Chapter 17 Section A: Earth's Atmosphere
- Four billion years ago, the atmosphere was composed mainly of ammonia, methane, and water
- There was little to no oxygen
- UV radiation penetrated the atmosphere, triggering the chemical reactions that led to life
- Oxygen was produced by photosynthesizing plants and the break down of water vapor by UV light (photodecomposition)
- Over time the reactive gases ammonia and methane largely disappeared
- Biological processes determine to a great extent the concentrations of atmospheric composition
- Currently the atmosphere is mostly made up of nitrogen and oxygen
- Total mass = 5.3 x 10^18 kg

Composition of Dry Air

Nitrogen (N2)
78.084% (780,840 ppmv)
Oxygen (O2)
20.946% (209,460 ppmv)
Argon (Ar)
0.9340% (9,340 ppmv)
Carbon dioxide (CO2)
0.0383% (383 ppmv)
Neon (Ne)
0.001818% (18.18 ppmv)
Helium (He)
0.000524% (5.24 ppmv)
Methane (CH4)
0.0001745% (1.745 ppmv)
Krypton (Kr)
0.000114% (1.14 ppmv)
Hydrogen (H2)
0.000055% (0.55 ppmv)
Nitrous oxide (N2O)
0.00003% (0.3 ppmv)
Xenon (Xe)
9x10-6% (0.09 ppmv)
Ozone (O3)
0%-7x10-6% (0.0 to 0.07 ppmv)
Water vapor (H2O)
~0.40% over full atmosphere, typically 1%-4% at surface's_atmosphere

Composition of Wet Air

Layers of the Atmosphere

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