Tropospheric ozone depletion events are phenomena that reduce the concentration of ozone in the earth's troposphere. Ozone (O3) is a trace gas which has been of concern because of its unique dual role in different layers of the lower atmosphere. [1] Apart from absorbing UV-B radiation and converting solar energy into heat in the stratosphere, ozone in the troposphere provides greenhouse effect and controls the oxidation capacity of the atmosphere. [1]
Ozone in the troposhere is determined by photochemical production and destruction, dry deposition and cross-tropopause transport of ozone from the stratosphere. [2] In the Arctic 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). [3] Nitrogen oxides play a key role in recycling active free radicals (such as reactive halogens) in the atmosphere and indirectly affect ozone depletion. [4] 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. [5]
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. [6] These processes are favored by light and low temperature conditions. [4] Since their discovery in the late 1980s, research on these ozone depletion events has shown the central role of bromine photochemistry. The exact sourcs 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. [7] 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. [3] 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. [7] Due to the autocatalytic nature of the reaction mechanism, it has been called bromine explosion.
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 and iodine) are also activated through mechanisms coupled to bromine chemistry. [6] 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. [6] 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. [4]
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. [6] The deposition of reactive gaseous mercury (RGM) in snow from oxidation by enhanced halogens increases the bioavailability of mercury. [4] 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. [4] Increment in background levels of nitrogen oxide apparently strengthens the consumption of ozone and the enhancement of halogens.
The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere. The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth's atmosphere as a whole is about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 15 to 35 kilometers (9 to 22 mi) above Earth, although its thickness varies seasonally and geographically.
Ozone depletion consists of 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, and a much larger springtime decrease in stratospheric ozone around Earth's polar regions. 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 stratosphere is the second layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air high in the sky and the cool layers of air in the low sky, close to the planetary surface of the Earth. The increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet (UV) radiation by the ozone layer. The temperature inversion is in contrast to the troposphere, and near the Earth's surface, where temperature decreases with altitude.
The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere, which are the lowest two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer that marks the end of the troposphere, and is approximately 17 kilometres (11 mi) above the equatorial regions, and approximately 9 kilometres (5.6 mi) above the polar regions.
Peroxyacetyl nitrate is a peroxyacyl nitrate. It is a secondary pollutant present in photochemical smog. It is thermally unstable and decomposes into peroxyethanoyl radicals and nitrogen dioxide gas. It is a lachrymatory substance, meaning that it irritates the lungs and eyes.
Ground-level ozone (O3), also known as surface-level ozone and tropospheric ozone, is a trace gas in the troposphere (the lowest level of the Earth's atmosphere), with an average concentration of 20–30 parts per billion by volume (ppbv), with close to 100 ppbv in polluted areas. Ozone is also an important constituent of the stratosphere, where the ozone layer (2 to 8 parts per million ozone) exists which is located between 10 and 50 kilometers above the Earth's surface. The troposphere extends from the ground up to a variable height of approximately 14 kilometers above sea level. Ozone is least concentrated in the ground layer (or planetary boundary layer) of the troposphere. Ground-level or tropospheric ozone is created by chemical reactions between NOx gases (oxides of nitrogen produced by combustion) and volatile organic compounds (VOCs). The combination of these chemicals in the presence of sunlight form ozone. Its concentration increases as height above sea level increases, with a maximum concentration at the tropopause. About 90% of total ozone in the atmosphere is in the stratosphere, and 10% is in the troposphere. Although tropospheric ozone is less concentrated than stratospheric ozone, it is of concern because of its health effects. Ozone in the troposphere is considered a greenhouse gas, and may contribute to global warming.
Dinitrogen pentoxide is the chemical compound with the formula N2O5. It is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen. It exists as colourless crystals that sublime slightly above room temperature, yielding a colorless gas.
Paul Jozef Crutzen was a Dutch meteorologist and atmospheric chemist. He and Mario Molina and Frank Sherwood Rowland were awarded the Nobel Prize in Chemistry in 1995 for their work on atmospheric chemistry and specifically for his efforts in studying the formation and decomposition of atmospheric ozone. In addition to studying the ozone layer and climate change, he popularized the term Anthropocene to describe a proposed new epoch in the Quaternary period when human actions have a drastic effect on the Earth. He was also amongst the first few scientists to introduce the idea of a nuclear winter to describe the potential climatic effects stemming from large-scale atmospheric pollution including smoke from forest fires, industrial exhausts, and other sources like oil fires.
A circumpolar vortex, or simply polar vortex, is a large region of cold, rotating air; polar vortices encircle both of Earth's polar regions. Polar vortices also exist on other rotating, low-obliquity planetary bodies. The term polar vortex can be used to describe two distinct phenomena; the stratospheric polar vortex, and the tropospheric polar vortex. The stratospheric and tropospheric polar vortices both rotate in the direction of the Earth's spin, but they are distinct phenomena that have different sizes, structures, seasonal cycles, and impacts on weather.
The hydroperoxyl radical, also known as the hydrogen superoxide, is the protonated form of superoxide with the chemical formula HO2, also written HOO•. This species plays an important role in the atmosphere and as a reactive oxygen species in cell biology.
Vanadium bromoperoxidases are a kind of enzymes called haloperoxidases. Its primary function is to remove hydrogen peroxide which is produced during photosynthesis from in or around the cell. By producing hypobromous acid (HOBr) a secondary reaction with dissolved organic matter, what results is the bromination of organic compounds that are associated with the defense of the organism. These enzymes produce the bulk of natural organobromine compounds in the world.
Iodine oxides are chemical compounds of oxygen and iodine. Iodine has only two stable oxides which are isolatable in bulk, iodine tetroxide and iodine pentoxide, but a number of other oxides are formed in trace quantities or have been hypothesized to exist. The chemistry of these compounds is complicated with only a few having been well characterized. Many have been detected in the atmosphere and are believed to be particularly important in the marine boundary layer.
Equivalent effective stratospheric chlorine (EESC) provides an estimate of the total effective amount of halogens in the stratosphere. It is calculated from emission of chlorofluorocarbon and related halogenated compounds into the troposphere and their efficiency in contributing to stratospheric ozone depletion, and by making assumptions on transport times into the upper atmosphere (stratosphere). This parameter is used to quantify man-made ozone depletion and its changes with time. As a consequence of the Montreal Protocol and its amendments phasing out ozone-depleting substances (ODS), the EESC reached maximum in the late 1990s and is now slowly decreasing.
Costas Varotsos is a Greek physicist known from his contribution to the global climate-dynamics research and remote sensing.
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The bromine cycle is a biogeochemical cycle of bromine through the atmosphere, biosphere, and hydrosphere.
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