Air Quality Strategy Pollutants

Introduction

The original National Air Quality Strategy (NAQS) published in 1997 (DOE 1997) set out a framework of standards and objectives for the air pollutants of most concern (SO2, PM₁₀, NOx, CO, lead, benzene, 1,3-butadiene and tropospheric ozone). The aim of the strategy was to reduce the air pollutant impact on human health by reducing airborne concentrations. Different pollutants have differing timescales associated with human health impacts. Therefore concentrations during episodes (both Winter and Summer) are important for some pollutants, but less so for others.

The NAQS identified air quality standards for 8 priority pollutants based on the recommendations of the Expert Panel on Air Quality Standards (EPAQS) or WHO guidance where no EPAQS recommendation existed. EPAQS was set up by the Secretary of State for the Environment in 1991, and is a panel created to “advise, as required, on the establishment and application of air quality standards in the UK… taking account of the best available evidence of the effects of air pollution on human health…”. The NAQS has been subject to periodic review, with consultation documents being published in 1998 and 2000 (DETR 1998a, 2000).

The NAQS then evolved into the Air Quality Strategy for England, Scotland, Wales and Northern Ireland (AQS for ESWNI), with the same goals. A second edition of the strategy was published in 2000 (DETR 2000), identifying further revisions and focused on the incorporation of air quality limit values in European Directives, and the impacts of devolution.

Following a consultation exercise new objectives for: particles, benzene, carbon monoxide and polycyclic aromatic hydrocarbons were announced for England in August 2002. More details can be found at the Defra website (Defra news release 323/02). The situation for Scotland differs slightly- the Air Quality (Scotland) Amendment Regulations came into force on 12 June 2002. More detailed information can be found on the Scottish Executive website (Scottish Executive News Release SEen057/2002 on www.scotland.gov.uk). For Wales, the consultation paper on the same subject is available from the Welsh assembly web pages (http://www.wales.gov.uk).

In addition to the above, Local Authorities in the UK have a duty, under the 1995 Environment Act: Part IV, to review and assess air quality in their areas. The Air Quality Regulations 2000 define a staged process of review and assessment on the basis of guidance provided by Defra and the Devolved Administrations. The first stage primarily involves the collection of existing data on air quality measurements and emission sources for the 8 pollutants of interest in the AQS for ESWNI. These data are then used to define whether there is likely to be an air quality problem in a specific future year (depending on pollutant). The second and third stages require the use of increasingly sophisticated monitoring and modelling tools to identify hotspots of pollution and to determine whether or not the relevant air quality objectives will be met in each area.

The NAEI is being used as an important source of data for the compilation of appropriate local inventories. Table 4.1 summarises the total 2000 emissions of the 8 priority pollutants covered by the AQS for ESWNI.

Table 4.1 Total UK Emissions of AQS for ESWNI Pollutants

Pollutant	Total 2000 emission (kt)
PM10	171
Carbon Monoxide	4167
Benzene	16.4
1,3 Butadiene	5.7
Nitrogen oxides	1512
Sulphur dioxide	1156
Tropospheric Ozone	NS^1
Lead	0.50
PAH	2.26

¹ No significant ozone emissions from anthropogenic sources

The following sections provide a discussion of the UK emissions for particulate matter, carbon monoxide, benzene and 1,3-butadiene whilst a full discussion of the other pollutants is included in other chapters of this report as indicated in Table 4.2.

Table 4.2 Location of Emissions and Discussion of AQS for ESWNI Pollutants

Pollutant	Location
PM10	Chapter 4: AQS Pollutants
Carbon Monoxide	Chapter 4: AQS Pollutants
Benzene	Chapter 4: AQS Pollutants
1,3 Butadiene	Chapter 4: AQS Pollutants
Nitrogen oxides	Chapter 5: Acidifying Gases & Ozone Precursors
Sulphur dioxide	Chapter 5: Acidifying Gases & Ozone Precursors
Tropospheric Ozone	Chapter 5: Acidifying Gases & Ozone Precursors
Lead	Section 6.3: Heavy Metals
PAH	Section 6.2: Persistent Organic Pollutants

Particulate Matter

Introduction

Historically, interest in particulate matter focused mainly on smoke which can cause health problems especially in combination with other pollutants. The classic example was emissions of smoke and sulphur dioxide leading to the London smogs in the 1950s and early 1960s when several thousand excess deaths were recorded. Smoke emissions have fallen significantly as a result of the Clean Air Acts eliminating domestic coal combustion in many urban areas. However, there is increasing interest in the measurement of fine particles, such as those arising from the combustion of diesel fuel in the transport sector, and aerosol concentrations in the atmosphere from other sources which may have harmful effects. Recent epidemiological evidence is also linking concentrations of particles in the atmosphere with human health effects. Indeed, current ambient mass concentrations are thought to be sufficient to lead to increased mortality and morbidity (EPAQS, 1995).

Particles can vary widely in size and composition. Particles larger than about 30 mm (a mm is a "micrometre", or one thousandth of a millimetre) fall rapidly under gravity and those larger than about 100 mm fall out of the atmosphere so rapidly they are not usually considered. At the other end of the size scale are particles less than a tenth of a mm which are so small they do not fall under gravity appreciably, but coagulate to form larger particles that are then removed from the atmosphere.

The US PM₁₀ standard was a monitoring standard designed to measure the mass of particles less than 10 mm in size (more strictly, particles that pass through a size selective inlet with a 50% efficiency cut-off at 10 mm aerodynamic diameter). This corresponds to the International Standards Organisation thoracic convention for classifying those particles likely to be inhaled into the thoracic region of the respiratory tract. The epidemiological evidence of the effects of particulates shows good correlation in the UK between PM₁₀ concentrations and mortality or morbidity (EPAQS 1995, 2001). Therefore PM₁₀ has become the generally accepted measure of particulate material in the atmosphere in the UK and in Europe. There is also an increasing interest in the correlation between PM_2.5 and health indicators. PM₁₀ measurements are being made in the UK (Bower et al, 1994, 1995 a & b, 1996, 1997) and their emissions have been included in the NAEI since 1995.

For many years the monitoring of particulate levels was based on the measurement of “Black Smoke”. Levels were estimated using a simple non-gravimetric reflectance method in which air is sampled through a filter and the resulting blackening measured. The method was calibrated for domestic coal smoke. When most of the emissions come from coal combustion the blackening should be approximately proportional to the mass concentrations. In the 50s and 60s, domestic coal combustion was the dominant source of black smoke and hence this method gave an indication of the concentration. The NAEI estimates of black smoke emissions were extended in 1988 to include emissions from all fuel combustion. Prior to 1988 only emissions from coal combustion had been estimated and published in the DOE Digest of Environmental Statistics. However, there have been no recent emission measurements to confirm these factors are still accurate.

Smoke from different sources has a different blackening effect and so there is no simple relationship between black smoke and the mass of particulate emissions. For example, typically diesel emissions have a blackening effect three times greater, mass for mass, than coal emissions, while petrol emissions are effectively an order of magnitude less. Black smoke is a poor indicator of the concentrations of particulates in the atmosphere current interest and the NAQS is focused on PM₁₀ (particulate matter less than 10mm i.e. 10 millionths of a metre) and smaller size fractions (EPAQS, 1995). However, black smoke has been shown to have relationships with health effects and is still used as an indicator.

For completeness the following sections present emission estimates and discussion for PM₁₀ , PM_2.5 , PM_1.0 , PM_0.1 and Black Smoke.

PM₁₀

Sources of emissions

PM₁₀ in the atmosphere arises from two sources. The first is the direct emission of particulate matter into the atmosphere from a wide range of sources such as fuel combustion, surface erosion and wind blown dusts and mechanical break-up in, for example, quarrying and construction sites. These are called ‘primary’ particulates. The second source is the formation of particulate matter in the atmosphere through the reactions of other pollutants such as sulphur dioxide, nitrogen oxides and ammonia to form solid sulphates and nitrates, as well as organic aerosols formed from the oxidation of VOCs. These are called ‘secondary’ particulates. This inventory only considers primary sources. For further information on secondary particulate see the third Quality of Urban Air Review Group report (QUARG, 1996) and the report from the Airborne Particles Expert Group (APEG, 1999)- see http://www.aeat.co.uk/netcen/airqual/reports/quarg/q3intro.html and http://www.defra.gov.uk/environment/airquality/airbornepm/index.htm respectively.

There is currently a programme sponsored by Defra and the Society of Motor Manufacturers and Traders (SMMT) to measure PM₁₀ emissions from road transport. This programme has developed measurement techniques and aims to produce improved PM₁₀ emission factors, particulate characterisation and particle size distribution.

The main sources of primary PM₁₀ are briefly described below:

· Road Transport. All road transport emits PM₁₀. However diesel vehicles emit a greater mass of particulate per vehicle kilometre than petrol-engined vehicles. Emissions also arise from brake and tyre wear and from the re-entrainment of dust on the road surface. Emission estimates for the re-entrainment of dust have been made. However this does not fall within the UN/ECE reporting format and consequently has been reported separately.

· Stationary Combustion. Domestic coal combustion has traditionally been the major source of particulate emissions in the UK. However, the use of coal for domestic combustion has been restricted in the UK by the Clean Air Acts, and as a result other sources are more important nationally. Domestic coal can still be a significant source in some smaller towns and villages, in Northern Ireland and in areas associated with the coal industry. Other fossil fuels emit PM₁₀, but emissions from gas use are small. In general, particles emitted from combustion are of a smaller size than from other sources.

· Industrial Processes. These include the production of metals, cement, lime, coke, and chemicals, bulk handling of dusty materials, construction, mining and quarrying. Emissions from these sources are difficult to quantify due to the contribution of fugitive emissions (i.e. those diffuse emissions which are released directly into the atmosphere from a process rather than being collected in a controlled manner and then vented to atmosphere). Few UK measurements are available for these fugitive releases. Nonetheless, there have been substantial improvements in the estimation of PM₁₀ emissions from industrial processes in recent years. Usually a substantial fraction of the particles from these sources is larger than 10 mm but the large quantities emitted ensure that the fraction less than 10 mm is still substantial.

PM₁₀ Emission estimates

Emissions of PM₁₀ are shown in Table 4.3 and Figure 4.1. Emissions of PM₁₀ from the UK have declined since 1970. This is due mainly to the reduction in coal use. Domestic emissions have fallen from 222 tonnes (41% of the total emission) in 1970 to 28 tonnes (16%) in 2000.

Emission estimates for the resuspension of dust from roads is not included in the standard UN/ECE reporting format (and hence not included in Table 4.3). However for completeness it is given in Table 4.4 below. Estimates for resuspension are based on the deposition of primary particles form all UK sources (including vehicle tailpipes and from brake and tyre wear) that are returned to the air from the turbulence of passing vehicles. As such, resuspension to some extent double counts the emissions, but is important in reconciling road side concentration measurements.

Table 4.3 UK Emissions of PM₁₀ by UN/ECE Source Category and Fuel (kt)

	1970	1980	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
BY UN/ECE CATEGORY^1
Comb. in Energy Prod.
Public Power	67	76	70	70	66	55	49	38	35	24	25	20	22	13%
Petroleum Refining Plants	5	5	3	4	4	4	4	4	4	4	4	3	3	1%
Other Comb. & Trans.	16	2	0	0	0	0	0	0	0	0	0	0	0	0%
Comb. in Comm/Inst/Res
Residential Plant	222	102	52	55	51	53	43	33	35	33	33	34	28	41%
Comm/Pub/Agri Comb.	22	11	8	8	8	7	7	6	6	7	6	5	5	3%
Combustion in Industry
Iron & Steel Comb.	8	2	1	1	1	1	1	1	1	1	1	1	1	0%
Other Ind. Comb.	89	44	39	39	41	39	38	35	32	30	27	24	20	11%
Production Processes
Processes in Industry	14	12	14	14	14	14	14	13	14	13	13	12	11	7%
Construction	8	3	4	4	3	3	3	4	4	4	4	4	4	2%
Quarrying	22	21	29	26	25	25	27	26	23	24	23	21	21	12%
Road Transport
Combustion	44	53	59	58	56	54	52	48	45	39	35	32	26	15%
Brake & Tyre Wear	2	3	4	4	4	4	4	4	5	5	5	5	5	3%
Other Trans/Mach	15	13	13	13	12	12	12	11	12	12	11	11	10	6%
Waste	1	4	3	3	2	2	3	2	2	1	1	1	1	1%
Agri. & Land Use Change	11	12	12	13	12	12	12	12	13	13	14	14	14	8%
By FUEL TYPE
Solid	359	199	138	142	135	124	107	84	80	68	65	60	53	31%
Petroleum	96	87	83	83	80	78	75	69	66	59	53	49	41	24%
Gas	5	6	7	7	7	8	8	8	9	9	10	10	10	6%
Non-Fuel	87	73	86	79	77	77	80	77	74	74	74	69	67	39%
TOTAL	546	365	313	311	299	285	270	238	230	209	201	188	172	100%

1 See Annex 1 for definition of UN/ECE Categories

Table 4.4 PM₁₀ Emission Estimates from Resuspension

	1970	1980	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
Resuspension from Road Trans	8.2	11.2	16.9	17.0	17.0	17.0	17.4	17.8	18.3	18.7	19.0	19.3	19.4

The geographical disaggregation of emissions is shown in Figure 4.2. There is a clear distinction between the important sources in rural and urban areas. Indeed, many of the sources do not occur inside towns and cities. While road transport accounts for only 20% of national emissions, it can account for up to 80% of primary emissions in urban areas such as London (Buckingham et. al., 1997).

Emissions from electricity generation have also recently been declining (since 1991) despite a significant growth in the electricity generated between 1970 and 2000. This is due to the move away from coal to natural gas and nuclear power for electricity generation and to improvements in the performance of electrostatic precipitators at coal-fired power stations. Also the installation of flue gas desulphurisation at two power stations have reduced particulate emissions further.

Figure 4.1 UK Emissions of PM₁₀

The one sector which has shown significant relative growth in emissions since 1970 is road transport, with a contribution to the total UK emission rising from 13% in 1970 to 20% in 2000. In urban areas with little industrial activity, where public power and industrial processes do not make a significant contribution, the contribution of road transport to emissions will be even higher; for example as much as 80% of primary emissions in London. The main source of road transport emissions is exhaust from diesel engined vehicles. Emissions from diesel vehicles have been growing due to the growth in heavy duty vehicle traffic and the move towards more diesel cars, (diesel cars have increased from 3% to 12% of the UK vehicle fleet between 1990 and 2000). Since around 1992, however, emissions from diesel vehicles have been decreasing due to the penetration of new vehicles meeting tighter PM₁₀ emission regulations.

Among the non-combustion and non-transport sources, the major emissions are from industrial processes, the most important of which is quarrying whose emission rates have remained fairly constant. Other industrial processes, including the manufacture of steel, cement, lime, coke, and primary and secondary non-ferrous metals, are collectively important sources of particulate matter although emissions from individual sectors are relatively insignificant.

Figure 4.2, Spatially Disaggregated UK Emissions of PM₁₀

Finer particulates: PM_2.5, PM₁ and PM_0.1

Inventories for PM_2.5, PM₁ and PM_0.1 have been estimated from the PM₁₀ inventory and the mass fractions in these size ranges available for different emission sources and fuel types. A total of 33 different size distributions covering PM_2.5 and PM₁ emissions from different source sectors were taken from the USEPA (1995) as being applicable to sources in the UK; a fewer number of sectors with size fractions in the PM_0.1 range were available from the study by the TNO Institute of Environmental Sciences, Energy Research and Process Innovation in the Netherlands for the Dutch National Institute of Public Health and Environment (RIVM) (TNO, 1997) who produced a particulates emissions inventory for Europe. In general, combustion processes emit a higher proportion of fine particles (<2.5 µm) than mechanical sources such as quarrying and construction. Gaseous fuels also tend to emit finer particles than petroleum and solid fuels.

Each of the detailed source sectors for which a PM₁₀ emission is estimated (a total of 236 individual sectors and sub-sectors) were allocated an appropriate size distribution and used to calculate emission inventories for PM_2.5, PM₁ and PM_0.1. The results are shown in Tables 4.5-4.7 in the same format as for the PM₁₀ inventory.

Figures 4.3-4.5 show trends in emissions of each particle size by source sector. The results show a comparable decline in emissions of each particle size since 1990, although the PM_0.1 size fractions indicate a larger decrease. Between 1990 and 2000, UK emissions of PM₁₀ fell by 40%, whereas emissions of PM_2.5 fell by 28%, PM₁ by 28% and PM_0.1 by 40%. There is a gradual change in the relative source contribution with particle size. This is illustrated in Figures 4.2 to 4.5 which show the contribution of each sector to PM₁₀, PM_2.5, PM₁ and PM_0.1 emissions from 1990 to 2000. Road transport becomes an increasingly important sector as the particle size decreases. In 2000, it accounted for 18% of PM₁₀ emissions, but 43% of PM_0.1 emissions.

Table 4.5 UK emissions of PM_2.5 by sector (ktonnes) estimated for the mass fraction of particles below 2.5 µm in each sector in the PM₁₀ inventory

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
Comb. in Energy Prod/Trans	41	37	34	37	33	34	35	35	36	39	38	19%
Comb. in Comm/Inst/Resid	94	83	71	70	67	57	55	56	52	53	46	23%
Combustion in Industry	70	61	53	55	50	44	44	43	41	43	33	17%
Production Processes	16	16	17	18	17	16	16	15	16	16	14	7%
Road Transport	40	41	43	44	43	43	45	45	46	47	48	24%
Other Transport	12	13	13	14	12	12	12	12	12	12	11	5%
Waste Treatment & Disposal	1	1	1	1	1	1	1	3	4	5	4	2%
Agriculture/Forestry	4	4	4	4	4	4	4	4	4	4	5	2%
TOTAL	277	255	236	244	227	212	212	213	211	218	198	100%

Table 4.6 UK Emissions of PM₁ by Sector (ktonnes) Estimated for the Mass Fraction of Particles below 1 µm in each Sector in the PM₁₀ Inventory

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
Comb. in Energy Prod/Trans	21	19	17	18	16	17	16	17	17	18	17	12%
Comb. in Comm/Inst/Resid	73	64	54	53	51	43	41	42	39	40	34	24%
Combustion in Industry	41	37	34	36	32	29	29	28	27	28	21	15%
Production Processes	8	7	8	8	8	7	8	7	7	8	7	5%
Road Transport	36	37	38	40	39	39	40	40	41	42	42	30%
Other Transport	11	11	12	13	11	11	11	11	11	11	10	7%
Waste Treatment & Disposal	1	1	1	1	1	1	1	3	3	5	3	2%
Agriculture/Forestry	4	4	4	4	4	4	4	4	4	4	5	3%
TOTAL	194	180	168	173	161	150	150	152	150	155	140	100%

Table 4.7 UK Emissions of PM_0.1 by Sector (ktonnes) Estimated for the Mass Fraction of Particles below 0.1 µm in each Sector in the PM₁₀ Inventory

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
Comb. in Energy Prod/Trans	6	6	6	5	5	4	4	3	3	3	3	10%
Comb. in Comm/Inst/Resid	3	3	3	3	3	3	3	3	3	3	3	8%
Combustion in Industry	8	8	8	8	8	7	7	6	6	5	5	15%
Production Processes	2	2	2	2	2	1	2	1	1	1	1	4%
Road Transport	25	25	24	24	23	22	20	18	16	15	13	43%
Other Transport	2	2	2	2	2	2	2	2	2	2	2	5%
Waste Treatment & Disposal	0	0	0	0	0	0	0	0	0	0	0	1%
Agriculture/Forestry	4	4	4	4	4	4	4	4	4	5	5	15%
TOTAL	51	51	50	48	47	43	42	38	36	34	31	100%

Figure 4.3 UK emissions of PM_2.5

Figure 4.4 UK emissions of PM₁

Figure 4.5 UK emissions of PM_0.1

Black Smoke

There has been less interest in the emissions of black smoke across recent years. This is because PM₁₀ has superceded black smoke as an indicator of particulate material in the air. As a result, black smoke emission estimates are presented here in less detail than previous years. The total emissions are included below in Table 4.8

Table 4.8 UN/ECE¹ Total Emission of Black Smoke (kt)

	1970	1980	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
TOTAL	1087	605	484	490	478	454	414	375	364	329	289	273	246

1 See Annex 1 for definition of UN/ECE Categories

Carbon monoxide emission estimates

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 1970-2000 emissions decreased by 53% 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 of CO 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	1999	2000	2000%
BY UN/ECE CATEGORY^2
Comb. in Energy Prod.
Public Power	117	121	114	113	110	99	106	104	102	71	73	61	60	1%
Petroleum Refining Plants	8	8	7	7	7	8	7	8	8	8	8	7	6	0%
Other Comb. & Trans.	45	24	22	22	20	20	20	22	23	23	26	27	28	1%
Comb. in Comm/Inst/Res														0%
Residential Plant	1237	608	345	369	347	369	324	260	268	246	239	244	215	5%
Comm/Pub/Agri Comb.	46	26	23	23	21	21	20	19	19	19	18	18	18	0%
Combustion in Industry
Iron & Steel Comb.	776	221	390	369	382	382	346	352	350	357	343	347	245	6%
Other Ind. Comb.	217	130	125	120	121	121	123	122	113	115	101	137	136	3%
Production Processes	173	135	178	171	163	163	168	172	177	178	163	142	135	3%
Extr./Distrib. of Fossil Fuels	2	2	7	3	3	2	3	3	3	3	3	1	1	0%
Road Transport	5409	5367	5240	5076	4858	4528	4271	4003	3966	3728	3508	3290	2881	69%
Other Trans/Mach
Off-Road Sources	584	512	428	445	453	439	428	407	408	407	407	406	405	10%
Other^3	30	28	32	30	29	28	26	26	26	25	23	22	21	1%
Waste	3	45	31	28	27	27	35	24	25	20	21	17	16	0%
Land Use Change	288	449	266	228	165	4	0	0	0	0	0	0	0	0%
By FUEL TYPE
Solid	2237	989	877	879	867	870	792	730	727	686	663	651	521	12%
Petroleum	6061	5930	5718	5569	5356	5012	4740	4450	4414	4172	3949	3727	3316	80%
Gas	43	35	40	40	43	49	54	57	61	58	60	66	60	1%
Non-Fuel	593	723	573	514	440	279	292	285	285	285	262	273	270	6%
TOTAL	8934	7677	7208	7002	6707	6210	5877	5522	5487	5201	4934	4718	4167	100%

1 UK emissions reported in IPCC format (Salway, 2001) differ slightly due to the different source categories used.

2 See Annex 1 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. Combustion-related emissions from the domestic and industrial sectors have decreased by 83% and 51% respectively 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 energy production accounts for only 1% of UK emissions.

Benzene

Studies have shown that exposure to benzene gives rise to an increase in the risk of developing leukaemia, and that benzene exerts its effect by damaging the genetic make-up of cells i.e. it is a genotoxic carcinogen. Consequently it is important to understand sources of benzene and their relative strengths, and ensure that emissions do not give rise to unacceptably high concentrations of benzene.

Benzene emissions arise predominately from the evaporation and combustion of petroleum products. Emissions of benzene are dominated by the road transport sector, accounting for 47% of the 2000 emission estimate total. As benzene is a constituent of petrol, emissions arise from both evaporative and combustion of petrol. Benzene emissions for 1990 to 2000 are given in Table 4.10 and Figure 4.8 below.

Benzene emissions also arise as stack emissions and, more importantly, fugitive emissions from its manufacture and use in the chemical industry. Benzene is a major chemical intermediate, being used in the manufacture of many important chemicals including those used for the production of foams, fibres, coatings, detergents, solvents and pesticides.

Benzene emissions have been steadily decreasing since 1990. These decreases are primarily due to the introduction of cars equipped with catalytic converters since 1991, although emissions from the domestic and industrial sectors are also falling. However, it is the increasing number of cars in the national vehicle fleet that are equipped with catalytic converters which maintains the decreasing total of benzene emissions as the reductions from other source sectors are small by comparison.

The most noticeable decrease between 1999 and 2000 arises from the road transport sector. This is because the benzene content of petrol was substantially decreasesd between 1999 and 2000- resulting in a corresponding decrease in emissions.

Table 4.10 UK emissions of Benzene by UN/ECE¹ Source Category (ktonnes)

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
BY UN/ECE CATEGORY^1
Comb. in Energy Prod.
Petroleum Refining Plants	0.00	0.00	0.01	0.01	0.00	0.01	0.01	0.01	0.01	0.01	0.01	0%
Other Comb. & Trans.	0.29	0.29	0.29	0.29	0.32	0.13	0.15	0.18	0.18	0.20	0.18	1%
Comb. in Comm./Inst/Res
Residential Plant	4.04	4.20	4.17	4.16	3.62	3.14	3.32	3.16	3.23	3.34	3.00	18%
Comm/Pub/Agri Comb.	0.11	0.13	0.13	0.12	0.13	0.14	0.15	0.14	0.14	0.15	0.16	1%
Combustion in Industry
Iron & Steel Comb.	0.31	0.32	0.30	0.30	0.23	0.24	0.24	0.26	0.24	0.23	0.22	1%
Other Ind. Comb.	0.41	0.39	0.37	0.37	0.53	0.56	0.59	0.60	0.57	0.54	0.60	4%
Production Processes	4.37	4.46	4.67	4.55	4.66	4.59	4.05	3.43	2.26	2.04	1.65	10%
Extr./Distrib. of Fossil Fuels	1.02	1.07	1.12	1.15	1.13	1.08	0.95	0.99	0.96	0.72	0.67	4%
Road Transport
Combustion	40.07	39.16	37.54	34.64	32.14	29.50	27.30	24.55	22.01	19.69	7.42	45%
Evaporation	2.41	2.38	2.33	2.17	2.01	1.84	1.72	1.58	1.34	1.22	0.29	2%
Other Trans/Mach	2.44	2.52	2.52	2.44	2.36	2.27	2.31	2.30	2.22	2.19	2.17	13%
Waste	0.10	0.10	0.10	0.09	0.09	0.08	0.08	0.08	0.07	0.07	0.06	0%
Land Use Change	0	0	0	0	0	0	0	0	0	0	0	0%
TOTAL	55.59	55.01	53.54	50.29	47.23	43.58	40.87	37.27	33.25	30.39	16.43	100%

1 See Annex 1 for definition of UN/ECE Categories

Figure 4.8 Time Series of Benzene Emissions

Spatially disaggregated emissions of benzene are shown in Figure 4.9. A high percentage of the total benzene emission arises from the road transport sector, and this is evident in the spatially disaggregated UK map. High emission densities (i.e. emission per 1x1 km grid square) may be found in areas of high population density, and it is apparent that the dominant emissions are arising from the road transport activities in these areas, although other urban sources do also make a significant contribution.

Road transport emissions of benzene fall with increasing speed (and then start rising at higher speeds). This results in relatively high emissions per km in urban areas, as can be seen in Figure 4.9. Although evident, motorways and other major roads are not particularly highlighted.

Figure 4.9 Spatially Disaggregated UK Emissions of Benzene

1,3-Butadiene

Studies have indicated that elevated concentrations of 1,3-butadiene give rise to a variety of cancers and damages the genetic structures of cells i.e. 1,3-butadiene is a genotic carcinogen. Atmospheric concentrations have been determined at which the risk of adverse impacts is considered acceptably small, and it is therefore important to be able to understand the major sources of 1,3-butadiene which contribute to the ambient concentration.

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 2000, contributing 81% of the total. Emissions of 1,3-butadiene for 1990 to 2000 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 61% from 1990 to 2000. 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, 2001) 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 ( but then increase at higher speeds). 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 emissions of NOx at higher speeds are more significant than those for benzene or 1,3-butadiene.

Table 4.11 UK Emissions of 1,3-butadiene by UN/ECE¹ Category (ktonnes)

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
BY UN/ECE CATEGORY^1
Comb. in Energy Prod.	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0%
Comb. in Comm/Inst/Res	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0%
Combustion in Industry	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.00	0.00	0%
Production Processes	0.91	0.79	0.90	0.70	0.58	0.70	0.52	0.42	0.33	0.52	0.36	6%
Extr./Distrib. of Fossil Fuels	0.16	0.15	0.14	0.12	0.11	0.09	0.08	0.07	0.06	0.03	0.02	0%
Road Transport	11.90	11.52	10.87	10.01	9.32	8.35	7.61	6.81	5.95	5.27	4.59	81%
Other Trans/Mach	0.74	0.75	0.78	0.76	0.73	0.70	0.72	0.69	0.70	0.67	0.69	12%
Waste	0.02	0.02	0.02	0.02	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0%
TOTAL	13.74	13.23	12.72	11.62	10.78	9.86	8.96	8.02	7.05	6.50	5.67	100%

1 See Annex 1 for definition of UN/ECE Categories

Figure 4.10 Time Series of 1,3-Butadiene Emissions

Figure 4.11 Spatially Disaggregated UK Emissions of 1,3- Butadiene

Accuracy of Emission Estimates of air quality strategy pollutants

Quantitative estimates of the uncertainties in emission inventories are based on calculations made using a direct simulation technique, which corresponds to the IPCC Tier 2 approach recommended for greenhouse gases and also the methodology proposed in draft guidance produced by the UN ECE Taskforce on Emission Inventories. This work is described in more detail by Passant (2002). Uncertainty estimates are shown in Table 4.12.

Table 4.12 Uncertainty of the Emission Inventories for Air Quality Strategy Pollutants

Pollutant	Estimated Uncertainty %
Carbon Monoxide	+/- 20%
Benzene	+/- 30%
1,3-butadiene	+/- 20%
PM10	-20% to +50%
PM2.5	+/- 20%
PM1	+/- 20%
PM0.1	+/- 20%
Black smoke	-50% to +80%

Carbon monoxide estimates

Carbon monoxide emissions occur almost exclusively from combustion of fuels, particularly by road transport. Emission estimates for road transport are highly uncertain, due to the relatively small number of measurements made of emissions which appear to be highly variable. Emissions from stationary combustion processes are also variable and 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. As a result of the high uncertainty in major sources, emission estimates for CO are much more uncertain than other pollutants such as NOx, CO₂ and SO₂ which are also emitted mainly from combustion processes.

Benzene and 1,3-Butadiene Estimates

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. However, due in particular to the uncertainty in the levels of both pollutants in VOC emissions from road transport and other combustion processes, the uncertainty in these inventories is much higher than the uncertainty in the VOC inventory.

Particulate Matter Estimates

The emission inventory for PM₁₀ has undergone considerable revision over the last three versions of the NAEI and must be considered significantly more robust now than, say, in 1997. Nonetheless, the uncertainties in the emission estimates must still be considered high. These uncertainties stem from uncertainties in the emission factors themselves, the activity data with which they are combined to quantify the emissions and the size distribution of particle emissions from the different sources.

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 either on the size distribution of the sample collected or, more usually, on size distributions given in the literature. Many sources of particulate matter are diffuse or fugitive in nature e.g. emissions from coke ovens, metal processing, or quarries. These emissions are difficult to measure and in some cases it is likely that no entirely satisfactory measurements have ever been made.

Emission estimates for combustion of fuels are generally considered more reliable than those for industrial processes, quarrying and construction. All parts of the inventory would need to be improved before the overall uncertainty could be reduced to the levels seen in the inventories for CO₂, SO₂, NOx, or VOC.

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 emission estimates for the smaller particles will be even more uncertain for a given source 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. Perhaps surprisingly, the inventories for the smaller particles are less uncertain overall than the PM₁₀ inventory. This is because the most uncertain PM₁₀ emissions are those from industrial processes, quarrying and construction and these sources emit very little of the finer particles, road transport dominating instead.

Black Smoke Estimates

Black smoke emissions are less accurate than those for PM₁₀ due to the fact that, since its importance as a policy tool has declined, the black smoke inventory has not been revised for many years and the relevance of the emission factors used in the inventory to current industrial technology is in doubt.