Although ozone is being destroyed in the stratosphere by chlorinated and brominated chemicals, the ozone problem in the lower atmosphere is one of overabundance. The gas is formed by the action of sunlight on primary pollutants, notably nitrogen oxides (NOx) and VOCs.
PORG's report contains the first comprehensive assessment of ozone levels in the UK. This is based on results from a network of monitoring stations established since the group first reported on ozone in 1986.
Background ozone concentrations are now in the range 10-30 ppb, twice the level 100 years ago. Levels are continuing to rise at rural sites at about 1ppm per year, consistent with a European trend.
Ozone levels are generally highest in south-east England where the weather is sunniest, but the uplands are also exposed to high concentrations because of meteorological effects.
Of particular concern are ozone peaks which occur downwind of sources in sunny, stable weather conditions. In the UK, such episodes can last for up to ten days in summer and produce ozone levels of 60-100 ppb. The WHO eight-hour guideline of 50-60ppb and an EC eight-hour guideline of 55 ppb have been breached at least once at every monitoring site in the UK over the last five years.
The report adds nothing new on the health affects of ozone. However, a report is expected shortly from another official advisory group recommending health-based air quality guidelines for the pollutant.
Ozone has a toxic effect on many plants, and in the USA it is estimated to reduce agricultural output by 5%. Sensitive crops such as onions and beans suffer 30% reductions in yields in some areas. The benefits of reducing ozone levels across the USA by 10% and 40% have been estimated at $0.8 billion and $2 billion per year, respectively.
Evidence of crop damage has been available in the UK since the early 1980s, but the costs involved have never been calculated. PORG's rough estimate of the benefits of reducing ozone levels suggests that these may not be very large - around £34 million per year, assuming a reduction in ozone concentrations from 35ppb to 25ppb during the growing season.
Ozone also affects trees and natural vegetation. The report concludes there is particularly good evidence of ozone damage to trees in southern England, and there are signs that natural vegetation in upland areas has also been affected.
Perhaps the most significant financial costs of ozone pollution are caused by its effects on materials, notably rubber, fabrics and paints. In the USA, the Environmental Protection Agency has estimated that a 10ppb reduction in average ozone levels would result in savings of $5.4 billion annually from reduced damage to elastomers and savings on antioxidant additives. PORG estimates the minimum cost of anti-ozone additives to be £20 million annually in the UK.
Ozone also reduces the life of paints and causes fading and weakening of fabrics. US estimates of the annual costs of these effects are $731 million and $248 million, respectively. PORG recommends that dose-effect studies and cost/benefit assessments of these effects are needed in the UK.
The report endorses the pan-European approach to controls on ozone precursors, notably two UN protocols on NOx and VOC emissions. However, modelling suggests these will be insufficient to achieve air quality targets for ozone. The 30% reductions in NOx and VOCs that these measures aim to achieve towards the end of the 1990s would produce only 11% reductions in peak and background ozone levels. Episodes of poor air quality will continue to be experienced in the UK during stable sunny weather, and little progress will have been made towards protecting ecosystems.
PORG also assessed the accuracy of the official VOC emissions inventory, using monitoring data and model calculations. It concludes that VOC emissions could be underestimated in the inventory by up to 35%. This is a worrying conclusion because the inventory formed the basis of the Government's recent strategy to reduce VOC emissions by 30% between 1988 and 1999, in line with the UN protocol (ENDS Report 226, p 35 ).