Measurement and Monitoring of Airborne Particles

13. Measurement of airborne particles in the United Kingdom has traditionally been by the Black Smoke method referred to above, and this continues to be the main method used for compliance with the European Community directive on Sulphur Dioxide and Suspended Particulates. In September 1995, Black Smoke was monitored in the United Kingdom at 255 sites, all of which have shown a marked downward trend in concentration over the years. As an illustration of this, measurements at sites in central London, Belfast, and Edinburgh between 1961 and 1994 are shown in Figure 3. However, with the changes in the main sources of urban pollution from coal burning to vehicle exhausts, Black Smoke is appropriate for measurement of soiling but is no longer regarded as the best method of measuring concentrations of airborne particles. PM₁₀ (as defined in paragraph 6 above) is increasingly used as the standard measurement. PM₁₀ is also currently regarded as the measurement best representing those particles most likely to penetrate into the lung and cause ill-health, although it is possible that advances in understanding may in the future indicate that another measure which excludes the larger particles (eg PM_2.5) may be more appropriate.
14. The Department of the Environment currently makes continuous measurements of PM₁₀ in 16 cities in the United Kingdom (see Figure 4), using automatic apparatus that determines the mass of the particles of the selected size range. Figure 5 shows the maximum daily average concentrations recorded for each month and site between 1992 and 1994. It can be seen that concentrations are highest in the winter months and lowest in the summer. However, the difference is less than that seen for other motor vehicle-derived pollutants such as carbon monoxide or oxides of nitrogen. This is because there is another important source of particles during the summer, the photochemical oxidation of sulphur dioxide and oxides of nitrogen to particulate sulphate and nitrate. Table 2 shows the number of days in each month of 1993 when daily average PM₁₀ concentrations exceeded 50 µg/m³ (see footnote 2). The greatest number of exceedences occurred in Belfast (where coal is still a major fuel), London, Birmingham and Bristol, and the smallest in Edinburgh.
15. Since a high proportion of PM₁₀ comprises fine particles that remain suspended for long periods, and since the stagnant weather conditions that give rise to winter pollution episodes often affect large areas of Britain, rises in particle concentrations often occur simultaneously in different parts of the country. Furthermore, such rises usually occur in association with rises in other traffic-related pollutants such as oxides of nitrogen. The daily average concentrations of PM₁₀ monitored at six urban sites in the United Kingdom during November 1993, are illustrated in Figure 6, which shows the way in which such concentrations rise and fall more or less concurrently in widely separated cities.

Table 2 Number of days in each month during 1993 when the daily average concentration of PM₁₀ exceeded 50 µm3

Site	London Bloomsbury	Edinburgh	Cardiff	Belfast	Birmingham Centre	Newcastle	Leeds	Bristol	Liverpool
January	1	0	2	5	0	0	1	0	-
February	7	0	1	8	0	1	4	2	-
March	2	0	4	3	2	1	5	2	-
April	4	0	6	4	7	2	0	4	2
May	3	0	3	6	3	0	0	3	1
June	3	0	1	3	3	2	1	1	-
July	1	-	0	0	0	0	0	0	0
August	1	0	-	0	0	-	1	0	0
September	0	0	-	1	1	-	1	1	1
October	0	0	-	8	0	0	1	0	0
November	7	2	2	7	8	6	9	12	13
December	0	0	0	0	0	0	0	0	0