Antarctic Ozone hole

How does Ozone Hole get created?

  • We all know that the Polar Regions get a much larger variation in sunlight than anywhere else and during the 3 months of winter spend most of time in the dark without solar radiation.
  • Temperatures reduce around or below-80°C for much of the winter and the extremely low Antarctic temperatures cause cloud formation in the relatively “dry” stratosphere.
  • These polar stratospheric clouds (PSCs) are composed of ice crystals that provide the surface for a multitude of reactions, many of which speed up the degradation of ozone molecules.
  • A complex interplay of chemistry, dynamics, and radiation lead to conditions conducive to significant ozone loss in the Polar Regions.
  • The sequence of events leading to the spring time depletion of ozone is initiated of the earth’s orbit at about 23.5° causes the polar regions to experience continual darkness during their winter season.
  • The air above the pole cools and a vortex is formed that isolates sets the stage for the rapid depletion of ozone by catalytic cycles. A catalytic cycle is a series of reactions in which a chemical family or a particular species is depleted, leaving the catalyst unaffected.

The odd-oxygen family is composed of ozone (O3) and atomic oxygen (O). In the presence of a chlorine atom, the net result is the conversion of an oxygen atom and ozone molecule into two molecules of molecular oxygen (O2).

Chlorofluorocarbon-bonds (CFCs) themselves are not involved in the catalytic process; upon reaching the stratosphere. They are subject to higher levels of ultraviolet radiation that decompose the CFCs and release atomic chlorine. The basic set of reactions that define the catalytic cycle involving chlorine and odd-oxygen appear below :

Cl+O3ClO+O2

ClO + OO3

Net result: O3+O2O2

  • Chlorine (Cl) is initially removed by reaction with ozone to form chlorine monoxide (ClO) in the first equation, but it is regenerated through reaction of CIO with any oxygen atom (O) in the second equation.
  • The net result of the two reactions is the depletion of ozone and atomic oxygen.

The catalytic cycle involving chlorine and ozone was not discovered until 1973. The suspected cause of the depletion was catalytic cycles involving chlorine and nitrogen. Further studies described the multiphase process involving polar found to be involved in the extensive ozone loss.

Field studies in 1987, which involved flights from south America over the Antarctic continent, showed the role of chlorine monoxide (ClO) in the ozone depletion picture.

A clear anti-correlation between chlorine monoxide and ozone concentration- that is, an increase in chlorine monoxide correlates to a decrease in ozone- inside the polar vortex has been reported. Similar processes lead to depletion in the arctic, but to a lesser limits formation of PSCs.

The potential for significant depletion in the arctic does exist, however measurements taken from the microwave limb sounder (MLS) on the upper atmosphere research satellite (UARS) platform in 1992 show concentrations of chlorine monoxide in the arctic similar to those found in the Antarctic vortex. The break-up of the polar vortex following polar spring time leads to mixing of ozones and pulses to down to lower latitude regions.

Important summary of the Antarctic Ozone hole

  • Antarctica is surrounded by oceans on all sides. Its unique geographic location causes the clouds in the stratosphere to be really cold. The coldness causes the formation of polar stratospheric clouds which provide an ideal surface for production of ozone depleting chlorine compounds. These polar stratospheric clouds are relatively lesser in the Arctic.
  • As mid-may brings on the onset of winter, the Antarctic stratosphere cools and descends closer to the surface. The
    Coriolis effect
    (caused by the earth’s rotation) sets up a strong westerly circulation around the south pole, forming an oblong vortex, which varies in size from year to year. Current theory holds that the vortex is like a semi-sealed reaction vessels with most of the Antarctic air staying trapped inside the vortex. As temperatures in the lower stratosphere cools below- 80°C, polar stratospheric clouds (PSCs) begin to form.
  • Most of the Antarctic chlorine ends up in reservoir compounds such as ClONO2 or HCl. Reservoir compounds are so named because they hold the atmospheric chlorine in an inactive form but can react later, usually after a hit by ultraviolet radiation, and release reactive chlorine molecules. On the surface of the PSC crystals, nitrogen compounds are readily absorbed and chlorine reservoir compounds are converted to far more reactive compounds such as Cl2 and HOCl.
  • The small amounts of visible light during the Antarctic winter are sufficient to convert much of the atmospheric Cl2 to Clo :
    • Cl2+light2Cl
    • Cl+O3ClO+O2
  • Ordinarily much of the ClO would be captured by atmospheric NO2 and returned to the ClONO2 reservoir, but the polar clouds have absorbed most of the nitrogen compounds such as NO2.
  • Spring brings an increase of ultraviolet light to the lower Antarctic stratosphere, providing the energy needed for the rapid catalytic breakdown of ozone by ClO and its dimer ClOOCl. Over 50% of the stratospheric ozone is destroyed by these two mechanisms, most of the damage occurring in the lower stratosphere.
  • Towards the end of spring (mid-December) the warm temperatures cause the vortex to break up; ozone-rich air from the surrounding area comes flooding in and masses of ozone-depleted air go wandering, temporarily lowering the ozone content in areas of south America and new Zealand by up to 10%.
  • One more important reason of that Antarctic Hole is bigger than Arctic Hole because Earth’s magnetic field directs more positively charged solar wind particles to Earth’s south pole. These are largely hydrogen, hydrogen oxidizes to water vapor, and water vapor both destroys ozone, and blocks one path of ozone production (not really important when UV-C is not available to make ozone anyway).

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