Ozone depletion

Image of the largest Antarctic ozone hole ever recorded over the South Pole in September 2006.
Layers of the atmosphere (not to scale). The Earth's ozone layer is mainly found in the lower portion of the stratosphere from approximately 20 to 30 km (12 to 19 mi).

Ozone depletion describes two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere(the ozone layer), and a much larger springtime decrease in stratospheric ozone around Earth's polar regions.[1] The latter phenomenon is referred to as the ozone hole. There are also springtime polar tropospheric ozone depletion events in addition to these stratospheric events.

The main cause of ozone depletion and the ozone hole is man-made chemicals, especially man-made halocarbon refrigerants, solvents, propellants, and foam-blowing agents (chlorofluorocarbon (CFCs), HCFCs, halons), referred to as ozone-depleting substances (ODS). These compounds are transported into the stratosphere by the winds after being emitted at the surface.[2] Once in the stratosphere, they release halogen atoms through photodissociation, which catalyze the breakdown of ozone (O3) into oxygen (O2).[3] Both types of ozone depletion were observed to increase as emissions of halocarbons increased.

Ozone depletion and the ozone hole have generated worldwide concern over increased cancer risks and other negative effects. The ozone layer prevents most harmful UVB wavelengths of ultraviolet light (UV light) from passing through the Earth's atmosphere. These wavelengths cause skin cancer, sunburn, and cataracts, which were projected to increase dramatically as a result of thinning ozone, as well as harming plants and animals. These concerns led to the adoption of the Montreal Protocol in 1987, which bans the production of CFCs, halons, and other ozone-depleting chemicals.

The ban came into effect in 1989. Ozone levels stabilized by the mid-1990s and began to recover in the 2000s. Recovery is projected to continue over the next century, and the ozone hole is expected to reach pre-1980 levels by around 2075.[4] The Montreal Protocol is considered the most successful international environmental agreement to date.

Ozone cycle overview

Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O
2
or diatomic oxygen), and ozone gas (O
3
or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after intaking ultraviolet photons. This converts a single O
2
into two atomic oxygen radicals. The atomic oxygen radicals then combine with separate O
2
molecules to create two O
3
molecules. These ozone molecules absorb ultraviolet (UV) light, following which ozone splits into a molecule of O
2
and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process that terminates when an oxygen atom recombines with an ozone molecule to make two O
2
molecules.

O + O
3
→ 2 O
2

The total amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination.

Ozone can be destroyed by a number of free radical catalysts; the most important are the hydroxyl radical (OH·), nitric oxide radical (NO·), chlorine radical (Cl·) and bromine radical (Br·). The dot is a notation to indicate that each species has an unpaired electron and is thus extremely reactive. All of these have both natural and man-made sources; at the present time, most of the OH· and NO· in the stratosphere is naturally occurring, but human activity has drastically increased the levels of chlorine and bromine. These elements are found in stable organic compounds, especially chlorofluorocarbons, which can travel to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are released from the parent compounds by the action of ultraviolet light, e.g.

CFCl
3
+ electromagnetic radiation → Cl· + ·CFCl
2

Ozone is a highly reactive molecule that easily reduces to the more stable oxygen form with the assistance of a catalyst. Cl and Br atoms destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle,[5] a chlorine atom reacts with an ozone molecule (O
3
), taking an oxygen atom to form chlorine monoxide (ClO) and leaving an oxygen molecule (O
2
). The ClO can react with a second molecule of ozone, releasing the chlorine atom and yielding two molecules of oxygen. The chemical shorthand for these gas-phase reactions is:

  • Cl· + O
    3
    → ClO + O
    2

    A chlorine atom removes an oxygen atom from an ozone molecule to make a ClO molecule
  • ClO + O
    3
    → Cl· + 2 O
    2

    This ClO can also remove an oxygen atom from another ozone molecule; the chlorine is free to repeat this two-step cycle

The overall effect is a decrease in the amount of ozone, though the rate of these processes can be decreased by the effects of null cycles. More complicated mechanisms have also been discovered that lead to ozone destruction in the lower stratosphere.

The ozone cycle
Global monthly average total ozone amount
Lowest value of ozone measured by TOMS each year in the ozone hole

A single chlorine atom would continuously destroy ozone (thus a catalyst) for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO
2
). Bromine is even more efficient than chlorine at destroying ozone on a per atom basis, but there is much less bromine in the atmosphere at present. Both chlorine and bromine contribute significantly to overall ozone depletion. Laboratory studies have also shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, fluorine atoms react rapidly with water and methane to form strongly bound HF in the Earth's stratosphere, while organic molecules containing iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities.

A single chlorine atom is able to react with an average of 100,000 ozone molecules before it is removed from the catalytic cycle. This fact plus the amount of chlorine released into the atmosphere yearly by chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) demonstrates the danger of CFCs and HCFCs to the environment.[6][7]

Other Languages
العربية: نضوب الأوزون
azərbaycanca: Ozon dəliyi
भोजपुरी: ओजोन कटाव
bosanski: Ozonska rupa
čeština: Ozonová díra
Deutsch: Ozonloch
eesti: Osooniauk
فارسی: کاهش ازون
ગુજરાતી: ઓઝોન અવક્ષય
한국어: 오존홀
हिन्दी: ओजोन ह्रास
hrvatski: Ozonske rupe
Bahasa Indonesia: Penipisan ozon
italiano: Buco nell'ozono
қазақша: Озон тесігі
Kreyòl ayisyen: Destwiksyon kouch ozòn
Кыргызча: Озон тешиги
Bahasa Melayu: Penipisan ozon
Nederlands: Gat in de ozonlaag
Simple English: Ozone depletion
slovenčina: Ozónová diera
slovenščina: Ozonska luknja
српски / srpski: Oštećenja ozonskog omotača
srpskohrvatski / српскохрватски: Oštećenje ozonskog omotača
suomi: Otsonikato
svenska: Ozonhål
Türkçe: Ozon deliği
українська: Озонова діра
Tiếng Việt: Sự suy giảm ôzôn