3. The Changing Atmosphere
The activities of humans during the last two centuries has produced changes in the atmosphere's gas composition that are often more rapid and sometimes of greater magnitude than past natural fluctuations. Some of the more important atmospheric gases that are being altered in their concentration by human activity include:
CO2 - carbon dioxide
CO - carbon monoxide
SO2 - sulphur dioxide
CH4 - methane
NOX - nitric oxide (NO) and nitrogen dioxide (NO 2)
N2O - nitrous oxide
CFxClx - chlorofluorocarbons
O3 - ozone
An examination of Canada's climate data over the last 100 years reveals patterns of change. For example, annual temperatures at most meteorological stations were appreciably colder at the beginning of this century. From the 1920s to the 1950s temperatures became warmer. Colder temperatures returned once again in the 1960s, and continued until the mid-seventies. A warming trend followed these cooler years, and has continued ever since with some of the warmest years in the last century being recorded in the 1980s and 1990s. Finally, these patterns mirror trends found in the global annual temperature records.
Over longer periods of time, non-instrumental climate data also indicates that weather has been quite variable in Canada and across the Earth. For example, over the past 800,000 years, there have been several great ice ages during which 30% of the continental surface of the Earth was covered by ice several kilometers thick. Each glacial period lasted about 100,000 years and was followed by a warmer interglacial period lasting 10,000 to 12,500 years.
For the past 10,000 years, the Earth has been enjoying the warm temperatures of the latest interglacial period. Temperatures today average globally about 15° Celsius. During this period the Earth's average global temperature fluctuated up and down by about 1° Celsius, over short periods of time (less than 500 years). These moderate and relatively slow fluctuations in climate have not led to drastic changes in the Earth's environment.
Changes in the Earth's climate can be the result of the following factors:
The chemical nature of the atmosphere is an important factor in determining the Earth's climate. Heat energy from the sun is trapped in the Earth's lower atmosphere by a natural process called the greenhouse effect. The amount of heat trapped depends primarily on the concentrations of the various greenhouse gases, such as carbon dioxide, water vapor, ozone, methane, and nitrous oxide (Table 1.2). The two greenhouse gases with the largest concentration in the troposphere are carbon dioxide and water vapor. Over the past 160,000 years, estimated levels of water vapor in the lower atmosphere have remained fairly constant while the levels of carbon dioxide have fluctuated naturally with the warming and cooling of the Earth.
Table 1.2 : Gases involved in the Greenhouse Effect: past and present concentration and sources.
Measurements indicate a 25% increase in the concentration of atmospheric carbon dioxide over the last 200 years (Figure 1.2). Much of this increase was caused by human activities such as the burning fossil fuels, conversion of natural habitats into agricultural fields, and deforestation. By the year 2050, scientists predict that carbon dioxide content in the atmosphere may increase by 80% because of the continued practice of these activities by a growing human population.
Studies of future climate indicate that the higher levels of carbon dioxide and other greenhouse gases in the atmosphere will most likely cause the planet to get warmer because of an enhancement of the Earth's greenhouse effect. Over the last century, the Earth has warmed by about 0.6° Celsius, and some scientists suggest that this may be evidence of an enhanced greenhouse effect. Other evidence, possibly indicating global warming, has shown that the arrival of spring has been getting earlier since 1940.
Computer simulations, of the climatic effect of continued increases in the concentration of the atmosphere's greenhouse gases, suggest an increase in the average global temperature by 1 to 3° Celsius by the year 2050. Possible environmental effects, particularly in Canada, that would arise from global warming include:
Finally, there is a lot of controversy over whether global warming is actually taking place. A minority of scientists have made the following challenges to this theory:
Air pollution occurs when the concentrations of certain substances become high enough to cause the atmospheric environment to become toxic. Air pollutants can be gaseous, liquid or solid in form, and can come from natural as well as human sources. Examples of natural sources of air pollution include forest fires, pollen, volcanic emissions, and dust. Human sources of air pollutants include emissions from industry, agriculture, forestry, transportation, power generation, and space heating.
In general, two types of air pollutants have been recognized:
Primary Pollutants: consist of materials (dust, gases, liquids and other solids) that enter the atmosphere through natural and human-made events (Figure 1.5). The main primary pollutants influencing our atmosphere in order of emission (by weight) are carbon monoxide, sulfur oxides, nitrogen oxides, volatile organic compounds, and particulate matter.
Volatile organic compounds are organic molecules that are mainly composed of carbon and hydrogen atoms (hydrocarbons). The most common volatile organic compound release into the atmosphere is methane. Methane poses no direct danger to human health, however, it does contribute to global warming through the greenhouse effect. Other volatile organic compounds release into the atmosphere include benzene, formaldehyde, and chlorofluorocarbons. Of these chemicals, benzene and formaldehyde are the most dangerous to human health because they are carcinogenic.
Particulate matter consists of liquid or solid particles that are small enough to remain suspended in the atmosphere for extended periods of time. Industrial activity and transportation are the major source of this type of air pollution. Particulate matter includes common irritants like smoke, pollen, and dust, which can affect the human respiratory system. In cities, particulate matter may also include particles composed of iron, copper, nickel, and lead. These particles influence the respiratory system immediately, and make breathing difficult for people with chronic respiratory disorders. Airborne lead, formed by the burning of lead gasoline, can accumulate in the tissues and bones of humans and other living organisms. At high levels lead can cause nervous system damage, convulsions, and even death. It is especially dangerous to children and the unborn.
Secondary Pollutants: consist of primary pollutants that have reacted with each other or with the basic components of the atmosphere to form new toxic substances. In cities, the emissions from cars and industries combine with the help of light energy from the sun to produce photochemical smog. Photochemical smog is extremely toxic to animal and plant life, and damages paint, rubber, and plastics.
Finally, air pollution can also occur indoors. In buildings, about 150 different indoor pollutants have been identified. Some of the more common indoor air pollutants include smoke from cigarettes and cooking, radon, formaldehyde, and asbestos. At high concentrations, these pollutants can cause dizziness, headaches, coughing, sneezing, nausea, burning eyes, chronic fatigue, and flu like symptoms. Some indoor pollutants, like asbestos and smoke, can cause disease and premature death.
The Industrial Revolution has been the central cause for the increase of pollutants in the atmosphere over the last three centuries. Before 1950, the majority of this pollution was created from the burning of coal for energy generation, space heating, cooking, and transportation. Under the right conditions, the smoke and sulfur dioxide produced from the burning of coal can combine with fog to create industrial smog. In high concentrations, industrial smog can be extremely toxic to humans and other living organisms. London is world famous for its episodes of industrial smog. The most famous London smog event occurred in December, 1952 when five days of calm foggy weather created a toxic atmosphere that claimed about 4000 human lives. Today, the use of other fossil fuels, nuclear power, and hydroelectricity instead of coal has greatly reduced the occurrence of industrial smog. However, the burning of fossil fuels like gasoline can create another atmospheric pollution problem known asphotochemical smog. Photochemical smog is a condition that develops when primary pollutants(oxides of nitrogen and volatile organic compounds created from fossil fuel combustion) interact under the influence of solar radiation to produce a mixture of hundreds of different and hazardous chemicals known as secondary pollutants. Table 1.3 below describes the major toxic constituents of photochemical smog and their effects on the environment. Development of photochemical smog is typically associated with specific climatic conditions and centers of high population density. Cities like Los Angeles, New York, Sydney, and Vancouver frequently suffer episodes of photochemical smog. In recent years, scientists have also noticed that smaller communities, like Kelowna and Kamloops, can develop similar pollution problems if conditions are right.
Table 1.3: Major Chemical Pollutants in Photochemical Smog: Sources and Environmental Effects
Certain conditions are required for the formation of photochemical smog. These conditions include:
1. A source of nitrogen oxides and volatile organic compounds. High concentrations of these two substances are associated with industrialization and transportation. Industrialization and transportation create these pollutants through fossil fuel combustion.
2. The time of day is a very important factor in the amount of photochemical smog present. The following diagram illustrates the daily variation in the key chemical players. The diagram suggests:
3. Several meteorological factors can influence the formation of photochemical smog. These conditions include:
4. Topography is another important factor influencing how severe a smog event can become. Communities situated in valleys are more susceptible to photochemical smog because hills and mountains surrounding them tend to reduce the air flow, allowing for pollutant concentrations to rise. In addition, valleys are sensitive to photochemical smog because relatively strong temperature inversions can frequently develop in these areas.
The previous section suggested that the development of photochemical smog is primarily determined by an abundance of nitrogen oxides and volatile organic compounds in the atmosphere and the presence of particular environmental conditions. To begin the chemical process of photochemical smog development the following conditions must occur:
If the above criteria are met, several reactions will occur producing the toxic chemical constituents of photochemical smog. The following discussion outlines the processes required for the formation of two most dominant toxic components: ozone (O 3) and peroxyacetyl nitrate (PAN). Note the symbol R represents a hydrocarbon (a molecule composed of carbon, hydrogen and other atoms) which is primarily created from volatile organic compounds.
Nitrogen dioxide can be formed by one of the following reactions. Notice that the nitrogen oxide (NO) acts to remove ozone (O 3) from the atmosphere and this mechanism occurs naturally in an unpolluted atmosphere.
O 3 + NO >>>> NO 2 + O 2
NO + RO 2 >>>> NO 2 + other products
Sunlight can break down nitrogen dioxide (NO 2) back into nitrogen oxide (NO).
NO 2 + sunlight >>>> NO + O
The atomic oxygen (O) formed in the above reaction then reacts with one of the abundant oxygen molecules (which makes up 20.94% of the atmosphere) producing ozone (O 3).
O + O 2 >>>> O 3
Nitrogen dioxide (NO 2) can also react with radicals produced from volatile organic compounds in a series of reactions to form toxic products such as peroxyacetyl nitrates (PAN).
NO 2 + R >>>> products such as PAN
It should be noted that ozone can be produced naturally in an unpolluted atmosphere. However, it is consumed by nitrogen oxideas illustrated in the first reaction. The introduction of volatile organic compounds results in an alternative pathway for the nitrogen oxide, still forming nitrogen dioxidebut not consuming the ozone, and therefore ozone concentrations can be elevated to toxic levels.
Photochemical smog can be a significant pollution problem in the Okanagan Valley. The Okanagan meets all the requirements necessary for the production of photochemical smog, especially during the summer months. During this time period there is an abundance of sunlight, temperatures are very warm, and temperature inversions are common and can last for many days. The Okanagan Valley also has some very significant sources of nitrogen oxides and volatile organic compounds, including:
The idea that the Okanagan is immune to the big city problems of photochemical smog may simply be wishful thinking. In fact, recent monitoring of ground level ozone has shown that the values between here and the Lower Mainland are quite comparable. In addition, research over a 4 year period (1985-1989) has shown that ozone levels can at times be higher over the Okanagan Valley than the Lower Mainland of British Columbia by almost 49%.
The ozone layeris a region of concentration of the ozone (O 3) molecule in the Earth's atmosphere. The layer sits at an altitude of about 10 to 50 kilometers, with a maximum concentration in the stratosphere at an altitude of approximately 25 kilometers. In recent years, scientists have measured a seasonal thinning of the ozone layer primarily at the South Pole. This phenomenon is being called the ozone hole.
The ozone layer naturally shields Earth's life from the harmful effects of the sun's ultraviolet (UV) radiation. A severe reduction in the concentration of ozone in the ozone layer could lead to the following harmful effects:
Ozone is created naturally in the stratosphere by the combining of atomic oxygen (O) with molecular oxygen (O 2). This process is activated by sunlight. Ozone is destroyed naturally by the absorption of ultraviolet radiation,
O 3 + UV >>>> O 2 + O,
and by the collision of ozone with other atmospheric atoms and molecules.
O 3 + O >>>> 2O 2
Since the late 1970s, scientists have discovered that stratospheric ozone amounts over Antarctica in springtime (September - November ) have decreased by as much as 60%. Satellite measurements (NIMBUS 7 - Total Ozone Mapping Spectrometer) have indicated a 3% decrease in ozone between 65 degrees North - 65 degrees South since 1978. A reduction of about 3% per year has been measured at Antarctica where most of the ozone loss is occurring globally. During late 1990s, large losses of ozone were recorded above Antarctica year after year in the months of September and August. In some years, spring levels of stratospheric ozone were more than 50% lower than the levels recorded months prior to the seasonal development of the hole. The following animation describes the change in ozone levels at the South Pole during the period 1978 to 1992:
It appears that human activities are altering the amount of stratospheric ozone. The main agent responsible for this destruction was human-made chlorofluorocarbons or CFCs. First produced by General Motors Corporation in 1928, CFCs were created as a replacement to the toxic refrigerant ammonia. CFCs have also been used as a propellant in spray cans, cleaner for electronics, sterilant for hospital equipment, and to produce the bubbles in styrofoam. CFCs are cheap to produce and are very stable compounds, lasting up to 200 years in the atmosphere. By 1988, some 320,000 metric tons of CFCs were used worldwide.
In 1987, a number of nations around the world met to begin formulating a global plan, known as the Montreal Protocol, to reduce and eliminate the use of CFCs. Since 1987, the plan has been amended in 1990 and 1992 to quicken the schedule of production and consumption reductions. By 1996, 161 countries were participating in the Protocol. The Montreal Protocol called for a 100% reduction in the creation and use of CFCs by January 1, 1996 in the world's more developed countries. Less developed countries have until January 1, 2010 to stop their production and consumption of these dangerous chemicals.
CFCs created at the Earth's surface drift slowly upward to the stratosphere where ultraviolet radiation from the sun causes their decomposition and the release of chlorine ( Cl ). Chlorine in turn attacks the molecules of ozone chemically converting them into oxygen molecules.
Cl + O 3 >>>> ClO + O 2
A single chlorine atom removes about 100,000 ozone molecules before it is taken out of operation by other substances. Chlorine is removed from the stratosphere by two chemical reactions:
ClO + NO 2 >>>> ClONO 2
Normally, these two reactions would quickly neutralize the chlorine released into the stratosphere. However, the presence of polar stratospheric clouds, rich in nitrogen , and sunlight facilitates a series of reactions which prolongs the reactive life of chlorine in the atmosphere. Interestingly, these polar stratospheric clouds require very cold air (approximately -85 degrees Celsius) for their formation. Stratospheric air of this temperature occurs normally every year above Antarctica in the winter and early spring months. Destruction of the ozone begins in Antarctica in the spring as this region moves from 24 hours of night to 24 hours of day. These clouds are less frequent in the Arctic stratosphere because winter cooling of the air in the stratosphere is less severe.
NASA's Earth Probe- Total Ozone Mapping Spectrometer home page has the latest images describing the current status of global stratosphere ozone levels in the atmosphere.
Acidic pollutants can be deposited from the atmosphere to the Earth's surface in wet and dry forms. The common term to describe this process is acid deposition. The term acid precipitation is used to specifically describe wet forms of acid pollution that can be found in rain, sleet, snow, fog, and cloud vapor. An acid can be defined as any substance that when dissolved in water dissociates to yield corrosive hydrogen ions. The acidity of substances dissolved in water is commonly measured in terms of pH (defined as the negative logarithm of the concentration of hydrogen ions). According to this measurement scale a solution with a pH of less than 7 is described as being acidic, while a pH greater than 7.0 is considered alkaline (Figure 1.7 ). Precipitation normally has a pH between 5.0 to 5.6 because of natural atmospheric reactions involving carbon dioxide. Precipitation is considered to be acidic when its pH falls below 5.6 (which is 25 times more acidic than pure water). Some sites in eastern North America have precipitation with pH values as low as 2.3 or about 1000 times more acidic than natural.
Acid deposition is not a recent phenomenon. In the 17th century, scientists noted the ill effects that industry and acidic pollution were having on vegetation and people. However, the term acid rain was first used two centuries later when Angus Smith published a book called 'Acid Rain' in 1872. In the 1960s, the problems associated with acid deposition became an international problem when fishermen noticed declines in fish numbers and diversity in many lakes throughout North America and Europe.
Acid deposition can form as a result of two processes. In some cases, hydrochloric acid can be expelled directly into the atmosphere. More commonly it is due to secondary pollutants that form from the oxidation of nitrogen oxides (NOx) or sulfur dioxide (SO 2) gases that are released into the atmosphere. Reactions at the Earth's surface or within the atmosphere can convert these pollutants into nitric acid or sulfuric acid. The process of altering these gases into their acid counterparts can take several days, and during this time these pollutants can be transferred hundreds of kilometers from their original source. Acid precipitation formation can also take place at the Earth's surface when nitrogen oxides and sulfur dioxide settle on the landscape and interact with dew or frost.
Emissions of sulphur dioxide are responsible for 60-70% of the acid deposition that occurs globally. More than 90% of the sulphur in the atmosphere is of human origin. The main sources of sulphur include:
After being released into the atmosphere, sulphur dioxide can either be deposited on the Earth's surface in the form of dry deposition or it can undergo the following reactions to produce acids that are incorporated into the products of wet deposition ( Figure 1.8):
SO 2 + H 2 O >>>> H 2 SO 3
SO 2 + 1/2O 2 >>>> SO 3 + H 2 SO 4
Some 95% of the elevated levels of nitrogen oxides in the atmosphere are the result of human activities. The remaining 5% comes from several natural processes. The major sources of nitrogen oxides include:
Acids of nitrogen form as a result of the following atmospheric chemical reactions (Figure 1.8):
NO + 1/2O 2 >>>> NO 2
2NO 2 + H 2 O >>>> HNO 2 + HNO 3
NO 2 + OH >>>> HNO 3
Finally, the concentrations of both nitrogen oxides and sulphur dioxides are much lower than atmospheric carbon dioxide which is mainly responsible for making natural rainwater slightly acidic. However, these gases are much more soluble than carbon dioxide and therefore have a much greater effect on the pH of the precipitation.
Acid deposition influences the environment in several different ways. In aquatic systems, acid deposition can affect these ecosystems by lowering their pH. However, not all aquatic systems are affected equally. Streams, ponds, or lakes that exist on bedrock or sediments rich in calcium and/or magnesium are naturally buffered from the effects of acid deposition. Aquatic systems on neutral or acidic bedrock are normally very sensitive to acid deposition because they lack basic compounds that buffer acidification (Figure 1.9 ). In Canada, many of the water bodies found on the granitic Canadian Shieldfall in this group. One of the most obvious effects of aquatic acidification is the decline in fish numbers. Originally, it was believed that the fish died because of the increasing acidity of the water. However, in the 1970s scientists discovered that acidified lakes also contained high concentrations of toxic heavy metals like mercury, aluminum, and cadmium. The source of these heavy metals was the soil and bedrock surrounding the water body. Normally, these chemicals are found locked in clay particles, minerals, and rocks. However, the acidification of terrestrial soils and bedrock can cause these metals to become soluble. Once soluble, these toxic metals are easily leached by infiltrating water into aquatic systems where they accumulate to toxic levels.
In the middle latitudes, many acidified aquatic systems experience a phenomenon known as acid shock. During the winter the acidic deposits can build-up in the snowpack. With the arrival of spring, snowpack begins to melt quickly and the acids are released over a short period of time at concentrations 5 to 10 times more acidic than rainfall. Most adult fish can survive this shock. However, the eggs and small fry of many spring spawning species are extremely sensitive to this acidification.
The severity of the impact of acid deposition on vegetation is greatly dependent on the type of soil the plants grow in. Similar to surface water acidification, many soils have a natural buffering capacity and are able to neutralize acid inputs. In general, soils that have a lot of lime are better at neutralizing acids than those that are made up of siliceous sand or weathered acidic bedrock. In less buffered soils, vegetation is effected by acid deposition because:
The combination of these effects can lead to plants that have reduced growth rates, flowering ability and yields. It also makes plants more vulnerable to diseases, insects, droughts and frosts.
The effects of acidic deposition on humans can be divided into three main categories. Acid deposition can influence human health through the following methods:
Acid deposition also influences the economic livelihoods of some people. Many lakes and streams on the eastern coast of North America are so acidic that the fish decline significantly in numbers. The reduced fish numbers then influence commercial fishermen and industries that rely on the sport fishing tourism. Forestry and agriculture are effected by the damage caused to vegetation. In some areas of eastern North America and Europe, large die-backs of trees have occurred.
Finally, acid deposition effects a number inanimate features of human construction. Buildings and head stones that are constructed from limestone are easily attacked by acids, as are structures that are constructed of iron or steel. Paint on cars can react with acid deposition causing fading. Many of the churches and cathedrals in Europe are under attack from the effects of acidic deposition.
The acid deposition problem in Canada tends to be localize east of the Manitoba border with Ontario. This area of Canada has a large number of industries that release acid causing substances into the atmosphere. This area is also influenced by pollution from industrial areas in eastern United States. Research has shown that almost half of the acid deposition that falls in southeastern Canada is due to combustion from factories in seven states: Ohio, Indiana, Pennsylvania, Illinois, Missouri, West Virginia, and Tennessee.
In general, high levels of aid deposition in eastern Canada have lead to the following environmental problems:
However, studies in the province of British Columbia have shown that at the present time there is no noticeable environmental damage is occurring from acid deposition. A habitat sensitivity map of the province British Columbia, based on the pH of water bodies and ability of soils and bedrock to neutralize acid deposition, have been constructed by the Ministry of Environment, Lands and Parks. This map indicates that habitats in the Rockies and central British Columbia have the highest potential to neutralize acidic inputs. Habitats on the west coast of Vancouver Island and coastal mountainous regions have the lowest potential for neutralization.
Large quantities of acid-forming pollutants are created in British Columbia. Most of the nitrogen oxide emissions are created from the combustion of fuels by vehicles. Many industries in British Columbia release sulphur dioxide into the atmosphere including natural gas processing plants, pulp mills, smelters, oil refineries, and power plants. Data from recordings stations in several cities indicate that sulphate deposition levels from these sources are well below the eastern Canadian standard developed for the protection of moderately sensitivity aquatic systems (Figure 1.10).
However, some researchers have suggested that a target value of 8-12 kilograms of wet sulphate deposition per hectare per year should be put in place to protect more sensitive habitats. Measurements of acidity indicate that precipitation falling in British Columbia generally has a pH similar to naturally occurring rainwater with no pollutants (Figure 1.11). Finally, a province wide survey of over 750 lakes between 1977 and 1986 found acidic 10 lakes. Subsequent investigations of these 10 lakes revealed that their acidic nature was due to natural causes.
There are several things that can be done in order to alleviate the problems of acid deposition. For lakes that have been acidified, the pH can be increased by a technique called liming. This process involves adding large quantities of hydrated lime, quick lime or soda ash to the waters in order to increase the alkalinity and pH. Areas that have employed this method have had some success with it. In West Wales, the pH of some lakes was increased from 5.5 to 7.0 and once again brown trout stocks can survive there. Liming, however, does not always work, as getting to the necessary lake may be impossible, the lake may be too big and therefore economically unfeasible, or the lake may have a high flush rate so that they quickly become acidified again after liming.
The best overall solution to the problems of acid deposition is to limit the emission of pollutants at their source. In eastern Canada, environmental regulations now limit the amount of sulfuric pollution that can now enter the atmosphere from industrial sources. Industrials have limited their emission of acidic pollutants through two methods. Many industries have switched to using fuels that have no sulphur or a low sulphur content. Other industries have used scrubbers installed on smokestacks to reduce the amount of sulphur dioxide being released into the atmosphere. The application of these two methods has created some promising results. For example, the once acidified Clearwater Lake near Sudbury, Ontario is now on its way to recovery. The pH of this lake dropped to 4.1 before regulations were put in place. By 1986, the pH of the lake measured 4.7. The reduction in nitrogen oxides is a more difficult problem to deal with because this type of acidic pollution is primarily created from automobile exhaust. A drastic reduction in number of motor vehicles used in eastern Canada over the next few decades seems unlikely. However, emissions from this non-point source could be controlled by regulating the use of specially designed catalytic converters.
In 1991, Canada and the United States established the Air Quality Accord that controls the air pollution that flows across international boundaries. In this agreement, acid deposition causing emissions are permanently capped in both countries (13.3 million tonnes for the U.S. and 3.2 million tonnes of sulphur for Canada) and plans were implemented for the reduction of nitrogen oxides.