiers for fuel combustion. This decision tree applies in general for each of the fuel combustion activities and for each of the gases.

The measurements referred to in this decision tree should be considered as continuous measurements. Continuous measurements are becoming more widely available and this increase in availability is in part driven by regulatory pressure and emissions trading. The decision tree allows available emission measurements to be used (Tier 3) in combination with a Tier 2 or Tier 1 estimate within the same activity. Measurements will typically be available only for larger industrial sources and hence only occur in stationary combustion. For CO2, particularly for gaseous and liquid fuels, such measurements should in most cases preferably be used to determine the carbon content of the fuel before combustion, whereas for other gases stack measurements could be applied. For some inhomogeneous solid fuels, stack measurements might provide more precise emission data.

Particularly for road transport, using a Tier 2 or Tier 3 technology-specific method for estimating N2O and CH4 emissions will usually bring large benefits. However, for CO2 in general, a Tier 1 method based on fuel carbon and fuel amount used will often suffice. This means that the generalized decision tree might result in different approaches for different gases for the same source category. Since emission models and technology-specific methods for road transport might be based on vehicle kilometres travelled rather than on fuel used, it is good practice to show that the activity data applied in such models and higher tier methods are consistent with the fuel sales data. These fuel sales data are likely to be used to estimate CO2 emissions from road transport. The decision tree allows the inventory compiler to use sophisticated models in combination with any other Tier methodology, including measurements, provided that the model is consistent with the fuel combustion statistics. In cases where a discrepancy between fuel sales and vehicle kilometres travelled is detected, the activity data, used in the technology-specific method should be adjusted to match fuel sales statistics, unless it can be shown that the fuel sales statistics are inaccurate.

Figure 1.2 Generalised decision tree for estimating emissions from fuel combustion

Figure 1.2 Generalised decision tree for estimating emissions from fuel combustion

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. Relation to other inventory approaches

The IPCC Guidelines for National Greenhouse Gas Inventories are specifically designed for countries to prepare and report inventories of greenhouse gases. Some countries may also be required to submit emission inventories of various gases from the Energy Sector to United Nations Economic Commission for Europe (UNECE) Long Range Transboundary Air Pollution (LRTAP) Convention2. The UNECE has adopted the joint European Monitoring Evaluation Programme (EMEP)/CORINAIR Emission Inventory Guidebook3 for inventory reporting.

Countries which are Parties to different Conventions have to use the appropriate reporting procedures when reporting to a specific Convention. The IPCC approach meets UNFCCC needs for calculating national totals (without further spatial resolution) and identifying sectors within which emissions occur, whereas the EMEP/CORINAIR approach is technology based and includes spatial allocation of emissions (point and area sources).

Both systems follow the same basic principles:

• complete coverage of anthropogenic emissions (CORINAIR also considers natural emissions);

• annual source category totals of national emissions;

• clear distinction between energy and non-energy related emissions;

• transparency and full documentation permitting detailed verification of activity data and emission factors.

Considerable progress has been made in the harmonisation of the IPCC and EMEP/CORINAIR approaches. UNECE LRTAP reporting now has accepted a source category split that is fully compatible with the UNFCCC split as defined in the Common Reporting Framework (CRF). Differences only occur in the level of aggregation for some specific sources. Such differences only occur in the energy sector in the transport source categories, where UNECE LRTAP requires further detail in the emissions from road transport.

The CORINAIR programme has developed its approach further to include additional sectors and sub-divisions so that a complete CORINAIR inventory, including emission estimates, can be used to produce reports in both the UNFCCC/IPCC or EMEP/CORINAIR reporting formats for submission to their respective Conventions. Minor adjustments based on additional local knowledge may be necessary to complete such reports for submission.

One significant difference between the approaches that remain is the spatial allocation of road transport emissions: while CORINAIR, with a view to the input requirements of atmospheric dispersion models, applies the principle of territoriality (emission allocation according to fuel consumption), the 2006 IPCC Guidelines follow what is usually the most accurate data: fuel sales (usually fuel sales are more accurate than vehicle kilometres). In the context of these IPCC Guidelines, countries with a substantial disparity between emissions as calculated from fuel sales and from fuel consumption have the option of estimating true consumption and reporting the emissions from consumption and trade separately using appropriate higher tier methods. National totals must be consistent with fuel sales.

Since both approaches are now generally well harmonised, the 2006 IPCC Guidelines will concentrate on emissions of direct greenhouse gases, CO2, CH4 and N2O with some advice on NMVOCs where these are closely linked to emissions of direct greenhouse gases (non-energy use of fuels, CO2 inputs to the atmosphere from oxidation of NMVOCs). Users are referred to the EMEP/CORINAIR Emission Inventory Guidebook for emission estimation methods for indirect greenhouse gases and other air pollutants.

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