Layer which contains 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 Antarctic temperatures also remain below PSC formation temperatures for much longer periods during winter.

These and other meteorological differences occur because of variations between the hemispheres in the distributions of land, ocean, and mountains at middle and high latitudes. As a consequence, winter temperatures are low enough for PSCs to form somewhere in the Antarctic for nearly the entire winter about 5 months , and only for limited periods 10—60 days in the Arctic for most winters. Isolated conditions. Stratospheric air in the polar regions is relatively isolated for long periods in the winter months.

The isolation is provided by strong winds that encircle the poles during winter, forming a polar vortex , which prevents substantial transport and mixing of air into or out of the polar stratosphere. This circulation strengthens in winter as stratospheric temperatures decrease.

The Southern Hemisphere polar vortex circulation tends to be stronger than that in the Northern Hemisphere because northern polar latitudes have more land and mountainous regions than southern polar latitudes.

This situation leads to more meteorological disturbances in the Northern Hemisphere, which increase the mixing in of air from lower latitudes that warms the Arctic stratosphere. Since winter temperatures are lower in the Southern than in the Northern Hemisphere polar stratosphere, the isolation of air in the polar vortex is much more effective in the Antarctic than in the Arctic.

Once temperatures drop low enough, PSCs form within the polar vortex and induce chemical changes such as an increase in the abundance of ClO see Q8 that are preserved for many weeks to months due to the isolation of polar air. Polar stratospheric clouds. PSCs form in the ozone layer during winters in the Arctic and Antarctic, wherever low temperatures occur see Figure Q The clouds often can be seen with the human eye when the Sun is near the horizon. Reactions on PSCs cause the formation of the highly reactive gas chlorine monoxide ClO , which is very effective in the chemical destruction of ozone see Q7 and Q8.

Polar stratospheric clouds PSCs. Reactions on the surfaces of liquid and solid PSCs can substantially increase the relative abundances of the most reactive chlorine gases.

The abundance of ClO increases from a small fraction of available reactive chlorine to comprise nearly all chlorine that is available. With increased ClO, the catalytic cycles involving ClO and BrO become active in the chemical destruction of ozone whenever sunlight is available see Q8. As a result, PSCs are often found over large areas of the winter polar regions and over significant altitude ranges, with significantly larger regions and for longer time periods in the Antarctic than in the Arctic.

The most common type of PSC forms from nitric acid HNO 3 and water condensing on pre-existing liquid sulfuric acid-containing particles. Some of these particles freeze to form solid particles. PSC particles grow large enough and are numerous enough that cloud-like features can be observed from the ground under certain conditions, particularly when the Sun is near the horizon see Figure Q PSCs are often found near mountain ranges in polar regions because the motion of air over the mountains can cause localized cooling in the stratosphere, which increases condensation of water and HNO 3.

When average temperatures begin increasing in late winter, PSCs form less frequently, which slows down the production of ClO by conversion reactions throughout the polar region. When temperatures rise above PSC formation thresholds, usually sometime between late January and early March in the Arctic and by mid-October in the Antarctic see Figure Q , the most intense period of ozone depletion ends.

Nitric acid and water removal. Once formed, the largest PSC particles fall to lower altitudes because of gravity. This process is called denitrification of the stratosphere. As a result, ClO remains chemically active for a longer period, thereby increasing chemical ozone destruction.

Significant denitrification occurs each winter in the Antarctic and only for occasional winters in the Arctic, because PSC formation temperatures must be sustained over an extensive altitude region and time period to lead to denitrification see Figure Q If ice particles grow large enough, they can fall several kilometers due to gravity.

As a result, a significant fraction of water vapor can be removed from regions of the ozone layer over the course of a winter. This process is called dehydration of the stratosphere. Because of the very low temperatures required to form ice, dehydration is common in the Antarctic and rare in the Arctic. The removal of water vapor does not directly affect the catalytic reactions that destroy ozone. Discovering the role of PSCs.

Ground-based observations of PSCs were available many decades before the role of PSCs in polar ozone destruction was recognized. The geographical and altitude extent of PSCs in both polar regions was not known fully until PSCs were observed by a satellite instrument in the late s. The role of PSC particles in converting reactive chlorine gases to ClO was not understood until after the discovery of the Antarctic ozone hole in Our understanding of the chemical role of PSC particles developed from laboratory studies of their surface reactivity, computer modeling studies of polar stratospheric chemistry, and measurements that directly sampled particles and reactive chlorine gases, such as ClO, in the polar stratosphere.

The first decreases in Antarctic total ozone were observed in the early s over research stations located on the Antarctic continent. Total ozone was lower in these months compared with previous observations made as early as The results became widely known to the world after three scientists from the British Antarctic Survey published their observations in the prestigious scientific journal Nature in Currently, the formation and severity of the Antarctic ozone hole are documented each year by a combination of satellite, ground-based, and balloon observations of ozone.

Very early Antarctic ozone measurements. The first total ozone measurements made in Antarctica with Dobson spectrophotometers occurred in the s following extensive measurements in the Northern Hemisphere and Arctic region.

Total ozone values observed in the Antarctic spring were found to be around Dobson units DU , lower than those in the Arctic spring. The Antarctic values were surprising because the assumption at the time was that the two polar regions would have similar values. We now know that these s Antarctic values were not anomalous; in fact, similar values were observed near the South Pole in the s, before the ozone hole appeared see Figure Q Antarctic total ozone values in early spring are systematically lower than those in the Arctic early spring because the Southern Hemisphere polar vortex is much stronger and colder and, therefore, much more effective in reducing the transport of ozone-rich air from midlatitudes to the pole compare Figures Q and Q Severe depletion of the Antarctic ozone layer was first reported in the mids.

Antarctic ozone depletion is seasonal, occurring primarily in late winter and early spring August—November. Peak depletion occurs in early October when ozone is often completely destroyed over a range of stratospheric altitudes, thereby reducing total ozone by as much as two-thirds at some locations.

In most years the maximum area of the ozone hole far exceeds the size of the Antarctic continent. The depletion is attributable to chemical destruction by reactive halogen gases see Q7 and Q8 , which increased everywhere in the stratosphere in the latter half of the 20th century see Q Conditions in the Antarctic winter and early spring stratosphere enhance ozone depletion because of 1 the long periods of extremely low temperatures, which cause polar stratospheric clouds PSCs to form; 2 the large abundance of reactive halogen gases produced in reactions on PSCs; and 3 the isolation of stratospheric air, which allows time for chemical destruction processes to occur.

The severity of Antarctic ozone depletion as well as long-term changes can be seen using satellite observations of total ozone and ozone altitude profiles. Antarctic ozone hole. Total ozone values are shown for high southern latitudes between 21 and 30 September as measured by a satellite instrument. Minimum values of total ozone inside the ozone hole are close to Dobson units DU compared with Antarctic springtime values of about DU observed in the early s see Figure Q The area of the ozone hole is usually defined as the geographical region within the DU contour see white line on total ozone maps.

In late spring or early summer November—December , these atmospheric winds weaken and the ozone hole disappears due to the transport of ozone-enriched air masses towards the pole. Antarctic ozone hole features. Four representations chemists use for ozone molecules. Each ozone molecule has three oxygen atoms, unlike normal molecular oxygen which only has two atoms per molecule. Ozone is a special kind of oxygen molecule.

Normal oxygen molecules O 2 , the kind we need to breathe, have two oxygen atoms. Ozone molecules O 3 have three oxygen atoms. Ozone is found in two different layers in Earth's atmosphere. According to the Environmental Protection Agency, exposure to ozone levels of greater than 70 parts per billion for 8 hours or longer is unhealthy [1].

Such concentrations occur in or near cities during periods where the atmosphere is warm and stable. The harmful effects can include throat and lung irritation or aggravation of asthma or emphysema. National Aeronautics and Space Administration. Goddard Space Flight Center. Topics What is Ozone? What is a Dobson Unit? What is the Ozone Hole? Each natural reduction in ozone levels has been followed by a recovery. Beginning in the s, however, scientific evidence showed that the ozone shield was being depleted well beyond natural processes.

When chlorine and bromine atoms come into contact with ozone in the stratosphere, they destroy ozone molecules. One chlorine atom can destroy over , ozone molecules before it is removed from the stratosphere. Ozone can be destroyed more quickly than it is naturally created.

Some compounds release chlorine or bromine when they are exposed to intense UV light in the stratosphere. These compounds contribute to ozone depletion, and are called ozone-depleting substances ODS ODS A compound that contributes to stratospheric ozone depletion.

ODS include chlorofluorocarbons CFCs , hydrochlorofluorocarbons HCFCs , halons, methyl bromide, carbon tetrachloride, hydrobromofluorocarbons, chlorobromomethane, and methyl chloroform.

ODS are generally very stable in the troposphere and only degrade under intense ultraviolet light in the stratosphere. When they break down, they release chlorine or bromine atoms, which then deplete ozone.

ODS that release chlorine include chlorofluorocarbons chlorofluorocarbons Gases covered under the Montreal Protocol and used for refrigeration, air conditioning, packaging, insulation, solvents, or aerosol propellants. Since they are not destroyed in the lower atmosphere, CFCs drift into the upper atmosphere where, given suitable conditions, they break down ozone. These gases are being replaced by other compounds: hydrochlorofluorocarbons, an interim replacement for CFCs that are also covered under the Montreal Protocol, and hydrofluorocarbons, which are covered under the Kyoto Protocol.

All these substances are also greenhouse gases. See hydrochlorofluorocarbons, hydrofluorocarbons, perfluorocarbons, ozone depleting substance. CFCs , hydrochlorofluorocarbons hydrochlorofluorocarbons Compounds containing hydrogen, fluorine, chlorine, and carbon atoms. Although ozone depleting substances, they are less potent at destroying stratospheric ozone than chlorofluorocarbons CFCs.

They have been introduced as temporary replacements for CFCs and are also greenhouse gases. See ozone depleting substance. HCFCs , carbon tetrachloride carbon tetrachloride A compound consisting of one carbon atom and four chlorine atoms.

Carbon tetrachloride was widely used as a raw material in many industrial uses, including the production of chlorofluorocarbons CFCs , and as a solvent. Solvent use ended when it was discovered to be carcinogenic. It is also used as a catalyst to deliver chlorine ions to certain processes. Its ozone depletion potential is 1. Methyl chloroform is used as an industrial solvent. Its ozone depletion potential is 0. ODS that release bromine include halons halons Compounds, also known as bromofluorocarbons, that contain bromine, fluorine, and carbon.