The Development of Earth's Atmosphere
Earth's modern atmospere is composed of 78% nitrogen (N2), 20% oxygen (O2), 0.93% Argon (Ar), 0.002% Neon (Ne). These elements are part of what called the non-variable protion of the atmosphere, meaning that their relative abundances do not change. Our atmosphere also contains variable constituents that, although very low in abundance, have major impacts on geologic and biologic process. These variable constituents include compounds such as 0.1-4% water (H2O), 0.034% Carbon Dioxide (CO2) and 0.0006%Ozone (O3).
Earth's very earliest atmosphere was most likely composed of the same gases contained within the solar nebula (hydrogen and helium). However, before Earth differentiated this early atmosphere would have been lost to space since Earth's gravitational attraction alone is insufficient to retain gases with such low molecular weights. Also, without a magnetic field these gases would have been blown away by the solar wind from the Sun. Once Earth differentiated and the magnetosphere was present, an atmosphere began accumulating as a result of outgassing during volcanism. Outgassing is the process by which volatile compounds are released from solids apon heating. This process is most visible during volcanic eruptions where large ammounts of gasses are released to the atmosphere.
Today water vapor is the most abundant volcanic gas followed by carbon dioxide, sulfur dioxide, carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen. Archean volcanoes would have emitted the same gases, and as result these gases would begin to accumulate in the atmosphere following differentiation. It should be noted that this newly created atmosphere would be lacking free oxygen and ozone. Reactions between these volcanic gases in the early atmosphere also formed ammonia (NH3) and methane (CH4).
This early oxygen-deficient but carbon dioxide-rich atmosphere persisted throughout the Archean. Evidence for this early atmosphere can be found in the rocks. Archean deposits lack minerals such as hematite and instead contain abundant pyrite (FeS2). Due to the lower reactivity of Sulfur, pyrite can only form in oxygen deficient environments. Oxidized iron (hematite) becomes increasingly common in Proterozoic rocks, as free oxygen begins to accumulate in the atmosphere.
Two processes account for introducing free oxygen into the atmosphere. The first, photochemical dissociation, involves ultraviolet radiation from the Sun disrupting water molecules in the upper atmosphere. During this process both oxygen and hydrogen are released to the atmosphere. Because of its low mass the hydrogen is typically lost to space where the heavier oxygen is retained by Earth's gravity. This process may have supplied 2% of the present-day oxygen level and although much lower than todays levels this amount of free oxygen can begin producing ozone, which creates a barrier against ultraviolet radiation. Even more important tp the accumulation of free oxygen is the process of photosynthesis. Photosynthesis is a metabolic process in which organisms use carbon dioxide and water to make organic molecules, and release oxygen as a waste product. Even so, probably no more than 1% of the free oxygen level of today was present by the end of the Archean, 2.5 billion years ago.