|Chapter 32: Tropospheric ozone and other photochemical oxidants — The problem|
Photochemical oxidants, especially ozone (O3), are among the most important trace gases in the atmosphere with respect to their physical and chemical functions. Their distributions show signs of change due to increasing emissions of ozone precursors (nitrogen oxides, volatile organic compounds or VOCs, methane and carbon monoxide). World Health Organization (WHO) air quality guidelines for ozone, the one-hour short-term limit value of 75 to 100 ppb for health protection and the long-term value of 30 ppb over the growing season for protection of vegetation, are frequently exceeded in most parts of Europe. The EC has established a directive on air pollution by ozone (92/72/EEC) which sets thresholds for ozone concentrations in air to protect human health and vegetation and to inform or warn the public. There is no compound in the troposphere where the difference between actual atmospheric levels and toxic levels is so marginal as is the case for ozone. Tropospheric ozone is increasing particularly in the northern hemisphere due to the increases in anthropogenic emissions there. Comparisons of pre-industrial ozone data with modern data have shown that surface ozone has more than doubled over the past century. Ozone concentrations in the middle troposphere over Europe and North America appear to have increased slightly over the last two decades. This increase has been witnessed both at ground level stations in the planetary boundary layer of the lower part of the troposphere (Bojkov, 1986; Volz and Kley, 1988; Anfossi et al, 1991) and at mountain tops in the elevated background troposphere (Marenco et al, 1994; see Figure 4.6). Episodic ozone peak hourly concentrations of 100 ppb and more occur frequently over Central Europe every summer. In the northern UK and Nordic countries such episodes very seldom occur and are mainly a consequence of long-range transport. In Southern Europe, local and subregional oxidant formation may be more important than regional long-range transport.
In the lower troposphere, close to the ground, ozone is a strong oxidant that at elevated concentrations is harmful to human health, materials and plants. In the upper troposphere, ozone is an important greenhouse gas and contributes greatly to the oxidation efficiency of the atmosphere. These effects are to be added to the detrimental effects of other pollutants such as sulphur dioxide, nitrogen oxides, carbon monoxide and particulates on human health in urban areas, and of acidifying compounds damaging ecosystems.
Photochemical-oxidant precursor emissions are widely distributed spatially, and originate from various sectors of activity. Thus, control strategies have been more difficult to establish than for acidification, and ozone has proved to be one of the most difficult pollutants to control both in Europe and North America. The trend of increasing activity in the main sectors causing ozone formation (ie, transport and the petrochemical industry) means that this prominent environmental problem may worsen and could be around for a long time.
Long-term records of other photochemical oxidants, such as hydrogen peroxide or peroxyacetylnitrate (PAN), are extremely sparse. PAN measurements in The Netherlands show an increase of almost a factor of three over the past decade. It has been argued that the often observed spring ozone maximum at background stations, instead of a summer maximum concomitant with maximum photochemical activity implied by maximum sunlight in summer, is the result of accumulation of NOx and VOCs during winter, and the subsequent formation of ozone by photochemical activity when spring arrives (Penkett et al, 1986). A Greenland ice core record has shown evidence that hydrogen peroxide concentrations in the atmosphere have increased by 50 per cent over the past 200 years, with most of the increase occurring in the past 20 years, thus indicating that human activities may be responsible for such a clear change (Sigg and Neftel, 1991).
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