Composition of Atmosphere of Earth, its Importace and Structure
There are three types of constituents in the atmosphere: major, minor, and trace. For pollution-free dry air at ground level, the composition may be expressed as percent by volume. The parameters of the atmosphere vary considerably with altitude. The density of the atmosphere shows a sharp decrease with increasing altitude.
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The Earth’s atmosphere plays a vital role in sustaining life on this planet by acting as the protective blanket that supports life on earth. It is a source of oxygen, carbon dioxide, and nitrogen. Oxygen is needed for respiration by animals, plants, and some bacteria. Nitrogen is an inert gas that limits the quantity of oxygen accessible for natural material oxidation, hence limiting spontaneous combustion (burning) of flammable materials and metal corrosion. “Nitrogen-fixing” bacteria use nitrogen to generate chemicals that are beneficial to plant growth. Photosynthesis is the process through which plants absorb carbon dioxide from the air and release oxygen. Carbon dioxide and water vapor are “greenhouse gases,” which keep the Earth warm enough for life to thrive. Water vapor in the air is a component of the water cycle, which results in precipitation (such as rain and snow) that refills soil moisture. Water vapor also keeps exposed live tissue from drying out. As a basic part of the hydrologic cycle, the atmosphere transports water from the ocean to land, thus acting as the condenser in a vast solar-powered still. Unfortunately, the atmosphere also has been used as a dumping ground for many of the pollutants- ranging from Sulphur dioxide to refrigerant Freon-a a practice that causes damage to vegetation and other materials, shortens human life, and alters the characteristics of the atmosphere itself.
Furthermore, many parts of the atmosphere provide protection at a distance. The atmosphere absorbs the majority of cosmic rays from space, shielding organisms from their harmful effects. It also absorbs most of the electromagnetic radiation from the sun, allowing transmission of significant amounts of radiation only in the regions of 300-2500 nm (near UV, visible, and near IR radiation) and 0.01-40 m (radio waves). For instance, the ozone layer absorbs UV radiation below 300 nm that can damage the tissues and genetic material of living organisms. Furthermore, because it reabsorbs much of the IR radiation by which absorbed solar energy is re-emitted to space, the atmosphere stabilizes the earth’s temperature. The mesosphere, which burns up millions of comets per day, shields the Earth’s surface from the constant bombardment of these falling objects. The magnetosphere, which extends well beyond the atmosphere, shields the earth from the solar wind’s harmful downpour of charged particles.
The atmosphere is separated into four distinct zones, each of which has its own characteristics. following are the Composition of Atmosphere of Earth
The troposphere is the region nearest to the ground or earth’s surface. It contains 70% of atmospheric mass. Temperature decreases with height. The average temperature drops from 15°C at sea level to -56.5°C at 11 km above sea level. Mixing of the air molecules due to their constant movement (winds) keeps the composition of the gases more or less homogenous throughout the troposphere. The troposphere contains 99 % of the water vapors in atmosphere. Most water vapors evaporate from the surface of the Earth and are found in the lower troposphere. The troposphere is where most of the weather happens. When it comes to density and temperature, air is far from uniform. With rising altitude, density falls exponentially. With rising altitude, the temperature in the troposphere lowers equally. The cold layer (-56 °C) at the top of the troposphere is tropopause
The lapse rate is the rate at which temperature changes with height. A negative lapse raterefers to the drop in temperature as altitude increases in the troposphere. The temperature inversion occurs when the lapse rate changes from negative to positive at the tropopause.
The region above the tropopause is called the stratosphere. It is the quiescent layer having a positive lapse rate. The temperature increases with an increase in altitude, until 50 km with a maximum of -2 °C, at the upper limit of the stratosphere. Temperature increases in this region due to ozone which absorbs solar UV radiations. Ozone is present in the stratosphere that forms a protective shield against hostile outer space environments. The air in this region is dry and clouds in the troposphere normally do not penetrate it. This region reflects sound waves from the earth’s surface back into the environment. The stratosphere is the second transition layer and is relatively warmer.
Because of the slow mixing in the stratosphere, the residence times of molecules or particles in this region are quite long. If the pollutants can reach or inject into the stratosphere, they pose ling term global hazards compared to their impact in the much denser troposphere. The ozone layer also serves as a source of heat for separating the quiescent stratosphere from the turbulent troposphere. The region immediately above the stratosphere (above 50 m height) is called stratopause, which is the second transitional layer that is relatively warm.
The mesosphere is the region above the stratosphere where temperature decreases with height; it exhibits a negative lapse rate. This decrease in temperature is due to low levels of ozone and other UV absorbing species in this region. The average temperature decreases to 92°C at an altitude of around 85-90 km. Mesopause is the top of the mesosphere, which is the coldest temperature in the atmosphere (about -100°C) and is the transition layer between Mesosphere and Thermosphere.
The thermosphere is the region above the mesosphere. The temperature in the thermosphere increases with increasing height, the maximum attainable temperature is 1200°C but there are not many molecules in this layer. Atmospheric gases (e.g., Oxygen and nitric oxide) in this region absorb solar radiation and undergo ionization. The region from 50-100 km is called ionosphere due to the presence of positive ions (O2+, O+, NO+) and electrons. The air becomes less and less dense as we reach space.