HUMAN IMPACT ON THE
MODIFICATION OF THE ATMOSPHERE
Human activities are increasingly altering the Earth’s climate. Human impacts on the climate system include increasing concentrations of atmospheric greenhouse gases (e.g., carbon dioxide, chlorofluorocarbons and their substitutes, methane, nitrous oxide, etc.), air pollution, increasing concentrations of airborne particles, and land alteration. A particular concern is that atmospheric levels of carbon dioxide may be rising faster than at any time in Earth’s history, except possibly following rare events like impacts from large extraterrestrial objects.
Atmospheric carbon dioxide concentrations have increased since the mid-1700s through fossil fuel burning and changes in land use, with more than 80% of this increase occurring since 1900. Moreover, research indicates that increased levels of carbon dioxide will remain in the atmosphere for hundreds to thousands of years. It is virtually certain that increasing
atmospheric concentrations of carbon dioxide and other greenhouse gases will cause global surface climate to be warmer.
Atmospheric changes induced by man may be grouped as:
a) Pollutants in the Atmosphere
To city-dwellers the most obvious way in which man has affected the atmosphere is through pollution. Pollutants include particulate matter, both solid and liquid particles, and gaseous substances such as sulphur dioxide (SO2), oxides of nitrogen (NO, NO2, NO3), carbon monoxide (CO) and hydrocarbon compounds. But not all man-made pollution comes from cities. Isolated industrial activities frequently create a footprint of atmospheric pollution in areas of countryside downwind from the industrial site: particularly infamous examples in Britain include smelters and brickworks. Mining and quarrying activities also send large amounts of mineral dust into the air. Even man-induced forest and grass fires as well as bonfires, can greatly add to particulate pollution at certain times of year.
Atmospheric pollutants are conducted upward from the emission sources by rising air currents as part of the normal convective processes. Larger particles settle under gravity and return to the ground as fallout. Smaller suspended particles are brought to the Earth by precipitation as washout. By a combination of the two processes the atmosphere tends to be cleaned of pollutants, and in the long run a balance is achieved between the input and output of pollutants, although there are large fluctuations in the quantities stored in the air at a given time. Pollutants are also eliminated from the air over their source areas by winds which disperse the particles into large volumes of clean air in the downwind direction. Smoke stacks are intended to take as much advantage of this as possible. The passage of a cold front accompanied by strong winds is usually very effective in sweeping away pollutants from an urban area, but during stagnant anticyclonic conditions concentrations may rise to high values, sometimes producing a smog.
Once in the atmosphere, the primary pollutants undergo a number of chemical reactions, generating a secondary group of pollutants. For example, sulphur dioxide combines with oxygen and
suspended water droplets to produce sulphuric acid. This acid is harmful to organic tissues and is also very corrosive. Photochemical reactions are brought about by the action of sunlight: for example, sunlight acting on nitrogen oxides and organic compounds produces ozone. Another toxic chemical produced by photochemical action is ethylene.
The harmful effects of atmospheric pollution on plant and animal life are manifold. For example, there are many technologies or devices burn wood, coal, or oil inside buildings such as woodstoves, boilers, furnaces, ovens and heaters. When these devices are used, they must be probably vented to the outside because the gases that result from combustion can have a serious impact on the ability of humans to breathe.
Carbon monoxide is one such gas that often results from combustion and it is becoming more common for carbon monoxide monitors or alarms to be installed within homes and buildings. Carbon monoxide, or CO, is a colorless, odorless gas that results from incomplete combustion or burning of fuel. Normally, the atmosphere contains a very small amount of carbon monoxide, about 200 parts per billion (ppb), or .02 parts per million (ppm). If the concentration of carbon monoxide in the air a person breathe increases slightly to 9 parts per million, the person may begin to have difficulty breathing. A healthy person may be just barely affected by CO exposure of 9 ppm, but older individuals and asthmatics, whose lung function may be already compromised, are likely to feel a greater level of effect. Carbon monoxide reduces the ability of the body’s blood to absorb oxygen. It is also colorless and odorless making detection difficult. Inhaling low levels of carbon monoxide can result in fatigue and chest pain, particularly in individuals with chronic heart disease. Increased exposure to CO can result in headaches, dizziness, sleepiness, nausea, vomiting, and disorientation. At very high levels, inhalation of carbon monoxide can cause loss of consciousness and death. An increase from .02 to 9 ppm in carbon monoxide may seem like a large relative increase, but a change of this magnitude is a change of only 0.000088% in the total concentration of gases in the air we breathe. In addition to carbon monoxide, there are many other chemicals, substances, and gases which can be harmful to human health. These chemicals, substances, or gases as a group are called indoor air pollutants. Indoor pollutants are
not as easily dispersed or diluted as outdoor pollutants are. As a result, concentrations can often be many times higher than outdoors. Indoor pollution occurs in a wide range of indoor environments including homes, schools, factories, office buildings, and commercial workplaces. Excessive noise, dust, odors and fumes can all serve to lower worker productivity and adversely affect human health. Pollutants found indoor include asbestos, biological contaminants, formaldehyde, fumes from household products, lead, nitrogen dioxide, particulates, pesticides, radon, and tobacco smoke.
Environmental Tobacco Smoke (ETS) is a major
source of indoor air pollution because it contains carbon monoxide, formaldehyde and many other harmful gases. ETS is often referred to as ‘second
hand smoke’ and the exposure to ETS is called
‘passive smoking’. Some building materials like asbestos, furnishings and household products like aerosol sprays, adhesives, paints etc. release volatile organic pollutants continuously which cause diseases ranging from scamming of lung tissues to visual disorder, abdominal cancer and memory impairment. Other sources includes, use of unvented stoves or space heater, solvents for cleaning products and housekeeping also release pollutants intermittently.
Further the number of automobiles is increasing day by day and has become a cause of air pollution and degradation of the environment. The automobile, with its internal combustion engine, emits poisonous gases that are harmful to human health and is the most serious pollution of the technological age. Exhaust emissions from diesel engines include carbon monoxide, hydrocarbon oxides, organic acids etc. The two primary pollutants are carbon monoxide and nitrogen dioxide, both of which are extremely poisonous gases. Lead is a toxic compound and its main source in the environment is believed to be from leaded gasoline used as fuel for internal combustion engines. The presence of lead in the atmosphere is a threat to the environment as well as for all living organisms.
The rapid rate of industrialization has resulted in more and more air pollution. Various industrial processes release almost all types of pollutants into the air. Some industries like cement, iron and steel, fertilizer, petrochemical, etc. are of great concern because of the difficulty in controlling the emission of pollutants from them. Acid rain has become a great threat to the
environment. The use of solvents is increasing with the growing use of paints, spray, polish, etc. Due to presence of hydrocarbons in these materials, air pollution is caused which is dangerous for health. Similarly, spray of pesticides in agriculture is also responsible for air pollution even in rural areas.
b) Changes in Atmospheric Gas Levels
Of the main natural constituent gases in the atmosphere, carbon dioxide (CO2) and oxygen (O2) are the most critical from an environmental viewpoint; both are inextricably involved in the biochemical cycles between atmosphere and the surface of the Earth. Although nitrogen comprises four fifths of the atmosphere, its inert chemical nature relegates it to a minor role in this respect. Oxygen and carbon dioxide are naturally added to the atmosphere by ‘outgassing’ from the Earth’s interior. The work of plants has been essential in removing carbon dioxide from the atmosphere and storing it as coal and other fossil organic substances. Before the Industrial Revolution, carbon dioxide levels appear to have been about 290 parts per million (p.p.m.) in the atmosphere. But in the last hundred years or so, this amount has increased by about ten per cent, largely because of man’s use of fossil fuels. It has been suggested that, in contrast to the effect of solid particles, an increased level in carbon dioxide content will increase the temperature of the atmosphere, since the gas is an absorber of long-wave radiation and will lead to global warming.
It has been pointed out also that man’s large- scale combustion of hydrocarbon fuels requires a large quantity of oxygen to be withdrawn from the atmosphere and converted into carbon dioxide and water vapour. There is therefore the possibility of a lowering of the oxygen content of the atmosphere to levels which might have a detrimental effect on animal life.
Changes in water vapour levels brought about by man through combustion and alterations to the vegetation cover could in theory markedly affect global radiation and heat balances in the same manner as changes in carbon dioxide levels. But water vapour content varies greatly from place to place and it is difficult to measure global changes. It seems unlikely that there would be a general build-up of excess atmosphere water vapour through combustion, as it would rapidly return to the oceans as
precipitation. A special case, however, is the emission of water vapour and various other substances by jet aircraft. These emissions occur in the stratosphere, where the water vapour content is normally very small. The condensation trails (contrails) of aircraft can often be observed to spread laterally and develop into cirrus clouds. These clouds are highly reflective and can have an effect on the Earth’s albedo.
Gases that contribute to the greenhouse effect include:
Water vapour: The most abundant greenhouse gas, but importantly, it acts as a feedback to the climate. Water vapour increases as the Earth’s atmosphere warms, but so does the possibility of clouds and precipitation, making these some of the most important feedback mechanisms to the greenhouse effect.
Carbon dioxide (CO2): A minor but very important component of the atmosphere, carbon dioxide is released through natural processes such as respiration and volcano eruptions and through human activities such as deforestation, land use changes, and burning fossil fuels. Humans have increased atmospheric CO2 concentration by a third since the Industrial Revolution began. This is the most important long-lived “forcing” of climate change.
Methane: A hydrocarbon gas produced both through natural sources and human activities, including the decomposition of wastes in landfills, agriculture, and especially rice cultivation, as well as ruminant digestion and manure management associated with domestic livestock. On a molecule-for-molecule basis, methane is a far more active greenhouse gas than carbon dioxide, but also one which is much less abundant in the atmosphere.
Nitrous oxide: A powerful greenhouse gas produced by soil cultivation practices, especially the use of commercial and organic fertilizers, fossil fuel combustion, nitric acid production, and biomass burning.
Chlorofluorocarbons (CFCs): Synthetic compounds of entirely of industrial origin used in a number of applications, but now
largely regulated in production and release to the atmosphere by international agreement for their ability to contribute to destruction of the ozone layer. They are also greenhouse gases.
c) Ozone Depletion
Ozone is formed in the stratosphere when oxygen molecules photo-dissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This converts a single O2 into two atomic oxygen ions. The atomic oxygen ions then combine with separate O2 molecules to create two O3 molecules. Further these ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom “recombines” with an ozone molecule to make two O2 molecules.
O + O3
→
2O2 chemical equation The overall amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination.But because of increasing man-made pollution and the release of gases like CFCs (chlorofluorocarbons) the natural balance is broken and the amount of ozone destroyed is far higher than the amount naturally formed.
The average amount of ozone in the atmosphere is about 300 Dobson Units, ozone holes are the areas where the concentration drops to an average of about 100 Dobson Units.
The Antarctic ozone hole is an area of the Antarctic stratosphere in which the ozone levels
drops to as low as 33% of their pre-1975 values. The ozone hole occurs during the Antarctic spring, from September to early December, as strong westerly winds start to circulate around the continent and create an atmospheric container.
Before the initiation of Antarctica Spring season the Polar winters are dark, consisting of three months without solar radiation (sunlight). The lack of sunlight decreases the temperature around or below “80 °C. These low temperatures form cloud particles polar stratospheric clouds (PSCs).
There are three types of PSC clouds — nitric acid trihydrate clouds, slowly cooling water-ice clouds, and rapid cooling water-ice (nacerous) clouds — that provide surfaces for chemical reactions that lead to ozone destruction.
Usually most of the chlorine in the stratosphere resides in stable “reservoir” compounds, primarily hydrochloric acid (HCl) and chlorine nitrate (ClONO2). But during the Antarctic winter and spring, reactions on the surface of the polar stratospheric cloud particles convert these “reservoir” compounds into reactive free radicals (Cl and ClO). However the reactions are slow due to absence of sunlight during winters.
During the spring season, the sun energy drives photochemical reactions and melts the polar stratospheric clouds, releasing the trapped compounds which lead to ozone depletion at its maximum.
Whereas as the temperature increases with time the polar stratospheric clouds (PSCs) are destroyed, the ozone depletion process shuts down, and the ozone hole closes.