MYP Chemistry Atmospheric Composition Notes

What Are the Main Gases in the Atmosphere Today?

1. Atmospheric composition means the gases that make up the air around Earth, and their relative proportions.
2. A key idea is that the most abundant gases are not necessarily the most reactive or the ones that most strongly affect climate.

Today, dry air at the Earth’s surface is mainly:

1. Other trace gases include:
   1. Neon (Ne)
   2. Helium (He)
   3. Methane (CH₄)
   4. Krypton (Kr)
2. Water vapour (H₂O) is also an important atmospheric gas, but its percentage varies a lot with location, weather and altitude.
3. It is essential for cloud formation and plays a big role in the greenhouse effect.

How Has the Atmosphere Changed Over Geological Time?

The Earth did not start with the atmosphere we breathe today. Its composition has changed dramatically over billions of years.

Early Earth (about 4.6 billion years ago)

1. Intense volcanic activity released large amounts of:
   1. Carbon dioxide (CO₂)
   2. Water vapour (H₂O)
   3. Nitrogen (N₂)
   4. Small quantities of other gases (e.g. SO₂)
2. There was very little oxygen (O₂) in the atmosphere.

Formation of Oceans and Removal of CO₂

1. As the Earth cooled, water vapour condensed to form oceans.
2. CO₂ began to dissolve in seawater, reducing its concentration in the atmosphere.
3. Over time, marine organisms formed shells and skeletons made of calcium carbonate (CaCO₃).
   1. When these organisms died, their shells were buried and became sedimentary rocks (e.g. limestone) and, eventually, part of fossil fuel deposits.
   2. This process locked away carbon and further reduced atmospheric CO₂.

The Rise of Oxygen – The Great Oxidation Event

1. At first, the atmosphere still contained almost no oxygen. That changed when photosynthetic organisms evolved.
   1. Early autotrophs (e.g. cyanobacteria) used photosynthesis to:
      1. Take in CO₂ and water
      2. Release oxygen as a by-product
2. Over hundreds of millions of years, this led to:
   1. A major decrease in CO₂
   2. A gradual increase in O₂

Today’s atmosphere is the result of this long history of:

1. Volcanic outgassing
2. Ocean formation
3. Rock formation
4. Photosynthesis and respiration

How Do Natural Processes Maintain or Change Atmospheric Composition?

The atmosphere is not static; it is constantly being modified and balanced by natural processes.

The Carbon Cycle – Linking Photosynthesis and Respiration

The carbon cycle is the continuous movement of carbon between:

1. The atmosphere
2. Living organisms
3. Oceans
4. Rocks and soils

Key processes

Photosynthesis

1. Plants, algae and some bacteria absorb CO₂ and use sunlight to make glucose.
2. They release O₂ as a by-product. $$6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \xrightarrow{\text { light }} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2$$
3. Result:
   1. Decreases atmospheric CO₂
   2. Increases atmospheric O₂

Respiration and Decomposition

1. Animals, plants and microorganisms respire:
2. They use O₂ to release energy from glucose
3. They produce CO₂ and water
4. When organisms die, decomposers break down their remains, also releasing CO₂.