Composition of the Atmosphere 

The atmosphere is a mixture of suspended liquid and solid particles that vary from place to place throughout the time. The atmosphere closest to earth (or ‘air’) is primarily composed (~99% by volume) of Nitrogen (N) and Oxygen (O) with N and O usually comprising ~78% and ~21% respectively. Water vapour is present in varying quantities usually>1%.  The remainder is made of other gases such as argon (0.9%) and carbon dioxide (CO2 – 0.04% and rising) as well as trace levels (<0.002%) of other gas such as neon, helium, methane, nitrous oxide, sulphur dioxide, krypton etc. Although N and O comprise the main part of the earth’s ‘dry air’, they are the least significant in relation to weather and climatic changes. Carbon dioxide, on the other hand, is the main component that influences meteorological variations. This is because it plays a significant role in the earth’s temperature fluctuations as it absorbs the energy released from the earth, and because it is an important factor in the earth’s heat transfer. 

Many of the air’s components such as water vapour (source of clouds and precipitation), solid and liquid particles (e.g. dust articles, smoke, ocean salts, pollen, ash, microscopic organisms and other ‘aerosols’) and ozone (molecules of O3 found in the stratosphere) can substantially affect weather and climate patterns. 

Vertical Structure of the Atmosphere

The atmosphere is composed of masses of gas that surround the earth. These gases are exposed to different gravitational pulls from the surface of the earth. This applies different pressure to the air on the layers just below. For example, at sea level, the pressure is approximately 1 Bar (1000 mbar ~ = 100 kPa), while at the top of Mt. Everest we can say the pressure is around 300 mbar. This way, we can say that the air pressure is lower in higher altitudes than closer to sea level.
Naturally, there are no ‘layers’ that separate the masses of air found in the atmosphere - they just gradually become thinner the closer to outer space this air is found. However, there is a vertical classification that divides these layers which, depending on their thermal differences (this varies with altitude). They are divided into the following atmospheric layers:


Troposhere 

The bottom-most layer, closest to earth, is where most of the weather events occur. In this layer, there is a notable decrease in temperature with respect to the altitude (the higher the altitude, the lower the temperature).  This change (decrease) is referred to as the environmental lapse rate. The ‘normal’ lapse rate averages a worldwide temperature rate of 6.5oC for every 300 m. Due to changes in humidity and pressure associated with the vertical movements in the air, this value is constantly changing (it is not uniform with respect to height) and should be frequently monitored. This information can be gathered by an instrument package called ‘radiosonde’. 
The density or ‘thickness’ of the air masses found in the troposphere varies depending on its location (latitude) as well as the season of the year (temperature dependent). But generally, this layer will reach a height of 12 kilometres before the temperature constant starts changing; this next layer is called the ‘Stratosphere’.

Stratosphere

 The stratosphere is the layer in between the Troposphere and Mesosphere. Here, due to a more stable environment in terms of air commotions, the temperature can remain relatively constant for the first 20 km above the earth’s surface before it begins to slowly increase with respect to altitude. This increase in temperature is directly related to the presence of the ozone layer found at around 20-30 km height, which absorbs the heat from the sun’s ultraviolet rays. 

Mesosphere 

The Mesosphere begins at a height of 50 km above the earth’s surface and ends approximately 80 km high. The temperature here drastically decreases, reaching temperatures as cold as -90 oC, and it has some of the lowest temperatures occurring compared to other atmospheric layers.

Thermosphere 

The Thermosphere experiences an increase in temperature the higher it goes. As this layer has limited atmospheric masses, this increase in temperature is caused by the heat that is absorbed from ‘direct’ solar radiation (i.e. short wave length and highly energetic sun beams). Temperatures are almost totally dependent on solar activity, and can reach as high as 2000oC. However, this heat is not felt from the surface of the earth as the atoms of oxygen and nitrogen coming from the sun are bounced off the air rather than being absorbed. Atmospheric particles cause the radiation in this layer to become electrically charged. This allows radio waves to bounce off and be received beyond the horizon. 

Exosphere

The exosphere is directly above the thermosphere and begins at approximately 550 km height. It has no ‘clearly defined’ starting point but it is considered as the uppermost layer due to the types of gases present here. The gases found here are mainly ionized hydrogen, helium and CO2, among others, and are considered some of the lightest gases in the atmosphere. This layer gives way to planets, moons and satellites, as it is an area with weakened pressure and gravity due mainly to the wide distance these gases are found from one another.

 

Purpose of the Atmosphere 

The atmosphere’s influence on the earth’s temperature, nitrogen storage, vital cycles, and dissipation/ decomposition of human influences such as pollution and emissions, is extremely important. The atmosphere protects the earth from outer space events and occurrences as well as UV and other radiation. It also helps to keep the planet warm and insulates it against extremes of night-day heating and cooling. It is also significant as the oceans’ energy source. Without its crucial regulatory role, the earth could not sustain life.

 

Movement of air/gases around the Earth

Air movement is caused by pressure or temperature differences and is experienced as wind. A pressure gradient develops when there are differences of pressure between two places, and air moves from a high-pressure region to a low-pressure region. This movement of air follows a spiralling route, outwards from high pressure and inwards towards low pressure. This is due to the rotation of the Earth beneath the moving air, which causes an apparent deflection of the wind to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. The deflection of air is caused by the Coriolis force. Consequently, air blows anticlockwise around a low-pressure centre (depression) and clockwise around a high-pressure centre (anticyclone) in the Northern Hemisphere. This situation is reversed in the Southern Hemisphere.

Pollutants emitted into the atmosphere do not stay in the atmosphere or even directly above the polluting source forever. They can be moved by wind, deposited as dry particles or in rain; they can react with other particles and change in nature. There is vertical mixing and changes through phenomena such as lightning. The most important feature of atmospheric chemistry is photochemical reactions, resulting from the absorption of light photons by molecules.  One very important photochemical reaction is responsible for the presence of ozone in the stratosphere. This occurs when the O2 molecule absorbs highly energetic ultraviolet radiation in the stratosphere to form oxygen atoms, which react further to produce ozone, O3.