National Atmospheric Emissions Inventory

4.3 Carbon monoxide Emission Estimates

Carbon monoxide arises from incomplete fuel-combustion and is of concern mainly because of its effect on human health. It leads to a decreased uptake of oxygen by the lungs and can lead to a range of symptoms as the concentration increases.

The UK emissions of carbon monoxide are shown in Figure 4.7 and Table 4.8 disaggregated by source and fuel. Over the period 1970-1996 emissions decreased by 39% reflecting significant reduction in emissions from road transport, domestic and agricultural sectors.

Figure 4.7 Time Series CO Emissions

The spatial disaggregation of CO emissions is shown in Figure 4.8. The observed pattern of emissions is clearly dominated by road transport emissions. A large proportion of road transport emissions are from vehicles travelling at slow speeds on urban or minor roads, hence the map shows high emissions in urban conurbations.


Figure 4.8 Spatially Disaggregated Emissions of CO

4.3.1 Transport

The most important source is road transport and in particular petrol driven vehicles. Emissions from road transport were fairly constant between 1970 and 1990 but in recent years have declined steadily. This is due primarily to the increased use of catalytic converters and to a lesser extent to fuel switching from petrol cars to diesel cars. The next largest source is emissions from off-road sources, in particular petrol engined machinery. This includes portable generators, fork lift trucks, lawnmowers and cement mixers. The estimation of emissions from such machinery is very uncertain since it is based on estimates of equipment population and annual usage time.

Table 4.8 UK Emissions of Carbon Monoxide by UNECE1 Source Category and Fuel (kt)

  1970 1975 1980 1985 1990 1993 1994 1995 1996 1996%
By UNECE Category
Comb. in Energy Prod & Transf. 325 304 333 292 315 267 240 230 207 4%
Comb. in Comm/Inst/Resid/Agri
    Domestic 1203 732 574 531 310 320 273 210 219 5%
    Other 41 24 23 22 19 17 16 15 16 0%
Combustion in Industry 114 62 46 42 44 43 39 37 34 1%
Production Processes 38 54 50 47 51 44 44 45 45 1%
Extr./Distrib. of Fossil Fuels 2 2 2 4 7 2 3 3 3 0%
Solvent Use 0 0 0 0 0 0 0 0 0 0%
Road Transport 4589 4644 4666 4485 4822 4164 3893 3599 3299 71%
Other Transp & Mach.
    Off-Road 1093 1002 949 860 780 801 781 736 756 16%
    Other 41 42 42 41 42 42 41 41 41 1%
Waste Treatment & Disp. 2 3 43 28 29 26 33 24 25 1%
Agricult/Forest/Land Use Change 204 210 365 416 266 4 0 0 0 0%
Nature 0 0 0 0 0 0 0 0 0 0%
By Fuel
Solid 1557 997 869 771 576 493 419 334 321 7%
Petroleum 5774 5737 5688 5421 5675 5064 4762 4425 4135 89%
Gas 26 26 25 30 30 43 47 54 60 1%
Non-fuel 295 319 511 546 406 130 134 126 129 3%
Total 7652 7080 7093 6768 6687 5730 5362 4940 4645 100%

1 UK emissions reported in IPCC format (Salway, 1998) differ slightly due to the different source categories used.
2 Railways, civil aircraft, shipping, naval vessels and military aircraft.

4.3.2 Other sources

Other emission sources of CO are small compared with transport and off-road sources. Emissions from the domestic sector have decreased by 82% since 1970 due to the decline in the use of solid fuels in favour of gas and electricity. The sudden decline in emissions from the agricultural sector reflects the banning of stubble burning in 1993 in England and Wales.

Power stations account for 4% of UK emissions; data have been recently revised to take account of lower emissions from low NOx burners and higher emission data from combined cycle gas turbines in line with the data available from the CRI (Environment Agency, 1997).

4.3.3 Accuracy of CO estimates

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 NOx emissions. It is estimated that CO emissions are accurate to +/- 40%.