The atmosphere was formed by outgassing of the Earth. Gases from the inside issued, for example during volcanic eruptions. In one million years 80 to 85 percent of the atmosphere was outgassed. The rest liberated during the next 4.4 billion years. As the result of photosynthesis the content of oxygen increased.

• carbon dioxide (certainly most abundant)
• nitrogen
• water vapor

trace amounts of:

• methane
• amonnia
• sulfur dioxide
• hydrochloric acid

the composition of contemporary atmosphere:

• nitrogen 78,09%
• oxygen 20,95%
• argon 0,93%
• carbon dioxide 0,03%

less than 0,01%

• neon
• helium
• methane
• krypton
• hydrogen
• nitrogen suboxide
• xenon
• ozone