What is the second most abundant gas in Earth's atmosphere? Oxygen is the second most abundant gas in Earth's atmosphere.
Dry air is composed of about 78.08% nitrogen and 20.95% oxygen by volume, making oxygen the clear second-place constituent. Argon follows at roughly 0.93%, while carbon dioxide makes up only a few hundredths of a percent. These figures describe dry air; water vapour varies greatly with location and weather.
Oxygen is essential for aerobic respiration, the process cells use to release energy from nutrients. It also supports combustion, although oxygen itself is not a fuel. Most free atmospheric oxygen is replenished by photosynthesis, especially by plants, algae and cyanobacteria, which release oxygen while converting carbon dioxide and water into organic matter.
A common mix-up is confusing atmospheric abundance with abundance in Earth's crust or the universe. Oxygen is the most abundant element by mass in Earth's crust because it is bound in minerals and oxides, but nitrogen is the most abundant gas in the atmosphere. Atmospheric oxygen also was not always plentiful: it accumulated substantially after the Great Oxygenation Event, about 2.4 billion years ago.