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, 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.

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

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 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.

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 223 tonnes (42% of the total emission) in 1970 to 38 tonnes (20%) in 1999.

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.

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 1999. 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 1999. 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 across 1990 to 1999). 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.

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 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 smaller size fractions indicate less of a decrease. Between 1990 and 1999, UK emissions of PM₁₀ fell by 39%, whereas emissions of PM_2.5 fell by 36%, PM₁ by 33% and PM_0.1 by 32%. 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 1999. Road transport becomes an increasingly important sector as the particle size decreases. In 1999, it accounted for 20% of PM₁₀ emissions, but 46% 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 1999. Emissions from non-combustion sources show the opposite trend.

The UK emissions of black smoke are shown in Table 4.8. Emissions have declined over the period 1970-1999 by 75%. 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 48% of the total. Road transport emissions rose by over a factor of 2 from 1970 to the first half of the 1990s, 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.

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.