Chapter 1. Introduction

Transcription

1 Chapter 1 Introduction The Earth is surrounded by an envelope of gases which provide many functions including, protection from harmful radiation, moderating the surface temperature and providing a medium (which we call air) that allows organisms to exchange gases in order to survive (breathing). Any substantial change in the nature or contents of the atmosphere has a direct consequence on how well the atmosphere performs these tasks. Air is of fundamental significance to our existence. If the quality of the air we breath is degraded then our health will directly suffer and our standard of living decrease. This is one of the great paradoxes of modern society. In general the higher the degree of sophistication of society (some might call this civilisation), the lower the quality of the air that they are exposed to. Indeed the lifestyle and extent of civilisation of societies on the Earth directly relate to the type of atmospheric degradation which is present. Historically air pollutants of greatest concern have been total suspended particulates (TSP), and oxides of sulfur, but as our processing industries become more sophisticated the list of significant pollutants grows. Now we commonly include oxides of nitrogen and photochemical oxidants (smog & ozone) as routine pollutants, and often include particulate lead, asbestos, mercury, sulfuric acid and many others that require careful monitoring. Concern over the atmospheric concentrations of substance not normally considered pollutants has also become much greater in the latter part of the twentieth century. A good example of this is CO 2. The levels of this gas have increased by as much as 5% over the last two decades of the twentieth century. Most of this is thought to be due to the combustion of fossil fuels. This has been targeted as a major environmental problem by most of the world s nations and strategies have been put in place to reduce the amount being discharged into the atmosphere. It is expected that the levels of CO 2 in the atmosphere will peak around 2040 so the problems associated with degradation of the atmosphere in general will take a long time to fix. This needs to be considered by nations that are emerging as economic powers. Often environmental concerns are dismissed in the interest of short-term profit a strategy which does not work in the long term as may be seen by the example of East Germany which required many billions of dollars to fix up pollution problems after re-unification with West Germany. The Earth s atmosphere has never been completely pure. It has always contained waste materials from many natural sources such as bushfires, decay from plant and animal life, and windblown dust particles to mention a few sources. Pollutants also arise from unexpected natural sources. For example rainforests are regarded as essential for the health of the atmosphere in that they remove CO 2 and replace it with O 2. For this reason

2 they are thought of as the Earth s lungs. Rainforests and other types of forests also produce natural hydrocarbon products however, which can undergo photochemical oxidation to produce pollutant haze which we would term air pollution. This formation of wastes is part of the cycling and recycling of matter in natural ecological systems. Wastes released into the atmosphere are diluted and dispersed in the air, and are processed and recycled through a variety of natural physical, chemical and biological mechanisms. For example, many particles of waste are removed by settling or are washed out by rain. Many gaseous wastes are oxidised to form particles, which then settle out or are washed out while others are converted to other gases in the atmosphere or absorbed by plants or other soil micro-organisms. In this way the atmosphere is continually cleaned of these substances. Atmospheric problems are made worse when weather conditions such as a lack of rain or wind cannot disperse pollutants. This not only holds them in the air, increasing human exposure, but also allows them to undergo chemical reactions to produce secondary pollutants that are often far more dangerous than those substances from which they were made. The residence time of waste products in the atmosphere before they are broken down or brought back to the ground varies between a few hours and many years. Air pollution problems occur whenever any of these normal functions of the atmosphere are disturbed or overloaded. Global Emissions form Natural & Man Made Sources % Emissions from Natural Sources % Emissions from man made sources 20 0 SOx CO NOx NH3 H/C Dust Pollutant Figure % of emissions from global and man made sources

3 Figure 1.1 shows that compared to natural sources, man s activities produce a much smaller amount of global pollution. Indeed natural pollutants may pose a serious threat to air quality when they are generated in large amounts near human settlements. Examples of this include dust storms, bush fires and volcanic eruptions. Natural pollution generally has a low impact on human wellbeing as the levels of pollutants associated with most natural pollutants are relatively low, the sources are generally well separated from large human populations and natural forces responsible for the pollution occur at infrequent intervals. One major exception to this rule is the biogenic emission of photochemically active hydrocarbons such as isoprene, α-pinene and other terpene molecules. These natural hydrocarbons play a major role in the formation of photochemical smogs, and are major contributors to haze. This is especially a problem in Australia where heat produces large amounts of eucalyptus haze from gum trees. This does not mean that pollution from man made sources is not a problem. A closer look at air pollution will show that dispersal of pollutants is a very important consideration as the atmosphere is not homogeneous. This means that pollutants tend to concentrate in specific areas most of which are near where large human populations reside. This means that pollutant levels around residential areas are often much greater than would be expected in ambient air. Natural sources on the other hand are in general more evenly spread, but there are exceptions such as extremely high levels of dust and acidic gases associated with volcanic activity. Man's activities (also called anthropogenic) release heat, gases, aerosols and other wastes into the atmosphere. These are particularly significant in that our wastes are discharged in such high concentrations that we overload the natural dispersal, dilution and recycling systems on a local, regional and global scale which causes damage to plants, animal life, and materials. The ecological systems themselves are degraded, and the services they provide are reduced. In addition to this overload due to increased concentrations of waste products in particular areas, we are now releasing quantities of new or previously rare substances into the environment. Very little is known about the dispersal processes and the passage through ecological systems of these substances. Many are resistant to degradation, some are cumulative and harmful. Air pollution is not an easy thing to define. The World Health Organisation defines air pollution as Air is polluted when one or several pollutants are present in the atmosphere at such a concentration and for so long a time that they are harmful to man, animals, plants or material property, cause harm or reduce well-being or disturb appreciably its application. Whilst this is fairly complete in its cover, it is also very complex hence simpler definition are often used. The definition of air pollution for legal purposes is defined in the NSW Protection of the Environment Operations Act as any deviation from the natural combination of gases in our atmosphere. What this definition fails to mention is that the natural combination of gases in our atmosphere must be taken as dry air at sea level. This is necessary as it is not possible to quantitatively define pure air because it will change according to altitude and location. This also means that theoretically air pollution can also arise from the removal of gases from the atmosphere.

4 Neither of the definitions listed above completely cover other factors that we might also call pollution such as the release of energy, radiation, odour or noise. Most air pollution concerns are associated with ambient air, that is air that is outdoors and free flowing hence most control programs focus on ambient air pollution, but significant pollution now occurs in occupational environments which are indoors. Especially important here are the effects of cigarette smoke which may contain large amounts of carbon monoxide and extremely toxic polycyclic aromatic hydrocarbons. The Atmosphere The Earth s atmosphere is approximately 160 kilometers deep, but 95% of its air mass lies within 20 kilometers of the surface. Moving up from the earth s surface the density of the atmosphere decreases rapidly and the air thins. Hence the atmosphere is neither uniform, nor static in nature. Its characteristics vary widely with altitude, season, location and solar flare activity. Air within a few kilometers of the earth s surface will typically contain the components shown in table 1.2. The most obvious conclusion that may be drawn from this table is that the pollutants with which we have the most problems make up an extremely small part of the atmosphere. Figure 1.1 shows sulfur dioxide to be the air pollutant produced in the greatest amount by a long way, yet it forms an insignificant proportion of the atmosphere (a fortunate thing from our point of view). The figures in Table 1.1 are distorted somewhat by the fact that in polluted city areas these % concentrations will change markedly for some pollutants. A few other important facts should be stated about the data in Table 1.1. The concentrations of nitrogen, oxygen, argon, neon, helium, krypton, hydrogen and xenon remain essentially constant. These gases are fairly inert and play little or no role in atmospheric chemistry. Table 1.1 Components of a normal atmosphere Element % (by volume) in the atmosphere Total Mass in the Atmosphere (x10 12 tonnes) nitrogen oxygen argon carbon dioxide Neon Helium Methane

5 Krypton Carbon Monoxide Ozone Nitrogen Dioxide Sulfur Dioxide water (normal range 1-3) Varies according to location Even though it is the most abundant gas, nitrogen has little effect on major atmospheric processes or to the sustenance of life forms. It does however, serve as a precursor for other species such as NO - 3, as well as amino acids and nucleic acids (amongst others) which are essential for life. Nitrogen reacts with oxygen the second most abundant gas in the atmosphere to form oxides of nitrogen (NO x ). These include NO, NO 2, N 2 O 4 and N 2 O. The NO x compounds in the atmosphere are found in trace levels and their concentration varies with place and time of day. Nitrous oxide (N 2 O) was thought to remain constant in atmospheric concentration, but synthetic fertiliser compounds containing nitrogen have been found to increase the amount of nitrification by soil bacteria, which has in turn increase the amount of N 2 O being generated by natural sources. As the great majority of species on Earth use an oxidative metabolism, it comes as no surprise that oxygen is the most important atmospheric gas for the nurturing of life. Its significance as an atmospheric gas goes far beyond sustaining life however. Oxygen is also present in the atmosphere as ozone (O 3 ), which exists in high concentrations in the upper atmosphere and acts as a heat and radiation shield for the planet maintaining fairly constant temperatures that allow life to exist. At 0.035%, the concentration of carbon dioxide in the atmosphere is very low compared to molecular oxygen or nitrogen. It is still of enormous significance however as it is the raw material used by plants for carbon fixation to produce the compounds used for energy by almost all forms of life. It is also a significant greenhouse gas which serves to keep the planet warm. Water vapour is the most variable of all atmospheric components (varies from ,000ppm), but it is also of huge significance. Water rapidly changes phase from liquid to gas or solid in response to atmospheric conditions. This means that it allows the transport of energy around the planet. Water vapour also condenses to form clouds that are responsible for the Earth s albedo the ability of the Earth to radiate sunlight back into space which is another factor in controlling the Earth s surface temperature. The atmosphere also contains trace gases produced from biological or geological processes. These include ammonia, methane, hydrogen sulfide, carbon monoxide and sulfur dioxide. Ammonia, methane and hydrogen sulfide are primarily produced by bacterial decomposition, whilst carbon monoxide and sulfur dioxide are produced mostly by geological processes.

6 To help understand the significance of air pollutants requires an analysis of what a person breathes. The average person breathes 20,000 litres of air per day, 995 of which is nitrogen or oxygen. The other 1% is a mixture of gases and particulates, many of which might be termed pollutants. This means that we breathe as much as 200litres of pollutants per day!

7 Stratification of the Atmosphere As we move away from the Earth s surface the temperature drops (as well as the pressure). This causes stratification or layering of the atmosphere The lowest layer of the atmosphere is known as the troposphere, and extends to an area of between 10 and 16 kilometres above the earth. This is where our weather and almost all of our pollution problems occur. Its composition fairly homogeneous due to the weather producing circulating air masses that allow constant mixing. 95% of the atmosphere s air mass is found in the troposphere. The upper troposphere has a temperature of -56ºC. At the top of the troposphere is the tropopause layer. It serves as a barrier to prevent water vapour rising much higher as it causes ice formation. Water vapour cannot pass through it. If it did, it would photodissociate in contact with ultraviolet radiation, and the hydrogen would be lost to space. The tropopause separates the stratosphere from the troposphere. Above the tropopause layer is the stratosphere, between 10 and 50 km above the earth.. Moving up in this layer we find the temperature rises, and may reach up to -2ºC., The ozone layer is within the stratosphere, and reaches levels of up to 10ppm in the middle of the stratosphere. The stratosphere can reach temperatures of up to -2 C due to the absorption of ultraviolet radiation by the ozone. The mesosphere is above the stratosphere, covering an area of between 50 and 85km above the earth. At 85km the temperature is -92 C. Above the mesosphere is an area called the exosphere where molecules and ions can be lost to space. An area extending far into space is called the thermosphere, an area of high temperatures (1200 C) due to the absorption of radiation from space. Figure 1.2 The layers of the atmosphere

8 History of Air Pollution Air pollution probably became a serious problem from the time man discovered how to use fire. The smoke which is associated with the incomplete combustion of hydrocarbon materials was probably as much a problem to cave men as it is today. Reports of air pollution and decimation of forests have been recorded since the fourteenth century. These were mostly linked with the burning of soft coal with a high sulfur content. This activity produces smoke, sulfur dioxide and particulate matter containing hydrocarbons. By the late nineteenth century London had become famous for its "pea souper" fogs. It was around this time that the word "smog" was coined to describe the thick combination of smoke from combustion, and fog which often rolled in from the North Sea. These smogs were marked by extreme reductions in visibility, strong odours of sulfurous gases and generally increase rates of illness and death. This type of smog is grey in colour and is generally referred to as London type smog. When the term smog is used today it is broadly applied to any pollution condition associated with a significant reduction in visibility - it does not pollutant type, source or the inclusion of fog. By the mid-twentieth century the first smog related deaths were occurring. In 1930 in the Meuse Valley of Belgium a temperature inversion trapped pollutant gases above the deep river valley where an industrial area was housed. The excessively high levels of air pollution resulted in 60 deaths. In 1948, 21 people were killed in Donora, Pennsylvania, by heavy smog. A temperature inversion trapped fog and pollutants in the basin in which the town was located for four days. Most victims died of lung and breathing disorders. Of the survivors, 7000 of the total population of became seriously ill. London suffered a similar disaster in 1952 when a "killer smog" descended on the city for more than a week. In this case the smog was so heavy that visibility during daylight hours was cut to only a few meters. SO 2 concentrations were raised to seven times their normal level, whilst CO was twice the normal level. During this period the death rate in London soared dramatically with the number of deaths occurring in the city closely following the concentration of sulfur dioxide in the atmosphere. It is estimated that this event killed 4000 people. These events lead to the development of anti-pollution regulations (such as the Clean Air Acts), which are still being modified to this day. There has been a great reduction in smoke in cities in the western world in the past few decades. This is due to the replacement of coal as a source of energy by fuels such as natural gas and oil. Smog still remains a serious problem however, as motor vehicle and industrial emissions have pumped many tonnes of hydrocarbons into the atmosphere and these in turn have produced photochemical smogs. These are particularly serious in cities such as Los Angeles, Tokyo and Sydney. Los Angeles type smogs is like the London type smog in that it produces a significant reduction in visibility, but the conditions which produce this smog are very different to those in London. In Los Angeles the climate is semi-arid, with many hot sunny days. It is also part of weather system that causes air to descend over the city greatly restricting the dispersal and movement of air. The very large number of motor vehicles used in the Los Angeles area produce relatively large amounts of nitrogen dioxide and unburnt

9 hydrocarbons, which then react in the atmosphere in the presence of UV light and oxidants to form a brown photochemical smog. Haze or Smog? One term that is often used in association with air pollution is haze, but how is it different to smog? Both smog and haze are typified by s reduction in visibility, but the difference is the intensity varies. Haze refers to a condition where the reduction in visibility is not great, and is generally applied to describe the atmospheric conditions over a very large area. Smog on the other hand is characterised by significant reductions in visibility - generally in metropolitan areas. Factors Likely to increase the Levels of Air Pollution Certain conditions help make air pollution worse. Basically these are factors which prevent air circulation, and concentrate air effluent into areas. Examples of these include; calm conditions low level emission sources temperature inversions high buildings and narrow streets By contrast other conditions are known to lower air pollution levels in general. These are those conditions that encourage circulation or remove pollutants from the atmosphere. Examples include; windy or turbulent conditions high levels of vegetation high level emission sources such as smoke stacks rain The Protection of the Environment Operations Act This act specifies all legal requirements for the control of air pollution in NSW. It attempts to reduce air pollution by prescribing standards for emission of pollutants. These are generally linked to accepted world health standards used in other countries. Some premises are licensed under the act to pollute. This does not mean that air pollution is encouraged, rather it is a means of controlling emission from major sources such as power stations etc.. These premises are charged a fee for a licence to discharge pollutants, the levels of which are carefully monitored. Any contravention of the licence means that the company responsible will be charged in the Land & Environment Court and generally fined.