Early atmosphere~4.5 billion years ago Mostly Hydrogen and HeliumEscaped into space

Outgassing of water vapor and CO2 created a secondary atmosphere with some nitrogen

As liquid water formed the oceans and CO2 was removed from atmosphere The atmosphere became dominated by nitrogen.

~ 2 billion years ago oxygen increased in the atmosphere because of plant life and photosynthesis.

Oxygen reach it’s present level, making up 21% of the dry atmosphere about 500 million years ago.

78 % Nitrogen N2

20.95 % Oxygen O2 Dry atmosphere

0.93 % Argon Ar

H2O 0 to 4 %

Carbon Dioxide CO2 0 to .037 % (or 370 ppm)Methane CH4 1.8 ppmNitrous Oxide N2O .3 ppmOzone 0.04 ppm or 40 ppb)Aerosols (particles) highly variable

N2O Nitrous Oxide