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Size Distribution and Chemical Nature of Airborne Particles
Summary
In its 1995 report the Panel described briefly the sources and physico-chemical composition of airborne particles. We explained that the likelihood of a particle remaining suspended in the air depends upon its size, shape and density, and that those particles most likely to be inhaled into the lung are usually below about 10 µm in diameter. Below this size, particles differ in their chances of being deposited on the inner surface of the lung’s tissue and therefore in their potential to cause harm.
The size distribution of particles in the urban air is conventionally characterised by three modes. The smallest of these, below 0.1 µm in diameter, is called the nucleation mode and is formed by condensation of hot vapour from combustion sources and from chemical conversion of gases to particles in the atmosphere. Particles of this size have a high chance of deposition in the gas-exchanging (alveolar) part of the lung; they are relatively short-lived and grow into larger particles between 0.1 and about 1 µm in diameter, known as the accumulation mode. These particles remain suspended for up to several weeks in the air, and are not readily removed by rain. The third, coarse, mode comprises particles greater than about 2 µm in diameter. These are generally formed by break-up of larger matter, and include wind-blown dust and soil, particles from construction and sea spray. Their size means that they remain in the air for relatively short periods, but they make (in relation to their numbers) a disproportionate contribution to PM10 mass when this is measured close to a source.
The chemical composition of ambient airborne particles varies both in time and space, depending on the activity of local and distant sources and meteorological conditions. Combustion processes produce particles based on carbon, carrying a variety of metal compounds and organic chemicals derived from the fuel burnt. Photochemical reactions produce mainly ammonium sulphate and nitrate particles, derived in part from gases produced by combustion sources and in part from ammonia derived from animal sources. Distinct differences in the proportions of these components occur in different seasons. In addition to this fine component, the coarser fraction may contain a wide variety of chemical substances including salt, silicates and biological particles, depending on local sources.
Particle Sizes
5. Typical particles from an urban and industrial area are shown in Figure 1. Reviews of the many thousands of measurements that have been made of the size distributions of suspended particulate matter in urban atmospheres have been synthesised to yield idealised relationships between the number of particles, their surface area, volume and mass, and their particle diameters. Figure 2 shows a schematic representation of a typical size distribution for atmospheric particles, indicating some formation pathways. Figure 3 shows the idealised particle size distribution of urban aerosols in the USA, in the form of three curves showing the number-, area-, and volume-size distributions, taken from Whitby (1978). Figure 3a shows the fraction of the number of particles in each size range in the idealised urban aerosol. The curve shows a predominant single peak with a shoulder towards higher particle sizes. This number distribution peaks at about 0.013 µm (13 nm) with a total number density of about 100,000 particles per cm3. Particles in this size range below about 0.1 µm are said to be in the nucleation mode.
Figure 2 Schematic representation of a typical size distribution for atmospheric particles, indicating some formation pathways.
6. Nucleation mode particles have been emitted into the atmosphere as primary particles by combustion sources (primary particulate material), both stationary and mobile. These particles include elemental carbon from diesel vehicles and metals from smelting operations. However, some nucleation mode particles are formed by condensation of gaseous material through gas-particle conversion processes (secondary particulate material). Few vapours can readily form totally new nucleation mode particles directly, but sulphuric acid vapour, formed by the photochemical oxidation of sulphur dioxide, has this capacity. In urban areas, nucleation mode particles are the most numerous of all the particles due to constant replenishment by fresh emissions.
7. Figure 3b shows the fraction of total surface area of the particles in each size range in the same idealised urban aerosol as Figure 3a. In this case the curve shows a strong single peak with shoulders to both higher and lower particle sizes. The area distribution peaks at about 0.1 µm. Particles in the size range 0.1 to 1 µm are said to be in the accumulation mode. Few accumulation mode particles are actually emitted into the atmosphere in this particle size range, rather they form by coagulation. Because this mode contains the bulk of the surface area of the particles (see Figure 3b), these particles offer the largest target area for adsorbing gaseous material. The process of coagulation reduces the number of particles. The process of adsorption of gaseous material from the atmosphere onto pre-existing nucleation or accumulation mode particles leaves the number of particles unchanged but causes the area, volume and mass of the suspended particulate material to grow rapidly. The accumulation mode is therefore the ageing region for all small particles because growth is rapid and loss processes are at a minimum in this size range. Typically there might be about 10,000 particles per cm3in the accumulation mode in an urban area. Coarse and nucleation mode particles make a relatively small contribution to the total surface area of the urban aerosol.
8. Figure 3c shows the fraction of the total volume (and hence mass) of the particles in each size range in the same idealised urban aerosol as Figure 3a. In this case the curve shows a bimodal distribution and the almost complete absence of a shoulder to lower particle sizes. The volume (or mass) distribution peaks at particle sizes of about 0.3 and 6 µm. The volume (or mass) of the urban aerosol appears to be in two modes: the accumulation mode and the coarse mode, with the latter containing particles in the size range 2 µm and above. The nucleation mode particles, although the greatest in number, contain a negligible fraction of the aerosol volume and mass.
9. Most of the particles in the coarse mode are formed by the frictional processes of comminution, such as wind-suspended soil dust or sea spray from breaking waves (primary suspended particulate material) and from the slow growth of particles from the accumulation mode (secondary particulate material). Typically, there might be a few tens or hundreds of particles per cm3 in the coarse mode in an urban area. However, not all of the coarse mode particles pass through the PM10 sampling and monitoring system and are measured, as some are too large. Coarse mode particles generally account for about 20-50% of the urban background PM10 mass in the UK.10. Different types of atmospheric particles have different particle size distributions. At around 1 µm there is a saddle point that separates the suspended particulate material formed by storms, the oceans and volcanoes, from the fine particulate matter formed by combustion and other chemical reactions in the atmosphere. Suspended particulate matter formed by storms and volcanoes may not only comprise coarse mode material. Studies of Saharan dust storms have shown that a second mass peak of submicron particles appears when the dust concentration (i.e., the wind strength) increases above the threshold of erosion, probably as a result of the comminution of particle aggregates by a sandblasting process (Gomes et al 1990). Recent work has indicated that ash-falls from certain volcanic eruptions may also be in the form of aggregates of particles with an already substantial submicron, as well as coarse mode, component and resuspension of the deposited material may then occur by winds and human activity.
Chemical Composition
11. Rapid response measurements of particle concentrations in the air, when taken close to major sources, such as road traffic, show variations on a timescale of minutes or even seconds as emissions from individual vehicles reach the sampling device. When a site is influenced by many sources, as is the case with particulate matter, not only will the concentration of particles vary quite rapidly with time, the chemical composition will also fluctuate similarly. As monitoring times are increased, so the average concentrations and composition become more reflective of longer term influences of the various sources upon the site, with day-to-day, and even year-to-year changes in concentration and composition dependent on weather conditions and variations in emissions.
12. Measurements of the chemical composition of airborne particulate matter on an hourly timescale can provide valuable insights into atmospheric processes and the sources of particulate matter. In general, however, measurements on an hourly timescale are highly resource intensive and are made only as part of specialised research. Investigations of the health effects of exposure to airborne particles are most commonly based on either daily or annual average measurements. On these timescales, measured concentrations of individual components reflect the pollution climate of the location as influenced by the major sources and prevailing meteorological conditions. Daily data are readily interpretable in terms of contributions from the individual major source categories; annual data show only minor year-to-year variation due to weather conditions, and data from periods of around ten consecutive years can be used to identify long-term trends.
13. Airborne particulate matter is of two distinct kinds. Primary particles, as outlined above, come directly from emission sources and are therefore spatially variable, depending on the proximity and magnitude of emissions. Secondary particles comprising mainly ammonium sulphate, ammonium nitrate and secondary organic compounds are formed slowly as air masses move over long distances and therefore vary on spatial scales of tens or hundreds of kilometres rather than tens or hundreds of metres as for primary particles. Thus, secondary particles show no appreciable gradient between urban and rural areas, whilst primary particles show strong spatial differences.
14. Figure 4, taken from the third report of the Quality of Urban Air Review Group (QUARG 1996), illustrates very approximately the major component composition of particulate matter sampled in urban areas. The components and their sources are as follows:
- Sulphate, mainly present in the atmosphere as ammonium sulphate, is a secondary pollutant formed from sulphuric acid vapour, itself formed in the atmosphere from oxidation of sulphur dioxide.
- Nitrate, present in the atmosphere largely as ammonium nitrate but with some sodium nitrate, is a secondary pollutant formed from nitric acid vapour, itself formed in the atmosphere from oxidation of oxides of nitrogen.
- Ammonium, present mainly as a component of ammonium sulphate and ammonium nitrate, is derived from ammonia emissions, mainly from agriculture, which react with sulphuric and nitric acids.
- Carbonaceous matter is the term used to describe the primary soots emitted from combustion processes, together with some secondary particulate organic matter formed in the atmosphere. Sources burning fossil fuels, such as petrol and diesel engines, emit particles, which comprise mainly carbon in its elemental form (termed black carbon) in combination with low volatility hydrocarbons, which typically make up a liquid coating on the particle surfaces. During the summer months atmospheric photochemical oxidation of volatile organic compounds leads to the formation of less volatile organic compounds which associate with airborne particles. These are termed secondary organic matter.
- Compounds of sodium and magnesium, which arise mainly from sea spray (which can be carried long distances inland by the wind) and road de-icing salt.
- Compounds of calcium and potassium, which are crustal in origin, associated with rocks and soils that enter the atmosphere mainly from resuspension of surface dusts. Other metals can arise from sources such as smelters, but are usually present only at trace levels.
- Chloride, which is mainly associated with sodium in sea salt and road de-icing salt but which can also arise through hydrogen chloride emissions from coal burning, and incineration processes.
- Insoluble minerals, such as clays, arising from rocks, soils and surface dusts.
- Biological materials, not explicitly included in Figure 4, include bacteria, some fungal spores, pollen fragments, and small fragments of leaf material. Their contribution to PM10 mass is currently not well quantified.
15. It may be seen from Figure 4 that there is a substantial difference between the composition of the coarse (15-2.5 µm) and fine fractions with a predominance of carbonaceous material and secondary ammonium sulphate and ammonium nitrate in the fine particles, whilst insoluble minerals associated with wind-blown soils and surface dusts tend to predominate in the coarse fraction (15-2.5 µm). It should also be noted that airborne particulate matter contains many other components, some of which are important in their own right. Among these are metals and specific organic compounds such as polycyclic aromatic hydrocarbons contained within the primary and secondary organic matter above, but not mentioned explicitly by name.
Figure 4 Approximate composition of urban particles in the UK, as measured in Leeds (Clarke et al 1996). 16. The National Multi-Element survey measures a range of transition metals1at five urban sites (DETR 1997). Of these, only iron comprises a major proportion of PM10 mass, with annual mean concentrations in 1995/6 ranging from 0.34 µg/m3in Glasgow to 0.95 µg/m3in Central London. The other metals measured are present only at trace levels, with zinc around 0.05 µg/m3and the other elements (cadmium, chromium, copper, manganese, nickel, vanadium and cobalt) typically below 0.02 µg/m3. Whilst iron arises substantially from windblown soils and dust and is therefore present largely in the coarse particles, the other metals derive mainly from combustion processes and reside largely in the fine fraction. Since the emissions of many of the transition metals arise mainly from high temperature processes such as smelting and fuel combustion, the metals become incorporated in the particles by condensation during the cooling of the combustion gases prior to emission. For this reason the more volatile metals are often enriched in the surface layer of particles. It should be noted that both coal and heavy fuel oil, which are major sources of sulphur dioxide emissions, also contain significant levels of trace metals, a proportion of which enters the atmosphere on combustion. Consequently, the concentrations of some trace metals, which are themselves primary pollutants, are found to be correlated with the concentrations of secondary sulphates and nitrates derived from sulphur and nitrogen oxide emissions from the same combustion sources (QUARG 1996).
17. Important factors when considering the potential toxicity of airborne particles are the solubility and acidity of individual components. The secondary inorganic components, sulphate, nitrate and ammonium, together with primary sodium chloride, are highly water soluble. On the other hand, carbonaceous combustion particles and soil-derived minerals are relatively insoluble. Primary organic carbon is of low solubility but the more oxidised secondary organic component is substantially water soluble. Some compounds of transition metals are highly water soluble (e.g., nitrates and chlorides in general), whilst others such as oxides and sulphides, are typically less so. Sulphuric acid is a strong acid and when present in airborne particles can lead to appreciable acidity. In the presence of ammonia gas, this acidity is rapidly neutralised and lost. Owing to the abundance of ammonia in the UK atmosphere, concentrations of this particle strong acidity are now generally extremely low, in contrast to the 1950s and 1960s when high sulphur dioxide concentrations meant that the capacity of the atmosphere to generate strong acid far exceeded its capacity to neutralise it. In North America, there are contrasting air pollution climates on the west and east coasts. In the west coast and especially California, concentrations of nitrate and ammonium are typically very high whilst concentrations of sulphate are low. On the east coast, however, sulphates generally predominate, and because of relatively low emissions of ammonia, high strong acid concentrations commonly occur.
18. A number of distinct airborne particle climates have been identified in the ambient environment of the UK. These include the roadside, urban background, coalburning and industrial pollution climates, which are separable from those in rural and remote rural locations. Regional variations are important; for example, coal burning is an important source of airborne particles in parts of Northern Ireland and in coal mining areas. Regional patterns of weather conditions favour increased secondary particle concentrations in the south and east of Britain with decreased levels in the north and west. In contrast, the roadside particle climate is highly localised to within a few tens of metres of the most heavily trafficked roads.
19. An important issue is the extent to which these variable influences on the chemical composition of airborne particles are or are not mutually correlated. To an extent, most common air pollutants show similar variations with time of day and time of year, when averaged out over extended periods. This is particularly true of primary pollutants but is not generally the case with secondary pollutants, and this has led to the characterisation of so-called wintertime and summertime pollution episodes. The former tend to involve a wide range of primary pollutants trapped under a shallow inversion layer while the latter involve sunlight-driven chemical reactions producing secondary components through atmospheric chemical reactions. Secondary pollutants, and to a lesser extent primary pollutants, are often subject to long range transport over hundreds or thousands of kilometres and pollutants deriving from emissions in continental Europe are frequently sampled in the UK atmosphere when wind directions are appropriate.
20. Most measurements of the chemical composition of particles are either made in short, intensive campaigns, or involve long averaging periods. It is only for sulphate and Black Smoke, which is an indirect measure of elemental carbon, that daily measurements are routinely made. When plotted as annual time series (as in APEG 1999) abrupt day-to-day changes in composition are sometimes seen, with concentrations doubling, or halving from one day to the next. More typically, however, pollutant mixes, as reflected by these pollutants, tend to fluctuate on a longer, approximately weekly, timescale. Hourly measurements of sulphate, nitrate and ammonium made during pollution episodes can show rapid concentration fluctuation as polluted air masses pass the measurement site, with widely fluctuating sulphate/nitrate ratios. There are also seasonal influences on particulate matter composition, with a generally greater coarse particle contribution to PM10 in summer than winter (APEG 1999) arising from an enhancement in crustal components (soils and dusts), together with the greater contributions of primary emissions to pollution episodes in winter and secondary pollutants (sulphates and nitrates) in summer, as noted above. An analysis of sulphate data for 1990-1992 across all UK Environmental Monitoring and Evaluation Programme (EMEP) sites showed the months April to July to have the highest sulphate concentrations (QUARG 1996). Chloride, mainly in coarse particles, shows a distinct winter maximum.
References
APEG. Source apportionment of airborne particulate matter in the United Kingdom. Report of the Airborne Particles Expert Group. London: Department of the Environment, Transport and the Regions, 1999.
Clarke AG, Andrews GE, Azadi-Bougar GA, Bartle KD, Toledi M, Askey SA. Sources and chemistry of atmospheric particles. Final report to Department of the Environment, Research Contract EPG 1/5/44, November 1996.
DETR. Digest of Environmental Statistics No. 19. London: The Stationery Office, 1997.
Gomes L, Bergametti G, Coude-Gassen G, Rognon P. Submicron desert dusts: a sandblasting process. J Geophys Res 1990; 95: 13,927-13,935.
QUARG. Airborne particulate matter in the United Kingdom. Third report of the Quality of Urban Air Review Group. London: Department of the Environment, 1996.
Whitby K. The physical characteristics of sulphur aerosols. Atmos Environ, 1978; 12: 135.
1The chemical structure of transition metals is such that most, in some forms, have a propensity to undergo oxidation and reduction reactions that can lead to the generation of free radicals. This is discussed further in the chapter on Possible Mechanisms of Toxicity.
Published 17 May 2001
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