Ozone layer and ozone hole dynamics

In 1985, atmospheric scientists discovered that stratospheric ozone over Antarctica had been reduced to half its natural level. This local loss, termed the Antarctic ozone hole, was traced to destruction of stratospheric ozone by human-made chemicals, especially chlorofluorocarbons (CFCs; artificial compounds consisting of chlorine, fluorine, and carbon and widely used as refrigerants and aerosol spray propellants). Other evidence indicates that ozone levels potentially declining over other regions, though nowhere as drastically as over Antarctica.

The ozone hole covers an area over the Antarctic continent, the surrounding ocean, the southern tip of South America in which stratospheric ozone begins to diminish every August (at the beginning of the Southern hemisphere's spring season), reaches a minimum of less than 50% of its natural value in October, and returns to normal levels by the beginning of December.

Essential to the formation of the Antarctic ozone hole is the polar vortex, which forms every winter over the South Pole. The pole is in 24-hour darkness in midwinter, so the air above it becomes very cold. Cooling air lowers its pressure. Air nearer the equator, warmer and therefore at higher pressure, is sucked toward the pole by the low pressure there. As this warm air moves southward it is twirled into a circular wind by the spin of the earth. This circular wind, the polar vortex, sits over the South Pole like a halo, isolating the air over the pole and allowing it to become even colder. Intermittently, the stratosphere over the pole becomes cold enough to form clouds . The droplets and ice crystals in these stratospheric clouds accelerate the breakdown of ozone by chlorine, essentially eliminating ozone from the lower stratosphere and allowing twice the usual amount of UV-B to reach the surface.

No ozone hole forms at the North Pole because the north-polar winter vortex is smaller and warmer than the southern one. There is nevertheless a 30% decline in north-polar ozone every March. Ozone levels have also declined by 3–6% over the inhabited (middle) latitudes, allowing more UV-B to reach the surface and increasing skin cancer rates.

The ozone layer protects the earth by absorbing UV-B, which can cause skin cancer and eye damage. Low-altitude ozone, however, blocks little UV-B and is toxic to plant and animal life.

Ozone (O₃) is a trace ingredient of the atmosphere that stops most solar radiation in the 280–315-nm ultraviolet (UV-B) band from reaching the ground. Ozone is produced in the stratosphere by the breakup of molecular oxygen (O₂) by solar radiation. It is also produced artificially in the lower atmosphere (troposphere ) by the burning of coal and gasoline. Ninety percent of the atmosphere's ozone is concentrated in the lower stratosphere, about 6–30 miles (10–50 km) up; this concentration of ozone is the ozonosphere or ozone layer.

Ozone is formed in the stratosphere when an O₂ molecule is split by a photon in the 175–242-nm ultraviolet band (1 nm[nanometer] = 10⁻⁹ m.) Each O then joins with an O₂ to form an O₃ (ozone) molecule. Ozone converts the energy it gains from absorbing ultraviolet (and infrared) photons into heat, supplying an average 15 watts of power to every square meter of the stratosphere. This ozone-driven heating defines the temperature-versus-altitude structure of the stratosphere.

Because ozone is created by sunlight it forms more rapidly over the tropics, where there is more sunlight per square meter. Some ozone created at tropical latitudes circulates through the upper stratosphere to the polar regions, but natural polar ozone levels remain lower than tropical levels. This contributes to the greater vulnerability of the polar regions to ozone depletion by CFCs and other chemicals, discussed below.

Ozone is destroyed primarily by the ClO (chlorine oxide) radical that is produced by the breakdown by sunlight of more complex chlorine-bearing molecules. ClO facilitates the reaction, participating as a catalyst. ClO radicals are free to facilitate reactions again and again. This catalytic persistence explains how minute concentrations of a human-made substance can alter the chemistry of an entire layer of the atmosphere: ozone is a million or so times more abundant in the stratosphere than ClO, but each ClO radical destroys thousands of ozone molecules.

Not all chlorine-containing compounds threaten the ozone layer, because not all are capable of reaching the stratosphere. Only non-water-soluble compounds such as CFCs, carbon tetrachloride (CCl₄), and methyl chloroform (CH₃CCl₃) can evade water capture in the troposphere and eventually circulate to the ozone layer. There they last anywhere from 5 years (methyl chloroform) to 100 years (CFC-12). CFC-F11 (CCl₃F), the primary contributor to stratospheric chlorine and therefore to ozone loss, has a lifetime of 45 years in the stratosphere.

CFCs are not the only compounds that affect stratospheric ozone; nitrous oxide (N₂O), the bromine-containing compounds termed halons, and methane (CH₄), also do so. Sulfur dioxide (SO₂) injected into the stratosphere by violent volcanic eruptions , such as that of Mt. Pinatubo in 1991, can cause significant, albeit temporary, drops in global stratospheric ozone.

In 1987, over 100 nations signed an international agreement to reduce emissions of CFCs and other ozone-depleting chemicals, the Montreal Protocol. Later amendments to the Protocol greatly increased its effectiveness, and today scientists estimate that with strict observance of the Protocol, and barring unforeseen side effects of global climate change, stratospheric ozone will cease to decline at some point in the next 10–20 years and recover to 1980 levels by about 2050.

See also Atmospheric chemistry; Atmospheric circulation; Atmospheric composition and structure; Atmospheric pollution; Atmospheric pressure