What is the history of gases in the atmosphere?

Answer 1

Earth is described as having three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere. The second atmosphere, rich in greenhouse gases but poor in oxygen, is a result of an impact, which molten the Earth releasing that released volatile gases and later more gases were released by volcanoes. Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga. In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from the Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known asimpact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would therefore have started to form even as the Earth formed. The new atmosphere probably contained water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.

PS: Ga (for gigaannus), is a unit of time equal to 109 years (one billion on the short scale, one milliard on the long scale).