Impacts of Photochemical Oxidants (Ozone)

Photochemical oxidants are the products of reactions between NOx and a wide variety of volatile organic compounds (VOCs). The most well known 'oxidants' are ozone (O3), peroxyacetyle nitrate (PAN) and hydrogen peroxide (H2O2). The main impact on the natural environment is mostly due to elevated O3. Excessive concentrations of tropospheric (ground level) O3 have toxic effects on both plants (Davison & Barnes, 1998; Ashmore, 2002; Long & Naidu, 2002) and human health.

The focus in earlier analyses of ground level ozone was on the peak O3 concentrations which occur under warm, sunny conditions, and thus largely occur in the UK from April to September. Policy actions to date across Europe have reduced the emissions of NOx and volatile organic compounds (VOCs) (ozone precursors). These emission controls have reduced peak ozone concentrations by typically 30 ppb in the UK, but over the last 20 years mean concentrations have been increasing in urban areas due to reductions in local depletion of O3 by NO, and in rural areas due to increases in the hemispheric background O3 concentration. The increases in background ozone concentrations and decreases in peak values are common to rural areas throughout the UK and therefore are potentially important for effects on sensitive vegetation across the country (RoTAP, 2012).

Effects on vegetation include visible injury, early senescence of leaves, and reduction of crop yield (PORG 1997). Experiments with open-top chambers in various parts of Europe (including the UK) show that exposure of plants to concentrations above 40 ppb for several weeks can reduce growth and the yield of sensitive crops species. However, it is difficult to translate this kind of information into effects on crops growing in the field and on natural communities. The most comprehensive information is available for wheat, and the evidence indicates that yields are reduced in some parts of Britain in high ozone years. For a discussion of the problems and challenges of assessing the effects on natural vegetation see Davison & Barns (1998).

AOT40 is the Accumulated Concentration Over a Threshold of 40 ppb. If an hourly average ozone concentration exceeds 40 ppb the difference between the concentration and 40 ppb is added to a running total. For the critical levels the AOTs are summed during daylight hours over fixed time periods (Table below). To reduce the effect of inter-annual variability in ozone concentrations the critical level is assessed on the 5-year average AOT40 rather than an AOT40 for individual years. The AOT40 value is calculated as the concentration at canopy height. The suggested default canopy height for forest trees is 20 metres while for semi-natural vegetation it is calculated at a height of 0.2 metres (20cm).

For forests the AOT40 is summed for April to September daylight hours inclusive, while for arable crops it is summed from June to August inclusive of daylight hours. There are two AOT40 critical levels for semi-natural vegetation. For semi-natural vegetation dominated by annual, the AOT40 is calculated over three months, and for those dominated by perennialsan AOT40 period from mid April to mid October is used (see table below). In APIS we present data based on the critical level of AOT40 5000 ppb hours (based on semi-natural vegetation communities dominated by perennials, 6 month growing season mid April- mid October). The alternative critical level may also be relevant. Daylight hours are defined as when solar radiation exceeds 50 W m-2. This is when vegetation's stomata should be open and so it will be absorbing ozone through them as it respires. The calculation of AOT40 is based on ozone concentrations at the top of the canopy as described in Section 3.4.2 of the CCE report.




Time period

Critical level (ppb h)

Agricultural crops

Yield reduction


3 months (June to August)


Horticultural crops

Yield reduction


3.5 months (between April and September)


Forest trees 

Growth reduction 


6 months (April to September)


(Semi-)natural vegetation communities dominated by annuals

Growth reduction and/or seed production


3 months (mid April to mid July, or growing season if shorter)


(Semi-)natural vegetation communities dominated by perennials

Growth reduction


6 months (mid April to mid October)


An important weakness of the present critical levels is that they refer to the exposure of the plant to the ozone (concentration x time) but the effects depend on how much is taken in through the stomata. Because stomatal conductance is affected by several environmental factors, notably humidity and soil water deficit, the critical level over estimates the amount of ozone absorbed. For example, in warm sunny weather ozone tends to be high, but if there is also a lack of moisture the stomata close, reducing ozone uptake. In that situation the critical level may be exceeded but the plants may show no effect because it did not take up enough ozone to be toxic. Furthermore, most studies of ozone have used in open-top chambers where the ozone concentration is easily maintained at a specific concentration. These were originally adopted because it was thought that they produce an environment that is similar to the open air, but it is now recognised that the turbulence, temperature and humidity, do not accurately reflect field conditions so the plants do not respond to ozone in the same manner as they would in a natural environment.

In open-top chamber experiments a well defined threshold concentration for damage to the vegetation is usually found (PORG, 1997), but in field experiments plants often show damage appearing over a wide range of concentrations (Gruenhage et al 1993). This inconsistency between chamber and field experiments is probably due to differences in ozone uptake.

The increasing body of evidence, both statistical and biological, indicate that an index based on the accumulated O3 flux through the plant stomata (Phytotoxic Ozone Dose above a threshold of Y, PODY) is better related than AOT40 to well-recognised adverse effects of O3 on crop yield and forest growth. For example, evidence of effects across Europe collated by Mills et al., 2011, for the ICP Vegetation suggests that a flux-based evaluation is a better representation of the spatial distribution of impacts on vegetation across Europe than AOT40 (RoTAP, 2012).

The Royal Society (Royal Society, 2008) reported on necessary geographical scale for effective control of ground level ozone, which has been shown to be a hemisphericscale environmental issue. Regional or country-scale control measures have limited ability to regulate ground level ozone exposures within the control regions (RoTAP, 2012).

The effects of ozone on vegetation and human health have until recently been the main motivation for research and monitoring of ground level ozone. While these two effects remain the main policy drivers, the role of ozone as a contributor to the direct radiative forcing of global climate has grown in importance (IPCC, 2007). In addition, the recent recognition that the effects of O3 on carbon sequestration through its effects on primary productivity of vegetation is emerging as an additional reason for interest in ground level ozone (Sitch et al., 2007) and the effects on vegetation are the primary focus of interest in ground level ozone.

The Review of Transboundary Air Pollution (RoTAP) report includes several chapters on Ozone and its effects including:

  • Section 5.4.7:  Scientific basis of ozone risk assessment
  • Section 5.4.8 Assessment of current impacts of ozone in the UK
  • Section 7.3.1 Trends in ozone concentration
  • Section 7.3.2 Evidence of impacts of ozone in Europe


Ashmore, M.R. 2002 Effects of oxidants at the whole plant and community level Air Pollution and Plant Life pp 89-118
Davison, A.W.; Barnes, J.D. 1998 Effects of ozone on wild plants New Phytologist 139 135-151
Grunhage, L.; Dammgen, U.; Haenel, H.D.; Jager, H.J.; Holl, A.; Schmitt, J.; Hanewald, K. 1993 A new potential air-quality criterion derived from vertical flux densitites of ozone and from plant response Angewandte Botanik 67 9-13
Mills, G.; Hayes, F.; Simpson, D. ; Emberson, L. ; Norris, D.; Harmens, H. ; Buker, P. 2011 Evidence of widespread effects of ozone on crops and (semi-)natural vegetation in Europe (1990–2006) in relation to AOT40- and flux-based risk maps. Global Change Biology 17 592-613
PORG, 1997 Ozone in the United Kingdom 1997 Fourth Report of the United Kingdom Photochemical Oxidants Review Group
Sitch, S. ; Cox, P.M. ; Collins, W.J. ; Huntingford, C. 2007 Indirect radiative forcing of climate change through ozone effects on the land-carbon sink Nature
Solomon, S. ; Qin, D. ; Manning, M. ; Chen, Z. ; Marquis, K.B. ; Averyt, M. ; Tignor, M. ; Miller, H.L. 2007 Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007