Short answers, pulled from the story.
Dry air contains 78.08% nitrogen and 20.95% oxygen by mole fraction. Argon makes up 0.93% of the mixture while carbon dioxide accounts for just 0.04%. Trace gases like neon, helium, krypton, and xenon exist in minute quantities below one part per million each.
The troposphere extends from Earth's surface to an average height of about 11 kilometers. The stratosphere sits above the troposphere and contains the ozone layer where temperatures rise again. The mesosphere follows next dropping to an average temperature of minus 90 degrees Celsius near its top. The thermosphere begins around 80 kilometers and sees temperatures climb due to solar radiation absorption. The exosphere marks the outermost boundary extending hundreds of thousands of kilometers into space.
Free oxygen did not accumulate until about 2.4 billion years ago during the Great Oxygenation Event. Cyanobacterial photosynthesis began producing oxygen roughly 2.7 billion years ago according to stromatolite fossils. Before that time any produced oxygen reacted with ferrous iron or sulfur on the surface.
The total mass of Earth's atmosphere weighs approximately 5.15 quadrillion tonnes. The total mass below 100 kilometers represents 99.99997% of the entire atmosphere. Scientists measure these concentrations using instrumented balloon soundings that rise through the sky.
Since 1750 human activity has increased concentrations of greenhouse gases including carbon dioxide and methane. Global average surface temperatures rose 1.1 degrees Celsius higher between 2011 and 2020 compared to 1850. Carbon dioxide emissions coupled with land development drive the current warming trend affecting all regions.