Composition of the Atmosphere: Gases, Water Vapour, Dust Particles

At a Glance

• Nitrogen 78.08 per cent
• Oxygen 20.95 per cent
• Argon 0.93 per cent
• Carbon dioxide About 0.04 per cent
• Water vapour 0 to 4 per cent, variable
• Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This is the first topic of the macro on climatology. It describes what the air is made of, before its layers, its heat and its movement are explained in the topics that follow. Previous topic: Rates and Forces of Plate Movement. Next topic: Structure of the Atmosphere: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

What Is the Composition of the Atmosphere?

The atmosphere is a mixture of gases, water vapour and solid particles. Up to about 80 km it is mixed by turbulence and has a nearly constant composition of its permanent gases.

Gases such as nitrogen and oxygen make up 99 per cent of the dry air, and the remaining part is argon, carbon dioxide and traces of many others.

What Are the Permanent Gases and Their Roles?

These proportions stay the same up to about 80 km, in a lower region called the homosphere, because winds keep the gases well mixed. Above that height, in the heterosphere, the gases separate into layers by weight. The level of carbon dioxide has risen from about 280 parts per million before industry to over 420 now, and the latest value should be checked in the annual reports of the monitoring agencies.

Which Components Are Variable and Why Do They Matter?

• Carbon dioxide: a minor but very important gas; it is transparent to incoming sunlight but absorbs outgoing heat, so it is a greenhouse gas. Its concentration was about 280 parts per million before the Industrial Revolution and is now over 420 parts per million because of the burning of fossil fuels and deforestation, which is the main cause of global warming.
• Water vapour: varies from almost zero in dry deserts to about 4 per cent in the warm humid tropics, and decreases with height and with latitude. It is the source of clouds and precipitation, and it also absorbs heat. Its role in weather is in Humidity, Saturation, Dew Point, Evaporation and Condensation.
• Ozone: found mainly at about 15 to 35 km, in the stratosphere, where it absorbs harmful ultraviolet radiation. At ground level it is a pollutant.
• Dust and aerosols: solid particles from soil, sea salt, smoke, volcanoes and pollution. They act as hygroscopic nuclei on which water vapour condenses to form clouds, and they scatter light, causing red sunsets and haze.

How Do Humans Alter the Atmosphere?

Burning of fossil fuels, farming, industry and land clearing have raised greenhouse gases, including carbon dioxide, methane and nitrous oxide, leading to warming of the climate. Chlorofluorocarbons, used in refrigeration, destroyed ozone in the stratosphere and caused the Antarctic ozone hole. The Montreal Protocol of 1987 phased out ozone-depleting substances, and the ozone layer is slowly recovering, with the Antarctic ozone hole expected to return to the 1980 levels around the middle of the century. The Kigali Amendment of 2016 phases down hydrofluorocarbons, which warm the climate. India has ratified the Kigali Amendment and set out a plan to phase down HFCs from 2028. As an analytical point, the atmosphere is a thin layer, with half of its mass within about 5.6 km of the surface, so small changes in its composition can have large effects, as the ozone hole and global warming show. The layers of this thin envelope are described in Structure of the Atmosphere: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

How Do Water Vapour and Dust Shape Weather?

• Water vapour: varies from almost zero over deserts and poles to about 4 per cent by volume in the warm, humid tropics. It decreases quickly with height and latitude. It is the source of clouds and rain, stores latent heat and is the strongest natural greenhouse gas.
• Dust particles and aerosols: tiny solid and liquid particles of pollen, smoke, sea salt, volcanic ash and soot, found mostly in the lower atmosphere. They act as hygroscopic nuclei around which water vapour condenses to form cloud drops, so rain could not form without them.
• Scattering of light: the particles and the air molecules scatter sunlight, which makes the sky blue by day and red at sunrise and sunset, and gives twilight.
• Pollution: aerosols from industry and fires reduce visibility and affect health and climate. The structure of the air in layers is in Structure of the Atmosphere: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

As an analytical point, the atmosphere is a thin envelope, but its composition controls the temperature of the Earth, the supply of oxygen and the shield against ultraviolet rays. Human activity is now changing it faster than natural processes did.

Why Is This Topic Important?

• Composition Percentages asked in Prelims
• Climate Greenhouse gases
• Policy Montreal and Kigali

What Came Next?

The next topic describes the five layers of the atmosphere. Next topic: Structure of the Atmosphere: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

Key People and Terms

• Outgassing Release of gases from the interior
• Greenhouse gas Gas that traps outgoing heat
• Ozone layer Ultraviolet-absorbing layer
• Aerosol Fine particles in air
• Hygroscopic nuclei Particles where water condenses
• Great Oxidation Event Rise of oxygen about 2.4 billion years ago

Exam Corner

Mains Angle

Questions link composition and climate. These points add depth.

• Origin Volcanoes and life
• Minor gases Large effect
• Policy Montreal and Kigali