Source: Frank Spooner Pictures
| 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).
| THE CAUSES |
The formation of photochemical oxidants is much more complex chemically than the formation of acidifying compounds. Ozone is not directly emitted but is a secondary pollution component formed in the atmosphere by the action of sunlight on ozone precursors (nitrogen oxides, VOCs, methane and carbon monoxide) depending on many factors, including hydrocarbon reactivity, time-scale, amount of sunshine, ambient temperature, competition for hydroxyl radicals (OH) between hydrocarbons, concentration of nitrogen oxides and altitude. With the exception of VOCs, ozone precursor emissions are dominantly anthropogenic, and more so in the northern hemisphere where there is more industrial activity (see Table 32.1).
Regional ozone concentrations have been thoroughly investigated in northwestern Europe (eg, Derwent et al, 1991), whereas the situation in Southern and Eastern Europe still needs more investigation. It is noteworthy that most of the world's natural atmospheric ozone resides not in the troposphere but in the stratosphere, where it provides a crucial shield against biologically harmful ultraviolet radiation. The depletion, due to man-made CFCs, of this beneficial stratospheric ozone is another major environmental problem (treated in Chapter 28). However, there is no way in which the depletion of 'good' stratospheric ozone and the build-up of 'bad' tropospheric ozone can compensate each other.
The concentration of ozone in the troposphere is determined by the balance of downward transport of ozone-rich air from the stratosphere, photochemical production and loss within the troposphere, and removal by deposition to the ground and vegetation. Increasing anthropogenic emissions of ozone precursors have led to increased photochemical production of ozone in the troposphere. There is now strong evidence from observations that ozone concentrations have increased since the last century. This trend has to be related to a shift in the balance of ozone sources and sinks.
First assessment of the EUROTRAC/TOR ozone network (Beck and Grennfelt, 1993) has shown that the internal production of ozone within Europe contributes substantially to long-term mean concentrations.
Slow moving high-pressure systems with predominantly clear skies and elevated temperatures set the stage for the photochemical formation and accumulation of ozone and other oxidants over wide regions during episodes which last for several days. Such highly elevated ozone levels were first observed in the Los Angeles smog and were scientifically explained in the early 1950s. As a consequence of the rise of the background long-term ozone concentration, episodic values will inevitably reach higher values.
In the remote troposphere and clean continental sites where NOx levels are low, NOx is a limiting factor to ozone formation. Here the background levels of carbon monoxide, methane and natural hydrocarbons provide the fuel for ozone formation. Since carbon monoxide and methane have relatively long atmospheric lifetimes, they will determine the long-term average ozone concentration. At higher levels of nitrogen oxides (more than a few ppb), ozone formation is more dependent on the VOC availability.
Emissions of ozone precursors are increasing (see Chapter 14). Fuel combustion is the most important source of anthropogenic nitrogen oxides, while fuel combustion and evaporative emissions from motor vehicles and the use of solvents are the main sources of anthropogenic VOCs in Europe. Motor vehicles account for a considerable fraction of the total emissions of nitrogen oxides and VOCs to the atmosphere in Europe. VOC emissions are more uncertain than emissions of other major air pollutants, and the contribution of individual VOC sources to photochemical oxidant formation varies with their chemical composition (VOC speciation), and the time and place of release. Methane is released mainly from rumen fermentation in cattle, landfill sites, leaks in the distribution of natural gas and rice paddies, while carbon monoxide is emitted mostly from road transport.
VOCs may be classified according to their ability to form ozone. The concept of photochemical ozone creation potentials (POCPs) has been developed (Derwent and Jenkin, 1991) and recently refined (Andersson-Sköld et al, 1992) for taking into account several effects: the contribution to peak ozone values the same day and one day later; the maximum contribution to ozone formation; and the average contribution to ozone formation over 48 and 96 hours, including the amount of ozone deposited during transport. Under typical conditions for a high-pressure situation in southern Sweden, POCPs show that ethylene and acrolein are the most efficient ozone producers, followed by higher alkenes, aromatics, alkanes and ethers, while chlorinated species, alcohols and ketones are very weak producers of ozone. Nevertheless, the relative POCPs differ depending on the method of calculation and the chemical environment (Andersson-Sköld et al, 1992).
Following the Sofia Protocol to the Long-Range Transboundary Air Pollution (LRTAP) Convention (see Chapter 31), emissions of nitrogen oxides in Europe are expected to decrease in the next ten years by 20 to 30 per cent, but not homogeneously in every country. Implementation of the UNECE VOC protocol, which is now being ratified (signed in 1991 by 20 European countries, the EC, the USA and Canada, and ratified by ten by 1994), should lead to a 15 per cent European emission reduction of VOCs by 1999 with respect to values in the late 1980s. Reductions in Western Europe are expected to be larger than those in Central and Eastern Europe.
| THE CONSEQUENCES |
The increase of ozone levels is of concern because of ozone's adverse effects on human health (Lippmann, 1989), materials and ecosystems. The transient health effects are more closely related to length of exposure than to one-hour peak concentrations. The effects of long-term chronic exposure to ozone remain poorly defined but epidemiologic and animal inhalation studies suggest that current ambient levels are sufficient to cause premature ageing of lungs. The effects of ozone on transient functional changes are sometimes greatly enhanced by the presence of other environmental pollutants. Other photochemical oxidants cause a range of acute effects including eye, nose and throat irritation, chest discomfort, cough and headache. These have been associated with hourly oxidant levels of about 100 ppb.
Effects on the respiratory tract (impaired lung function, increased bronchial reactivity) are deemed the most important health effects of ozone. Pulmonary function decrements in children and young adults while undertaking exercise have been reported at hourly average ozone concentration in the range 80 to 150 ppb. Increased incidence of asthmatic attacks, and respiratory symptoms, have been observed in asthmatics exposed to similar levels of ozone. Maximum ozone concentrations are generally found in the afternoon.
Ozone contributes to damage to materials such as paint, textile, rubber and plastics. However, in general only little damage occurs to materials either because they are resistant enough after preventative measures (addition of anti-oxidant) or because their useful life is, in any event, rather short. However, works of art may be damaged after lengthy exposure to ozone.
In the case of crops and some sensitive natural types of vegetation or plant species, exposure to ozone will lead to leaf damage and loss of production. Generally ozone does not react with the surface of the plant but enters mainly through its open stomata (the pores on the underside of leaves). Ozone can affect all levels of biological organisation (Skärby and Selldén, 1984). At the cell level, by:
These changes, in turn, affect higher levels of plant organisation, leading to:
Tropospheric photochemical oxidants also need to be considered in relation to other environmental problems. Photochemical oxidants do not normally interact simply with acidification in ecosystems, since oxidants affect primarily the canopy, while the other damages are caused through soil changes (Grennfelt et al, 1993). There are, however, a number of investigations showing synergistic effects between ozone and acidifying compounds (Guderian and Tingey, 1987). It has been suggested that the combined stress from acid deposition and photochemical oxidants may increase the potential for forest damage. Acid fog deposition has been shown to interact with ozone, causing increased leaching from coniferous canopies.
As a second consequence of further increases in global trace gas emissions, a further decrease is expected to occur of the self-cleansing capacity of the troposphere (ie, the ability of the hydroxyl radical to react with most pollutants). This would result in longer atmospheric residence times of trace gases and, consequently, an enhanced greenhouse effect and an increased influx of ozone-depleting trace gases into the stratosphere.
| GOALS |
Observed and forecast trends in both ozone precursor emissions and tropospheric ozone concentrations may have serious consequences. For the assessment of environmental effects from ozone, control of both peak values and long-term values are important. To limit the effect of tropospheric photochemical oxidants on human health, vegetation, agricultural crops and forests, regional scale episodes of photochemical oxidant formation need to be controlled. This has been reflected in the UNECE development of critical levels for ozone, where short-term (one half to eight hours) as well as long-term (vegetation season) averages are considered. On the other hand, the stronger impact on human health of cumulative daily exposure, rather than one-hour peak concentrations, should be used as a guide when setting priorities.
The concept of critical level for ozone assumes a threshold below which no or small effects occur. The observed effects are related to the ozone dose expressed in ppb-hours above the threshold level. Recommendations have been made (UNECE, 1993) for critical levels for crops of 5300 ppb-hours above a reference level of 40 ppb (during daylight over three months) and for forests of 10 000 ppb-hours above 40 ppb (24 hours during six months). The reference level of 40 ppb is exceeded all over Europe, although the exceedance is much less in the north of Scandinavia than in Central Europe. The significance of ozone precursor emission reduction will thus vary within Europe (see Table 32.2).
The long-term average ozone concentrations at ground level are closely related to the ozone concentrations in the troposphere. Model calculations show that uniform percentage reductions in European emissions of nitrogen oxides and/or VOCs leads to stronger reduction of the high percentile concentration values (90, 95 and 98 percentiles) than of average ozone levels. In general the influence of European emissions on the 98 percentile oxidant values is two to three times as large as on the averaged concentrations. This again demonstrates that long-term averaged ozone concentrations depend more on the upper tropospheric oxidant (ozone, nitrogen dioxide, PAN, etc) concentrations, whereas high episodic peak concentrations are determined mainly by photochemical production over Europe (De Leeuw and van Rheineck Leyssius, 1989).
The global emissions of carbon monoxide, methane, and nitrogen oxides are expected to increase in the next decade by 0.9, 0.9 and 0.8 per cent per year respectively. As a consequence, and assuming that the chemical system stays linear, ozone concentrations in the northern-hemispheric troposphere are expected to keep rising at a rate of more than 1 per cent per year, leading to increased exposure of people and ecosystems to ozone. No explicit goals have yet been set with regard to tropospheric ozone. In order to halt these increases, it could be recommended that global nitrogen oxide emissions should at least be stabilised, since tropospheric concentrations of nitrogen oxides are a limiting factor for tropospheric ozone formation. Nitrogen oxide emissions from aircraft contribute strongly to tropospheric nitrogen oxide concentrations, and therefore to ozone formation; these aircraft emissions are expected to rise in the next decade by more than 3 per cent per year.
Control of background tropospheric ozone levels should be made with reference to their direct contribution to the mean level of ground level ozone and to the enhanced greenhouse effect. This latter effect cannot be appraised by just assuming an equivalent increase of carbon dioxide, as is normally done in various scenario assessments, because of the short residence time of ozone and its non-uniform distribution vertically, horizontally and temporally (Marenco et al, in press). This requires new research.
| STRATEGIES |
Because of the general growth in road transport (both private cars and other vehicles), there are indications that the actions already undertaken will not be sufficient to reduce photochemical oxidant formation in Europe. This is particularly the case for nitrogen oxides. For VOCs, the situation is more optimistic, but reductions are taking place mainly for those VOCs released by evaporation of solvents and other evaporative emissions, which are not necessarily the most photochemically active.
Agreed emission reductions will probably result in only a 5 to 10 per cent decrease in ozone peak concentrations, which is not sufficient to avoid exceeding current air quality standards. Part of this decrease could even be counteracted by the expected continuing increase in background tropospheric ozone. In order to avoid the present exceedances of the 75 ppb ozone level (see Chapter 4), simultaneous reductions in the European emissions of nitrogen oxides and VOCs of at least 70 per cent would be necessary. For nitrogen oxides, such reductions are technically feasible. For VOCs, such reductions could be achieved by Europe-wide introduction of improved three-way catalytic converters for cars, diesel engines with reduced VOC-emissions, reduced use and substitution of organic solvents and various add-on techniques in storage, transport and distribution of organic products (RIVM, 1992).
The further increase of global emissions of ozone precursors will also lead to global problems (such as decrease of the self-cleansing capacity of the troposphere, enhanced greenhouse effect, and increased influx of ozone-depleting trace gases into the stratosphere). In order to prevent or reverse this trend, global emissions of carbon monoxide, methane and low reactive hydrocarbons would need to be reduced. In Europe, transport emissions of carbon monoxide and VOCs can be substantially reduced by further introduction of catalytic converters in cars. Industrial carbon monoxide emissions in Central and Eastern Europe can be reduced by implementing more advanced technology.
Since emissions of most compounds have common and linked effects, it is important to consider all effects together for setting emission reduction strategies. Through combined reductions in pollutant emissions, a wider range of adverse environmental impacts can be dealt with, a positive synergism in terms of reduction of adverse impacts can be obtained, and more flexible opportunities are made available (Grennfelt et al, 1993). For instance, a combined emission reduction programme may favour energy efficiency or fuel substitution, and could lead to a reduction in the amount of greenhouse gases emitted into the atmosphere. On the other hand, when it involves emissions of nitrogen oxides, a combined emission reduction can lead to locally increased ozone formation in areas of high emissions, or simply to transferring the location of peak ozone concentrations. Also, a reduction in global emissions of nitrogen oxides can lead to a global reduction in the hydroxyl radical and thus an increased lifetime of methane and CFC substitutes, with a consequent increase in the global warming potential of these species (see Chapter 27).
Table 32.3 shows the strength of the relationship between a change in the precursor emissions of ozone and acidifying compounds, and the concentration of different compounds or visibility effect. Concentrations of ozone, hydroxyl and hydrogen peroxide, intermediaries in the photchemical process, are particularly important for enhancing the rate of transformation of sulphur dioxide into sulphate, nitrogen oxide into nitrate and ammonia into ammonium. It appears from Table 32.3 that nitrogen oxide emissions stand out as a controlling factor for almost all parameters listed. Methane emissions are important for long-term changes in ozone, hydroxyl and hydrogen peroxide. Carbon monoxide has a controlling effect on the loss of hydroxyl. VOCs have a controlling effect on the peak episodic ozone concentrations, but otherwise VOCs (with the exception of methane) are thought to play only a minor role in atmospheric oxidation. All these relationships can be quantified through model calculations.
The reductions of nitrogen oxides and VOC emissions required to reduce ground-level and tropospheric ozone levels would also have additional environmental benefits: