The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth’s gravity. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth’s surface, absorbing ultraviolet solar radiation, warming the surface through heat retention, and reducing temperature extremes between day and night, which is sometimes referred to as the diurnal temperature variation.

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.

Air composition, temperature, and atmospheric pressure vary with altitude, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth’s troposphere and in artificial atmospheres. The atmosphere has a mass of about 5.15×10¹⁸ kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) 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 100 km (62 mi), or 1.57% of Earth’s radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

Atmospheric Levels

Talk about the different layers of Earth’s atmosphere! How are they defined?!? Physical Properties!

• The Exosphere is the outermost layer of Earth’s atmosphere, which extends from 700km above sea level to 10,000km above sea level. This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen, and carbon dioxide. At these levels above sea level, there are no meteorological phenomena that occur; however, there are opportunities for the aurora to occur in the lower altitudes of this atmospheric layer. 
• The Thermosphere extends from 80km to a height of 500-1000 km above the surface, which varies due to solar activity. The lower part of this layer contains the ionosphere. This layer is completely free of water and clouds, but can sometimes be host to the aurora.
• The Mesosphere extends 50 km to the height of 80–85 km above sea level.
• The Stratosphere extends from the top of the troposphere at roughly 12 km above Earth’s surface to the stratopause at an altitude of about 50 to 55 km. This layer contains the ozone layer.
• The Troposphere is the lowest layer of Earth’s atmosphere, which extends to the height of about 12 km and also contains about 80% of the mass of Earth’s atmosphere. The troposphere is denser than all its overlying atmospheric layers because a larger atmospheric weight sits on top of the troposphere and causes it to be most severely compressed. Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth’s weather takes place. This layer has basically all the weather-associated cloud genus types generated by active wind circulation.

Secondary Atmospheric Layers

Within the five principal layers above, that are largely determined by temperature, several secondary layers may be distinguished by other properties:

Ozone Layer is contained within the stratosphere and ozone concentrations are about 2 to 8 parts per million.
Ionosphere is a region of the atmosphere that is ionized by solar radiation. It is responsible for auroras.
The magnetosphere is a region of space surrounding an astronomical object in which charged particles are manipulated or affected by that object’s magnetic field
The planetary boundary layer is the part of the troposphere that is closest to Earth’s surface and is directly affected by it, mainly through turbulent diffusion. 
The surface-based homosphere includes the troposphere, stratosphere, mesosphere, and the lowest part of the thermosphere, where the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.
The heterosphere, which includes the exosphere and most of the thermosphere. Here, the chemical composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. T

Optical Properties Of The Atmosphere

• Scattering is a general physical process where some forms of radiation are forced to deviate from a straight trajectory by one or more paths due to localized non-uniformities in the medium through which they pass.
• Absorption is how matter takes up a photon’s energy — and so transforms electromagnetic energy into internal energy of the absorber.
• Emission is the opposite of absorption, it is when an object emits radiation.
• Refractive Index is a number that describes how fast light travels through the material.

Atmospheric Evolution

• The earliest atmosphere consisted of gases in the solar nebula, primarily hydrogen, which could have probably been simple hydrides.
• The second atmosphere was formed by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids, which consisted largely of nitrogen, carbon dioxide, and inert gases. The majority of this “second atmosphere” was formed by nitrogen.
• The third atmosphere saw plate tectonics influence the long-term evolution of the atmosphere by transferring carbon dioxide to and from large continental carbonate stores. This meant that large amounts of oxygen could become free in the atmosphere, created by methods other than photosynthesis.
• Introduction Of Air Pollution is the when chemicals, particulate matter or biological materials that cause harm or discomfort to organisms were introduced into the atmosphere.

The study of Earth’s atmosphere and its processes is called atmospheric science, which is sometimes called aerology.

References And Further Readings

https://www.space.com/17683-earth-atmosphere.html

https://spaceplace.nasa.gov/atmosphere/en/