Without atmospheric oxygen, life on Earth would be extremely different. The increase in atmospheric oxygen to present levels, and the corresponding increase in atmospheric levels of ozone, with their ability to absorb biologically harmful high energy radiation from the Sun, allowed life to evolve and to emerge from the oceans to the land, undergoing an amazing evolution to its present diversity.

Oxygen is one of the most abundant elements on our planet. It is the most abundant element by mass in the Earth's crust and is found in most rocks. Oxygen also accounts for 89% of the mass of the oceans. After molecular nitrogen, oxygen is the second most abundant element in the Earth's atmosphere, with molecular oxygen accounting for 20.95% of the atmospheric content. There is nearly uniform mixing of molecular oxygen in the atmosphere until above the mesosphere, roughly 80 km above the Earth's surface. Therefore, because of the nearly exponential decrease in pressure with altitude, the bulk ofmolecular oxygen is found in the first few kilometers above the Earth's surface. However, although the turbulent mixing in the lower atmosphere keeps the molecular oxygen nearly in constant mixing ratio (relative to the total air density), measurements indicate that there are seasonal latitudinal variations of as much as 15 ppm (parts per million molecules of air). These seasonal variations are most pronounced at high latitudes in the Northern Hemisphere, where the seasonal cyclic variations in photosynthesis and respiration are most strongly felt.

While there are many gases and particles in the atmosphere containing oxygen, the other gas that needs to be discussed as part of the oxygen cycle because of its great importance both to the atmosphere and to life on Earth is ozone. While atmospheric concentrations for ozone are much smaller (ppm levels) than that for molecular oxygen, ozone is important for several reasons: (1) it absorbs biologically harmful levels of ultraviolet (UV) radiation, keeping this radiation from reaching the Earth's surface; (2) it is a 'greenhouse' gas that influences the Earth's climate; and (3) direct contact with ozone pollution in the lower atmosphere can be harmful to plants, animals, and humans.

The following sections discuss the oxygen cycle in more detail, focusing on the processes affecting molecular oxygen and ozone in the atmosphere, the historical changes in the amounts of these important gases, and their projections for the future.

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