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Facts of the Matter
Richard Brill
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Correct ozone-oxygen cycle crucial to life on Earth
OZONE WAS discovered in 1840 by Christian Friedrich Schonbein, who named it after the Greek word for smell ("ozein"), from the peculiar odor in lightning storms. Ozone is a form of oxygen, but with very different properties from the oxygen gas that is one-fifth of the air we breathe. The interaction between sunlight and three forms of oxygen consisting of one, two, or three atoms is one of the most important processes on the planet. It is very doubtful that there would be life at all on land without the ozone-oxygen cycle.
Ozone is present in extremely low concentrations throughout Earth's atmosphere. It is concentrated in the stratosphere, where it protects us from being fried by sunlight, and accumulates near the surface where sunlight forges it into the key ingredient of smog.
Despite the name, the ozone layer is not composed of ozone. Fifteen miles up, in the middle of the stratosphere where the its concentration is greatest, ozone is present at a mere 10 parts per million; only ten molecules out of every million are ozone. The rest are predominantly nitrogen and oxygen molecules just as here at the surface.
Ten ppm is not much, but it is 100 times its concentration in the atmosphere in general.
NEW YORK TIMES
Ozone forms when reactive gasses and organic compounds are energized by sunlight. Photochemical smog is the result, which leaves airborne particles and ground-level ozone.
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The ozone-oxygen cycle, driven by sunlight, is an important factor to life on Earth because ozone filters deadly ultraviolet radiation (UV).
More correctly it is dual processes of ozone being both made and destroyed that absorbs the UV.
Because of their electron structure, oxygen atoms form two bonds. A molecule of oxygen gas has two atoms of oxygen glued together by two chemical bonds. The two atoms form the molecule O2. Ozone molecules consist of three oxygen atoms in a triangular structure, with each atom sharing a single bond with each of its two neighbors.
UV radiation is electromagnetic in nature, a form of invisible light. The spectrum of wavelengths of electromagnetic radiation spans an unfathomable range of sizes from a 10-millionth of a nanometer to 30 million meters. A nanometer (nm) is one one-billionth of a meter. Visible light occupies only the minuscule range of 400 to 700 nm. UV is just a form of light with wavelengths from 100 to 400 nm, shorter than our eyes can perceive.
The quantum energies of UV photons just happen to lie in the same range as the strength of chemical bonds between atoms in the chemicals of life, especially DNA.
UV radiation is subdivided into three types based on wavelength: UVA (315-400 nm), UVB (280-15 nm), and UVC (100-280 nm). A photon associated with light of shorter wavelengths carries a larger quantum of energy.
The sun emits ultraviolet radiation in all three bands, three-fourths of it as "soft" long-wave UVA. By the time it reaches the surface, 95 percent is UVA and 5 percent is UVB. No measurable UVC from solar radiation reaches Earth's surface, because the shorter UVC wavelengths are completely absorbed by the ozone-oxygen cycle in the stratosphere.
AN OZONE MOLECULE'S existence begins when UVC energy breaks apart a few oxygen molecules into two free oxygen atoms. Some of the unattached atoms react with other oxygen molecules to form ozone molecules. Collisions with an oxygen or nitrogen molecule carries off excess energy in the form of heat.
UVC is completely absorbed by these reactions in the upper half of the stratosphere and none remains below 20 miles altitude.
The ozone molecules formed by the above reaction then absorb UVB, which breaks them apart again into a molecule of oxygen gas and a free oxygen atom, once again with a third molecule that carries off the excess energy as heat. The UVB penetrates deeper so these recombination reactions occur lower in the stratosphere.
The ozone layer is very effective at screening out UVB; for radiation with a wavelength of 300 nm, the intensity at Earth's surface is 350 million times weaker than at the top of the atmosphere
UVA photons are only slightly impeded by the ozone-oxygen cycle.
UVB causes most sunburns and skin cancers, but its is responsible for the production of vitamin D, which is an essential for human health.
The breaking of bonds in the ozone-oxygen cycle converts chemical energy into kinetic energy of molecular motion. The overall effect is to convert the UV into heat, without any net loss of ozone. This cycle keeps the ozone layer in balance, heats the stratosphere from the top down and protects DNA from damage by UVC.
Ozone near Earth's surface is a different story. Ozone is a hazardous and corrosive chemical and pollutant. It forms when nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs) react in the atmosphere when energized by sunlight. Motor vehicle exhaust, industrial emissions, and chemical solvents are the sources of these potently reactive gases.
Ozone may form directly at the source or many kilometers downwind.
Photochemical smog is the result, which leaves airborne particles and ground-level ozone. Ozone is a powerful oxidizing agent that readily reacts with other chemical compounds to make many possibly toxic oxides.
Complex chemical reactions produce microdroplets of other noxious compounds that hang in the air.
In addition to the growing emissions of smog-producing chemicals in the near-surface atmosphere, there has been a steady decline in stratospheric ozone over the past three decades and a larger seasonal decrease in the polar skies, referred to as the "ozone hole."
The overall amount of ozone in the stratosphere is determined by a balance between production by solar radiation, and removal by recombination, but certain free radicals act as catalysts to speed the recombination reaction and reduce the number of ozone molecules.
The main source of these atoms in the stratosphere is breaking down by sunlight of man-made molecules that contain chlorine and bromine. Each chlorine or bromine atom can catalyze the destruction of tens of thousands of ozone molecules before it is removed from the stratosphere.
Above and beyond human and animal health and esthetic factors of smog, there are a variety of possible biological consequences of the thinning stratospheric ozone such as increases in skin cancer, damage to plants, extinction of UV-sensitive species, and reduction of plankton populations in the oceans' photic zones.
Ozone is a double-edged chemical sword, a toxic necessity, and ironically we are removing it from where we need it and adding it to where we can't afford it.
Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to
rickb@hcc.hawaii.edu