Nitrogen makes up slightly more than 78 percent of dry air by volume. Oxygen accounts for nearly 21 percent of the volume of air. Water in the form of vapor is mixed with the gases. Air also carries tiny particles of solid materials, such as dust. Water and solid particles are not considered part of the air, although they have an effect on its condition and quality.
Air also contains small amounts of several other gases.
They include neon, helium, krypton, xenon, hydrogen, ozone, carbon dioxide, nitrous oxide, and methane. In some parts of the world, the air contains traces of ammonia, carbon monoxide, and hydrogen sulfide. Air, with all its inclusions and contaminants, forms the earth's atmosphere. The atmosphere acts as an insulation blanket around the earth, admitting some energy from the sun, reflecting some, and reflecting back radiant energy given up by earth during times of darkness. The troposphere is the layer closest to the earth.