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Oxygen and Ozone
The lowest 50 km of the atmosphere is relatively well stirred by
convection and turbulent processes, so the mixture of atmospheric gases
is quite homogenous over this region. Masses of nitrogen and oxygen
decrease exponentially with height throughout the troposphere. Ozone
is produced by photochemical processes in the stratosphere, so its
concentration increases from the base of the stratosphere to a maximum
around 30 km and then decreases. Monatomic oxygen and monatomic
hydrogen exist at low concentrations above these levels.
Perhaps one of the most famous experiments in science was done by Stanley Miller who took water, methane, and ammonia put it in a jug and subjected it to solar radiation. His discovery of the development of complex molecules from such a situation suggests that these are the ingredients for life to form. Similarly, about 3 billion years ago, oxygen began appearing on Earth. Figure 10 gives the reactions in the earth's prebiotic atmosphere that allow an initial atmosphere of H2O and CO2 to form O2. Solar radiation decomposes water into H and OH. Carbon monoxide and OH give CO2 back again plus H. The OH can give water and monatomic oxygen, and the monatomic oxygen together with a third species (M) can produce diatomic oxygen and hydrogen which could then escape. So it would be theoretically possible for sunlight in an atmosphere with water and CO2 to produce oxygen, but probably not more than a few tenths of a percent of what we now find.
Figure 11 starts at 100 million years before the present and goes back in time to show what happened to oxygen and ozone over time. Two processes began to occur: first, nitrogen could be fixed, and secondly CO2 could be absorbed by plants, like green algae, thereby producing oxygen allowing biological activity to expand. Ocean plants appeared first, then land plants, ocean animals, and land animals. Land animals did not appear until the oxygen concentration of the atmosphere reached some critical level to feed cells by diffusion processes. Microbial organisms played a dominant role in the evolution of the early atmosphere of the Earth. Note that the abundance of ozone (O3) relative to the present atmosphere is high compared to diatomic oxygen (O2) in this early part of the record. We later will see the critical role of ozone in protecting the biosphere from ultraviolet radiation, and that this early-evolutionary protection was important for subsequent plant development.
Therefore, living matter gradually produced oxygen over a period of 1.5 billion years, leading to a situation on earth unique from other planets in our solar system. In the process of producing the oxygen, plants absorbed CO2 in the green plant cycle and converted atmospheric nitrogen to plant nitrogen, thereby driving down the atmospheric levels of both of these constituents.
Another way of getting an overview of the different forms and transformations of oxygen is to look at the oxygen cycle (Figure 12). In future summary informations we will examine cycles of other molecules, but the oxygen cycle is one of the most interesting. Circles in the accompanying figure represent present estimates of flows, and boxes represent present estimates of reservoirs. The atmosphere itself is a large reservoir, 1019 moles, but an even larger reservoir exists in sedimentary rocks. Oxygen may be chemically combined in these reservoirs whereas in the atmosphere it's free. The reservoir of oxygen in fossil fuels is about 3 times larger than that of the biosphere, which consists of plants and animals - both living and dead - at the Earth's surface.
The largest flows of oxygen, photosynthesis and respiration/decay, are about 1016 moles per year. The atmosphere gains oxygen by weathering of rocks, and a comparable amount is lost from the surface by burial, such as marine plant parts and animal skeletons that drift to the bottom of the deep ocean. Burning of fossil fuels (oil, coal, natural gas) in production of energy represents a loss of oxygen for the atmosphere. Finally, a small amount of O2 is gained by the atmosphere when water vapor is broken down by sunlight (photolysis), with hydrogen released to space in the process.
The concept of a material cycle is very helpful in evaluating the
impact of human activity in comparison with natural processes. In
future learning units we will apply this concept to global
distributions of carbon, nitrogen, sulfur, and water substance.
Similar reasoning will be used to evaluate global flow of energy, and
ultimately we will see how these cycles or budgets of materials and
energy all are connected in the earth/atmosphere/ocean/ice system.
