Serious Science
This month’s serious science topic comes from Dr Stefan Reis (Centre for Ecology & Hydrology), where he provides us with a closer look at ground level ozone - a common air pollutant that can cause damage to the lungs and irritate the throat if inhaled, and is the main component of smog.
Urban Ozone
What is ground level ozone?
Ozone, often referred to as ground level ozone or tropospheric ozone, is formed in the lower atmosphere (the troposphere) by the action of sunlight on nitrogen dioxide (NO2). Nitrogen dioxide is naturally generated in the air by lightning or released from the burning of biomass and soil emissions. In developed regions the main source of man-made NO2 is from burning fossil fuels. Ozone formation is accelerated by the presence of organic gases, so-called volatile organic compounds (VOCs), both naturally occurring and man-made (see Fig. 1). High concentrations of ozone at ground level are of particular interest, as they have adverse effects on human health, plant growth and production of agricultural crops. Ground level ozone is a short-term climate forcing pollutant so also contributes to climate change.
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) in the air to form NO2. Both NO2 and NO are typically summarised as nitrogen oxides (NOx). A comprehensive analysis of our current understanding of ozone formation and trends has recently been compiled by Monks et al. (2015).
Ozone trends
Before industrialisation, ground level ozone concentrations were approximately 10 ppb (parts per billion, approximately 20 μg m-3) globally. Man-made emissions of the precursor gases have increased this background concentration to 20-30 ppb. However, elevated concentrations of ozone can be much larger than the ‘background’ because of the accumulation of local or regional variations in precursor emissions. These elevated concentrations can vary by factors of 10 to 100 over a typical city and its suburban to rural surroundings.
Future challenges
Traditionally, high levels of ground level ozone were most often observed downwind of cities in rural environments, as pollution masses dispersed out from the cities and the mix between NO2 and NO accumulated in favour of O3 formation. This generated a difference in ground level ozone between the more polluted city centre and the suburban and rural areas, resulting in lower background concentration in city centres.
More recent effects of policy measures aimed at reducing the emissions of ozone precursor pollutants has resulted in changes in atmospheric composition across cities that favour ozone formation. This presents challenges for air pollution control policies, as significant further reductions of NO x will be required to substantially reduce urban ozone concentrations, and as a result, population exposure. In addition, any measures implemented to reduce emissions of air pollutants and greenhouse gases need to be assessed with regard to their effect on ozone concentrations, to avoid unintended consequences.
 In Europe and the UK, maximum ozone concentrations have been seen to slowly decline over the past decade, while mean concentrations have been level or shown a slight increase due to global background ozone concentrations continuing to rise. While the mechanisms underlying these trends are not yet completely understood, emissions of ozone precursors in the growing economies in East and South Asia and hemispheric transport of these precursors may be a main contributor.
References
European Environment Agency (2016a) http://ec.europa.eu/eurostat/web/products-datasets/-/tsdph380 (Fig.3)
European Environment Agency (2016b) http://www.eea.europa.eu/data-and-maps/figures/ds_resolveuid/SN5BJGYVE7 (Fig.4)
Monks, P. S., Archibald, A. T., Colette, A., Cooper, O., Coyle, M., Derwent, R., Fowler, D., Granier, C., Law, K. S., Mills, G. E., Stevenson, D. S., Tarasova, O., Thouret, V., von Schneidemesser, E., Sommariva, R., Wild, O., and Williams, M. L.: Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer, Atmos. Chem. Phys., 15, 8889-8973, doi:10.5194/acp-15-8889-2015, 2015. http://www.atmos-chem-phys.net/15/8889/2015/
Salvo A and Geiger F M 2014 Reduction in local ozone levels in urban Sao Paulo due to a shift from ethanol to gasoline use Nature Geoscience 7 450-8. http://www.nature.com/ngeo/journal/v7/n6/abs/ngeo2144.html
Scotland's Environment website. http://www.environment.scotland.gov.uk/get-informed/air/ (Fig.2)
Van Vuuren et al (2011) The Representative Concentration Pathways: An Overview. Climatic Change, 109 (1-2), 5-31.
Volz-Thomas, A., Mihelic, D. (1990) Ozonproduktion in Reinluftgebieten. Einfluß von Schadstoff-Konzentrationen, Gesellschaft Österreichischer Chemiker (Hrsg.), Tagungsband zum Symposium "Bodennahes Ozon" in Salzburg, Bd. 11 der Schriftenreihe "Umweltschutz". (Fig.1)
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