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.