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Not all air pollution is caused by people. Both particulate matter and gases are released into the Earth's atmosphere by volcanoes and by forest and grass fires ignited by lightning strikes. Strong winds can lift particles from the surfaces of the oceans and land and can carry them through the air. The process of organic decay can also release certain gases into the atmosphere.
When a volcano erupts, particulate matter, ash and gases are propelled high into the Earth's atmosphere. The strong upward force can cause intermixing between the troposphere and the stratosphere, the two lowest layers of the atmosphere. Materials that reach the upper troposphere and the stratosphere can affect all areas of the Earth and can persist for years. One effect of upper atmospheric debris is the blocking and scattering of radiation from the sun. The debris from a large Indonesian volcanic eruption in 1815 created a drop in global temperature that led to what has been called "the year without a summer."
Forest fires and grass fires caused by lightning strikes are a part of the Earth's natural history and are important factors in the life cycles of many plants and animals. Although they lack the vertical force of volcanoes, major fires can spread ash and smoke over thousands of square miles, and they contribute to the atmospheric load of carbon dioxide. Forest and grass fires also cause pollution by redistributing any industrial pollutants that have been deposited on the plants. This sort of relay effect can help transport pollutants thousands of miles from their sources.
Humans cause about half the air pollution that may hurt us. We have chosen lifestyles that include cars, plastics, appliances and many other items that make our lives easier and more comfortable. From their production we get two forms of ambient air pollution, particulate matter and gases.
Particulate matter is the finely divided solid or liquid material found in a flue gas. Typical terms are dust, fume, smoke, mist, and spray. The terms dispersion aerosol and condensation aerosol are also used.
Particles can be formed by mechanical action, chemical reaction, condensation or atomization. . Incomplete combustion leads to the emission of solid particles of carbon or particles from the unburnable minerals in the fuel.
A characteristic of particles is their size. In any sample of particles obtained from a flue gas, the particles will have different sizes and shapes. Particles formed by condensation or chemical processes are generally spherical. Particles formed in grinding or crushing operations are usually irregular in shape. In sources where particulate emissions are controlled by baghouses or electrostatic precipitators, particle sizes typically range from 0.1 to 1 um.
Particle size also has great implications with regard to human health. Particles greater than 10 um in diameter tend to collect in the upper part of the respiratory system. Here, particles are filtered by nasal hair. Particles in the range of 1 to 10 um in diameter tend to collect in the middle part of the respiratory system, the tracheobronchial region. Particles in the size range of 1 um tend to deposit at locations where the bronchi of the lungs begin to branch off into the smaller bronchioles. Here the particles impact the walls of the bronchi because they can't make the turn into the bronchioles. The smaller, submicron size particles (<1 um) can make the turn and penetrate down into the air sacs (the alveoli, where oxygen is transferred to the blood).
Sulfur Oxides are produced when metal sulfide ores are roasted and fossil fuels containing appreciable inorganic sulfides and organic sulfur are combusted. Of the four known monomeric sulfur oxides, only SO2 is found at appreciable levels in the gas phase in the troposphere. Sulfur trioxide is emitted directly into the atmosphere in ore smelting and fossil fuel combustion and is produced by the oxidation of SO2 in the atmosphere. Because it has a high affinity for water, it is rapidly converted to sulfuric acid.
Nitrogen Dioxide is a colored gas which is light yellowish orange to reddish brown at relatively low and high concentrations, respectively. It has a pungent, irritating odor. It is also relatively toxic and because of its high oxidation rate it is extremely corrosive as well. Because NO2 is colored, it absorbs light. This light absorption results in photolysis which initiates a complex spectrum of photochemical events.
See previous Wind article for information concerning Carbon Monoxide and Ozone.
Volatile Organic compounds happen when we burn fossil fuels in cars, power plants and factories. Chemicals mix with the leftover carbon from the fuel and oxygen in the air to make VOCs. VOCs also come from solvents, paints, glues and the chemicals from dry cleaners. They also escape from factories where chemicals are made. VOCs help make ground-level ozone and other air pollutants. Some of these gases can be toxic and some may cause cancer. They may also damage plants.
Each of these pollutants will be described separately and in more depth over the next few months. Also included will be a discussion of health effects and federal regulations that apply to each pollutant and affected industries.