to expunge water in the form of rain, snow, or fog. The heat gained and lost by water through these processes increases turbulence in the lower atmosphere, especially at mesoscale and microscale. The Brewer–Dobson circulation is a theory of large-scale ozone circulation. Concentrations of other trace gases are higher near natural and artificial sources. This includes pollution from human activity (especially agriculture, industry, and transportation), natural gas fields, radon produced by radioactive decay in certain minerals, volcanic gas, emissions from limnic eruptions. Oxygen is emitted and carbon dioxide is absorbed by plants and microorganisms performing photosynthesis, but CO2 levels are most strongly affected by wild fires and human activity. References Atmosphere of Earth