Atmospheric pollution describes harmful gases, solid particles, and liquid droplets that are present in the atmosphere in quantities above natural ambient levels. The main gases in the atmosphere are nitrogen (78.08%), oxygen (20.95%), and argon (0.93%). Carbon dioxide, a greenhouse gas, accounts for 0.038%. Given this composition, pollutants are present in very small proportions, usually measured as the mixing ratio of the number of molecules of the pollutant per total number of air molecules, such as parts per million (ppm) or parts per billion (ppb), or as a mass concentration per unit volume of air, such as micrograms per cubic meter (µg/m3). Each year, millions of tons of pollutants are emitted into the atmosphere, and the effects on the health of the planet and its ...