Low level ozone (O3) is produced following the reaction in warm sunlight of nitrates (NOx) and Volatile Organic Compounds (VOCs) both of which derive from vehicle and shipping emissions (Caporn & Emmet 2009). Unlike the atmospheric pollutants of nitrogen and sulphur, atmospheric levels of O3 are increasing by around 0.2 ppb per annum or by 4ppb in the last 20 years (RoTAP 2012).
Concentrations of O3 are often higher in rural areas than urban ones. In the spring and summer night time levels in the uplands of the UK can remain high whilst in the lowlands they follow a cyclical pattern reducing to very low night time levels. As a result during sunny spells upland areas can be exposed to very high levels of continuous O3 exposure for many days. Over the past 20 years, peak concentrations of O3 have decreased but annual mean concentrations have increased (RoTAP 2012).
These maps show the O3 concentrations in the UK between March to May and May to July. It is clear that upland areas (amongst others) receive high levels of O3 during these months. O3 concentrations on Dartmoor are amongst the highest in the country (RoTAP 2012).
O3 is a well-known phytol-toxic gas and Ashmore (2005) provides a comprehensive overview of its significant adverse effects on human health, crop yields, forest growth and species composition and damage in semi-natural vegetation. For example, 1.2 million tonnes of lost wheat production in 2000 (which accounted for 7% of the total) was reported in the UK (RoTAP 2012).
Mills et al (2007) published data on individual species responses to O3, they reported that 80.4% of species in raised and blanket bog, 60% in valley and transition mires and 51.7% in temperate shrub heathland were O3 sensitive.
Franzaring et al (2000) in an experiment in the Netherlands found that after 28 days exposure to increase O3 concentration Purple Moor Grass (Molinia caerulea) showed significantly increased shoot weights and increased root to shoot ratios.
Hayes et al (2006) studied the response of various upland plant species grown in solardomes with varying concentrations of O3. They found that the sedge Star Sedge (Carex echinata) suffered leaf injury symptoms, the grass Red Fescue (Festuca rubra) also showed signs of leaf injury and premature senescence, the grass Yorkshire Fog (Holcus lanatus) and the sedge Common Yellow-sedge (Carex demissa) were unaffected and the forb Heath Bedstraw (Galium saxatile) whilst the grass Mat Grass (Nardus stricta) showed signs of biomass loss the following spring.
As reported above the peak concentrations of O3 have decreased but the means have increased. Hayes et al (2010) found that in a simulated situation upland species still responded detrimentally when the peak concentrations were lowered and the means increased.
Wedlich et al (2012) showed than in upland meadows in the Pennines increased O3 concentrations did not affect grasses but did significantly reduce the forb community, thus favouring the grass species.
In another experiment the effects of increased O3 concentrations on Heather (Calluna vulgaris) were tested (Foot et al 1996). They reported that Heather can be adversely affected by prolonged O3 episodes particularly if these are followed by or co-incide with frosting temperatures.
JNCC published a review of the impacts of O3 on nature conservation (Morrissey et al 2007) and concluded that ‘Ozone is, and is likely to remain, a significant threat to many BAP Priority Habitats. However, the knowledge base on which to assess these specific risks is extremely small, and a targeted programme of research to address these gaps is urgently needed.’
The environmental impacts of O3 are complex, in addition to the impacts described above O3 is also a greenhouse gas in its own right along with carbon dioxide and methane (Caporn and Emmett 2009) and additionally Wyness et al (2011) reported that enhanced nitrogen deposition exacerbates the negative effect of increasing background O3 in the grass Cocksfoot (Dactylis glomerata) but not in the forb Meadow Buttercup (Ranunculus acris).
Mills et al (2013) reviewed the impact of rising O3 levels in the atmosphere on ecosystem services and found that O3 had the potential to detrimentally effect all of the services. For example, high O3 concentrations have the potential to reduce carbon sequestration and speed up global warming, increase methane emissions, reduce crop productivity, reduce biodiversity and worsen air quality.
Natural England is beginning to acknowledge the implications of increased O3 and published the following (NE 2015)
Ground level ozone is a toxic atmospheric pollutant of growing concern, with potentially harmful effects on plant communities (Morrissey et al. 2007). It is formed in the lower atmosphere in the presence of sunlight by complex photochemical reactions between pollutants from a range of sources including traffic. Critical levels for ozone effects on vegetation are already widely exceeded and background emissions of precursors in the northern hemisphere are increasing (Natural England 2008; RoTAP 2012).
The implications for biodiversity of increasing background levels of ground-level ozone. Background ozone levels have now increased to a level where exposure to ozone may cause adverse effects in semi-natural vegetation, especially in the spring months in upland Britain.
However unlike for nitrogen deposition (see here) no specific measures for action have been set out yet.
AQEG (2009) conclude that O3 levels are likely to continue to rise in rural and urban areas and are likely to pose an increased threat to human health and the environment generally. Whilst measures taken in the UK to reduce O3 precursor compounds, which are methane, non-methane volatile organic compounds (VOC), oxides of nitrogen and carbon monoxide, can be beneficial, it will take action on a northern hemisphere scale if effective control of O3 levels is to be achieved.
AQEG (2009) Ozone in the United Kingdom. Air Quality Expert Group for Defra, Scottish Executive, Welsh Assemble Government and DoE NI.
Ashmore M.R. (2005) Assessing the future global impacts of ozone on vegetation. Plant, Cell and Environment 28: 949-964.
Bonn A., Allott T., Hubacek K. & Stewart J. (2009) Drivers of Environmental Change in Uplands. Routledge. London.
Caporn S.J.M. & Emmett B.A. (2009) Threats from air pollution and climate change to upland systems. In Bonn et al (2009) pp34-58
Franzaring J., Tonneijck A.E.G., Kooijman A.W.N. & Dueck Th.A. (2000) Growth responses to ozone in plants species from wetlands. Environmental and Experimental Botany 44: 39-48.
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.
Hayes F., Mills G., Jones L. & Ashmore M. (2010) Does a simulated upland grassland community respond to increasing background, peak or accumulated exposure of ozone? Atmospheric Environment 44: 4155-4164.
Hayes F., Mills G., Williams P., Harmens H. & Büker P. (2006) Impacts of summer ozone exposure on the growth and overwintering of UK upland vegetation Atmospheric Environment 40: 4088–409.
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.
Mills, G., Wagg, S., Harmens, H. (2013). Ozone pollution: Impacts on ecosystem services and biodiversity. ICP Vegetation Programme Coordination Centre, Centre for Ecology and Hydrology, Bangor, UK.
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.
Natural England (2008) State of the Natural Environment 2008. Natural England Reports No 85. Sheffield: Natural England. URL: http://publications.naturalengland.org.uk/publication/31043
Natural England (2015) Natural England Access to Evidence Information Note EIN009 Summary of evidence: Land use. http://publications.naturalengland.org.uk/publication/6684569048711168
RoTAP (2012). Review of Transboundary Air Pollution: Acidification, eutrophication, ground level ozone and heavy metals in the UK. Centre for Ecology and Hydrology, Edinburgh. Available from: http://www. rotap.ceh.ac.uk/
Wedlich K.V., Rintoul N., Peacock S. Cape J.N., Coyle M. Toet S, Barnes J. & Ashmore M. (2012) Effects of ozone on species composition in an upland grassland. Oecologia 168: 1137-1146