Emissions of 1,3 butadiene arise from the combustion of petroleum products and its manufacture and use in the chemical industry. 1,3-Butadiene is not present in petrol but is formed as a by-product of combustion- hence it is not present in road transport evaporative emissions. The road transport sector dominates the UK emissions in 1999, contributing 85% of the total. Emissions of 1,3-butadiene for 1990 to 1999 are given in Table 4.11 and Figure 4.10 below

As with benzene, the introduction of catalytic converters in 1991 has had a significant impact on the emissions from the road transport sector, causing a reduction in emissions of 56% from 1990 to 1999. Emissions from other significant combustion sources, such as other transportation and machinery, have not significantly decreased.

1,3-Butadiene emissions also arise as stack and, more importantly, fugitive emissions from its manufacture and extensive use in the chemical industry. 1,3- Butadiene is used in the production of various forms of synthetic rubber. Reported emission estimates for the chemical industry sectors (Environment Agency, 1999) have been incorporated into the inventory.

Spatially disaggregated emissions of 1,3-butadiene are shown in Figure 4.11. Emissions of 1,3-butadiene arise almost exclusively from road transport activities, and an interesting comparison may be drawn with the UK emissions map for benzene (Figure 4.9), where other sources make a significant contribution to the total emissions. As with benzene, emissions of 1,3-butadiene per km from road transport decrease with increasing speed. Consequently the emissions density is high in urban areas, and the major roads (such as motorways) are not particularly highlighted.

Benzene and 1,3-butadiene emission maps (Figures 4.9 and 4.11) may be contrasted with mapped NO_x emissions (Figure 5.2)- as the latter increase with increasing vehicle speed.

1 See Appendix 4 for definition of UN/ECE Categories