Ozone is a relatively unstable form of molecular oxygen containing three oxygen atoms (O3) and its production occurs in two layers of the atmosphere, i.e. troposphere and stratosphere. The production of ozone in low-level atmosphere (troposphere) is regarded negatively because the ozone in the layer comes from pollutants, which result from industrial activities, transport, and some natural sources. On the other hand, the natural production of ozone in the stratosphere is, however, necessary because it plays a key role in protecting life on Earth from the ultraviolet rays of the sun.
The problem we are facing now is the decreasing level in the stratosphere and the increasing level of ozone in the troposphere due to anthropogenic activities. This piece of writing will attempt to discuss the deteriorating conditions occurred in stratospheric and tropospheric layers as the result of anthropogenic activities, and how it affects life on Earth. Thus the writing will be divided into two sections. The first part will describe the condition of stratospheric ozone layer, i.e. the natural process of ozone production and destruction, polar winter, and the effects of ozone destruction. The last section will identify the production of tropospheric ozone caused by pollutants the resulted effects.
Stratosphere is a part of atmospheric layer above the troposphere and extends from approximately 10 km to 50 km above the Earth and about 90 percent of Earth’s ozone production occurs in this layer. It is naturally formed and destroyed at a constant rate. Ozone molecules are constantly created in chemical reactions caused when the ultraviolet radiation from the sunlight strikes the stratosphere. The theory of the formation and depletion of ozone molecules is known as the Chapman’s theory.
Apart from their destruction by absorbing ultraviolet light, ozone molecules are also constantly being destroyed by natural compounds containing nitrogen, which comes from soil and the ocean, hydrogen that comes mostly from atmospheric water vapour, and chlorine from the oceans, also, it is destroyed when absorbing ultraviolet light.
The average amount of ozone in the stratosphere remains fairly constant when the creative and destructive forces occur naturally. But this natural ozone has gradually been depleted in recent decades by various human activities that release ozone-destroying chemicals into the atmosphere. The chemicals released into the atmosphere by industrial activities include chlorocarbon compounds (e.g. CCl4 and CH3Cl3), chlorofluorocarbon compounds or CFCs (e.g. CFCl3 and CF2Cl2) and halon compounds (e.g. CF3Br and CF2ClBr). Most of these chemical substances will remain unchanged long enough to drift up to the stratosphere because they are chemically stable compounds containing halogen atoms, i.e. chlorine or bromine.
CFCs are used for a wide variety of industrial purposes, e.g. in refrigeration systems, air conditioners, aerosols, solvents and in the production of some types of packaging, because they are relatively inexpensive, highly effective, stable in the atmosphere and non-toxic to humans. But once CFCs reach the stratosphere, the solar ultraviolet radiation will strike them and release chlorine (Cl), which acts as a catalyst. The chlorine atom repeatedly combines with and breaks apart ozone molecules and forming the single oxygen molecule and one chlorine monoxide molecule (ClO). Then, the chlorine monoxide molecule can combine with an oxygen atom to form an oxygen molecule and release the chlorine to begin the process all over again. Through this cycle, one chlorine atom can destroy up to 100.000 ozone molecules and deplete ozone much faster than nature can replace it. Other common industrial solvents, which contains lower chlorine such as methyl chloroform and carbon tetrachloride and halons containing bromine act in a similar way.
In Antarctica, the ozone-depleting compounds have worsened the stratospheric ozone condition for the few decades. The key ingredients for polar ozone losses are high chlorine and bromine levels, cold temperatures during the late winter, and relative isolation of the polar region from the midlatitudes. During winter in the austral or southern hemisphere (May-July), the stratospheric air over the Antarctic gets colder than the air anywhere else because it is isolated from warmer, lower-altitude air by a strong set of confining winds known as the polar cortex. The polar cortex isolates the air until it gets so cold down to -80°C and then develops polar stratospheric clouds (PSCs). Through chemical reactions, particle surfaces in these clouds can create more reactive halogen chemicals. Thus when the first light of austral spring (August-November) reaches these molecules, they break apart and release massive levels of the chlorine and bromine atoms that drive catalytic destruction cycles.
In the northern hemisphere, the Arctic polar vortex is much weaker than the Antarctic vortex. Also, the temperatures in the northern hemisphere are substantially warmer during midwinter and it remains warm by early spring. Because of these warmer temperatures, the formation of PSC is much less there. As a result, heterogeneous chemistry and the activation of chlorine are much less frequent in northern hemisphere. Therefore, the warmer temperatures of arctic pole prevent the massive losses of ozone compared to the Antarctic pole.
As the stratospheric ozone layer is depleted, higher ultraviolet radiation reaches the earth’s surface and harms human health, freshwater and marine ecosystems, reduce crop yields, and affect forests. The impacts of the increased UV levels include the increasing cases of skin cancers, cataracts, and impaired immune systems; decreasing growth of some crops, such as canola, barley, oats and soybeans; decreasing the amount of single-celled plants, known as phytoplankton in the ocean, which could ultimately affect fish populations; reducing the construction materials used outdoor; increasing the resistance of trees that grow at higher elevations, where UV is naturally stronger.
In order to cope with the stratospheric ozone depletion, a series of international agreements on the reduction and elimination of production and use of ozone depleting substances are implemented, e.g. the Montreal Protocol. These efforts will result in recovery of the ozone layer in the next 50 years.
Tropospheric Ozone
Located 0-15 km above the Earth, the troposphere is known as the ground-level atmosphere layer where all weather takes place. The ozone produced in this ground level is known as bad ozone. This ground-level ozone has two major sources, i.e. intrusion from the stratosphere and production from photochemical reactions. The tropospheric ozone plays several key roles in the atmosphere because although it oxidises many chemical substances in troposphere and controls tropospheric chemistry, it is also a gaseous pollutant harmful for human being and crops; it oxidises many chemical substances in troposphere and controls tropospheric chemistry; and it is a green house gas that contributes to the global warming.
Tropospheric ozone is formed by the interaction of sunlight, particularly ultraviolet light, with volatile organic compounds (VOCs) and nitrogen oxides (NOx), which are emitted by automobiles, gasoline vapours, fossil fuel power plants, refineries, and certain other industries. VOCs are organic compounds that contain carbon and hydrogen and can evaporate easily, e.g. octane, butane and sugar, which are mostly emitted by motor vehicles, vegetation, industry, commerce, dry cleaners, and paints. While, nitrogen oxides like nitric oxide (NO) and nitrogen dioxide (NO2) are released into the atmosphere when fossil fuels like oil, coal and natural gas are burned. It is mostly emitted by motor vehicles, power plants, industrial facilities, biomass burning and lighting. When released into the atmosphere, both VOCs and NOx can produce ozone and other harmful pollutant that lead to smog, which is sometimes called as photochemical smog or photochemical air pollution. The air pollutants typically found in smog include oxidized organic compounds (usually aldehydes, e.g. formaldehyde) ozone, sulphur oxides, and nitrogen oxides.
As mentioned previously, tropospheric ozone is also a greenhouse gas (GHG) because it absorbs upward directed terrestrial radiation. It plays a key role in atmospheric chemistry. As a strong oxidant, it affects the lifetimes and the concentrations of most atmospheric trace gases, including CH4 and the replacements of the CFCs such as hydro chlorofluorocarbons (HCFCs) and hydro fluorocarbons (HFCs), which, in turn, are greenhouse gases that have implications for global climate.
The increased ground-level ozone causes some harmful impacts to humans, plants, and materials. Tropospheric ozone is harmful to human health because it reacts readily with the membranes of the eye and those lining the lung's air passages. It may cause human health problems, e.g. eye irritation, breathing problems, lung damage, coughing, wheezing, chest tightness or pain, dry throat, headache or nausea, intensification of asthma symptoms, reduced resistance to infection; reduced functioning of the immune system, tired feeling, and increased cardio-respiratory deaths. Also, the ozone can harm plants, trees, and crops by preventing the plant from being able to use the sun's energy by reacting with the molecular links between the carbon atoms (called the carbon-carbon bonds) in the plant's photosynthetic mechanism. Furthermore, ground-level ozone affects materials by deteriorating and reducing the strength of products made of rubber and certain fabrics because ozone is a strong oxidant.
The problem of tropospheric ozone is not easy to solve. Tighter controls on vehicle and industrial emissions of NOx and VOCs and a reduction in biomass burning and traffic levels would reduce emissions of ozone precursors and ozone formation.
The problems of stratospheric ozone depletion and tropospheric ozone production are mostly due to anthropogenic activities that release manmade chemicals containing millions of tonnes of ozone depleting substances and tones of air pollutants. Those activities are not only causing ozone problems, but also harmfully affect human’s health and plants. In order to avoid those harmful effects, a series of international agreements to reduce the pace of ozone depletion in stratosphere and ozone production in troposphere have been held, e.g. the 1985 Vienna Convention on the Protection of the Ozone Layer and the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, which are arranged to freeze and decrease the production CFC to certain levels.
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