De viktigste menneskeskapte inngrep i atmosfærens tilstand

Forfatter: Øystein Hov

Det er hovedsakelig fire miljøspørsmål som er koblet til de kjemiske endringene i atmosfæren, skriver Øystein Hov: luftkvalitet, avsetning, klimaforandringer, UV-stråling og stratosfærisk ozon.

Air quality refers mainly to urban and conurban air pollution due to industrial and domestic emissions and to road transportation.

Deposition includes environmental problems linked to emission, transformation and transport of minor constituents which, by further deposition at the surface will induce detrimental environmental effects.

The climate change issue is more global in nature and is related, when considering the particular aspect of atmospheric chemistry to the potential role, in the additional greenhouse effect, of trace constituents which have a chemical activity in the atmosphere. These include long lifetime gases such as methane, nitrous oxide, and halocarbons, which will directly act as greenhouse gases. It also includes precursors of ozone formation (carbon monoxide, nitrogen oxides, volatil organic compounds) and of aerosols particles, which in turn will also impact the radiative balance of the troposphere and of the Earth’s surface. If climate change is one of theses issues, it only constitutes part of the potential field of application of atmospheric chemistry.

Finally, the depletion of stratospheric ozone, with its potential effect on UV-B radiation increase at the Earth’s surface, is emphasized by the most recent results showing negative trends over the last two decades in the polar and the middle latitude regions of both hemispheres.

Man-made activities can clearly be indentified as the main causes of these four environmental issues: aircraft emissions, road transport, energy production, waste handling, atmospheric fate of CFC substitutes, algriculture emissions, alternative fuels, solvents. The links between the man-made activites and the environmental issues can be identified as described in the following table.

Local pollution, acid deposition, ozone formation, eutrophication, changing oxidation efficiency, ozone layer depletion and climate change are linked together. Therefore, the solution which can be brought to one specific problem in a given area, could lead to new issues elsewhere. Some of the man made waste products that enter the atmosphere, contribute to many of the environmental problems, and a given source of pollution can first represent a local pollution problem. Later, the pollutants are passed to act on larger scales through transport and transformation (from the local to urban, regional, continental, hemispheric and global scales). Atmospheric pollution problems are thus intrinsically linked.

Therefore, several spatial scales are involved in these environmental issues which vary from the local or urban scale, to the regional, continental and global scales. The impact of each major environmental issue at these various scales differs in magnitude and importance. Some issues are more closely linked to shorter scales (air quality and deposition) while others are more closely linked with larger scales (climate change and stratospheric ozone depletion). The important relations between the environmental issues and the spatial scales can then be summarized as follows:

• air quality – local and regional
• deposition – regional
• climate change – continental and global
• stratospheric ozone and UV – regional and continental

Influence of anthropogenic activities

The importance of the environmental problems and of the related science issues is obviously linked to their direct economic impact. Human activities (aircraft emissions, road transport, energy production, waste handling, atmospheric fate of CFC substitutes, agriculture emissions, alternative fuels, solvents emissions) are linked to the environmental issues as indicated in the first table above.

Aircraft emissions

Through the emission of gases and particles from aircraft engines, air traffic contributes to changes in the in air quality at the Earth’s surface, in the deposition of acids, in the anthropogenic greenhouse effect and climate, in the chemical processes in the upper troposphere and lower stratosphere, and in the stratospheric ozone concentration, thus affecting the UV-B radiation at the surface. The question of how large the emissions and their effects are, today and in the coming decades, is of importance with respect to environmental policy, air carriers, and aviation industry. Because of the long- duration effects, the long life-time of aircraft, and the long development times of new technologies, these questions are urgent.

The present fleet of subsonic aircraft consumes about 130 to 160 Tg (i.e. millions of tons) of fuel per year and emits CO₂, H₂, NO_x, particles (mainly soot), sulphur oxides, carbon monoxide, various hydrocarbons (HC), and radicals such as OH. Though the absolute amounts of the emissions are small compared to other anthropogenic global emissions (2% for CO₂, 2-3% for NO_x), these emissions occur in the critical altitude region below and above the tropopause, between 9 km and 14 km altitude, and are concentrated mainly in the latitude regions between 40°N and 60°N. Furthermore, global air traffic is increasing rapidly by 6 to 8% annually, with largest growth-rates in the Far East. Fuel consumption is increasing by 3 to 4%, and emissions will increase accordingly unless serious attempts are made to reduce the specific emissions. Moreover, some nations are considering the development of a fleet of supersonic aircraft cruising at altitudes between 16 km and 22 km.

Subsonic airliners fly in a range of altitudes where the background concentrations of the emitted species H₂ and NO_x are small, where the residence time in the atmosphere is of the order of weeks to months, and hence much longer than near the Earth’s surface, where temperature is low, favouring particle formation processes and changes to the radiation budget, and where the observed changes in ozone concentration are not yet explained by models.

The present knowledge suggests that subsonic aviation emitting NO_x causes increases in ozone in the upper troposphere of about 4 to 8% in summer and 2 to 4% in winter in the zonal mean, and also causes an increase in ozone in the lower stratosphere. These changes are smaller than the natural ozone variations from year to year. The fact that the ozone concentrations are decreasing on the long term in the lower stratosphere and that their increase in the upper troposphere over Europe has stabilized during the last decade, may be due either to larger non-aviation effects or to the possibility that the sign of aviation impact on ozone near the tropopause is not yet understood.

Aviation also contributes to atmospheric particles formation. Measurements have shown that approximately 1016 particles get formed in the wake of modern aircraft per kilogram of burned fuel. This implies 10 to 30% change in background concentrations of condensation nuclei, which opens the possibility for changes in natural cloudiness and feedback on radiation and climate. The particle formation process depends strongly on the conditions in the combustion, and on the environmental conditions in the diluting exhaust plume. For example, aircraft flying in very cold and humid regions might cause many more particles than those in warm and dry air masses. The consequences of the particle emissions are far from being understood, and they can have important climate effects through changes in the cirrus cloud cover.

Supersonic aviation will increase the abundances of NO_x, H₂O, soot, sulphur compounds, the particle formation in the lower stratosphere, and, by increase in fuel consumption, CO₂ emissions. Past studies have concentrated on the possible photocatalytic ozone destruction by the emitted NO_x. Within the last ten years it has been learned that this impact is smaller than thought before, mainly because of interactions with the chlorine chemistry, lasting for the next decades. Today, the main concern comes from the potential of supersonic aircraft emissions to cause increases in particle abundances (of water, nitric acid, sulphuric acid, soot, perhaps also hydrocarbons), which dehydrate and denoxify the atmosphere and provide the surface for heterogeneous reactions. At present, this impact cannot yet be conclusively assessed.

European research has concentrated on subsonic traffic, but has also contributed to the international assessments of future supersonic aviation. Much progress has been made with respect to gas phase aspects of the NO_x impact on ozone. A recent intercomparison of different models shows a relatively narrow spread of the predicted ozone response. However, one cannot exclude that they share major deficiencies. In the United States, a large scale research programme has been started for assessment of the impact of subsonic aviation. The subsonic issue turned out to be the more difficult one, because of the more complex atmospheric motions near the tropopause, and a lack of data on background concentrations and trends of ozone, H₂O, NO_x, aerosols, and because of interactions with clouds. On the other hand, the atmospheric impact of present subsonic aviation can be measured, and some important datasets have been provided recently, in particular by European projects.

Road transport

Road transport is by far the major source of environmental degradation in urban centres. Transport trends, planning policies and traffic management schemes can have significant impacts, not only on local but also on global environmental conditions. Since motor vehicles emit several pollutants classified as probable or definite carcinogens, including benzene, formaldehyde, acetaldehyde, 1,3-butadiene and particulate matter (or soot/smoke, especially from diesel vehicles), the expected growth in traffic raises still further the importance of clarifying the level of total traffic emission.

Recent studies (AUTO-OIL programme) predict that, in general, European urban road transport pollutants emissions will decline significantly from the 1990 level by the year 2010. Based on the “already agreed” measures on vehicle emissions, the European average reduction of NO_x emissions is predicted to be about 70%, the reduction of CO, VOCs and benzene about 40%, and the reduction in total particulate matter (TPM) emissions about 70%. Furthermore, by 2010, the emission reduction resulting from these measures will bring a considerable improvement with regard to regional ozone pollution.

However, it appears that the predicted average reductions are not representative for all countries. The expected reductions will not be sufficient for achieving stringent air quality standards for some of the pollutants (NO₂, benzene and PM₁₀) and thus further emission reduction measures should be applied. These measures include fuel quality changes, vehicle technology changes, and local technical and non-technical measures (traffic bans, speed regulation, improvement of public transport, alternative fuels, taxation etc.). Concerning the reduction of regional ozone pollution it must be noted that the benefit of the additional measures will be rather small.

Although there is an increasing evidence that particulate material may contribute significantly to patterns of morbidity and mortality (especially the smaller size fractions) our knowledge is still limited. Road transport is thought to be the major source of respirable particles in urban areas and in particular diesel engines emit at a much higher rate than petrol engines. Concerning these emissions from diesel vehicles, only limited reliable data are available for small vehicles, and less is known about emissions from other vehicles, such as buses, coaches and trucks. Also, the available data for petrol engine emissions on particulate matter are rather sparse. In addition further emissions have to be considered to engine emissions, as particulate matter is also emitted from tyre wear and brake wear, while dust is entrained the atmosphere by the motion of the vehicle along the road. Obviously, quantitative estimates of the emission from these sources are difficult.

The amount of ozone produced within urban plumes as well as of regional ozone depends crucially on the amount of traffic emissions of VOCs and NO_x. As the potential of each hydrocarbon for ozone production is different and thus the speciation of the hydrocarbons emitted from road transport, there is limited experimental information available on this matter. The lack of speciation data is more pronounced when future emissions are to be estimated because of the possible differences in the fuel composition and/or the use of alternative fuels. The atmospheric degradation pathways of VOCs may be rather complex and some steps are not fully understood, as is the case for aromatic compounds.

The greatest uncertainties concerning the relations between road transport and air quality are the related to the determination of the accuracy of the emission inventories (spatial and temporal distribution of sources, emission factors etc..), the fact that fuel packages offer differing potentials for reducing different pollutants, the lack of integrated methodologies for estimating the effect of local technical and non-technical measures (e.g. traffic management), the lack of consistency in the existing air quality data at the various locations (monitoring site location, integration time, instrumentation etc.), and the non availability of traffic models interfaced with emission models and atmospheric models.

Energy production

The present pattern of energy production is mainly based on the use of fossil fuel, with exceptions being found in Belgium and France where nuclear power plants contribute to a large part of the power production. The main emissions of fossil power plants consist of carbon dioxide, sulfur dioxide, nitrogen oxide, fly ashes and lower amounts of hydrochloric HCl and hydrofluoric acid HF, volatile elements and boron compounds. Carbon dioxide from power plants contributes to the increasing concentrations of radiatively active gases, sulfur dioxide and nitrogen oxide contribute to acid deposition and nitrogen oxide also to the formation of oxidants. The other compounds are mainly important in terms of local air pollution. The VOC emission of power plants are in general very low.

The carbon dioxide emitted by power plants is an important part of the total anthropogenic emissions and is the best known component of the carbon cycle. The role of sulfur dioxide and nitrogen oxide are studied as part of the total atmospheric loads, important for acid deposition and oxidant formation.

The application of wet scrubbers has significantly contributed to decrease the sulfur emissions in Northern and Western Europe and to the downward trend in sulfur concentrations observed in Europe. It has also led, to a large extent, to a reduction in the emissions of other elements, such as hydrochloric and hydrofluoric acid and boron compounds. In the same way, low NO_x burners and catalytic converters have strongly reduced NO_x emissions, but these reductions have not a large impact as most of the NO_x emissions are caused by traffic anyway. The emissions of fly ashes of modern plants equipped with scrubbers and electrostatic precipitators is also quite low.

Reductions of carbon dioxide can only be achieved by shifts in fuel (e.g. from coal to gas, from fossil fuels to renewable sources), or by measures to increase efficiency as the application of high efficiency turbines and coupling of electricity and heat generation. It must be stated, however, that measures for emission abatement have been applied in limited areas of Europe only. Germany, Scandinavia and the Netherlands are good examples. In other areas of Europe, emissions of power plants still constitute a large contribution to oxidant formation and acid deposition. Local impact, by way of enhanced dry deposition or local wash-out of plumes, is also a problem in these areas and this problem is not always sufficiently well characterised.

Evaluation of the effect of abatement measures should receive attention whenever technological abatement measures are planned. In the eastern part of Germany, acid deposition was enhanced on the short term by abatement measures as first dust was removed from all emissions. This alkaline dust neutralised to a large extent sulfuric and nitric acid, produced by the emissions of power plants, so the first effect after implementation of abatement measures was an increase of acidic compounds in wet precipitation. This further emphasizes the requirement for a global approach of the atmospheric pollution problems.

Waste handling

The problems associated with industrial and municipal waste makes one of the major challenges for modern society. In Europe about 104 millions of tons of municipal waste, 240 millions of tons of industrial waste and 60 millions of tons of sediment sludge are produced annually. 65% of them are landfilled, 24% incinerated and only a minor fraction composted or recycled. Hazardous and/or toxic components are causing contamination of soil, groundwater (leachate) and atmosphere (emissions), thus having a possible negative impact on human society.

In Europe about 60 000 to 120 000 landfill sites, occupying 800 km² to 1700 km² , are recorded. These sites generate 12.5 x 109 m3 of landfill gas annually, of which about 755 x 106 m3 are presently recovered. Landfill gas contains carcinogenic and pathogenic components and contributes 7 to 20% of global anthropogenic sources of methane emission and 2% of the total emission of greenhouse gases. The relative composition of a landfill gas varies from one site to another, but in general one would expect about 40-70% in volume of methane, 30-50% of carbon dioxide, less than 1% of nitrogen, hydrogen, oxygen each and about 1% of trace gases such as hydrocarbons, halogenated organics, nitrogen and sulphur-containing organic compounds. The concentrations of the trace gases can vary considerably from one site to another.

Incineration is by far the most commonly used process for destroying organic compounds in industrial wastes. Despite the aim of complete destruction of the organic compounds, minor concentrations of products of incomplete combustion are present in the emissions. Thus the flue gas may contain a numerous amount of organic compounds such as chlorobenzenes, chlorophenols, polychlorinated biphenyls (PCB), chloronaphthalenes, nitroaromatic compounds, polycyclic aromatic hydrocarbons (PAH), phthalate esters, phosphoric acid ester, polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). Incineration does not remove heavy metal contamination. Volatile metals, particularly mercury, will tend to be released into the flue gas. Other inorganic effluents are SO₂, NO_x, CO and halogenated acids (HCl, HF).

Composting is an industry related recycling method which becomes more and more popular. It is a process of microbial decay of the organic fraction of mainly urban waste and agricultural residues taking place in controlled conditions and generating a material called compost, sufficiently stabilised to be handled, stored and/or applied to soil. The major problem of the composting facilities are the release of contaminants in the biogas. Also, the odour presents a serious problem for compost plants

Atmospheric fate of CFC replacement compounds

Chlorofluorocarbons (CFCs) are unreactive in the troposphere and are transported to the stratosphere where they are photolysed, releasing chlorine atoms. This results in catalytic loss of ozone. Manufacture of CFCs is now regulated under the Montreal Protocol and production of many ozone depleting substances has ceased. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have been selected as replacement compounds since they contain at least one hydrogen atom and are therefore susceptible to reaction with OH radicals in the troposphere. It has been shown that since HFCs contain no chlorine, they will not give rise to ozone loss. Although HCFCs contain chlorine, scavenging by OH radicals in the troposphere largely prevents diffusion of these compounds into the stratosphere.

Agricultural Emissions

The main agricultural emissions concern nitrous oxide, methane and carbon dioxide. The IPCC assessment concludes that the emissions of nitrous oxide from agriculture are much greater than previously assumed. The global emission from this source is now estimated to be 3.5 Tg N₂O-N per year, not far short of that from the largest terrestrial source, i.e. tropical forests. The higher estimate for agriculture stems from a reassessment of the percentage of nitorgen applied to agricultural land as fertilisers and manures that is lost as N₂O, and it includes the indirect (“offsite”) losses as well as the direct ones from agricultural fields. In spite of this increase in the estimated size of the agricultural source, the global budget for N₂O is still not balanced. Unless some major source has not yet been identified, one or more of the known sources is still being underestimated. This could well be the agricultural one.

Agricultural emissions of methane in Europe come predominantly from enteric fermentation in ruminant livestock. Worldwide, emissions from rice grown under flooded conditions are a second significant agricultural source. Aerobic soils act as a sink for methane, much smaller than the atmospheric sink, but comparable with the estimated annual increase in atmospheric methane. Intensification of land use particularly conversion of forest to agricultural land, greatly reduces this sink, but the impact on the global budget is minor.

Agricultural practices which cause a decline in soil organic matter increase the net emission of CO₂ to the atmosphere. If drier, warmer conditions result from global change, this trend is likely to be promoted. Drainage of northern peatlands for agriculture and forestry, though potentially reducing methane emissions, will increase emissions of CO₂ and N₂O.

Alternative and reformulated fuels

Although the implementation of the three-way catalyst has been a very important step towards emission reduction from traffic, the fuel composition is thought to influence motor vehicle emissions significantly, and these emission sources still make a major contribution to the tropospheric ozone formation and climate change.

The use of alternative fuels, for example methanol, ethanol and rape seed-methyl ester (RSME), aims at lowering CO₂ emissions to reduce greenhouse gas forcing. Additionally, the reformulation of fuels by adding oxygenated compounds is becoming more and more important on the market. Some oxygenated compounds (especially branched aliphatic ethers) enhance the fuel octane number, thus providing a possibility to reduce the aromatic fraction in the fuel and, consequently, reduce benzene emissions. Other compounds like organic carbonates and smaller ethers are still under development for application as additives.

Solvents emission

Organic solvents are employed in a large number of industrial processes including surface cleaning, painting and coating printing, dry cleaning and chemical synthesis. These compounds also have domestic uses mainly in paints, cleaning agents and cosmetics. Important agriculture uses include solvents for pesticides and herbicides. Paints, industrial and domestic, and coating processes represent nearly half of the organic solvent activity in Europe.

The three main types of solvents in use at the present time are hydrocarbons, chlorinated and oxygenated hydrocarbons. Due to their volatility, solvents are emitted into the atmosphere where they can have detrimental health effect, directly or indirectly through their oxidation products. They can also have atmospheric impact on a global scale (stratospheric ozone depletion, greenhouse effect), or on regional and local scales as precursors of ozone and other photooxidants.

These health and atmospheric impacts have already led to regulation policies for the emissions of solvents. In this respect, chlorinated solvents such as 1,1,1-trichlorethane and CFCs are being phased out under the Montreal Protocol as species which deplete stratospheric ozone. Policies have been been enacted in the United States to reduce VOC emissions. In Europe, a Council Directive limits the emissions of organic compounds arising from the use of organic solvents in certain processes and industrial installations. The contribution to man-made VOC emission from total organic solvent use is in the order of one third, while emissions from transportation account for about 50 %.

The solvent regulations are likely to favor aqueous formulations, as well as no-solvent technologies and small-volume alternative solvents. Alternative solvents are essentially aliphatic and cycloaliphatic hydrocarbons, oxygenated hydrocarbons and possibly hydrofluorocarbons (HFCs). Among these solvent substitutes, oxygenated compounds are certainly the largest class of compounds which will be developed as alternative solvents. These compounds include alcohols, ketones, ethers, esters, glycol ethers and glycol ether esters.

Although these oxygenated compounds are generally much less volatile than the solvents used to date, the fraction emitted into the atmosphere will be oxidized and contribute to ozone formation in the troposphere. Oxidation can also produce other phototoxic species within the group of photooxidants which affect human health or are phytotoxic. Potential CFC replacements such as HFCs may have sufficiently long lifetimes that they may contribute to the greenhouse effect. These different classes of compounds oxidize in the atmosphere by a complex multi-step mechanism initiated mainly by the OH radical reaction. The efficiency of formation of ozone and possible other toxic species is dependent on the details of the degradation pathways. The rate of the initial OH reaction determine the persistence of the primary VOC in the atmosphere, while the ozone forming potential is a strong function of the degradation mechanism in the troposphere.