ch17a

**History ** - 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
 * Chapter 17 Section A: Earth's Atmosphere **

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 * 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 ||

**Composition of Wet Air** []

** Layers of the Atmosphere ** []

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