AIR POLLUTION

 

Air pollution is the presence of chemicals in the atmosphere in concentrations high enough to harm organisms, ecosystems, or human-made materials, or to alter climate. Note that almost any chemical in the atmosphere can become a pollutant if it occurs in a high enough concentration. The effects of air pollution range from annoying to lethal.

Sources:

Air pollutants come from natural and human

sources. 

Natural sources include wind-blown dust, pollutants from wildfires and volcanic eruptions, and volatile organic chemicals released by some plants. Most natural air pollutants are spread out over the globe or removed by chemical cycles, precipitation, and gravity. But in areas experiencing volcanic eruptions or forest fires, chemicals emitted by these events can temporarily

reach harmful levels.

Most human inputs of outdoor air pollutants occur in industrialized and urban areas with their higher concentrations of people, cars, and factories. These pollutants are generated mostly by the burning of fossil fuels in power plants and industrial facilities (stationary sources) and in motor vehicles (mobile sources). However, human inputs began long before the industrial era, and have steadily increased.

Types of pollutants:

Scientists classify outdoor air pollutants into two categories:

Primary pollutants are chemicals or substances emitted directly into the air from natural processes and

human activities at concentrations high enough to cause harm. 

While in the atmosphere, some primary pollutants react with one another and with other natural components of air to form new harmful chemicals, called secondary pollutants.

Major air pollutants:

Carbon monoxide can combine with hemoglobin in red blood cells, which prevents the normal binding of oxygen with hemoglobin molecules. This in turn reduces the ability of blood to transport oxygen to body cells and tissues. Long-term exposure can trigger heart attacks and aggravate lung diseases such as asthma and emphysema. At high levels, CO can cause headache, nausea, drowsiness, confusion, collapse, coma, and death.

Carbon dioxide (CO2) is a colorless, odorless gas. About 93% of the CO2 in the atmosphere is the result of the natural carbon cycle. The rest comes from human activities, mostly the burning of fossil fuels and the clearing of CO2-absorbing forests and grasslands. Until recently it has not been classified in many countries as an air pollutant. But there is considerable and growing scientific evidence that increasing levels of CO2 caused by human activities are contributing to atmospheric warming and projected climate change, because CO2 is being added to the atmosphere faster than it is removed by the natural carbon cycle. This can contribute to human health problems such as heat exhaustion and to the reduction of food supplies in some areas, while causing water shortages, prolonged drought, or

excessive flooding in other areas.

Nitric oxide (NO) is a colorless gas that forms when nitrogen and oxygen gas react under high-combustion temperatures in automobile engines and coal-burning power and industrial plants. Lightning and certain bacteria in soil and water also produce NO as part of the nitrogen

cycle. 

In the air, NO reacts with oxygen to form nitrogen dioxide (NO2), a reddish-brown gas. Collectively, NO and NO2 are called nitrogen oxides (NOx). Some of the NO2 reacts with water vapor in the air to form nitric acid (HNO3) and nitrate salts (NO3–), components of harmful acid deposition. Both NO and NO2 play a role in the formation of photochemical smog—a mixture of chemicals formed under the influence of sunlight in cities with heavy traffic. 

Nitrous oxide (N2O), a greenhouse gas, is emitted from fertilizers and animal wastes, and is produced by the burning of fossil fuels. At high enough levels, nitrogen oxides can irritate the eyes, nose, and throat, aggravate lung ailments such as asthma and bronchitis, suppress plant growth, and reduce visibility when they are converted to nitric acid and nitrate salts.

Sulfur dioxide (SO2) is a colorless gas with an irritating odor. About one-third of the SO2 in the atmosphere comes from natural sources as part of the sulfur cycle. The other two-thirds (and as much as 90% in some urban areas) come from human sources, mostly combustion of sulfur-containing coal in power and industrial plants, oil refining, and smelting of sulfide ores.

Factors decreasing air pollution:

Five natural factors help reduce outdoor air pollution.

First, particles heavier than air settle out as a result of gravitational attraction to the earth. 

Second, rain and snow partially cleanse the air of pollutants. 

Third, salty sea spray from the oceans washes out many pollutants from air that flows from land over the oceans. 

Fourth, winds sweep pollutants away and mix them with cleaner air. 

Fifth, some pollutants are removed by chemical reactions. For example, SO2 can react with O2 in the atmosphere to form SO3, which reacts with water vapor to form droplets of H2SO4 that fall out of the atmosphere as acid precipitation.