Emission factors are generally based on a few measurements on an emitting source which is assumed to be representative of the behaviour of all similar sources. Emission estimates for PM₁₀ are based whenever possible on measurements of PM₁₀ emissions from the source, but sometimes measurements have only been made on the mass of total particulate matter and it has been necessary to convert this to PM₁₀ based on the size distribution of the sample collected.

It is not easy to quantify the accuracy of the national emission estimates, but it is possible to give a qualitative indication of the overall reliability of the estimates and to rank them by source sector. In order of decreasing reliability of emission estimates, the ranking by source is broadly:

1) Road transport

2) Stationary combustion

3) Industrial processes

4) Mining & quarrying and construction.

The most reliable emission estimates are from diesel cars and are based on a number of detailed measurements. Contributions from sources such as mining and quarrying and construction are subject to great uncertainty although there has been considerable work to improve the emission estimates.

Although the major sources are thought to be included in the national PM₁₀ inventory, there are still some sources which have not been included either because of lack of data on emission factor measurements for conditions pertinent to the UK or because of lack of appropriate activity data. Sources that are notably missing from the inventory are:

• Entrainment of road dust. Whilst USEPA data exists for calculating emissions from this source, they are not considered appropriate for UK road conditions. Further measurements of dust resuspension on UK-typical roads are required to provide data for emission inventory application. Until then, resuspension of road dust remains a major area of uncertainty in the UK particulate emission inventory.
  
• Agricultural sources. Emissions from ploughing and harvesting activities may, in particular, be significant sources. Certain livestock farming activities may also be important. Emission factors relevant to UK agricultural activities are not available to estimate emissions from these sources at present. Further measurements of particulate emissions from different agricultural activities in the UK are required to provide data for emission inventory applications.
  
• Biological sources. Emission factors are not currently available for this source.

The approach adopted for estimating emissions of the smaller particle sizes, while it is currently the only one available, includes a number of assumptions and uncertainties. The approach depends on the PM₁₀ emission rates estimated for each sector which themselves have great uncertainties. The inventories for the smaller particles will be even more uncertain as there are additional uncertainties in the size fractions and their applicability to individual emission source sectors. The relevance of US and Dutch size fraction data to UK emission sources can also be questioned. The figures should give a good indication of the finer particle sources and the relative magnitude of emissions, but there is clearly a need for more measurements of emission rates and particle size distributions of UK sources.

Black smoke emissions are less accurate than those for SO₂ due to the nature of the measurement technique employed. Similar Black Smoke emissions, arising from the combustion of different fuels, will give rise to differing levels of blackening. This measurement uncertainty gives rise to uncertainties in the Black Smoke measurements, and hence the estimation of Black Smoke emission factors. Accuracy is likely to be in the range ± 20-25%.

CO emissions depend on the technology employed and the specific combustion conditions. The emission factors used in the inventory have been derived from relatively few measurements of emissions from different types of boiler, hence they are less accurate than CO₂ and SO₂ emissions. Likewise CO emissions from road transport are subject to the same uncertainties surrounding NO_x emissions. It is estimated that CO emissions are accurate to ± 40%.

There has been much improvement in the benzene and 1,3-butadiene emission estimates in recent years. Information gained in speciating the emissions of NMVOC (see Section 5.5) has helped the generation of more robust emission inventories for both benzene and 1,3-butadiene. It is difficult to assess the levels of uncertainty of the current emission inventories, but the uncertainties associated with the emissions of these two pollutants may be regarded as high.