The Production and Destruction of Ozone

Without human intervention, the stratospheric ozone layer would be produced and destroyed through natural processes. The amounts of ozone in the stratosphere vary naturally throughout the year as a result of production and destruction processes, and as a result of winds and other transport processes that move the ozone molecules around the planet. In addition, changes in ozone occur associated with changes in the solar radiation reaching the Earth during the 11-year solar cycle and with various events such as large explosive volcanic eruptions.

Production of ozone in the stratosphere results primarily from photodissociation of oxygen, O2, molecules. The breaking of the molecular bond by high energy solar photons at wavelengths less than 242 nm results in oxygen atoms that generally react rapidly with an oxygen molecule to form ozone. The sequence of reactions to form ozone is represented as:

where hv represents a photon, A is wavelength, and M is a third atmospheric gas, normally N2 or O2, the primary components of air.

Since the atmosphere is uniformly filled with oxygen molecules, most of the ozone is generated where there is a balance between the decreasing atmospheric density with altitude and where there is available UV solar radiation in the wavelength region less than 242 nm. This primarily occurs in the upper stratosphere.

The high-energy solar radiation needed to produce ozone is largely absorbed in the stratosphere resulting in the production of the ozone layer. Too little UV radiation reaches the troposphere for it to be a major cause of ozone production in this region. In contrast, the lesser amount of ozone in the troposphere is largely formed through a series of chemical reactions, generally referred to as smog reactions. The primary source of ozone in the troposphere (and in the smog in urban areas) comes through the conversion of nitric oxide, NO, to nitrogen dioxide, NO2, which then photolyzes at visible wavelengths to release an oxygen atom that produces ozone through reaction (see eqn [2]). The transport of stratospheric ozone to the troposphere is also important to the budget of tropospheric ozone.

If there was no natural ozone destruction, most of the oxygen in the stratosphere, and perhaps throughout the atmosphere, would eventually be converted to ozone. Such concentrations of ozone would be intolerable to many forms of life on Earth, both because of the direct toxicity of ozone and because of the resultant elimination of any UV radiation reaching the Earth's surface.

Ozone photodissociates at UV and visible wavelengths to produce O and O2. However, because the oxygen atom will generally react to reform ozone, this mechanism produces no net change in the amount of odd oxygen. The actual destruction of ozone and odd oxygen in the stratosphere occurs mainly through catalytic reactions with other gases. In the stratosphere, important catalysts for ozone destruction include nitric oxide (NO), hydroxyl (OH), chlorine (Cl), and bromine (Br). Such gases can be directly substituted for X in the following catalytic mechanism,

which results in the net reaction of O + O3 being converted to two oxygen molecules. As gas X is recycled through these reactions, it continues to destroy ozone until some other reaction converts X to a less reactive form, such as HNO3 or HCl. In addition to the mechanism described by reactions [3] and [4], there are a number of other catalytic mechanisms also affecting ozone. In this way, a single chlorine atom can lead to the net destruction of thousands of ozone molecules. Because of such cyclic reaction mechanisms, relatively small concentrations of reactive chlorine in the stratosphere can have a significant impact on the amount and distribution of stratospheric ozone.

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