Particulate matter is classified by size, which is directly linked to the distance it will penetrate into the lung, the smaller the particle the further it will penetrate. PM10 is particulate matter below 10 micron diameter. PM10s are designated as the upper limit of the respirable fraction, that is, the mass fraction of inhaled particles which penetrate to the unciliated airways of the lung and as such, are thought to be a major health hazard.
The National Air Quality Strategy (NAQS) has set a maximum level for PM10 of 50 microgram/m³ over a 24-hour period measured as the 99th percentile, (i.e. 4 exceedance days allowed each year) to be achieved by 2005. It is estimated that in order to achieve the objectives urban emissions need to fall by around 60% from 1995 levels. Policies already in place will not be sufficient to achieve this scale of reductions.
Particulate matter in the ambient atmosphere can arise from a variety of sources; wind-raised dust from spoil heaps, open fields and construction sites, sea spray, industrial activity, emissions from animals and plants (microbial and fungal spores), volcanoes, forest fires, traffic and combustion processes, residues from the evaporation of sprays, mists and fogs, and photochemical conversion of gas to particles (mainly sulphates and nitrates). The main sources are however, road transport, stationary combustion and industrial processes.
The atmospheric lifetime of particulate matter is strongly related to particle size. Larger particles (say 10 micron diameter) may be airborne for up to 20 hours and may travel 20 to 30 km while smaller particles may travel several thousand km. A proportion of particles escape from the boundary layer to higher levels in the troposphere. The smallest fraction that reaches the troposphere (8 to 15 km high) may remain airborne for up to a year.
While healthy individuals are now unlikely to experience acute effects at typical air pollution levels, there is some evidence of associations with advanced mortality, chronic illness and discomfort for sensitive groups. At UK levels there is no evidence of effects caused by increased levels of particulate matter on people in good health. It is however likely that particles < 10 microns from any origin can worsen heart and breathing problems in sensitive groups. Effects range from days of restricted activity to mortality.1 In some local areas emissions from traffic, industry and other sources can still affect the quality of life for all.
Air pollution can also significantly damage the general environment e.g. trees, wildlife and buildings and not least of all can alter the climate. It is estimated that surface soil suspension causes the introduction of 150 million tonnes of dust per year into the Northern Hemisphere atmosphere and around double if the Sahara Desert is included (reference). Atmospheric aerosol in the troposphere can influence climate in two ways: directly through the reflection and absorption of solar radiation and indirectly through the modification of the optical properties and lifetime of clouds.
1. Information taken from 3rd QUARG report on Airborne Particulate Matter in the United Kingdom. 1996. DoE
PM10 IN CORNWALL
The CAQF was formed in 1995. As part of the Department of Transport and the Regions (DETR ) programme to test the draft proposals in the National Air Quality Strategy (NAQS) the CAQF monitored PM10 between March 1997 and October 1997 simultaneously at three sites in the St Austell area. In October 1997 the programme was completed with the production for the DETR of the final report in a series of three.
During the DETR programme the NAQS standard was breached only six times on 11th April 1997, peaking at 51 mg/m³. During 1998 the standard was breached on three days in March 1998 (47 exceedences) and two days in May 1998 (27 exceedences). These total 74 exceedences. The highest 24-hour daily running averages for the year were sorted into order, invalid readings removed and the 99th percentile taken. For 1998 the 99th percentile reading is 58 mg/m³. Consequently the NAQS standard was exceeded. As shown below, (graph 1) the March exceedences occurred during a nation-wide air quality event.
After the successful completion of the DETR programme, one of the particulate monitors was relocated to Tregonning Hill in the Mounts Bay area. This has enabled the collection of data which will be used to identify the various sources of PM10 in Cornwall. Analysis of the sample composition from this site is in progress at the time of writing.
The monitoring of particulate matter has also been initiated in Wadebridge in conjunction with road traffic monitoring.
Graph 1 shows PM10 levels at a variety of UK sites, (both rural and urban), for March 1998. Levels recorded at St Dennis (SD) were of the same order as those at the other sites. As has been reported elsewhere such events are typically associated with easterly and south-easterly winds(2) but the March event shown in graph 1 was during a period of westerly winds. Morphological studies of filters from SD indicate large amounts of material which would typically be associated with emissions from internal combustion engines. Given that there is little road traffic associated with SD it seems that the particulate matter events must be dominated by transboundary pollution.
The annual average PM10 measured at St Dennis for the period 1st April 1997 – 31st March 1998 was 21 mg/m³. This compares with values of 24 mg/m³ from Bristol, 23 mg/m³ from Nottingham, and 21 mg/m³ from London Brent. Hence in spite of its rural location St Dennis has an annual average level of PM10 loading which is comparable with many urban sites across the UK.
Graph 2 compares the average diurnal variations in PM10 for March 1997 and March 1998 at SD with those from Bristol. It can be seen that levels at SD peak in the evening between 16.00 and 21.00 whilst those at Bristol peak in the morning between 07.00 and 11.00.
We identify the former with domestic coal fire emissions in SD and Graph 3 supports this suggestion by comparing the average diurnal variation in PM10 emissions at SD in summer and winter – for the months May to September (summer) there is no evening peak but for October to April (winter) there is. SO2 monitoring is being undertaken in the area to investigate this.
Although particulate matter is perceived as being essentially an urban pollution problem, and although atmospheric deposition is recognised as having serious health implications there is also a significant impact on the coastal zone, where levels are as high as any in the UK. Little work has been done to assess the consequences for Cornwall of long-term exposure of sensitive habitats to air pollutants – not the least of these consequences being the economic implications for agriculture and tourism. It is important that the continuous monitoring of Air Quality in Cornwall is maintained in order to build a long-term picture of levels of particulate matter concentration, deposition and composition across the County.
2. King A and Bennett S, Comparisons of Non-urban Levels of Particulate Matter, Urban Network and Meteorological Data, ETSU Report Number ETSU N/01/0033/REP, DTI, 1997