Ozone in the UK: Recent Trends and Future Projections

Ozone near the planetary surface is considered an air pollutant because of its adverse effects on human health, crop yields, and ecosystems more broadly. In addition, ozone in the free troposphere is a greenhouse gas with climate impacts. Ozone is a relatively long-lived air pollutant so can be transported for periods of days to several weeks over large distances. The concentrations that arrive over the UK in the prevailing westerly winds from the Atlantic are influenced by the cumulative effects of emissions that occur throughout the mid-latitudes of the Northern Hemisphere.UK population and ecosystem exposure to ozone is frequently lower than the north Atlantic baseline due to dominant ozone sink processes of removal by reaction with local emissions of NO and deposition to the terrestrial surface. In spring and summer, net ozone production from UK emissions, in addition to advection from mainland Europe, leads to episodes of elevated concentrations.Control of surface ozone is complex since it is not emitted directly from any source but is formed in the atmosphere in a non-linear manner from reactions involving nitrogen oxides (NOx), methane, volatile organic compounds (VOCs) and sunlight. Air quality policies have been in place to reduce surface ozone since the 1980s, through control of emissions of NOx and VOCs from sources such as vehicles, combustion, fuels and solvents. The 1990s saw reductions in peak concentrations and fewer severe ozone pollution events – a consequence of substantial reductions in emissions implemented internationally through agreements such as the Gothenburg protocol. Overall, since the UK is a net sink for ozone, changes in ozone concentration are controlled by the changes in the chemical and deposition sinks as well as changes in baseline concentration and local chemical production.The distribution of ozone across the UK shows highest concentrations over upland and rural locations, with annual average concentrations of >60 μg m-3 widespread over rural areas in Scotland, Northern Ireland, Wales, south-west England and the Pennines. Urban and suburban concentrations are typically ~15 μg m-3 lower than rural values, although this is very sensitive to local emissions from road transport, and in heavily-trafficked roadside locations ozone concentrations can locally be close to zero. Ozone typically reaches its highest concentrations in the UK during April and May, although anticyclonic weather patterns in summer can also lead to significantly elevated concentrations.Since 2000 there has been little observed change in UK rural ozone concentrations, or ozone in the air arriving from the wider Atlantic region to the UK. There have however been some upwards trends in suburban and urban ozone (average increases of the order 5 – 9 μg m-3 over the 20-year period 2000-2019). These upwards trends in suburban and urban ozone are a consequence of reductions in primary emissions of NO; the NO has the effect of locally suppressing ozone.All areas of the UK have been compliant with the EU target value for the protection of human health for at least the last 12 years (≤25 days per year with daily maximum 8-hour mean ozone concentration >120 μg m-3, averaged over 3 years). Substantial parts of the UK are however routinely not compliant with the EU long-term objective of zero exceedance of this metric, although the number and geographical extent of these exceedance days has generally been declining over the last 20 years. All areas of the UK have been compliant with the EU target value for the protection of vegetation for at least the last 12 years (AOT40 not exceeding 18000 μg m-3.hour per year, averaged over 5 years). The EU long-term objective for the protection of vegetation is lower (AOT40 The prediction of future surface ozone in the UK is complex and subject to multiple drivers. Wider hemispheric trends in ozone resulting from global trends in emissions, in particular of methane, will have a significant impact on UK concentrations, but the direction of change is unclear. Further UK reductions in NOx emissions are anticipated and these will likely lead to further increases in urban ozone but be beneficial more widely and reduce ozone at a transboundary scale. Projections indicate little change to UK VOC emissions in the coming decade and this will become a limiting factor for reducing ozone in an optimal manner. Climate change will have multiple effects on ozone precursor emissions, chemical production and loss, dispersion and deposition. Whilst climate change is likely to lead to periods of higher temperatures that are known to increase biogenic VOC emissions and to reduce the surface dry deposition of ozone, the net effect on ozone of all climate-change influences is not known.As other air pollutants in the UK continue to decline it seems likely that the importance of ozone will grow, especially as its contribution to climate provides further motivation for control measures.