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 m m fall rapidly under gravity and those larger than about 100 m m 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 m m 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 m m in size (more strictly, particles that pass through a size selective inlet with a 50% efficiency cut-off at 10 m m 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). 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. As black smoke is such 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).

There is currently a programme sponsored by DETR 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 smaller size than from other sources.

• Industrial Processes. These include bulk handling, construction, mining and quarrying. These sources are all difficult to quantify due to the nature of the sources and to the lack of appropriate UK measurements. However, there have been substantial improvements in the estimation of PM₁₀ emissions from these sources. Usually a substantial fraction of the particles from these sources is larger than 10 m m but the large quantities emitted ensure that the fraction less than 10 m m 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 216 tonnes (44% of the total emission) in 1970 to 26 tonnes (16%) in 1998.

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 the deposition of primary particles form all UK sources (including vehicle tailpipes and from break 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)

1 See Appendix 4 for definition of UN/ECE Categories

Table 4.4 PM₁₀ Emission Estimates from Resuspension

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 25% of national emissions, it can account for up to 80% of primary emissions in urban areas such as London (Buckingham et. al., 1997).

Figure 4.1 UK Emissions of PM₁₀

Figure 4.2, Spatially Disaggregated UK Emissions of PM₁₀

Emissions from electricity generation have also recently been declining (since 1991) despite a significant growth in the electricity generated between 1970 and 1998. 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.

The one sector which has shown significant growth in emissions since 1970 is road transport, with a contribution to the total UK emission rising from 9% in 1970 to 25% in 1998. 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, currently accounting for 11% of the car kilometres. 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 transport sources, the major emissions are from a range of industrial processes and quarrying whose emission rates have remained fairly constant.

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 µ m) 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 139 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 smaller size fractions indicate less of a decrease. Between 1990 and 1998, UK emissions of PM₁₀ fell by 41%, whereas emissions of PM_2.5 fell by 38%, PM₁ by 36% and PM_0.1 by 38%. 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 1998. Road transport becomes an increasingly important sector as the particle size decreases. In 1998, it accounted for 25% of PM₁₀ emissions, but 62% of PM_0.1 emissions. The increased contribution of traffic to emissions of the finer particles also explains why the finer particles have shown a smaller fall in UK emissions between 1990 and 1998. Emissions from non-combustion sources show the opposite trend.

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

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

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

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

Accuracy of Particulate Matter Estimates

Although the primary emissions inventory for PM₁₀ is continuously being improved, 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. It is also possible that not all the sources that exist have been considered and some sources which are known to exist have not been quantified because of the lack of relevant data for estimating the emissions.

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

The UK emissions of black smoke are shown in Table 4.8. Emissions have declined over the period 1970-1998 by 74%. The most significant cause of the reduction is the decline in the use of solid fuels in the domestic sector. Up until the late eighties, domestic emissions were the most important contribution to the total, however since then emissions from road transport have dominated and currently account for 55% of the total. Road transport emissions rose by over a factor of 2 from 1970 to the first half of the 1990’s, the emission largely composed of emissions from diesel engines. Road transport emissions have declined since 1992 due to stricter regulations on diesel engines. Emissions from off-road sources have been estimated from estimates of the amount of diesel oil and petrol consumed by these machines. These emissions account for 12% of the UK total. The decline in agricultural emissions up to 1993 is due to the banning of stubble burning.

Table 4.8 UK Emissions of Black Smoke by UN/ECE Source Category and Fuel (kt)

1 See Appendix 4 for definition of UN/ECE Categories

2 Including railways, shipping, naval vessels, military aircraft

It is likely that emissions of black smoke have fallen more than is indicated by these estimates as a result of improved abatement measures on large combustion plant and stricter regulation on diesel engines. However, little data is available on these control measures, including their effect on the blackening effect of particulates which is crucial to the black smoke inventory, and only the diesel emission reductions have been estimated.

Accuracy of Black Smoke estimates

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%.