What is the atmosphere primarily composed of?

Earth's atmosphere composition: nitrogen (N₂) ~78%; oxygen (O₂) ~21%; argon (Ar) ~0.9%; carbon dioxide (CO₂) ~0.04% (rising due to fossil fuels); water vapor variable (0-4%); trace gases (neon, helium, methane, krypton, hydrogen, ozone). Important: oxygen is only 21% — but it's enough to sustain life. Atmospheric layers (low to high): (1) Troposphere — 0-12 km; weather happens here; temperature decreases with altitude; (2) Stratosphere — 12-50 km; contains ozone layer (absorbs UV); temperature increases with altitude; (3) Mesosphere — 50-85 km; meteors burn up; coldest layer; (4) Thermosphere — 85-600 km; auroras occur; very low density but high temperatures; (5) Exosphere — beyond, gradually merges with space. Greenhouse effect: certain gases (CO₂, methane, water vapor) absorb infrared radiation, trapping heat; natural greenhouse effect makes Earth habitable; human enhancement is causing climate change. Ozone layer protects against UV-B radiation; depleted by CFCs (now banned under Montreal Protocol). Air pressure decreases with altitude. Atmospheric pressure at sea level: ~1 atm ≈ 101 kPa ≈ 14.7 psi.