Tropospheric Ozone Depletion Events
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Tropospheric ozone depletion events are phenomena that reduce the concentration of ozone in the Earth's lower atmosphere.
Ozone Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...
(O3) is a trace gas which has been of concern because of its unique dual role in different layers of the atmosphere. Apart from absorbing UV-B radiation and converting solar energy into heat in the
stratosphere The stratosphere () is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher ...
, ozone in the
troposphere The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the Atmosphere, planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the ...
provides
greenhouse effect The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source (as in the case of Jupiter) or ...
and controls the
oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
capacity of the atmosphere.


Sources of tropospheric ozone

Ozone in the troposhere is determined by photochemical production and destruction, dry deposition and cross-tropopause transport of ozone from the stratosphere. In the
Arctic The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...
troposphere, transport and photochemical reactions involving nitrogen oxides and volatile organic compounds (VOCs) as a result of human emissions also produce ozone resulting in a background mixing ratio of 30 to 50 nmol mol−1 (ppb). Nitrogen oxides play a key role in recycling active free radicals (such as reactive
halogens The halogens () are a group (periodic table), group in the periodic table consisting of six chemically related chemical element, elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and ten ...
) in the atmosphere and indirectly affect ozone depletion. Ozone depletion events (ODEs) are phenomena associated with the sea ice zone. They are routinely observed at coastal locations when incoming winds have traversed sea ice covered areas.


Halogen activation

During springtime in the polar regions of Earth, unique photochemistry converts inert halide salt ions (e.g. Br) into reactive halogen species (e.g. Br atoms and BrO) that episodically deplete ozone in the atmospheric boundary layer to near zero levels. These processes are favored by
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
and low
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
conditions. Since their discovery in the late 1980s, research on these ozone depletion events has shown the central role of
bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...
photochemistry. The exact sources and mechanisms that release bromine are still not fully understood, but the combination of concentrated sea salt in a condensed phase substrate appears to be a pre-requisite. Shallow boundary layers are also likely to be beneficial since they enhance the speed of autocatalytic bromine release by confining the released bromine to a smaller space. Under these conditions, and with sufficient acidity, gaseous hypobromous acid (HOBr) can react with condensed sea salt bromide and produce bromine that is then released to the atmosphere. Subsequent photolysis of this bromine generates bromine radicals that can react with and destroy ozone. Due to the autocatalytic nature of the reaction mechanism, it has been called bromine explosion.


Chemical destruction

It is still not fully understood how salts are transported from the ocean and oxidized to become reactive halogen species in the air. Other halogens (
chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
and
iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...
) are also activated through mechanisms coupled to bromine chemistry. The main consequence of halogen activation is chemical destruction of ozone, which removes the primary precursor of atmospheric oxidation, and generation of reactive halogen atoms/oxides that become the primary oxidizing species. The oxidation ability originally influenced by ozone is weakened, while the halogen species now holds the oxidation ability. This changes the reaction cycles and final products of many atmospheric reactions. During ozone depletion events, the enhanced halogen chemistry can effectively oxidize reactive gaseous elements.


Effects

The different reactivity of halogens as compared to OH and ozone has broad impacts on atmospheric chemistry. These include near complete removal and deposition of mercury, alteration of oxidation fates for organic gases, and export of bromine into the free troposphere. The deposition of reactive gaseous mercury (RGM) in snow from oxidation by enhanced halogens increases the bioavailability of mercury. Recent changes in the climate of the Arctic and state of the Arctic sea ice cover are likely to have strong effects on halogen activation and ozone depletion events. Human-induced climate change affects the quantity of snow and ice cover in the Arctic, altering the intensity of nitrogen oxide emissions. Increment in background levels of nitrogen oxide apparently strengthens the consumption of ozone and the enhancement of halogens.


See also

* Arctic haze * Free radical halogenation * Tropospheric ozone * Frost flower (sea ice)


References

{{Reflist Environmental chemistry Ozone depletion