The Physico-Chemical Composition of Airborne Particles

5. The ability of a particle to remain suspended in the air depends essentially on its size, shape and density. Large heavy particles fall rapidly, while fine light particles remain suspended for longer. The same properties determine where in the human respiratory tract a particle lands when inhaled, so small particles can penetrate further than can larger ones. In general, spherical particles below about 10 µm in diameter have the greatest likelihood of reaching the furthest parts of the lung, the air spaces or alveoli, where the delicate tissues involved in the exchange of oxygen and carbon dioxide, the essential processes of respiration, are to be found. Particles larger than this, up to about 20 µm, may be deposited in the nose, throat and airways of the lung. Not all particles that are inhaled are deposited in the lung. Almost all particles larger than 7 µm are deposited in the nose or throat, and only about 20-30% of particles between 1 and 7 µm are deposited in the lung's air spaces. However, up to 60% of very fine particles, below about 0.1 µm, are deposited in the air spaces.
6. Airborne particles may be measured in several different ways. The simplest is to suck high volumes of air through a filter and to weigh the mass that accumulates over a given period. This method measures total suspended particulate matter and includes airborne particles of all sizes. In the Black Smoke method, lower flow rates are used and the larger particles are not collected, the concentration of fine particles being estimated by measuring the blackness of the stain produced on the filter paper. In order better to reflect the hazard implicit in particulate matter reaching the lung's air spaces, other methods have been developed to measure only those particles below a certain size. That most commonly used for measuring air pollution relies on the use of a size-selective sampler which collects small particles preferentially, collecting 50% of 10 µm aerodynamic diameter particles, more than 95% of 5 µm particles, and less than 5% of 20 µm particles. The resultant mass of material is known as PM₁₀. Different inlets allow collection of particles of different size ranges, for example PM_2.5, which represents the collection of 50% of 2.5 µm diameter particles. In this report, we refer usually to particles measured as PM₁₀. They are thus defined solely by their physical characteristics, and no particular chemical composition is implied.
7. Particles in the air are conventionally described as occurring in three different size ranges, or modes. The smallest, below 0.2 µm in diameter, called the nucleation mode, is formed by condensation of hot vapours, as from incinerators and vehicle exhausts, and by chemical conversion of gases to particles in the atmosphere, as by oxidation of sulphur dioxide to sulphuric acid particles. These very small particles have a relatively short existence, since they coagulate into larger particles; nevertheless they are often the most numerous ones when particles are counted rather than expressed as mass. Particles between 0.2 and 2 µm in diameter make up the accumulation mode, and comprise those that have grown from the nucleation mode by coagulation or condensation of vapours. They are the most long-lasting of atmospheric particles, not readily being removed by rain and remaining in the air for some 7 to 30 days. Larger particles, greater than 2 µm, belong to the coarse mode; they are formed mainly by mechanical attrition and therefore consist mostly of minerals derived from soil, sea spray and industrial processes. These particles, because of their size, remain suspended in the air for only short periods.
8. The chemical composition of atmospheric particles clearly depends upon the major sources. For example, by the coast, sea spray may mean that salt is the main component, while close to a main road vehicle exhausts will determine the composition. Such variations need to be borne in mind when considering the results of epidemiological studies from different countries. Those particles of greatest concern, since the largest numbers of people are exposed to them, are those occurring in towns and cities, and some general indication can be given of their chemical composition. About 40-50% of the mass of airborne particles in the United Kingdom is soluble in water, and contains mainly sulphate, nitrate, chloride and ammonium ions. The insoluble fraction consists mainly of carbon, together with a range of minerals derived from soil. Many other elements and compounds are found in trace amounts, depending on local sources, and some of these, such as lead, dioxins and polycyclic aromatic hydrocarbons, may be toxic in their own right. The composition of the airborne particles differs markedly depending on the size fraction examined. Studies carried out in Leeds in the 1980s (see Figure 1) showed that, at that time, the finer particles, below about 2.5 µm diameter, comprised about 50% carbonaceous material from combustion processes and 40% ammonium sulphate and nitrate, while the coarser fraction, above about 2.5 µm, comprised about 60% insoluble soil and wind-blown minerals and 15% carbonaceous material, the residue being mainly salts.