11. Atmospheric Ozone Levels in Summer in Rural England
MGR Cannell and TH Sparks

Exposure to tropospheric ozone in summer (AOT40 for forests, see below) averaged for three rural sites in England (Harwell, Aston Hill and Ladybower) shown in relation to:
mean April-September temperatures in Central England. The AOT40 for forests is the Accumulated exposure Over the Threshold of 40 parts per billion during daylight hours in April-September.

Specific Indicator
Surprisingly, the ozone which causes photochemical smog, occurs in higher concentrations, and is less variable, in rural areas than in cities. Rural concentrations are monitored at 17 sites in the UK. The most meaningful measure is one which integrates exposure over the summer when plants can be adversely affected. The indicator chosen is the Accumulative exposure Over the Threshold of 40 ppb during daylight hours during the months of April-September, otherwise known as the AOT40 for forests, averaged for rural monitoring sites at Harwell, Aston Hill and Ladybower. These sites are in central England and ozone concentrations there are not so affected by NOx emissions from cities or by air masses from the continent as at sites elsewhere.
[Source: Institute of Centre for Ecology & Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB]

Relevance
Ozone is formed in the lower atmosphere by the action of sunlight on volatile organic compounds (hydrocarbons) in the presence of nitrogen oxides. This ozone is potentially harmful to vegetation and people (unlike that in the stratosphere which shields the earth from UVB radiation) and the damage done is dependent on both the concentration and the duration of exposure. The critical AOT40 (above which damage may occur) is 10000 ppb hours during April-September for forests, and 3000 ppb hours during May-July for crops and natural vegetation.

Sensitivity to climatic and other factors
The main factors influencing ozone concentrations in the UK are: variation in the Northern hemisphere background concentration, changes in emissions of hydrocarbons and nitrogen oxides (the precursors) from Europe and the UK, changes in the concentration of nitrogen oxide which can lead to ozone destruction, and changes in the annual pattern of meteorological conditions across Europe.
Meteorological conditions which favour ozone formation are:(i) high solar radiation; thus, near Edinburgh the average ozone concentration rises by about 2 ppb per 100 W/m2 increase in solar radiation over the range 0-800 W/m2; (ii) high concentrations of hydrocarbons and moderate concentrations of nitrogen oxides, (iii) warm temperatures; thus near Edinburgh concentrations can increase rapidly when temperatures rise above 20 _C; and (iv) stagnant or slow-moving air of continental European origin, which can enter the UK already polluted with photochemically-generated chemical species.

High ozone concentrations are also positively associated with other pollutants; thus, when ozone exceeds 60 ppb, the concentrations of sulphur dioxide and nitrogen dioxide are also substantially larger than their mean values.

The selected indicator (1987-2002) is closely correlated with the number of sunshine hours and temperatures in Central England.

Change over time
The indicator values have fluctuated between 2000 and 10000 ppb hours between 1987 and 2002, being greatest in years with warm, sunny summers. In the long term, there has been a tendency for rural ozone concentrations in the UK to increase by about +0.5 ppb/yr, but there appears to have been little change since 1990. There has, however, been a reduction in the magnitude of peak zone concentrations which occur during the summer, possibly as a result of changing meteorological conditions. Thus, in the period 1972-1985, maximum hourly average concentrations often exceeded 150 ppb and even 250 ppb in the hot summer of 1976. But in 1986-1995, the hourly maximum exceeded 150 ppb only three times at any of the UK monitoring sites. Peak ozone concentrations in the exceptionally hot summer of 1995 were not as high as expected, owing to reductions in emissions of hydrocarbons and nitrogen oxides in some European countries and air circulation patterns which frequently brought relatively clean air into the UK from N. Europe.
Clearly, rural ozone levels could increase if summers became warmer and sunnier and other conditions were favourable.