1 Evolution of Oxygen in the Atmosphere “Early” reactions driven by high energy sunlight H 2 O --- h,  H + OH CO h,  CO + O Recombination reactions 2OH -----> H 2 O + O 2O -- M ---> O 2 2H -- M --> H 2 (H 2 can be lost to space) 2H 2 + O > 2H 2 O This was a minor process!

2 To produce large amounts of O 2 we need LIFE BUG O 2 Sunlight What regulates the global Oxygen concentration?

3 Global scale oxygen feedback loop O2O2

4 O2O2 Deep, Cold, O 2 -rich Water

5 Global scale oxygen feedback loop O2O2 Deep, Cold, O 2 -rich Water Respiration

6 “Correct” Atmospheric Oxygen Photosynthesis = Respiration Too Little Atmospheric Oxygen Photosynthesis >> Respiration Too Much Atmospheric Oxygen Photosynthesis << Respiration Feedback on Atmospheric O 2

7 The rate of photosynthesis = respiration = 5 x mol/yr. The atmosphere is 20% oxygen, or 3.8 x mol. How fast is O 2 cycling in the environment?

8 The rate of photosynthesis = respiration = 5 x mol/yr. The atmosphere is 20% oxygen, or 3.8 x mol. Residence time for oxygen = 7600 years What does this mean?? Oxygen is well mixed in the atmosphere! How fast is O 2 cycling in the environment?

9 Nitrogen was outgassed early in Earth formation and constitutes 80% of the atmosphere. (1.4 x moles) Two major sinks: – –N 2 (g) + O 2 (g)  T ----> 2 NO(g) – –N 2 (g) + 3H 2 (g) ----  T, catalyst ---> 2 NH 3 (g) – –Total flux = 4.6 x moles/year Residence time = 3 x 10 7 years Nitrogen in the atmosphere

10 Major Constituents of the Atmosphere (atm.) N O Ar CO What about WATER? Variable concentration atm

11 p(H2O) = p(saturation)*(%RH)/100 Average amount of water = 7 x mol. average flux = 2.5 x mol./yr.. residence time = 10 days Water is poorly mixed! Temperature --> (oC)

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