Ozone Overview
1. Introduction.2. Ozone formation.
3. Trends in concentrations.
4. Effects of ozone on vegetation.
5. Ozone risk assessment
6. Ozone and climate change.
7. Further information.
8. References.
1. Introduction
Ozone (O3) is present throughout the atmosphere and can be either positive or negative depending on where it is found. The stratosphere (15 - 50 km) has the largest fraction and concentrations (90%, Colls 1997). Stratospheric O3 is important as it regulates the transmittance of ultraviolet light to the surface of the earth. Hence reductions in stratospheric O3 in Polar regions, particularly the Antarctic "ozone hole", are of concern regarding the health effects of exposure to increased levels of UV-B. In the troposphere (0-15 km) ozone is a toxic pollutant of global importance and it is a major constituent of photochemical smog.
2. Ozone formation
Mixing with stratospheric air provides a natural global average background of around 10-20 parts per billion (ppb), though there is some debate about the concentration. Additional quantities of tropospheric O3, a secondary pollutant, are formed by the action of sunlight on oxides of nitrogen (mainly from vehicle emissions and industry) and volatile organic compounds (VOCs) (mainly from vehicles, solvents and industry).The atmospheric chemistry involved in ozone formation is complex (see PORG 1997). It takes time for the ozone to accumulate as the chemical reactions involved are quite slow. Peak ozone formation takes place downwind of precursor sources in sunny weather with low wind speeds. Ozone concentrations are influenced by sources, meteorology and chemical reactions over local, regional and hemispherical distances. As a result there is considerable spatial and temporal variation in ozone concentrations. Although most ozone precursors are found in urban areas, concentrations of tropospheric ozone tend to be higher away from towns. Partly this is a consequence of nitrogen oxide (NO) reacting with ozone to produce nitrogen dioxide (NO2, itself a pollutant) and oxygen. In the UK, ozone concentrations are strongly influenced by emissions of precursors in Europe.
Figure 1. Distribution of mean ozone concentrations over the periods March-May (for comparison with woodland bulb data) and May-July (for comparison with grassland species data), using data for the five year period 2003-07.
[From RoTAP 2011: Review of Tranboundary Air Pollution: Acidification, Eutrophication, Ground Level Ozone and Heavy Metals in the UK. www.rotap.ceh.ac.uk]
3. Trends in concentrations
Information on ozone concentrations, both real time and historic can be found on the air quality archive. Generally, because of controls on emissions of precursor gases in the UK and Europe, there has been a reduction in the intensity of summer ozone episodes. However, there is evidence of an increase in background concentrations, as a result of increased global emissions. In addition, control of emissions of oxides of nitrogen (NOx) in the UK has led to an increase in ozone in urban areas.4. Effects of ozone on vegetation
There is evidence of widespread ozone damage to vegetation in Europe. Effects of ozone on vegetation include visible injury, early senescence of leaves, reduction of crop/forest yield and a reduction in biomass of sensitive plant species.
Figure 2 and 3: clover plant damaged by ozone and bean plant damaged by ozone
Under present day concentrations, ozone causes significant impacts on crops. Ozone-induced yield losses for 23 crops (mainly arable) in 47 countries in Europe were estimated to be €6.7 billion per year for year 2000 ozone concentrations.
There has been less research on the impacts of ozone on (semi-) natural vegetation. However, available evidence suggests that individual wild plants are as sensitive to ozone as the most sensitive crop species (Barnes and Davison, 1998). Recent research has shown several plant communities of (semi-)natural vegetation are potentially ozone-sensitive after calculating the percentage of ozone-sensitive species within each community (Hayes et al., 2007; Mills et al., 2007). In a report for JNCC and the conservation agencies, Morrisey et al (2007) reviewed the impacts of ozone on BAP habitats, drawing on information from major reviews, relevant experimental studies in the UK, and data syntheses defining the relative sensitivity of species and communities. Their findings are summarised below.
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For woodlands, the major focus of ozone research has been on trees. Beech and birch are sensitive to ozone, while oak and Scots Pine have also shown adverse effects at concentrations found in the UK.
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The direct and indirect effects of ozone on woodland ground flora are poorly understood, although there is evidence that these communities may be sensitive to ozone. Grasslands are the best studied habitats in terms of ozone sensitivity with several common positive indicator species reported to be ozone sensitive.
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Studies of ozone effects on grassland communities have reported changes in community composition at concentrations found in the UK. In one study, these effects of ozone led to a change in composition which was unfavourable from a conservation perspective.
- Other habitats, such as wetlands, heaths, montane and inland rock habitats are poorly studied although there is some evidence that montane habitats and bogs are sensitive to ozone.
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An assessment of the exposure to ozone of BAP Priority Habitats was carried out by comparing their national distributions with mean six-month AOT40 values.
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All but one BAP Priority Habitat (lowland fens) in England have over 80% of their national distribution in areas that are likely to exceed critical levels of ozone exposure. The highest exposures are of Priority Habitats with a primarily southern distribution.
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In Scotland and Wales, upland habitats have the highest exposure to ozone. In Wales, as in England, most Priority Habitats have the majority of their distribution in areas that are likely to exceed critical levels.
- Very little of the area of Northern Ireland experiences ozone exposures above the critical level although there is a need for improved monitoring in this region.
5. Ozone risk assessment
The first critical levels for ozone were based on the concept of AOT40 which summarises the cumulative exposure to concentrations above 40ppb over the period of concern. They have been set for forests, arable and horticultural crops and semi-natural vegetation.However, several important limitations and uncertainties have been recognised for using AOT40. In particular, the real impacts of ozone depend on the amount of ozone reaching the sites of damage within the leaf, whereas AOT40-based critical levels only consider the ozone concentration at the top of the canopy. Therefore, AOT40-based critical levels are replaced or supplemented with models of absorbed dose or flux into the leaf on the basis that these provide a more realistic basis for ozone risk assessment.
Flux-based critical levels and associated response functions are available for mapping and quantifying impacts at the local and regional scale, including effects on crops, forest trees, and impacts on the vitality of fodder-pasture and natural grassland species in (semi-)natural vegetation. AOT40 measures continue to be used where fluxed based models are not available.
APIS currently provides information on AOT40 critical levels and not fluxed based methods.
Morrisey et al (2007) compared a risk assessment for oak and productive grasslands in four locations in the UK. Although, this preliminary assessment was illustrative rather than predictive, they showed from risk assessment using flux models that BAP Habitats in Scotland and Wales are at greater risk of ozone impacts than is indicated by current assessments based on AOT40.
6. Ozone and climate change
Climate change will have important effects on ozone concentrations, because of the influence of meteorology on its formation, and the impacts of ozone. Exposure to ozone will also reduce the land carbon sink due to reductions in plant/tree growth, offsetting increased due increases in carbon dioxide concentrations. Ozone itself is a green house gas.7. Further information
The Royal Society published a major review in 2008 "Ground-level ozone in the 21st century: future trends, impacts and policy implications".The Review of Transboundary Air Pollution (RoTAP) reviews current evidence of impacts in the UK and the scientific basis for risk assessment.
The Air Quality Expert Group published a review of ozone in the United Kingdom in 2009. The report investigates the recent historic trends, current status and likely future changes to tropospheric ozone concentrations in the UK over the next 20 years.
The International Co-operative programme on Effects of Air Pollution on Natural Vegetation and Crops co-ordinates an international research programme under the Convention on Long-Range Transboundary Air Pollution investigating the impacts of air pollutants on crops and (semi-)natural vegetation. In 2006 it reviewed the evidence of ozone impacts across Europe.
8. References
Colls J. [Ed.] (1997): Air pollution an introduction. 1st ed. E & FN Spon, London. 341 pages.
Davison A.W. and Barnes J.D. (1998): Effects of ozone on wild plants. New Phytologist (Special Issue on Disturbance of the Nitrogen Cycle) 139, 135-151.
Hayes, F., Jones, M.L.M., Mills, G., Ashmore, M. (2007): Meta-analysis of the relative sensitivity of semi-natural vegetation species to ozone. Environmental Pollution 146: 754-762.
Morrissey, T., Ashmore, M.R., Emberson, L.D., Cinderby, S. and Büker, P. (2007): The impacts of ozone on nature conservation: a review and recommendations to research and policy. JNCC Report No. 403
PORG (1997): Ozone in the United Kingdom 1997. Fourth Report of the United Kingdom Photochemical Oxidants Review Group. Department of the Environment, London.
Mills, G., Hayes, F., Jones, M.L.M., Cinderby, S. (2007): Identifying ozone-sensitive communities of (semi-) natural vegetation suitable for mapping exceedance of critical levels. Environmental Pollution 146: 736-743.