Composition and Structure of the Earth’s Atmosphere

 

Earth is the only planet in the solar system with an atmosphere that can sustain life as we know it. Earth’s atmosphere not only contains the air that we breathe but also protects it from the blasts of meteors that are constantly being hurtled towards it. The pock-marked surface of the moon bears testimony to the fact that it is being constantly bombarded with meteorites in the absence of a protective atmosphere. The earth’s surface, by contrast, is relatively free of these craters on two counts –  all but the largest of them burn up in the atmosphere before reaching the surface, and the craters formed by those that do, are quickly eroded by weather generated in the atmosphere, and the evidence is washed away. The earth’s atmosphere also protects us from the blasts of heat and radiation emanating from the sun.

The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at an altitude of about 120 km, above which the atmosphere becomes too thin to support aircraft flight, is often thought to constitute the border between the atmosphere and outer space. However, atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km. At sea level, air pressure is about 1 kg/cm2, and decreases rapidly with altitude. At an altitude of 3 km the air pressure is about 0.7 kg/cm2. There is also very little oxygen at this altitude.

The earth's atmosphere also acts as a protective shield by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation). Were it not for the earth’s atmosphere, its daytime temperatures would be very high and night time temperatures very low. The atmosphere warms the planet by day and cools it at night.

Air, the chief constituent of the atmosphere, was held by the ancient Greek thought to be one of the four elementary substances (along with earth, fire and water) constituting the universe. However, by the early 1800s, it was recognized that the atmosphere was actually made up of several chemically distinct gases, notably nitrogen, oxygen and small amounts of argon. The development of spectrometry in the 1920s led to the discovery of much smaller concentrations of ozone and carbon dioxide. It was also discovered that the concentrations of these gases, while small, varied widely from place to place and time to time.

Composition of Earth’s Atmosphere:

Atmospheric gases are reckoned in two categories: major gases and trace gases. The major gases in Earth's atmosphere include nitrogen (78%), oxygen (21%), water vapour (1-4%) and argon (0.93%). Trace gases include carbon dioxide (380 ppm), neon (18 ppm), helium (5 ppm), methane (1 ppm), krypton (1 ppm), hydrogen (0.5 ppm), nitrous oxide (0.3 ppm), sulphur dioxide (0.1 ppm), and ozone (0.04 ppm). It is also an established fact that the major constituents of the atmosphere (except water vapour) remain more or less constant through time and space whereas that of the minor constituents varies with time and place. The Table below summarizes the gaseous composition of Earth’s atmosphere.

Constituent

Chemical symbol

Mole percent

Nitrogen

N2

78.084

Oxygen

O2

20.947

Argon

Ar

0.934

Carbon dioxide

CO2

0.0350

Neon

Ne

0.001818

Helium

He

0.000524

Methane

CH4

0.00017

Krypton

Kr

0.000114

Hydrogen

H2

0.000053

Nitrous oxide

N2O

0.000031

Xenon

Xe

0.0000087

Ozone*

O3

trace to 0.0008

Carbon monoxide

CO

trace to 0.000025

Sulfur dioxide

SO2

trace to 0.00001

Nitrogen dioxide

NO2

trace to 0.000002

Ammonia

NH3

trace to 0.0000003

 

Ref: Mackenzie, F.T. and J.A. Mackenzie (1995) Our changing planet. Prentice-Hall, Upper Saddle River, NJ, p 288-307.

 

Although both nitrogen and oxygen are essential to human life on the planet, they have little effect on weather and other atmospheric processes. The variable components, which make up far less than 1% of the atmosphere, have a much greater influence on both short-term weather and long-term climate. For example, variations in water vapor in the atmosphere are familiar to us as relative humidity. Water vapor, CO2, CH4, N2O, and SO2 all have an important property – they absorb heat emitted by Earth and thus warm the atmosphere, creating what we call the “greenhouse effect”. Without these so-called greenhouse gases, the Earth's surface would be about 30 degrees Celsius cooler – too cold for life to exist as we know it. By their percentage contribution to the greenhouse effect on Earth the four major gases are: water vapour (36–70%), CO2 (9–26%), CH4 (4–9%) and O3 (3–7%).

In addition to gases, the atmosphere also contains particulate matter such as dust, volcanic ash, rain, and snow. These are, of course, highly variable and are generally less persistent than gas concentrations, but they can sometimes remain in atmosphere for relatively long periods. The volcanic explosion of Krakatoa (west of Sumatra in Indonesia) in August 1883 threw up 20 cubic km of pulverized rock in the atmosphere which remained suspended in the atmosphere for the next 3 years. Volcanic ash from the 1991 eruption of Mt. Pinatubo in the Philippines, darkened skies around the globe for over a year.

Though the major components of the atmosphere vary little today, they have changed dramatically over Earth's history, (about 4.6 billion years). The early atmosphere was hardly the life-sustaining blanket of air that it is today; most geologists believe that the main constituents then were nitrogen gas and carbon dioxide, but no free oxygen. In fact, there is no evidence for free oxygen in the atmosphere until about 2 billion years ago, when photosynthesizing bacteria evolved and began taking in atmospheric carbon dioxide and releasing oxygen. The amount of oxygen in the atmosphere has risen steadily from 0% 2 billion years ago to about 21% today.

Structure of the Atmosphere:

The Earth’s atmosphere is divided into several concentric strata or layers. About 99% of the total atmospheric mass is concentrated in the first 30 km above Earth's surface. Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

Earth's atmosphere is divisible into five main layers – the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. The atmosphere thins out in each higher layer until the gases dissipate in space. As stated earlier, there is no distinct boundary between the atmosphere and space, but an imaginary surface about 120 km  from the surface, called the Karman line, which is usually considered by scientists to be the upper limit of the atmosphere.

1.      The troposphere is the layer closest to Earth's surface. It is 4 to 12 miles (7 to 20 km) thick and contains half of Earth's atmosphere. Air is warmer near the ground and gets colder higher up. Nearly all of the water vapor and dust in the atmosphere are in this layer and that is why clouds are found here.

2.      The stratosphere is the second layer. It starts above the troposphere and ends about 31 miles (50 km) above ground. Ozone is abundant here and it heats the atmosphere while also absorbing harmful radiation from the sun. The air here is very dry, and it is about a thousand times thinner here than it is at sea level. Because of that, this is where jet aircraft and weather balloons fly.

3.      The mesosphere starts at 31 miles (50 km) and extends to 53 miles (85 km) high. The top of the mesosphere, called the mesopause, is the coldest part of Earth's atmosphere with temperatures averaging about minus 130 degrees F (minus 90 C). This layer is hard to study. Jets and balloons don't go high enough, and satellites and space shuttles orbit too high. Scientists do know that meteors burn up in this layer.

4.      The thermosphere extends from about 56 miles (90 km) to between 310 and 620 miles (500 and 1,000 km). Temperatures can get up to 2,700 degrees F (1,500 C) at this altitude. The thermosphere is considered part of Earth's atmosphere, but air density is so low that most of this layer is what is normally thought of as outer space. In fact, this is where the space shuttles flew and where the International Space Station orbits Earth. This is also the layer where the auroras occur. Charged particles from space collide with atoms and molecules in the thermosphere, exciting them into higher states of energy. The atoms shed this excess energy by emitting photons of light, which we see as the colorful Aurora Borealis and Aurora Australis.

5.      The exosphere, the highest layer, is extremely thin and is where the atmosphere merges into outer space. It is composed of very widely dispersed particles of hydrogen and helium.

The figure below shows temperature variations within different layers of the Earth’s atmosphere.

 Role of the Atmosphere in Climate and weather:

Earth is able to support a wide variety of living beings because of its diverse regional climates, which range from extreme cold at the poles to tropical heat at the Equator. Regional climate is often described as the average weather in a place over more than 30 years. A region's climate is often described, for example, as sunny, windy, dry, or humid. These can also describe the weather in a certain place, but while the weather can change in just a few hours, climate changes over a longer span of time.

Earth's global climate is an average of regional climates. The global climate has cooled and warmed throughout history. Today, we are seeing unusually rapid warming. The scientific consensus is that greenhouse gases, which are increasing because of human activities, are trapping heat in the atmosphere.

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