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Which gases did early volcanic activity release into Earth’s atmosphere?

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abundance of oxygen — evolution of the atmosphere A “best guess” reconstruction of the abundance of O₂ in Earth's atmosphere as a function of time. The O₂ abundance axis is logarithmic.

Early volcanic activity and the process of planetary outgassing released a variety of volatile substances from Earth’s interior to form the planet's secondary atmosphere. These gases included:12

Water: Released primarily as water vapour, which later condensed to form the hydrosphere.13
Carbon gases: Including carbon dioxide, carbon monoxide, and methane.1
Sulfur gases: Primarily hydrogen sulfide, though sulfur dioxide was also present.13
Halogen compounds: Such as hydrochloric acid.1
Nitrogen: Released as molecular nitrogen (N $_{2}$ ).13

The specific composition of these gases depended on the oxidation state of the early crust. If the earliest outgassing products were equilibrated with metallic iron, hydrogen would have been released as a mixture of molecular hydrogen and water vapour, while carbon would have appeared as carbon monoxide and sulfur as hydrogen sulfide. Over time, as the surface cooled and chemical reactions occurred, these gases were modified, leading to the eventual accumulation of an atmosphere where molecular nitrogen became the most abundant gas. Notably, free oxygen was not a constituent of these early volcanic releases.31