Sources of Airborne Particulate Matter

9. Particles in the air may arise from a wide variety of sources, either natural or man-made. Of the former, forest fires and volcanic eruptions provide dramatic examples, while sea spray and the erosion of soil and rocks by wind are important sources in many localities. Biological sources are ubiquitous, and particularly in rural areas considerable numbers of pollen grains, fungal spores and their fragments contribute to the total mass of airborne particles. Man-made airborne particles result mostly from combustion processes, from the working of soil and rock, from many other industrial processes and from the attrition of road surfaces by motor vehicles. These types of particles, together with those derived from natural combustion sources, may be classified as either primary or secondary: the former, such as carbon particles from combustion, mineral particles derived from stone abrasion and salt from the sea, are released directly into the air, while the latter are formed in the atmosphere by the chemical reaction of gases, first combining to form less volatile compounds which in turn condense into particles. It is important to recognise that these particles, whatever their source or composition, are all measured as PM₁₀ if they fall within the appropriate size range, and that therefore the potential hazards of airborne particles may well be different in different places. For example, in country districts the components of PM₁₀ more likely to cause adverse effects may be fragments of pollen grains and fungal spores, causing allergic symptoms; whereas in cities the main components are likely to be derived from vehicle emissions and have quite different effects, discussed further in the sections on measurement and health effects.
10. Airborne particles arising from human activities come from a wide range of sources. The largest single source in urban areas is road traffic, and within this category the largest component is that derived from diesel exhaust, followed by lead-rich particles derived from cars running on leaded fuel. Currently, the cleanest type of vehicle (in terms of particle emissions) in common use is the petrol car with a catalytic convertor running on unleaded fuel. Inventory estimates suggest that in 1990 in Greater London about 86%, by weight, of primary PM₁₀ emissions were derived from vehicle exhaust, and a national inventory of emissions of primary PM₁₀ for the year 1993 ascribed 24%, by weight, of these particles to this source (see Table 1). As with many other pollutants, the proportionate contribution of road traffic to pollution in urban areas is significantly greater than in the total national inventory. These inventories are presented in more detail in the third report of the Quality of Urban Air Review Group.
11. Secondary particles are less easy to ascribe to their original sources. They comprise mainly ammonium sulphate and nitrate, originating from oxidation of sulphur and nitrogen oxides to acids which are then neutralised by atmospheric ammonia derived from agricultural sources. The chemical processes involved in the formation of these secondary particles are relatively slow and their persistence in the atmosphere is prolonged. Thus, while road traffic may be the main source of the original oxides of nitrogen, and coal and oil burning the main sources of sulphur oxides, the secondary particles are distributed more evenly throughout the air with less difference between urban and rural areas. They may also drift for considerable distances, and this results in transport of pollution across national boundaries. As an example, Figure 2 illustrates the concentrations of particulate sulphate as measured at 88 sites across Europe in 1993, showing a marked east to west gradient and also a gradient across the United Kingdom, with higher concentrations being observed in the south and east.
12. The national emissions inventory shown in Table 1 does not include secondary particles, or particles from natural sources, such as sea spray and wind blown dust. Analysis of data from particle samples collected in suburban Birmingham has given a useful indication of the sources of particles, although it should be noted that analysis was conducted on total suspended particulate matter (TSP) of a size range greater than PM₁₀ which may give undue prominence to particles larger than PM₁₀ from natural sources. The results, derived from two months of sampling, revealed six major source categories: airborne road dust and soil (comprising 32%) associated mainly with road surface dusts suspended by the movement of traffic, vehicle exhausts (25%), secondary particles (23%), coal combustion (11%), incineration and the metals industry (7%), and road salt plus marine aerosol (2%). Since these data were collected over a relatively short period in an area not wholly typical of UK cities and relate to TSP rather than PM₁₀, they should be viewed as indicative of source categories, rather than quantitatively defining the sources of particles in UK urban air. In wintertime pollution episodes in the major cities, the composition of PM₁₀ is dominated by particles emitted from traffic and domestic coal burning. Throughout the year however the composition of the smaller PM_2.5, fraction, which may eventually prove to be of greater health significance, is dominated by vehicle exhaust emissions and secondary particles.

Table 1 Estimated United Kingdom emissions of PM₁₀ by emission source, 1993, thousand tonnes per year

Source	Estimated Emissions*	Percentage of Total **
Power Stations (Fossil Fuelled)	40	15
Domestic Commercial/public service	37 5	14 2
Refineries Iron and Steel Other Industrial Combustion	7 20 18	3 8 7
Construction Industrial Processes Mining and Quarrying	4 30 29	2 11 11
Extraction and distribution of Fossil Fuels	0	0
Solvent Use	0	0
Road Transport: Diesel Petrol Non-exhaust (Tyres and Brakes)	49 13 4	19 5 2
Other Transport	7	3
Waste Treatment and Disposal	0	0
Agriculture	1	0
Total	263	100

*     Rounded to nearest thousand tonnes

**   Rounded to nearest 1%