Ozone pollution

 

Many people are well aware of the depletion of the ozone layer high in the atmosphere. Therefore, it is not surprising that people are often confused to learn that ozone can act as a pollutant. To avoid confusion, it is important to distinguish between ozone in the upper atmosphere (stratospheric ozone) and that which occurs at ground level (tropospheric ozone).

Stratospheric ozone

Stratospheric ozone occurs high in the atmosphere where it acts as a natural shield filtering out harmful ultra-violet rays and preventing them from reaching the earth’s surface. It has been well documented that some industrial pollutants such as chloroflurocarbons (CFCs), chlorine and bromine are damaging this layer. The result is a thinning of the protective layer, particularly over Antatartica - the so-called ozone hole. The concentration of the ozone-depleting chemicals in the atmosphere has now stopped rising, but the ozone layer is unlikely to recover until the middle of this century.

Ground level ozone

Ozone is also present at ground level where it is usually in low concentrations. However, in the presence of sunlight a series of chemical reactions with other forms of pollution can produce ozone. When these conditions occur background concentrations can become high enough to have important consequences for both humans and plants.

Background concentrations of ozone have been increasing slowly in recent years, although peak concentrations in summer have fallen. This upward trend for ozone is therefore different to other pollutants and is a result of a global rise in the emission of the principle precursors to its formation - nitrogen oxides (NOx) and volatile organic compounds (VOC’s).

Predictions are for ozone concentrations to continue to rise with this form of pollution becoming increasingly important through the 21st century.

How does ozone affect plants?

High concentrations of ozone can cause visible injury to the leaves of trees. Ozone injury varies from reddening and bronzing to a yellowing of the leaf surface and, ultimately, cell death (necrosis) and premature senescence.

Ozone diffuses into the leaf through stomata - the small openings in the leaf surfaces through which gas exchange occurs. Once inside the leaf it is transformed to a variety of reactive cell damaging compounds, here it can cause damage to internal structures and the photosynthetic system. Ozone can also make plants more susceptible to drought by affecting their ability to regulate water loss.

Ozone exposure in Britain regularly reaches the value at which damage to trees can occur and, across Europe as a whole, it has been estimated that ozone may reduce forest growth by about 10%.

Ozone pollution research at Forest Research

We are conducting research into the effects of ground level ozone on a range of tree species through the Impact Studies research programme. A number of these experiments have investigated how the impacts of ozone pollution can be modified by other environmental variables including water supply and atmospheric CO2 concentration. This work has contributed to the setting of critical levels for ozone through the UNECE, including new flux-based critical levels. 

Ambient ozone concentrations are monitored at eleven of the UK Level II Forest Health plots using passive samplers, with monitoring of visible injury to both the main tree species and to specific shrub species also carried out. Three of the Level II plots are also instrumented to estimate the ozone exposure of the canopy compared to that measured at a standard measurement height of 1.5 m.