Chemical reactions between two gases follow well-defined physical rules. Some of these reactions occur on the surfaces of polar stratospheric clouds formed in the winter stratosphere. Reactions have been studied that involve many different molecules containing chlorine, bromine, fluorine, and iodine and other atmospheric constituents such as carbon, oxygen, nitrogen, and hydrogen. These studies have shown that several reactions involving chlorine and bromine directly or indirectly destroy ozone in the stratosphere.
Computer models have been used to examine the combined effect of the large group of known reactions that occur in the stratosphere. These models simulate the stratosphere by including representative chemical abundances, winds, air temperatures, and the daily and seasonal changes in sunlight. These analyses show that under certain conditions chlorine and bromine react in catalytic cycles in which one chlorine or bromine atom destroys many thousands of ozone molecules.
Models are also used to simulate ozone amounts observed in previous years as a strong test of our understanding of atmospheric processes and to evaluate the importance of new reactions found in laboratory studies.
The response of ozone to possible future changes in the abundances of trace gases, temperatures, and other atmospheric parameters have been extensively explored with specialized computer models see Q Atmospheric observations have shown what gases are present in different regions of the stratosphere and how their abundances vary with respect to time and location.
Gas and particle abundances have been monitored over time periods spanning a daily cycle to decades. Observations show that halogen source gases and reactive halogen gases are present in the stratosphere at the amounts required to cause observed ozone depletion see Q7.
Ozone and chlorine monoxide ClO , for example, have been observed extensively with a variety of instruments. ClO is a highly reactive gas that is involved in catalytic ozone destruction cycles throughout the stratosphere see Q8.
Instruments on the ground and on satellites, balloons, and aircraft now routinely measure the abundance of ozone and ClO remotely using optical and microwave signals. High-altitude aircraft and balloon instruments are also used to measure both gases locally in the stratosphere see Q4.
Observations of ozone and reactive gases made in past decades are used extensively in comparisons with computer models to increase confidence in our understanding of stratospheric ozone depletion. Certain industrial processes and consumer products result in the emission of ozone-depleting substances ODSs to the atmosphere. ODSs are manufactured halogen source gases that are controlled worldwide by the Montreal Protocol.
These gases bring chlorine and bromine atoms to the stratosphere, where they destroy ozone in chemical reactions. Important examples are the chlorofluorocarbons CFCs , once used in almost all refrigeration and air conditioning systems, and the halons, which were used as fire extinguishing agents. Current ODS abundances in the atmosphere are known directly from air sample measurements. Halogen source gases versus ozone-depleting substances ODSs.
Those halogen source gases emitted by human activities and controlled by the Montreal Protocol are referred to as ODSs within the Montreal Protocol, by the media, and in the scientific literature. Halogen source gases such as methyl chloride CH 3 Cl that have predominantly natural sources are not classified as ODSs. The contributions of ODSs and natural halogen source gases to the total amount of chlorine and bromine entering the stratosphere, which peaked in and , respectively, are shown in Figure Q The difference in the timing of the peaks is a result of different phaseout schedules specified by the Montreal Protocol, atmospheric lifetimes, and the time delays between production and emissions of the various source gases.
Ozone-depleting substances ODSs. ODSs are manufactured for specific industrial uses or consumer products, most of which result in the eventual emission of these gases to the atmosphere. Total ODS emissions increased substantially from the middle to the late 20th century, reached a peak in the late s, and are now in decline see Figure Q A large fraction of the emitted ODSs reach the stratosphere, where they are converted to reactive gases containing chlorine and bromine that lead to ozone depletion.
ODSs containing only carbon, chlorine, and fluorine are called chlorofluorocarbons, usually abbreviated as CFCs. CFCs, along with carbon tetrachloride CCl 4 and methyl chloroform CH 3 CCl 3 , historically have been the most important chlorine-containing halogen source gases emitted by human activities. These and other chlorine-containing ODSs have been used in many applications, including refrigeration, air conditioning, foam blowing, spray can propellants, and cleaning of metals and electronic components.
As a result of the Montreal Protocol controls, the abundances of most of these chlorine source gases have decreased since see Figure Q With restrictions on global production in place since , the atmospheric abundances of HCFCs are expected to peak between and Another category of ODSs contains bromine.
The most important of these gases are the halons and methyl bromide CH 3 Br. Halons are a group of industrial compounds that contain at least one bromine and one carbon atom; halons may or may not contain a chlorine atom. Halons were originally developed to extinguish fires and were widely used to protect large computer installations, military hardware, and commercial aircraft engines.
As a consequence, upon use halons are released directly into the atmosphere. Halon and halon are the most abundant halons emitted by human activities. Methyl bromide is used primarily as a fumigant for pest control in agriculture and disinfection of export shipping goods, and also has significant natural sources. Halon reached peak concentration in and has been decreasing ever since, reaching an abundance in that was 8.
Changes in halogen source gases entering the stratosphere. A variety of halogen source gases emitted by human activities and natural processes transport chlorine and bromine into the stratosphere. Ozone-depleting substances ODSs are the subset of these gases emitted by human activities that are controlled by the Montreal Protocol.
These partitioned columns show the abundances of chlorine- and bromine-containing gases entering the stratosphere in and , when their total amounts peaked, respectively, and in The overall reductions in the total amounts of chlorine and bromine entering the stratosphere and the changes observed for each source gas are also indicated. The amounts are derived from tropospheric observations of each gas.
Note the large difference in the vertical scales: total chlorine entering the stratosphere is about times more abundant than total bromine. Both, however, are important because bromine is about 60 times more effective on a per-atom basis than chlorine at destroying ozone. Human activities are the largest source of chlorine reaching the stratosphere and CFCs are the most abundant chlorine-containing gases. Methyl chloride is the primary natural source of chlorine.
The largest decreases between and are seen in methyl chloroform, carbon tetrachloride, and CFC The HCFCs, which are substitute gases for CFCs and also controlled under the Montreal Protocol, have risen substantially since and are now approaching expected peak atmospheric abundances see Figure Q The abundance of chlorine-containing very short-lived gases entering the stratosphere has risen substantially since ; these compounds originate primarily from human activity, undergo chemical loss within the troposphere, and are not controlled by the Montreal Protocol.
For bromine entering the stratosphere, halons and methyl bromide are the largest contributors. The largest decrease between and is seen in the abundance of methyl bromide attributed to human activities, because of the success of the Montreal Protocol.
Only halon shows an increasing abundance relative to Methyl bromide also has a natural source, which is now substantially greater than the human source. Natural sources make a much larger fractional contribution to bromine entering the stratosphere than occurs for chlorine, and they are thought to have remained fairly constant in the recent past.
Natural sources of chlorine and bromine. There are a few halogen source gases present in the stratosphere that have large natural sources. These include methyl chloride CH 3 Cl and methyl bromide CH 3 Br , both of which are emitted by oceanic and terrestrial ecosystems. In addition, very short-lived source gases containing bromine such as bromoform CHBr3 and dibromomethane CH 2 Br 2 are also released to the atmosphere, primarily from biological activity in the oceans.
Only a fraction of the emissions of very short-lived source gases reaches the stratosphere because these gases are efficiently removed in the lower atmosphere. Volcanoes provide an episodic source of reactive halogen gases that sometimes reach the stratosphere in appreciable quantities.
Other natural sources of halogens include reactive chlorine and bromine produced by evaporation of ocean spray. These reactive chemicals readily dissolve in water and are removed in the troposphere.
The amount of chlorine and bromine entering the stratosphere from natural sources is fairly constant over time and, therefore, cannot be the cause of the ozone depletion observed since the s.
Other human activities that are sources of chlorine and bromine gases. Other chlorine- and bromine-containing gases are released to the atmosphere from human activities. Common examples are the use of chlorine-containing solvents and industrial chemicals, and the use of chlorine gases in paper production and disinfection of potable and industrial water supplies including swimming pools.
Most of these gases are very short-lived and only a small fraction of their emissions reaches the stratosphere. The Montreal Protocol does not control the production and consumption of very short-lived chlorine source gases, although the atmospheric abundances of some notably dichloromethane, CH 2 Cl 2 have increased substantially in recent years. Solid rocket engines, such as those used to propel payloads into orbit, release reactive chlorine gases directly into the troposphere and stratosphere.
The quantities of chlorine emitted globally by rockets is currently small in comparison with halogen emissions from other human activities. Lifetimes and emissions.
Estimates of global emissions in for a selected set of halogen source gases are given in Table Q Emission from banks refers to the atmospheric release of halocarbons from existing equipment, chemical stockpiles, foams, and other products. In the global emission of the refrigerant HCFC CHF 2 Cl constituted the largest annual release, by mass, of a halocarbon from human activities. The emission of methyl chloride CH 3 Cl is primarily from natural sources such as the ocean biosphere, terrestrial plants, salt marshes and fungi.
The human source of methyl chloride is small relative to the total natural source see Q After emission, halogen source gases are either naturally removed from the atmosphere or undergo chemical conversion in the troposphere or stratosphere.
Lifetimes vary from less than 1 year to years for the principal chlorine- and bromine-containing gases see Table Q The long-lived gases are converted to other gases primarily in the stratosphere and essentially all of their original halogen content becomes available to participate in the destruction of stratospheric ozone.
Gases with short lifetimes such as HCFCs, methyl bromide, and methyl chloride are effectively converted to other gases in the troposphere, which are then removed by rain and snow.
Therefore, only a fraction of their halogen content potentially contributes to ozone depletion in the stratosphere. The amount of an emitted gas that is present in the atmosphere represents a balance between its emission and removal rates. A wide range of current emission rates and atmospheric lifetimes are derived for the various source gases see Table Q The atmospheric abundances of most of the principal CFCs and halons have decreased since in response to smaller emission rates, while those of the leading substitute gases, the HCFCs, continue to increase under the provisions of the Montreal Protocol see Q In the past few years, the rate of the increase of the atmospheric abundance of HCFCs has slowed down.
In the coming decades, the emissions and atmospheric abundances of all controlled gases are expected to decrease under these provisions. Emissions of halogen source gases are compared in their effectiveness to destroy stratospheric ozone based upon their ODPs, as listed in Table Q see Q The calculations, which require the use of computer models that simulate the atmosphere, use as the basis of comparison the ozone depletion from an equal mass of each gas emitted to the atmosphere.
Halon and halon have ODPs significantly larger than that of CFC and most other chlorinated gases because bromine is much more effective about 60 times on a per-atom basis than chlorine in chemical reactions that destroy ozone. The gases with smaller values of ODP generally have shorter atmospheric lifetimes or contain fewer chlorine and bromine atoms. Fluorine and iodine. Fluorine and iodine are also halogens.
Many of the source gases in Figure Q also contain fluorine in addition to chlorine or bromine. After the source gases undergo conversion in the stratosphere see Q5 , the fluorine content of these gases is left in chemical forms that do not cause ozone depletion. As a consequence, halogen source gases that contain fluorine and no other halogens are not classified as ODSs. Iodine is a component of several gases that are naturally emitted from the oceans and some human activities.
Although iodine can participate in ozone destruction reactions, iodine-containing source gases all have very short lifetimes. The importance for stratospheric ozone of very short-lived iodine containing source gases is an area of active research.
Other non-halogen gases. Other non-halogen gases that influence stratospheric ozone abundances have also increased in the stratosphere as a result of emissions from human activities see Q Important examples are methane CH 4 and nitrous oxide N 2 O , which react in the stratosphere to form water vapor and reactive hydrogen, and nitrogen oxides, respectively.
These reactive products participate in the destruction of stratospheric ozone see Q1. Increased levels of atmospheric carbon dioxide CO 2 alter stratospheric temperature and winds, which also affect the abundance of stratospheric ozone. Should future atmospheric abundances of CO 2 , CH 4 and N 2 O increase significantly relative to present day values, these increases will affect future levels of stratospheric ozone through combined effects on temperature, winds, and chemistry see Figure Q Efforts are underway to reduce the emissions of these gases under the Paris Agreement of the United Nations Framework Convention on Climate Change because they cause surface warming see Q18 and Q Although past emissions of ODSs still dominate global ozone depletion today, future emissions of N 2 O from human activities are expected to become relatively more important for ozone depletion as future abundances of ODSs decline see Q Table Q The chlorine- and bromine-containing gases that enter the stratosphere arise from both human activities and natural processes.
When exposed to ultraviolet radiation from the Sun, these halogen source gases are converted to more reactive gases that also contain chlorine and bromine. Some reactive gases act as chemical reservoirs which can then be converted into the most reactive gases, namely ClO and BrO. These most reactive gases participate in catalytic reactions that efficiently destroy ozone. Halogen-containing gases present in the stratosphere can be divided into two groups: halogen source gases and reactive halogen gases see Figure Q Once in the stratosphere, the halogen source gases chemically convert at different rates to form the reactive halogen gases.
The conversion occurs in the stratosphere instead of the troposphere for most gases because solar ultraviolet radiation a component of sunlight is more intense in the stratosphere see Q2. Reactive gases containing the halogens chlorine and bromine lead to the chemical destruction of stratospheric ozone. Reactive halogen gases. The chemical conversion of halogen source gases, which involves solar ultraviolet radiation and other chemical reactions, produces a number of reactive halogen gases.
These reactive gases contain all of the chlorine and bromine atoms originally present in the source gases. The most important reactive chlorine- and bromine-containing gases that form in the stratosphere are shown in Figure Q These two gases are considered important reservoir gases because, while they do not react directly with ozone, they can be converted to the most reactive forms that do chemically destroy ozone.
The most reactive forms are chlorine monoxide ClO and bromine monoxide BrO , and chlorine and bromine atoms Cl and Br. A large fraction of total reactive bromine is generally in the form of BrO, whereas usually only a small fraction of total reactive chlorine is in the form of ClO. Conversion of halogen source gases.
Halogen source gases containing chlorine and bromine are chemically converted to reactive halogen gases, primarily in the stratosphere. Most of the halogen source gases are ozone-depleting substances.
The conversion requires solar ultraviolet radiation and a few chemical reactions. The shorter-lived gases undergo partial conversion in the troposphere. The reactive halogen gases contain all the chlorine and bromine originally present in the source gases before conversion.
The reactive gases can be grouped into the reservoir gases, which do not directly destroy ozone, and the most reactive gases, which participate in ozone destruction cycles see Q8. Reactive chlorine at midlatitudes. Reactive chlorine gases have been observed extensively in the stratosphere using both local and remote measurement techniques. The measurements from space displayed in Figure Q are representative of how the amounts of chlorine-containing gases change between the surface and the upper stratosphere at middle to high latitudes.
Total available chlorine see red line in Figure Q is the sum of chlorine contained in halogen source gases e. In the troposphere, total chlorine is contained almost entirely in the source gases described in Figure Q At higher altitudes, the source gases become a smaller fraction of total available chlorine as they are converted to the reactive chlorine gases. At the highest altitudes, available chlorine is all in the form of reactive chlorine gases. In the altitude range of the ozone layer at midlatitudes, as shown in Figure Q, the reservoir gases HCl and ClONO 2 account for most of the available chlorine.
The abundance of ClO, the most reactive gas in ozone depletion, is a small fraction of available chlorine. The low abundance of ClO limits the amount of ozone destruction that occurs outside of polar regions. Reactive chlorine in polar regions. Reactive chlorine gases in polar regions undergo large changes between autumn and late winter. Meteorological and chemical conditions in both polar regions are now routinely observed from space in all seasons.
Autumn and winter conditions over the Antarctic are contrasted in Figure Q using seasonal observations made near the center of the ozone layer about 18 km Ozone values are high over the entire Antarctic continent during autumn in the Southern Hemisphere. High HCl indicates that substantial conversion of halogen source gases has occurred in the stratosphere. In the s and early s, the abundance of reservoir gases HCl and ClONO 2 increased substantially in the stratosphere following increased emissions of halogen source gases.
HNO 3 is an abundant, primarily naturally-occurring stratospheric compound that plays a major role in stratospheric ozone chemistry by both moderating ozone destruction and condensing to form PSCs, thereby enabling conversion of chlorine reservoirs gases to ozone-destroying forms. The low abundance of ClO indicates that little conversion of the reservoir gases occurs in the autumn, thereby limiting catalytic ozone destruction.
Reactive chlorine gas observations. The abundances of chlorine source gases and reactive chlorine gases as measured from space in are displayed as a function of altitude for a range of latitudes. In the troposphere below about 12 km , all of the measured chlorine is contained in the source gases. In the stratosphere, the total chlorine content of reactive gases increases with altitude as the amount of chlorine source gases declines. This is a consequence of chemical reactions initiated by solar ultraviolet radiation that convert source gases to reactive gases see Figure Q In the ozone layer 15—35 km , chlorine source gases are still present and HCl and ClONO 2 are the most abundant reactive chlorine gases at midlatitudes.
By late winter September , a remarkable change in the composition of the Antarctic stratosphere has taken place. Low amounts of ozone reflect substantial depletion at 18 km altitude over an area larger than the Antarctic continent. Antarctic ozone holes arise from similar chemical destruction throughout much of the altitude range of the ozone layer see altitude profile in Figure Q The meteorological and chemical conditions in late winter, characterized by very low temperatures, very low HCl and HNO 3 , and very high ClO, are distinctly different from those found in autumn.
Low stratospheric temperatures occur during winter, when solar heating is reduced. This conversion occurs selectively in winter on PSCs, which form at very low temperatures see Q9. Low HNO 3 is indicative of its condensation to form PSCs, some of which subsequently descend to lower altitudes through gravitational settling.
High ClO abundances generally cause ozone depletion to continue in the Antarctic region until mid-October spring , when the lowest ozone values usually are observed see Q Similar though less dramatic changes in meteorological and chemical conditions are also observed between autumn and late winter in the Arctic, where ozone depletion is less severe than in the Antarctic. Reactive bromine observations. Fewer measurements are available for reactive bromine gases in the lower stratosphere than for reactive chlorine.
This difference arises is in part because of the lower abundance of bromine, which makes quantification of its atmospheric abundance more challenging. The most widely observed bromine gas is BrO, which can be observed from space. Estimates of reactive bromine abundances in the stratosphere are larger than expected from the conversion of the halons and methyl bromide source gases, suggesting that the contribution of the very short-lived bromine-containing gases to reactive bromine must also be significant see Q6.
Chemical conditions in the ozone layer over Antarctica. Observations of the chemical conditions in the Antarctic region highlight the changes associated with the formation of the ozone hole. Satellite instruments have been routinely monitoring ozone, reactive chlorine gases, and temperatures in the global stratosphere.
Results are shown here for autumn May and late winter September seasons in the Antarctic region, for a narrow altitude region near 18 km Ozone has naturally high values in autumn, before the onset of ozone destruction reactions that cause widespread depletion. When the abundance of ClO is low, significant ozone destruction from halogens does not occur.
Chemical conditions are quite different in late winter when ozone undergoes severe depletion. Temperatures are much lower, HCl has been converted to ClO the most reactive chlorine gas , and HNO 3 has been removed by the gravitational settling of polar stratospheric cloud particles. The abundance of ClO closely surrounding the South Pole is low in September because formation of ClO requires sunlight, which is still gradually returning to the most southerly latitudes. Ozone typically reaches its minimum values in early to mid-October see Q Note that the first and last colors in the color bar represent values outside the indicated range of values.
As a result, a single chlorine or bromine atom can destroy many thousands of ozone molecules before it leaves the stratosphere. In this way, a small amount of reactive chlorine or bromine has a large impact on the ozone layer.
Stratospheric ozone is destroyed by reactions involving reactive halogen gases, which are produced in the chemical conversion of halogen source gases see Figure Q The most reactive of these gases are chlorine monoxide ClO , bromine monoxide BrO , and chlorine and bromine atoms Cl and Br. These gases participate in three principal reaction cycles that destroy ozone. Cycle 1. Ozone destruction Cycle 1 is illustrated in Figure Q The net result of Cycle 1 is to convert one ozone molecule and one oxygen atom into two oxygen molecules.
In each cycle, chlorine acts as a catalyst because ClO and Cl react and are reformed. In this way, one Cl atom participates in many cycles, destroying many ozone molecules. For typical stratospheric conditions at middle or low latitudes, a single chlorine atom can destroy thousands of ozone molecules before it happens to react with another gas, breaking the catalytic cycle.
During the total time of its stay in the stratosphere, a chlorine atom can thus destroy many thousands of ozone molecules. Polar Cycles 2 and 3.
The abundance of ClO is greatly increased in polar regions during late winter and early spring, relative to other seasons, as a result of reactions on the surfaces of polar stratospheric clouds see Q7 and Q9. Cycles 2 and 3 see Figure Q become the dominant reaction mechanisms for polar ozone loss because of the high abundances of ClO and the relatively low abundance of atomic oxygen which limits the rate of ozone loss by Cycle 1. Cycle 2 begins with the self-reaction of ClO. The net result of both cycles is to destroy two ozone molecules and create three oxygen molecules.
Ozone destruction Cycle 1. The destruction of ozone in Cycle 1 involves two separate chemical reactions. The cycle can be considered to begin with either ClO or Cl. The net or overall reaction is that of atomic oxygen O with ozone O 3 , forming two oxygen molecules O 2. The cycle then begins again with another reaction of ClO with O. Chlorine is considered a catalyst for ozone destruction because Cl and ClO are re-formed each time the reaction cycle is completed, and hence available for further destruction of ozone.
Atomic oxygen is formed when solar ultraviolet UV radiation reacts with O 3 and O 2 molecules. Cycle 1 is most important in the stratosphere at tropical and middle latitudes, where solar UV radiation is most intense. Sunlight requirement. Sunlight is required to complete and maintain these reaction cycles. Cycle 1 requires ultraviolet UV radiation a component of sunlight that is strong enough to break apart molecular oxygen into atomic oxygen.
Cycle 1 is most important in the stratosphere at altitudes above about 30 km Cycles 2 and 3 also require sunlight. In the continuous darkness of winter in the polar stratosphere, reaction Cycles 2 and 3 cannot occur. Therefore, the greatest destruction of ozone occurs in the partially to fully sunlit periods after midwinter in the polar stratosphere.
Sunlight in the UV-A to nm wavelengths and visible to nm wavelengths parts of the spectrum needed in Cycles 2 and 3 is not sufficient to form ozone because this process requires more energetic solar UV-C solar radiation see Q1 and Q2. As a result, ozone destruction by Cycles 2 and 3 in the sunlit polar stratosphere during springtime greatly exceeds ozone production.
Other reactions. Global abundances of ozone are controlled by many other reactions see Q1. Reactive hydrogen and reactive nitrogen gases, for example, are involved in catalytic ozone-destruction cycles, similar to those described above, that also take place in the stratosphere.
Reactive hydrogen is supplied by the stratospheric decomposition of water H 2 O and methane CH 4. Methane emissions result from both natural sources and human activities. The abundance of stratospheric H 2 O is controlled by the temperature of the upper tropical troposphere as well as the decomposition of stratospheric CH 4. Reactive nitrogen is supplied by the stratospheric decomposition of nitrous oxide N 2 O , also emitted by natural sources and human activities.
The importance of reactive hydrogen and nitrogen gases in ozone depletion relative to reactive halogen gases is expected to increase in the future because the atmospheric abundances of the reactive halogen gases are decreasing as a result of the Montreal Protocol, while abundances of CH 4 and N 2 O are projected to increase due to various human activities see Q Polar ozone destruction Cycles 2 and 3. Significant destruction of ozone occurs during late winter and early spring in the polar regions when abundances of ClO reach large values.
The net reaction in both cases is two ozone O 3 molecules forming three oxygen O 2 molecules. The destruction of ozone by Cycles 2 and 3 is catalytic, as illustrated for Cycle 1 in Figure Q, because chlorine and bromine gases react and are re-formed each time the reaction cycle is completed.
Sunlight is required to complete each cycle and to help form and maintain elevated abundances of ClO. During polar night and other periods of darkness, ozone cannot be destroyed by these reactions.
Ozone-depleting substances are present throughout the stratospheric ozone layer because they are transported great distances by atmospheric air motions.
The very low winter temperatures in the Antarctic stratosphere cause polar stratospheric clouds PSCs to form. Special reactions that occur on PSCs, combined with the isolation of polar stratospheric air in the polar vortex, allow chlorine and bromine reactions to produce the ozone hole in Antarctic springtime. The ozone hole appears over Antarctica because meteorological and chemical conditions unique to this region increase the effectiveness of ozone destruction by reactive halogen gases see Q7 and Q8.
In addition to a large abundance of these reactive gases, the formation of the Antarctic ozone hole requires temperatures low enough to form polar stratospheric clouds PSCs , isolation from air in other stratospheric regions, and sunlight see Q8. Arctic and Antarctic temperatures. Air temperatures in both polar regions reach minimum values in the lower stratosphere in the winter season.
This occurs on average for 1 to 2 months over the Arctic and about 5 months over Antarctica each year see heavy red and blue lines. Reactions on liquid and solid PSC particles cause the highly reactive chlorine gas ClO to be formed, which catalytically destroys ozone see Q8.
The range of winter minimum temperatures found in the Arctic is much greater than that in the Antarctic. In some years, PSC formation temperatures are not reached in the Arctic, and significant ozone depletion does not occur. In contrast, PSC formation temperatures are always present for many months somewhere in the Antarctic, and severe ozone depletion occurs each winter season see Q Distribution of halogen gases. The abundances are comparable because most long-lived source gases have no significant natural removal processes in the lower atmosphere, and because winds and convection redistribute and mix air efficiently throughout the troposphere on the timescale of weeks to months.
Halogen gases in the form of source gases and some reactive products enter the stratosphere primarily from the tropical upper troposphere. Stratospheric air motions then transport these gases upward and toward the pole in both hemispheres.
Low polar temperatures. The severe ozone destruction that leads to the ozone hole requires low temperatures to be present over a range of stratospheric altitudes, over large geographical regions, and for extended time periods. Low temperatures are important because they allow liquid and solid PSCs to form. Reactions on the surfaces of these PSCs initiate a remarkable increase in the most reactive chlorine gas, chlorine monoxide ClO see below as well as Q7 and Q8.
Stratospheric temperatures are lowest in the polar regions in winter. In the Antarctic winter, minimum daily temperatures are generally much lower and less variable than those in the Arctic winter see Figure Q It is both a natural and a man-made product that occurs in the Earth's upper atmosphere the stratosphere and lower atmosphere the troposphere.
Depending on where it is in the atmosphere, ozone affects life on Earth in either good or bad ways. Stratospheric ozone is formed naturally through the interaction of solar ultraviolet UV radiation with molecular oxygen O2. The "ozone layer," approximately 6 through 30 miles above the Earth's surface, reduces the amount of harmful UV radiation reaching the Earth's surface.
Tropospheric or ground-level ozone — what we breathe — is formed primarily from photochemical reactions between two major classes of air pollutants, volatile organic compounds VOC and nitrogen oxides NOx.
These reactions have traditionally been viewed as depending upon the presence of heat and sunlight, resulting in higher ambient ozone concentrations in summer months. Within the last decade, however, high ozone concentrations have also been observed under specific circumstances in cold months, where a few high elevation areas in the Western U.
Ozone contributes to what we typically experience as "smog" or haze, which still occurs most frequently in the summertime, but can occur throughout the year in some southern and mountain regions.
Although some stratospheric ozone is transported into the troposphere, and some VOC and NOx occur naturally, the majority of ground-level ozone is the result of reactions of man-made VOC and NOx.
Significant sources of VOC are chemical plants, gasoline pumps, oil-based paints, autobody shops, and print shops. Nitrogen oxides result primarily from high temperature combustion. Significant sources are power plants, industrial furnaces and boilers, and motor vehicles.
Many people mistakenly believe that tropospheric ozone concentrations are high only in major urban areas, but high ambient ozone concentrations can and do occur anywhere. Ozone in the troposphere is poisonous substance Tropospheric ozone, on the contrary, is a so-called secondary pollutant , a photo oxidant which is formed by the interaction of solar radiation with primary polluting precursors.
It results from complex chemical reactions , with a substantial role for the following players: nitrogen oxides volatile organic compounds carbon monoxide temperature sunlight. Ozone O3 is mainly found in two layers of our atmosphere: the troposphere and the stratosphere. Troposphere, why is it an important atmospheric layer? Ozone hole over Antarctica, what is it and what causes it?
Ozone in the troposphere, formation of polluting gases.
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