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                        Oxygen Cycle
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           Sources of Oxygen:
                     1) photodisassociation of H2O vapor
                            2) photosynthesis

                   Since oxygen is so reactive its cycling is complex:
                     1) As a constituent of CO2 it circulates freely
                    throughout the biosphere.
                            2) Some CO2 combines with Ca to form carbonates.
                            3) O2 combines with nitrogen compounds to form
                            4) O2 combines with iron compounds to form ferric
                            5) Photosynthesis and respiration
                            6) O2 in the troposphere is reduced to O3 (ozone).
                    Ground level O3 is a pollutant which damages lungs.

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