Composition of the Atmosphere

The Earth's atmosphere is made up of approximately 78 percent nitrogen, 21 percent oxygen, and small amounts of argon, carbon dioxide, neon, helium, krypton, hydrogen, ozone and traces of other gases. Although its composition is the same at sea level and at great heights, except for water vapor and some impurities, the air becomes less dense at higher altitudes. In addition to these gases, the atmosphere is also full of inanimate matter such as dust, soot, volcanic ash, salt crystals from sea spray and fine sand grains raised by storms in the deserts. In addition, two to three thousand tons of cosmic dust falls from space and into the Earth's atmosphere each day. The atmosphere is full of life. An enormous number of plants and animals, including human beings, live in the densest bottom layer. It is a zone full of flowering, breathing and growing creatures. There is also an abundance of life above this layer. Up to a height of about six miles, the air is full of suspended bacteria, fungal spores and pollen grains. Between 2500 and 4500 feet, a cube of air the size of a lump of sugar contains an average of about 70 microorganisms, including mold spores, cocci and yeast species.

Formation of Clouds

Function of the Atmosphere

The atmosphere, a thin shell of gases surrounding the Earth, is a very effective protective shield that reflects and absorbs harmful radiation and objects like meteorites. The atmosphere protects the Earth from receiving too much radiation from the Sun. The Sun is a vast ball of exploding gases – mostly hydrogen that is converted to helium through a nuclear reaction. At its surface, the Sun reaches a temperature of 6,000 °C at the surface. The Sun's heat and light travel almost 150 million kilometers (93 million miles) passing Mercury and Venus, until reaching the Earth's atmosphere. An extremely important layer in the atmosphere lies between 25 and 50 kilometers (15 and 31 miles) above the ground. This is rich in a gas called ozone. The ozone layer filters out ultraviolet light from the Sun. Below this, the atmosphere becomes much more mixed. The air that we breathe is made up of 78.09 percent nitrogen, 20.95 percent oxygen, 0.93 percent argon, with the remaining 0.03 percent consisting of carbon dioxide and many other gases. The progress of this energy coming from the Sun is hampered even further in the 10 kilometers (6 miles) just above the Earth's surface – the area in which our weather exists. Clouds, water surfaces, snow and ice reflect energy back from the Earth. Ultimately, only one in every 2,000 million parts of the Sun's energy reaches the Earth. Without an atmosphere, the Earth would be extremely hot during the day and extremely cold at night. It has been calculated that if the Earth's atmosphere disappeared, temperatures at the equator would reach 80 °C during the day and fall to –140 °C at night.

Particles in the Atmosphere

Even when the air seems to be completely clear, it is full of atmospheric particles – invisible solid and semisolid bits of matter, including dust, smoke, pollen, spores, bacteria and viruses. Some atmospheric particles are so large that you will feel them if they strike you. However, particles this large rarely travel far before they fall to the ground. Finer particles may be carried many miles before settling during a lull in the wind, while still tinier specks may remain suspended in the air indefinitely. The finest particles are jostled this way and that by moving air molecules and drift with the slightest currents. Only rain and snow can wash them out of the atmosphere. These tiny particles are so small that scientists measure their dimensions in microns – a micron is about one 25-thousandth of an inch. They include pollen grains, whose diameters are sometimes less than 25 microns; bacteria, which range from about 2 to 30 microns across; individual virus particles, measuring a very small fraction of a micron; and carbon smoke particles, which may be as tiny as two hundredths of a micron.

Particles We Breathe

Particles are frequently found in concentrations of more than a million per cubic inch of air. A human being's daily intake of air is about 450,000 cubic inches. This means that we inhale an astronomical numbers of foreign bodies. Particles larger than about 5 microns are generally filtered from the air in the nasal passages. Other large particles are caught by hairlike protuberances in the air passages leading to the lungs and are swept back toward the mouth. Most of the extremely fine particles that do reach the lungs are exhaled again – although some of this matter is deposited in the minute air sacs within the lungs. From these air sacs, particles may go into solution and pass through the lung walls into the bloodstream. If the material is toxic, harmful reactions may occur when it enters the blood. Fine particles retained in the lungs can cause permanent tissue damage, as with Coalworkers' pneumoconiosis (black lung disease), caused by buildup of coal dust in the lungs, and with silicosis, which is caused by the buildup of silicon dust.

Microorganisms

Once, physicians were taught that infectious microorganisms quickly settle out of the air and die. Today, the droplets ejected, say, by a sneeze, are known to evaporate almost immediately, leaving whatever microorganisms they contain to drift through the air. Only a relatively small fraction of the microorganisms that human beings breathe cause disease. In fact, most bacteria are actually helpful. Some, for example, convert atmospheric nitrogen into usable plant food. Pathogenic, or disease-producing, microorganisms, however, can be very dangerous. Most propagate by subdivision-each living cell splits into two cells. Each of the new cells then grows and divides again into two more cells. Provided with ideal conditions, populations multiply quickly. Fortunately microorganisms do not thrive very well in the air. Unless there is enough humidity in the air, many desiccate and die. Short exposure to the ultraviolet radiation of the Sun also kills most microorganisms. Low temperatures greatly decrease their activity, and elevated temperatures destroy them rapidly. Still, many microorganisms survive in the air, despite these hazards. Among the tiniest of airborne particles are viruses, which are on the borderline between living matter and lifeless chemical substances.

Thickness of Ozone Layer

The ozone layer is a layer in Earth's atmosphere that has a high concentration of ozone molecules. The average thickness of the ozone layer is 50km. Although, if it was compressed to near sea level conditions, the ozone layer would only be 3 mm thick on average. The ozone layer protects us from harmful sun radiation. Protection given by the ozone layer has decreased due to man-made compounds released such as CFCs. The concentration of ozone in the ozone layer has decreased by about 3 % due to man-made emissions.

Exercises