| The above illustration demonstrates the decline of visibility in measurable forms called deciviews (DV), extinction coefficient (Bext), and visual range (V.R.) in kilometers (one kilometer=0.6 mile) |
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Primer on Air Pollution 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. Types of Air Pollution 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. 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 chlorofluoro-carbons. 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. Photochemical Smog (a). Introduction The burning of fossil fuels like gasoline can create another atmospheric pollution problem known as photochemical 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. The table 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 photo-chemical smog but in recent years, scientists have also noticed that smaller communities can develop similar pollution problems if conditions are right. Major chemical pollutants in photochemical smog: Sources and environmental affects.
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: Early morning traffic increases the emissions of both nitrogen oxides and VOCs as people drive to work. Later in the morning, traffic dies down and the nitrogen oxides and volatile organic compounds begin to react forming nitrogen dioxide, increasing its concentration. As the sunlight becomes more intense later in the day, nitrogen dioxide is broken down and its byproducts form increasing concentrations of ozone .At the same time, some of the nitrogen dioxide can react with the volatile organic compounds to produce toxic chemicals such as peroxyacetyl nitrates (PAN). As the sun goes down, the production of ozone is halted . The ozone that remains in the atmosphere is then consumed by several different reactions.3. Several meteorological factors can influence the formation of photochemical smog. These conditions include: Precipitation can alleviate photochemical smog as the pollutants are washed out of the atmosphere with the rainfall. Winds can blow photochemical smog away replacing it with fresh air. However, problems may arise in distant areas that receive the pollution. Temperature inversions can enhance the severity of a photochemical smog episode. Normally, during the day the air near the surface is heated and as it warms it rises, carrying the pollutants with it to higher elevations. However, if a temperature inversion develops pollutants can be trapped near the Earth's surface. Temperature inversions cause the reduction of atmospheric mixing and therefore reduce the vertical dispersion of pollutants. Inversions can last from a few days to several weeks. 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. (c). Chemistry of Photochemical Smog To begin the chemical process of photochemical smog development the following conditions must occur or be available: Solar radiation The production of oxides of nitrogen (NOx) The production of volatile organic compounds (VOCs) Temperatures greater than 18 degrees Celsius (64 degrees F) 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 (O3) 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. (1) Nitrogen dioxide can be formed by one of the following reactions. Notice that the nitric oxide (NO) acts to remove ozone (O3) from the atmosphere and this mechanism occurs naturally in an unpolluted atmosphere. O3 + NO »»» NO2 + O2 NO + RO2 »»» NO2 + other products (2) Sunlight breaks down nitrogen dioxide (NO2). NO2 + sunlight »»» NO + O (3) 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 (O3). O + O2 »»» O3 (4) Nitrogen dioxide (NO2) can also react with radicals produced from volatile organic compounds in a series of reactions to form toxic products such as peroxyacetyl nitrates (PAN) NO2 + R »»» products such as PAN It should be noted that ozone can be produced
naturally in an unpolluted atmosphere. However, it is consumed by
nitric oxide as illustrated in the first reaction.
The introduction of volatile
organic compounds in part (b)1 results in an alternative pathway for
the nitric oxide. Moreover, the introduction of VOCs at this stage
allows for the formation of nitrogen dioxide and not the consumption
of the ozone. As a result, ozone concentrations can be elevated to
toxic levels. |
© 2007, Ronald P. Walker
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