oxygen cycle - Ozone

The dynamic system of changes in the nature of oxygen-containing compounds circulating between the atmosphere, the soil, and living organisms. The oxygen cycle is interwoven with other cycles, such as the nitrogen cycle and the global water cycle. The main biological phase involves the use of gaseous oxygen during respiration in animals and plants, with the consequent production of water and carbon dioxide, and the use of these products by green plants during photosynthesis, resulting in the liberation of gaseous oxygen.

The main source of oxygen within the biosphere and atmosphere is photosynthesis which breaks down carbon dioxide and water to create sugars and oxygen:

CO2 + H2O + energy → CH2O + O2

An additional source of atmospheric oxygen comes from photolysis, whereby high energy ultraviolet radiation breaks down atmospheric water and nitrite into component molecules. The free H and N atoms escape into space leaving O2 in the atmosphere:

2H2O + energy → 4H + O2
2N2O + energy → 4N + O2

The main way oxygen is lost from the atmosphere is via respiration and decay mechanisms in which animal life consumes oxygen and releases carbon dioxide.

Table 1: Major reservoirs involved in the oxygen cycle

Reservoir	Capacity (kg O2)	Flux In/Out (kg O2 per year)	Residence Time (years)
Atmosphere	1.4 * 1018	30,000 * 1010	4,500
Biosphere	1.6 * 1016	30,000 * 1010	50
Lithosphere	2.9 * 1020	60 * 1010	500,000,000

Table 2: Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)

Gains
Photosynthesis (land) Photosynthesis (ocean) Photolysis of N2O Photolysis of H2O	16,500 13,500 1.3 0.03
Total Gains	~ 30,000
Losses - Respiration and Decay
Aerobic Respiration Microbial Oxidation Combustion of Fossil Fuel (anthropologic) Photochemical Oxidation Fixation of N2 by Lightning Fixation of N2 by Industry (anthropologic) Oxidation of Volcanic Gases	23,000 5,100 1,200 600 12 10 5
Losses - Weathering
Chemical Weathering Surface Reaction of O3	50 12
Total Losses	~ 30,000

Ozone

The presence of atmospheric oxygen has led to the formation of ozone and the ozone layer within the stratosphere.