To address the key challenges presented by ground-level (or tropospheric) ozone, the Centre for Ecology & Hydrology (CEH) plays a lead role in bringing the UK research community together. Ozone is a secondary pollutant, which forms as other pollutants react in the atmosphere and has adverse effects on human health, ecosystems and agricultural crops.

Substantive research and policy efforts to tackle these problems have been hampered by the complex interactions between the emission sources, atmospheric processes and meteorological factors and are discussed here.

Ozone is formed in the lower atmosphere by the action of sunlight on nitrogen dioxide (NO2), which is naturally present from lightning, biomass burning and soil emissions; man-made contributions to NO2 from burning fossil fuels dominate in developed regions. Ozone formation is accelerated by the presence of organic gases, both biogenic and man-made. Ozone is toxic to plants, animals and humans; toxic concentrations are found in polluted air, downwind of NO2 sources and especially in strong sunlight. Ozone is removed from the atmosphere by deposition to plants, and also by reaction with nitric oxide (NO) to form NO2.

Global Challenge Network on Tropospheric Ozone

Adverse effects of ground-level ozone are a global problem, requiring interdisciplinary collaborations to identify and develop viable solutions. In order to bring together researchers, industry and key stakeholders, a Global Challenge Network on Tropospheric Ozone has been funded by the Science & Technology Facilities Council (STFC) and coordinated by the NERC Centre for Ecology & Hydrology (CEH). The partners in this network use their joint expertise in instrumentation, engineering, data networking and modelling to provide the common factors missing from the ozone-related research in the UK. The network aimed at joining up disparate research areas, leading to shared knowledge and expertise, and a willingness to form new consortia and collaborations in meeting common challenges.

Key results on our current knowledge about the challenges presented by ground-level ozone have been summarised in a series of factsheets. Below, highlights from each factsheet are compiled, linking to the factsheets for in-depth information.

Factsheet 1: How much, where and why?

Before industrialization, tropospheric (lower atmosphere) ozone concentrations were approximately 10 ppb (or 20 μg m-3) globally. Man-made emissions of the precursor gases have increased this background concentration to 20-30 ppb. However concentrations of ozone can be much larger than the ‘background’, because of local or regional variations in precursor emissions and land-use.

Concentrations of ozone in the lowest part of the atmosphere, the surface or boundary layer, can vary by factors of 10 to 100 over a typical city and its suburban to rural surroundings. In order to warn of possible human health impacts and estimate the effects of ozone on environmental health, adequate monitoring is required, to quantify the spatial and temporal variations in ozone concentrations.

Factsheet 2: Modelling Ozone in the Lower Troposphere

Ground-level ozone interacts with plants and with other pollutants. These interactions are highly non-linear and may enhance or deplete ground level ozone so need to be properly represented by models. Model domains and scales (vertical and horizontal) are key limitations for the simulation of ground-level ozone. In areas with high emissions of nitrogen oxides (NOx, ie. large cities) reaction with NOx plays a major role in controlling ground-level ozone, whereas in less polluted areas hemispheric transport is a major driver of ground-level ozone.

Biogenic emissions are also a critical source of biogenic volatile organic compounds (VOCs) which are precursors of ozone formation. The next figure (below right) shows annual average surface ozone for the year 2008 calculated by the EMEP4UK model. Left panel shows the European domain at a 50 km x 50 km horizontal resolution and the right panel shows the UK domain at a 5 km x 5 km horizontal resolution.

Factsheet 3: Established & emerging findings on health effects of ground-level ozone

EMEP4UK Modelled annual mean concentrations of ground level ozone over the UK and Europe in parts per billion

Exposure to ground-level ozone damages human health, especially, but not only, people’s respiratory (lung system) health.

The strongest evidence comes from numerous studies of short–term exposure, i.e. of daily concentrations measured variously as daily 1 hour maximum or daily 8 hour maximum or 24 hour mean concentrations. Daily ozone concentrations increase all-cause, cardiovascular and respiratory mortality; respiratory hospital admissions; asthma severity; and, less clearly, cardiovascular hospital admissions.

A 2014 report by the European Environment Agency (EEA) calculated exposure to ozone leading to 16,000 premature deaths in the EU in 2011 (assuming a threshold of 35 ppb).

Number of days on which the 2012 target value for the protection of human health was exceeded (summers of 2012, 2013 and 2014)

Source: European Environment Agency

Factsheet 4: How does ground-level ozone affect agricultural crops and food production?

Ground-level ozone is a threat to food production as it has a negative impact on the yield and quality of important staple crops. Soybean, wheat and rice are the most sensitive to ozone, with maize and barley being moderately sensitive.

Example of ozone-induced leaf damage to an agricultural crop

Above: Example of ozone-induced leaf damage to an agricultural crop (Photo: D. Velissariou)

Factsheet 5: How does ground-level ozone affect vegetation?

Ground-level ozone damages ozone-sensitive vegetation. Effects include visible leaf injury, increased or premature die-back and reduction in growth and seed production of sensitive species. These can result in an increase in abundance of ozone-tolerant species and effects on the functioning of ecosystems and the services they provide.

Ozone damage on leaves in grassland
An ozone damaged leaf
Ozone damaged leaves on trees
Examples of ozone-induced leaf damage on trees and grassland
Photos: Gina Mills, Marcus Schaub

Additional information

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