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Synergistic impacts of ozone pollution and drought on vegetation: implications for water cycle, climate and air quality

Dr Steve Arnold (SEE), Harry Harmens (CEH), Felicity Hayes (CEH), Gina Mills (CEH)

Project partner(s): CEH Bangor

Contact email:

Ozone (O3) is a pollutant in the lower atmosphere (troposphere), which is harmful to human health (WHO, 2008) and to vegetation (Mills et al., 2016). Tropospheric O3 is also a greenhouse gas, contributing climate warming, with a global climate forcing of around 0.4 Wm-2, approximately 25% of net man-made climate warming. Ozone is a secondary pollutant, formed in-situ in the troposphere through complex sunlight-driven reactions involving nitrogen oxides (NO+NO2) and volatile organic compounds (e.g. CO, CH4).

The harmful effects of tropospheric O3 on vegetation result in substantial reductions in global crop yields (Mills and Harmens, 2011, Van Dingenen et al., 2009), with potential implications for future food security (Tai et al., 2014). In addition, reductions in vegetation productivity due to ozone damage result in a reduction in terrestrial uptake of atmospheric CO2 and O3, leading to an indirect warming effect on climate due to an increase in atmospheric CO2 and O3 (Sitch et al., 2007). This indirect CO2 radiative forcing resulting from vegetation damage caused by increases in O3 pollution since the industrial revolution may be as large as +0.4 Wm−2 (Sitch et al., 2007), equivalent to the direct O3 climate effect. Less well-explored effects of O3 uptake to vegetation are the responses of exchange of non-CO2 trace gases between plants and the atmosphere. Such responses are driven by the effects that O3 uptake has on the small pores in the leaf surfaces of plants called stomata. Observations have shown that stomatal opening can reduce in response to plant O3 uptake, reducing the exchange of trace gases between vegetation and the atmosphere, including water vapour and O3 itself. This has potential implications for the large-scale water cycle and atmospheric composition during exposure of plants to elevated O3.

Vegetation models assume a decrease in stomatal conductance with increasing ozone. However, recent data has shown that O3 pollution can cause stomatal sluggishness or even opening, so that stomata do not perform as expected in response to climatic stresses (Mills et al., 2016). There is a growing body of evidence demonstrating that the presence of ozone may change the response of stomata to drought. It is thought that stomata close in response to drought, to reduce water loss, however the presence of O3 appears to inhibit this stomatal closure (e.g. Mills et al., 2009, Hayes et al., 2012, Hoshika et al., 2015, Wagg et al., 2013). This can have implications for water cycling at the landscape scale (Sun et al., 2012). The implications of these effects have not been evaluated at a global scale, and the possible magnitude of such an effect as a modifier of global water cycling has not been determined. These effects may change our understanding of the net climate impact of tropospheric O3 pollution in present day.

Schematic showing trace gas exchange through leaf stomata.

This project will use new observations from plant physiologists and state-of-the-art Earth system modelling to give new insight into plant-mediated O3 pollution effects on water cycling at a global scale. Potential areas for investigation may include:

- Creating a parameterisation for interactive O3 and drought impact on plant stomatal conductance for a variety of plant species/plant functional types (PFTs).

- Model investigation of regions and periods where interactive effects of O3 and drought on stomatal may occur, and their implications for air quality and water cycling.

- Identify regions at highest risk of impact in the current and future climate.

- Investigating implications of drought-O3 interactions for hydrology, atmospheric circulation and surface temperature during heatwave episodes using model simulations.

- Evaluation of atmospheric chemistry, hydrology and surface temperature during e.g. European and N American drought episodes using in-situ observations from surface sites, aircraft and from satellite data.

- Human health and crop yield implications of surface O3 response to these interactions during heatwave episodes.

Potential for high impact outcome

This project will deliver improved understanding on fundamental interactions between air quality and climate. It has the potential to produce high impact first estimates of key interactions between air pollution and land surface / atmosphere exchange. Such interactions may be key to understanding hydrological and air quality response to drought episodes. These issues are of high interest to policy makers as well as the climate and atmospheric chemistry science communities. The work will contribute to new understanding regarding the role of atmospheric-composition climate feedbacks, which are priority topics for e.g. IPCC.

Solardome experimental facility at CEH Bangor. These experiments are used to understand the response of vegetation to ozone exposure (see

Training and research group

The student will benefit from expertise in both numerical atmospheric chemistry-climate modelling and analysis of complex plant physiological datasets. The project provides a high level of specialist scientific training in: (i) State-of-the-science application and analysis of global atmospheric models; (ii) interpretation and application of plant physiological datasets; (iii) analysis and interpretation of in-situ aircraft datasets and satellite datasets (iv) numerical modeling and use of supercomputers, (v) science communication with expert and non-expert groups through attendance at international workshops on pollutant research and impacts.

The student will join the Biosphere-Atmosphere Group (BAG) - a group of around 12 students and postdoctoral researchers within ICAS working on projects in atmospheric composition and its links to climate, air quality and the biosphere. The student will be supervised in Leeds by Steve Arnold. For more information see: We encourage interested applicants to get in touch and arrange an informal visit to Leeds to meet and talk with the group.

Partners and Collaborations

The student will be based in Leeds, but will work closely with CEH scientists in Bangor, who will provide novel data on O3 impacts on stomatal conductance, and interactions between O3 and drought on stomatal conductance for different plant functional types. CEH Bangor hosts the Programme Centre of the International Cooperative Programme on Effects of Air Pollution on Natural Vegetation and Crops (, a subsidiary body of the UNECE Convention on Long-range Transboundary Air Pollution. EH Bangor also has a range of unique exposure facilities to assess impacts of ozone on vegetation in a changing climate such as warming/heat waves and increased drought events (


Hayes, F., Wagg, S. et al. (2012). Ozone effects in a drier climate: implications for stomatal fluxes of reduced stomatal sensitivity to soil drying in a typical grassland species. Global Change Biology 18: 948-959.

Hoshika, Y., Katata, G. et al. (2015). Ozone-induced stomatal sluggishness changes carbon and water balance of temperate deciduous forests. Scientific Reports 5: 9871.

Mills, G., Harmens, H. (Eds) (2011). Ozone pollution: A hidden threat to food security. ICP Vegetation Programme Coordination Centre, CEH Bangor, UK. ISBN: 978-1-906698-27-0.

Mills, G., Harmens, H. et al. (2016). Ozone impacts on vegetation in a nitrogen enriched and changing climate. Environmental Pollution 208: 898-908.

Mills, G., Hayes, F. et al. (2009). Chronic exposure to increasing background ozone impairs stomatal functioning in grassland species. Global Change Biology 15: 1522-1533.

Sitch, S., Cox, P. M. et al. (2007). Indirect radiative forcing of climate change through ozone effects on the land-carbon sink. Nature 448(7155): 791-794.

Tai, A.P.K, Martin, M.V. et al. (2014). Threat to future global food security from climate change and ozone air pollution. Nature Climate Change 4: 817-821.

Van Dingenen, R., Dentener, F.J. et al. (2009). The global impact of ozone on agricultural crop yields under current and future air quality legislation." Atmospheric Environment 43: 604-618.

Wagg, S., Mills, G. et al. (2013). Stomata are less responsive to environmental stimuli in high background ozone in Dactylis glomerata and Ranunculus acris. Environmental Pollution 175: 82-91.

WHO (2008). Health risks of ozone from long-range transboundary air pollution. (Amann, M. et al.) World Health Organization.

Related undergraduate subjects:

  • Chemistry
  • Engineering
  • Environmental science
  • Meteorology
  • Physical science
  • Physics