4. Early atmosphere ~4.5 billion years ago Mostly Hydrogen and Helium Escaped 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 its present level, making up 21% of the dry atmosphere about 500 million years ago.

6. 78 % Nitrogen N 2 20.95 % Oxygen O 2 Dry atmosphere 0.93 % Argon Ar H 2 O 0 to 4 % Carbon Dioxide CO 2 0 to.037 % (or 370 ppm) Methane CH 4 1.8 ppm Nitrous Oxide N 2 O.3 ppm Ozone 0.04 ppm or 40 ppb) Aerosols (particles) highly variable

11. N 2 O Nitrous Oxide