Box 14A Emission inventory methodology developments

To provide a useful and informative emissions database, a clear explanation of the assumptions made to derive the estimates of emissions should be included in an emission inventory. Such an inventory should include the following types of information:

• emission measurements;
• emission factors;
• individual emission source specifications (location, activity, processes, controls, emission heights, conditions, etc);
• activity statistics (energy use, production, population, employment, transport, etc);
• references to the sources of the data included; and
• comments on the assumption made in deriving the emissions;

as well as emission estimates derived from the above data.

Emission inventories usually provide :

• the distribution of emissions across relevant technologies and socio-economic sectors;
• the spatial distribution of emissions; and
• trends in emissions over time.

National, and hence international, emission inventories have included very few direct measurements of emissions from individual sources but have derived emission estimates from activity statistics (for example fuel consumption, vehicle kilometres and production statistics) and representative scaling factors or emission factors. Large, localised sources of pollution, for example power-plants, refineries and factories, are often included separately in the inventory as point sources. Increasingly, point source emissions are based on plant-specific information, including on-site measurements, provided to the regulatory authorities by the plant operators in compliance with legal requirements (for example EU Large Combustion Plant Directive or various schemes for toxic release inventories). In contrast, smaller or more diffuse sources, such as domestic housing, motor vehicles and agriculture, are treated as area sources and accounted for on an administrative area (for example county, département or Land) or regular grid basis (for example 50 x 50 km) based on activity statistics and appropriate emission factors. In some inventories, where activity statistics are available on individual roads, motor vehicles can be treated as line sources.

The methodology of emission inventory developed steadily throughout the 1980s and certain fundamental requirements have now been recognised. These are completeness, consistency and transparency. An emission inventory should cover all known sources (completeness). To enable users to understand how the estimates were derived, inventories should include activity data and emission factors plus references to the source of these data as well as the emission estimates or measurements themselves (transparency). Finally, emission inventories need to use the same definitions and subdivisions (consistency) to allow comparisons to be made between national totals and contributions from sub-sectors. However, these requirements have seldom if ever been fully achieved in emission inventory systems established to date.