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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 O3 have toxic effects on both plants (Davison & Barnes, 1998; Ashmore, 2002; Long & Naidu, 2002) and human health.

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 probably reduced in some parts of Britain in high ozone years. With regard to the effects on natural vegetation, it has proved difficult to determine whether there are important ecological effects under UK conditions and more technically-challenging research is needed. For a discussion of the problems and challenges of assessing the effects on natural vegetation see Davison & Barns (1998).

In 1988 the UNECE accepted the critical levels/loads approach as the basis for abatement of transboundary air pollutants. The ozone critical level defines the maximum exposure, over a threshold of 40 ppb, below which there is no negative effect according to present knowledge. Ozone critical levels for Europe were initially agreed at a UNECE workshop in 1993 (Fuhrer & Achermann, 1994), revised at the Kuopio workshop in 1996 (Karenlampi & Skarby, 1996) and they have just been subject to further debate (UNECE meeting Goteborg, November 2002). The levels are defined using the AOT40 index which is the Accumulated Ozone concentration over a Threshold of 40 ppb, i.e., the sum of the difference between hourly mean concentrations and 40 ppb when the concentration exceeds 40 ppb, hence the units are ppb hours. The latest critical levels are:

Crops and semi-natural vegetation: 3,000 ppb hours during May to July daylight hours

Forests: 5,000 ppb hours during April to September daylight hours (where: daylight hours are when solar radiation exceeds 50 Wm-2, exceedance is assessed on a five year mean)

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. Therefore, although the AOT40 can indicate the potential for effects on vegetation it does to correlate well with the magnitude of effects, making it difficult to make economic assessments of ozone damage. The scientific community and the UNECE are well aware of these shortcomings and research is in progress that aims to provide a critical level that is expressed in terms of the absorbed dose. Some experiments are being done in the open air, without the use of chambers.

References:

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