Carbon monoxide

National Atmospheric Emissions Inventory

Carbon monoxide arises from incomplete fuel-combustion and is of concern mainly because of its effect on human health and its role in tropospheric ozone formation. 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 4.6 and Table 4.9 disaggregated by source and fuel. Over the period 1970-1998 emissions decreased by 43% reflecting significant reduction in emissions from road transport, domestic and agricultural sectors.

Figure 4.6 Time Series CO Emissions

The spatial disaggregation of CO emissions is shown in Figure 4.7. 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 4.7 Spatially Disaggregated UK Emissions of CO

Transport

The most important source is road transport and in particular petrol driven vehicles. Emissions from road transport fell only slightly between 1970 and 1990 but in recent years have declined more significantly. 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 emissions from off-road sources 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 4.9 UK Emissions of Carbon Monoxide by UN/ECE¹ Source Category and Fuel (kt)

	1970	1980	1990	1991	1992	1993	1994	1995	1996	1997	1998	1998%
BY UN/ECE CATEGORY²
Comb. in Energy Prod.
Public Power	117	121	114	113	110	100	106	104	102	70	73	2%
Petroleum Refining Plants	4	4	5	6	6	6	6	6	6	7	7	0%
Other Comb. & Trans.	45	24	22	22	20	20	20	22	23	23	26	1%
Comb. in Comm/Inst/Res
Residential Plant	1251	622	358	382	347	369	324	260	268	251	234	5%
Comm/Pub/Agri Comb.	46	26	22	22	21	21	20	19	19	19	18	0%
Combustion in Industry
Iron & Steel Comb.	220	63	108	102	106	105	97	98	98	100	96	2%
Other Ind. Comb.	140	80	77	73	73	72	73	69	62	62	52	1%
Production Processes	362	271	412	389	373	377	391	397	410	419	413	9%
Extr./Distrib. of Fossil Fuels	2	2	7	3	3	2	3	3	3	3	3	0%
Road Transport	5427	5378	5147	5026	4759	4465	4227	3969	3926	3665	3479	73%
Other Trans/Mach
Off-Road Sources	475	412	339	353	360	347	339	319	329	315	310	7%
Other³	29	28	31	30	30	30	29	29	30	29	29	1%
Waste	2	43	29	27	26	26	33	24	24	19	18	0%
Land Use Change	288	449	266	228	165	4	0	0	0	0	0	0%
By FUEL TYPE
Solid	1666	772	532	548	509	510	457	389	387	345	324	7%
Petroleum	5966	5839	5534	5427	5167	4859	4609	4332	4299	4020	3828	80%
Gas	42	34	37	37	40	46	50	53	56	53	55	1%
Non-Fuel	734	880	835	762	684	528	550	545	557	563	551	12%
TOTAL	8407	7525	6938	6774	6399	5943	5665	5320	5299	4982	4758	100%

1 UK emissions reported in IPCC format (Salway, 1999) differ slightly due to the different source categories used.

2 See Appendix 4 for definition of UN/ECE Categories

3 Including railways, shipping, naval vessels, military aircraft

Other sources

Other emission sources of CO are small compared with transport and off-road sources. Emissions from the domestic sector have decreased by 81% 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. Currently power stations account for only 2% of UK emissions.

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 NO_x emissions. It is estimated that CO emissions are accurate to ± 40%.