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Prepared at the request of the Department of the Environment November 1996 | |
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Published by:
Institute of Terrestrial Ecology (Edinburgh Research Station), Bush Estate, Penicuik, Midlothian, EH26 0QB, UK ISBN: 0 7058 1683 4 |
 | Ozone concentrations in the UK are consistent with those throughout northern Europe and Scandinavia. Over Europe as a whole, the mean and episode ozone concentrations increase from the British Isles south-east towards Central Europe. Episode frequency and peak concentrations are larger close to the Alps with peak concentrations of 180 ppb and hours above 60 ppb exceeding 400 per year. In the UK and southern Scandinavia by contrast, peak concentrations seldom exceed 100 ppb and annual exposure to 60 ppb seldom exceeds 200 hours. |
| Annual average ozone concentrations in the UK are in the range 10 to 30 ppb and are largest at elevated and coastal sites and smallest in urban areas (where ozone is depleted by reaction with nitric oxide emitted from vehicles and other combustion sources). |
| Photochemical episodes occur throughout the UK each year with concentrations typically reaching 60 to 80 ppb but occasionally exceeding 100 ppb. Such episodes are more common in southern England than northern England and Scotland with ozone concentrations exceeding 60 ppb for 150 to 200 hours during an average summer along the south coast of England while across Scotland the exposure to concentrations greater than 60 ppb is typically 40 to 80 hours each summer. |
| Exposure to episode ozone concentrations is considerably increased in upland areas throughout the country. The periods of exposure has been estimated from an analysis of the altitude dependence of ozone concentration. From this work it has been shown that episode ozone concentrations are doubled on land above 250 m (ASL) relative to low ground (<50 m) in the same region. For example land above 250 m (ASL) in the Pennine Hills and in the mountains of Cumbria and Wales experiences 150 to 180 hours above 60 ppb in an average summer while low ground in these regions experiences only 60 to 80 hours above 60 ppb. The higher ground (>400 m) in the north of England and Scotland also experiences much longer exposure to episode concentrations (>150 hours above 60 ppb) than low ground in the same region. |
| An increase in free tropospheric ozone has been observed over Europe during the last decade suggesting that ozone concentrations have increased throughout the northern troposphere. The magnitude of the trend is 1 2% per year. Model calculations show that these trends are consistent with precursor emissions. |
| Data from rural UK sites furthest from urban conurbations have shown an increase in mean ozone concentration over the period 1986 1991 of about 1 ppb per year. |
| Pronounced daily and annual cycles in ozone are apparent at rural sites, with daily maxima and minima at 1400 and 0500 respectively and annual maxima and minima in April May and Nov Dec respectively. |
| Hydrogen peroxide concentrations show seasonal and diurnal cycles with larger concentrations on summer days of typically 1.0 ppb. |
| Concentrations of peroxyacetyl nitrate are typically 0.1 ppb in winter and 0.5 ppb in summer and are largest during photochemical episodes when concentrations of PAN are typically 1% of those of ozone. |
| The annual number of exceedences of 90 ppb, above which air quality is described as poor by the DoE increases in the UK from north to south and with altitude. The average number of hours per year above 90 ppb exceeds 20 in many parts of southern and central Britain and on uplands from the Pennines to Dartmoor; it is estimated that about 30% of the population live in these areas. In recent warm summers, there have been more than 50 hours with concentrations above 90 ppb in some locations. |
| Concentrations of NO2, one of the main precursors for tropospheric ozone, have increased in most, but not all rural areas and have not increased in city centres. Two recent national surveys in 1991 and 1992 show NO2 concentrations 35 to 53% greater than in 1986. |
| Over large areas of the UK, motor vehicle emissions are responsible for more than half of NO2 concentrations near the ground; the vehicle contribution to ground level NO2 concentrations increases to 70 80% in the London area. |
| WHO guidelines for short term health effects of NO2 have been exceeded at most urban sites as have UNECE long-term critical levels for effects on vegetation. UNECE critical levels have also been exceeded in parts of rural southern and central England. |
| An atmospheric budget for NOx compounds over the UK has been constructed, which shows that 25% of UK emissions are deposited in rain and by dry deposition within the country, the remaining 75% of the UK NOx emissions are exported. |
| The total emission of VOCs in the UK is 2 million tonnes annually of which less than 5% is of natural origin, the remainder being of vehicle and industrial origin. There is, however, considerable uncertainty in the estimates of all VOC emissions. |
| Some of the hydrocarbons measured in the UK have carcinogenic properties, benzene and 1,3 butadiene in particular. A measurement programme for these and other hydrocarbon species in urban areas has been commissioned by the DoE. |
| Ozone is a toxic gas which, if delivered to the respiratory tract in sufficient quantities will produce damage. The concentrations of ozone experienced by people in the UK during photochemical oxidant episodes, especially for those taking heavy exercise, will have adverse effects on lung function, but these effects are unlikely to be indicative of irreversible lung damage. |
| A small proportion of the whole population, perhaps 10%, is particularly sensitive to ozone. It is not possible at present to identify in advance of experimental studies individuals with increased susceptibility. |
| There are north-south and altitude gradients in the annual number of exceedences of 90 ppb of ozone, above which air quality is described as poor by the DoE. The average number of hours per year above 90 ppb exceeds 20 in many parts of southern and central Britain and on uplands from the Pennines to Dartmoor; it is estimated that about 30% of the population live in these areas. In each of the recent warm summers, there have been more than 50 hours with concentrations above 90 ppb in some locations. |
| Experimental evidence suggests that current concentrations of ozone in the UK do, in certain summers, affect crop yield, tree physiology and growth, and the species composition of plant communities. |
| For European countries the phytotoxicity of ozone causes the major crop losses attributable to air pollution. |
| Preliminary critical levels proposed for ozone to protect the most sensitive plant species are exceeded throughout rural UK in most summers. The critical level is based on the seasonal cumulative dose above 40 ppb during daylight hours. |
| The longevity of a range of materials is reduced by ozone. The effects of ozone on materials has been estimated to cost $2500 million annually in the USA, the equivalent figure is not currently available for the UK. The acceptable level of ozone to prevent material damage is currently exceeded throughout the UK. |
| Ozone is a greenhouse gas and increases in its concentration in the troposphere lead to warming of the surface-troposphere system. Increases in tropospheric ozone would compensate to some extent, for any increased UV penetration of the stratosphere resulting from stratospheric ozone depletion. |
| Significant reductions in emissions of precursor gases will occur as a result of the VOC Protocol and the introduction of 3-way catalytic converters on new petrol vehicles from January 1993; |
| Despite projected reductions in European emissions of VOC and NOx, associated reductions in ozone will be insufficient to meet air quality targets for the year 2000 and beyond; |
| Control of NOx will be beneficial not only in the context of reducing peak episodic ozone levels, but is necessary also for reduction of global background tropospheric ozone, nitrogen dioxide in urban areas, terrestrial and marine eutrophication, and acid deposition; |
| An integrated approach, such as the critical loads approach, which takes account of biological sensitivity, geographical location, source-receptor relationships and the efficacy of control measures can be used to formulate control strategies. |
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