7. stratospheric chemistry - ucd · within the stratosphere the air is generally neutral or stable...

7. Stratospheric Chemistry

7. Stratospheric ChemistryThere is an abrupt change in concentrations of severalimportant trace constituents between the troposphereand stratosphere.

For example, water vapour decreases substantially andO3 increases by up to an order of magnitude.

This is because there is very little vertical mixing betweenthe relatively moist, ozone-poor troposphere and the dry,ozone-rich stratosphere.

Within the stratosphere the air is generally neutral or stablewith respect to vertical motions.

Also, the removal of aerosols and trace gases by precipita-tion (wash-out or wet deposition) is absent in the strato-sphere.

Materials that enter the stratosphere can remain there forlong periods of time, often as stratified layers.

Unperturbed Stratospheric OzoneDistribution

Ozone in the stratosphere is of great importance:

• It forms a protective shield that reduces the intensityof UV radiation from the Sun that reaches the Earth’ssurface.

• Due to the absorption of UV radiation, O3 determinesthe vertical profile of temperature in the stratosphere.

• It is involved in many stratospheric chemical reactions.

W. N. HartleyW. N. Hartley (1846-1913) was a renowned spectroscopistwho spent his working life in Ireland.

He was Professor of chemistry at Royal College of Science,Dublin.

In 1881 Hartley measured the UV radiation reaching theEarth’s surface and found a sharp cut-off at λ = 0.30 µm.

He correctly attributed this to absorption by O3 in thestratosphere.

Subsequent ground-based UV measurements, and the firstmeasurements of the concentrations of stratospheric O3 byballoons in the 1930s, placed the maximum O3 concentrationin the lower part of the stratosphere.

Shown in the figure below are the results of more recentmeasurements of O3. The presence of an ozone layer be-tween heights of ∼15–30 km is very clear.

Mean vertical distributions of ozone concentrations basedon measurements at different latitudes (given in degrees).

The O3 layer is highly variable. Its height and intensitychange with latitude, season, and meteorological conditions.The total ozone column abundance [in Dobson units (DU)]is indicated in the figure.

The largest column densities of O3 in the northern hemi-sphere occur in polar latitudes in spring.

In the SHS the spring maximum is at midlatitudes.

Since O3 is produced by photochemical reactions, the pro-duction is a maximum in the stratosphere over the tropics.

At any given point in the atmosphere, the balance betweenthe production and loss of O3 and its flux divergence deter-mines the O3 concentration.

The high concentration at polar and midlatitudes is at-tributed to meridional transport of O3 from the equator

Much of the change in the total column abundance of O3 isdue to differences in the profiles below 20 km.

Techniques for Measuring O3In situ measurements of vertical profiles of O3 may be ob-tained from ozonesondes, which can be carried on radiosondeballoons.

The O3 sensor is comprised of two electrolytic cells, eachcontaining a solution of potassium iodide (KI). The cellsare initially in chemical and electrical equilibrium.

However, when an air sample containing O3 is drawn throughone of the cells, the equilibrium is perturbed and an electriccurrent flows between the cells.

The amount of electric charge, which is proportional to thepartial pressure of the O3 in the ambient air, is continu-ously transmitted to a ground station along with the ambi-ent pressure and temperature.

In this way, a vertical profile of O3 is obtained, the integra-tion of which provides the O3 column from ground level upto the height of the balloon.

Brewer Spectrophotometer

Brewer SpectrophotometerThe O3 column from the ground to the top of the atmo-sphere can be measured by passive remote sensing using aninstrument called a Brewer spectrophotometer.

This is done by measuring the amount of UV sunlight thatreaches the ground, and deducing from this how much UVabsorption occurred due to O3. Absorption by O3 occurs inthe UV-B region (λ = 0.29− 0.32m).

However, clouds and some aerosol particles also absorb inthis wavelength band. Therefore, a region of the electro-magnetic spectrum where O3 absorbs only weakly, but cloudsand aerosol absorb similarly to the UV-B region, is alsomonitored [to provide a reference].

By taking the ratio of the two measured values, absorptionby O3 in the total vertical column can be obtained.

Since 1960 remote sensing measurements from satellites haveprovided a wealth of information on the global distributionof O3 and the variations in its vertical profiles and columnabundance.

Ozone can be derived from satellite observations using anyof the four passive techniques depicted in ithe following fig-ure:

• backscatter UV (BUV)

• occultation

• limb emission

• limb scattering

Since 1960 remote sensing measurements from satellites haveprovided a wealth of information on the global distributionof O3 and the variations in its vertical profiles and columnabundance.

Ozone can be derived from satellite observations using anyof the four passive techniques depicted in ithe following fig-ure:

• backscatter UV (BUV)

• occultation

• limb emission

• limb scattering

For a description of these methods, see Wallace & Hobbs.

Four passive techniques for measuring ozone from satellites.10

Chapman’s Theory

In 1930 Sydney Chapmanproposed a simple chemi-cal scheme for maintainingsteady state concentrationsof O3 in an “oxygen-only”stratosphere.

These reactions are called theChapman reactions.

The Chapman reactions are as follows:

• The dissociation of O2 by solar UV radiation

O2 + hν → 2O

• The reaction of atomic oxygen and molecular oxygen toform O3

O + O2 + M→ O3 + M

O2 + hν → 2O

O + O2 + M→ O3 + M

• The photodissociation of O3

O3 + hν → O2 + O

O2 + hν → 2O

O + O2 + M→ O3 + M

• The photodissociation of O3

O3 + hν → O2 + O

• The combination of atomic oxygen and O3 to form O2

O + O3 → 2O2

The O atoms produced by this set of reactions undergo nu-merous collisions with N2 and O2 molecules, most of whichdo not result in any permanent products.

Occasionally, however, a three-body collision occurs in whichM absorbs the excess energy of the collision and a stable O3molecule is produced.

Note that the excess energy acquired by M, which is in theform of thermal energy that warms the stratosphere, derivesprimarily from the energy of the incoming photon, that is,from solar energy.

Since the concentration of M decreases with increasing al-titude, the time constant for converting O atoms and O2molecules to O3 increases with altitude (e.g., from a fewseconds at 40 km to ∼100 s at 50 km).

Schematic illustration of the Chapman reactions.

Stratospheric O3 concentrations exhibit minor diurnal vari-ations (analogous to the day-to-day variations in weather inthe troposphere).

After sunset, both the source and sink of O3 are switchedoff, the remaining O atoms are then converted to O3 withina minute or so.

The Chapman reactions reproduce some of the broad fea-tures of the vertical distribution of O3 in the stratosphere.For example, they predict that O3 concentrations shouldreach maximum values at an altitude of ∼25 km.

However, the Chapman reactions overpredict the concen-trations of stratospheric O3 by about a factor of two in thetropics and they underpredict O3 concentration in middleto high latitudes.

There is an equator-to-pole circulation in the stratosphere,known as the Brewer-Dobson Circulation, which transportsO3 from its primary source in the tropical stratosphere tohigher latitudes.

The Ozone Hole

The Ozone HoleIn the 1970s it was suggested that catalytic cycles involvingreactive chlorine (Cl) and bromine (Br) could be importantin the destruction of O3.

The natural sources of Cl and Br in the stratosphere are thedestruction of methyl chloride (CH3Cl) and methyl bromide(CH3Br).

The destruction of stratospheric O3 by the various chemi-cal mechanisms discussed above, and many other possiblemechanisms not discussed here, are not simply additive be-cause the species in one cycle can react with those in an-other.

Therefore, advanced numerical models that consider all theknown suspects must be used to unravel the relative impor-tance of the various mechanisms for destroying O3 in thestratosphere.

This task is complicated further by the fact that concentra-tions of many of the prime suspects are changing with timedue to anthropogenic emissions.

Most of the catalytic reactions that have been proposed forthe removal of stratospheric odd oxygen are of the form

X + O3 → XO + O2

XO + O → X + O2

−−−−−− − −−−−−−Net: O + O3 → 2O2

This task is complicated further by the fact that concentra-tions of many of the prime suspects are changing with timedue to anthropogenic emissions.

Most of the catalytic reactions that have been proposed forthe removal of stratospheric odd oxygen are of the form

X + O3 → XO + O2

XO + O → X + O2

−−−−−− − −−−−−−Net: O + O3 → 2O2

If the concentration of catalyst X is increased significantlyby anthropogenic activities, the balance between the sourcesand sinks of atmospheric O3 will be disturbed and strato-spheric O3 concentrations can be expected to decrease.

One of the first concerns in this respect was a proposal inthe 1970s to create a fleet of supersonic aircraft flying in thestratosphere. This is because aircraft engines emit nitricoxide (NO), which can decrease odd oxygen.

However, the proposed fleet of supersonic aircraft was re-jected on both environmental (?) and economic grounds.

One of the first concerns in this respect was a proposal inthe 1970s to create a fleet of supersonic aircraft flying in thestratosphere. This is because aircraft engines emit nitricoxide (NO), which can decrease odd oxygen.

However, the proposed fleet of supersonic aircraft was re-jected on both environmental (?) and economic grounds.

Of much greater concern, with documented impacts, is thecatalytic action of chlorine, from industrially manufacturedchlorofluorocarbons (CFCs), in depleting stratospheric ozone.

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons (CFCs)Chlorofluorocarbons (CFCs) are compounds containing chlo-rine fluorine and carbon.

Halons are compounds that contain, in addition, bromine.

Compounds containing H, F and C are called hydrofluoro-carbons (HFCs)

Compounds containing H, Cl, F, and C are called hydrochlo-rofluorocarbons (HCFCs).

HFCs and HCFCs are considered to be more “environmen-tally friendly” than CFCs because they are partially de-stroyed by OH in the troposphere.

CFC-11 (CFCl3) and CFC-12 (CF2Cl2) are the most com-mon CFCs. CFCs were first synthesized in 1928, as theresult of a search for a nontoxic, nonflammable refrigerant.

During the 20th century CFCs were marketed under thetrade name Freon and became widely used as

• refrigerants

• propellants in aerosol cans

• refrigerants

• inflating agents in foam materials

• refrigerants

• solvents

• refrigerants

• solvents

• cleansing agents

Concern about the effects of CFCs on the atmosphere be-gan in 1973 when it was found that they were spreadingglobally and, because of their inertness, were expected tohave residence times of up to several hundred years in thetroposphere.

Such long-lived compounds eventually find their way intothe stratosphere where, near an altitude of ∼20 km, they ab-sorb UV radiation in the wavelength interval 0.19–0.22 µmand photodissociate:

Concern about the effects of CFCs on the atmosphere be-gan in 1973 when it was found that they were spreadingglobally and, because of their inertness, were expected tohave residence times of up to several hundred years in thetroposphere.

Such long-lived compounds eventually find their way intothe stratosphere where, near an altitude of ∼20 km, they ab-sorb UV radiation in the wavelength interval 0.19–0.22 µmand photodissociate:

CFCl3 + hν → CFCl2 + Cl

andCF2Cl2 + hν → CF2Cl + Cl

The chlorine atom released by these reactions can serve asthe catalyst X and destroy odd oxygen in the cycle

Cl + O3 → ClO + O2

ClO + O → Cl + O2

−−−−−− − −−−−−−Net: O + O3 → 2O2

The chlorine atom released by these reactions can serve asthe catalyst X and destroy odd oxygen in the cycle

Cl + O3 → ClO + O2

ClO + O → Cl + O2

−−−−−− − −−−−−−Net: O + O3 → 2O2

In 1990, ∼85% of the chlorine in the stratosphere originatedfrom anthropogenic sources. Because CFCs absorb stronglyin the infrared, they are also significant greenhouse gases.

The first evidence of a significant depletion in stratosphericO3 produced by anthropogenic chemicals in the stratospherecame, surprisingly, from measurements in the Antarctic.

In 1985, British scientists, who had been making ground-based, remote sensing measurements of O3 at Halley Bay(76◦S) in the Antarctic for many years, reported that therehad been about a 30% decrease in total O3 column abun-dance each October (i.e., in the austral spring) since 1977.

These observations were subsequently confirmed by remotesensing measurements from satellite and by airborne in situmeasurements.

Shown in the next Figure are a series of satellite measure-ments of the total O3 column in the southern hemisphere.

Satellite observations of the total ozone column (DU) in the southern

hemisphere during October for eight years from 1970-2002.

The high O3 values (red and orange colours) that encirclethe Antarctic continent and the lower O3 values (green) overthe continent itself in the 1970s (upper four images) areconsidered to be close to natural.

The Antarctic ozone hole, as it came to be called, is appar-ent in the lower four images in the Figure as the anoma-lously low O3 values (blue) over the continent in Octoberfrom 1999-2002.

The areal extent of the Antarctic ozone hole, derived fromsatellite measurements for the period September 7 to Octo-ber 13 1979-2002 are shown next.

From 1979 to 2001 the ozone hole grew progressively until itoccupied an area (∼25 million km2) similar to that of NorthAmerica.

Average areal extent of the ozone hole (<220 DU)from September 7-October 13, 1979-2003.

The presence of an ozone hole over the Antarctic raisedseveral intriguing questions.

• Why over the Antarctic?

• Why during spring?

• The magnitude of the measured decreases in O3 weremuch greater than predictions based solely on gas-phasechemistry of the type outlined above. Why?

• Why did the size of the ozone hole decrease sharply in2002?

The research that led to the answers to these questions,which is outlined below, was one of the triumphs of 20th

century science.

The research that led to the answers to these questions,which is outlined below, was one of the triumphs of 20th

century science.

Paul Crutzen, Mario Molina, and Sherwood Rowland wereawarded the Nobel Prize in Chemistry in 1995 for theirexplanation of stratospheric O3 loss by CFCs and nitrogen-containing gases.

Mechanism of O3 Hole

Mechanism of O3 HoleDuring the austral winter (June-September) stratosphericair over the Antarctic continent is restricted from interact-ing with air from lower latitudes by a large-scale vortexcirculation, which is bounded by strongly circulating winds.

Because of the lack of solar heating in the austral winter,the air within the vortex is extremely cold and thereforeslowly sinks.

High-level clouds, called polar stratospheric clouds (PSCs),form in the cold core of the vortex, where temperatures canfall below −80◦C.

In the austral spring, as temperatures rise, the winds aroundthe vortex weaken, and by November the vortex disappears.

The vortex serves as a giant and relatively isolated chemicalreactor in which unique chemistry can occur.

For example, although the concentrations of O3 in the vor-tex are normal in August, the concentrations of ClO in thevortex are ten times greater than just outside the vortex col-lar and, by September, O3 concentrations within the vortexdecrease dramatically.

There are also sharp decreases in the oxides of nitrogen andwater vapour when passing from the outside to the insideof the vortex collar.

These decreases are due, respectively, to the formation ofnitric acid (HNO3), and to the condensation of water at thevery low temperatures inside the vortex.

Polar Stratospheric Clouds

Polar Stratospheric CloudsType I PSCs. These probably consist of a mixture of liquid

and solid particles of nitric acid trihydrate [HNO3(H2O)3– NAT for short], water and sulfuric acid, which condenseat about −78◦C. These particles are ∼1 µm in diameter,so they sediment very slowly (∼10 m per day).

Type II PSCs. These consist of a mixture of ice and watertogether with some dissolved HNO3, form near ∼ 85◦C.Since these particles are > 10 µm in diameter, they settleout with appreciable speeds (∼1.5 km per day).

Type III PSCs. These “mother-of-pearl” clouds are producedby the rapid freezing of condensed water in air flow overtopography. However, they are of limited extent and du-ration and do not form over the South Pole.

Schematic of the polar vortex (blue) over Antarctica.

As the particles in PSCs slowly sink, they remove both wa-ter and nitrogen compounds from the stratosphere, referredto as dehydration and denitrification

On the surface of the ice particles that form PSCs, the fol-lowing heterogeneous reactions are thought to be important:

As the particles in PSCs slowly sink, they remove both wa-ter and nitrogen compounds from the stratosphere, referredto as dehydration and denitrification

On the surface of the ice particles that form PSCs, the fol-lowing heterogeneous reactions are thought to be important:

ClONO2(g) + HCl(s) → Cl2(g) + HNO3(s)

ClONO2(g) + H2O(s) → HOCl(g) + HNO3(s)

HOCl(g) + HCl(s) → Cl2(g) + H2O(s)

N2O5(g) + H2O(s) → 2HNO3(s)

N2O5(g) + HCl(s) → ClNO2(g) + HNO3(s)

[(s) indicates those compounds that are on (or in) ice par-ticles, and (g) indicates species that are released as gases.]

The above reactions convert the reservoir species ClONO2and HCl into Cl2, HOCl and ClNO2.

When the Sun rises in the Antarctic spring, these threespecies are photolyzed rapidly to produce Cl and ClO

Cl2 + hν → 2Cl

HOCl + hν → OH + Cl

ClNO2 + hν → Cl + NO2

andCl + O3 → ClO + O2

The above reactions convert the reservoir species ClONO2and HCl into Cl2, HOCl and ClNO2.

When the Sun rises in the Antarctic spring, these threespecies are photolyzed rapidly to produce Cl and ClO

Cl2 + hν → 2Cl

HOCl + hν → OH + Cl

ClNO2 + hν → Cl + NO2

andCl + O3 → ClO + O2

Ozone is then destroyed efficiently by

ClO + ClO + M → (ClO)2 + M

(ClO)2 + hν → Cl + ClOO

ClOO + M → Cl + O2 + M

2Cl + 2O3 → 2ClO + 2O2

−−−−−−−− − −−−−−−−−Net: 2O3 + hν → 3O2

Schematic of the polar vortex (blue) over Antarctica.Again!

Comments on Chemical Cycle

Comments on Chemical Cycle• It is a catalytic cycle in which ClO is the catalyst, because

two ClO molecules are regenerated for every two ClOmolecules that are destroyed.

• The reactions do not depend on atomic oxygen (which isin short supply).

• The Cl atom in the ClO derives from Cl released fromCFCs. However, as we have seen, the Cl atom is thennormally quickly tied up as HCl and ClONO2.

• In the presence of PSCs, Cl2, HOCl and ClNO2 are re-leased and, as soon as the solar radiation reaches sufficientintensity in early spring, Cl and ClO are released, leadingto the rapid depletion of O3 in the Antarctic stratosphere.

• The dimer (ClO)2 is formed only at low temperatures.Low enough temperatures are present in the Antarcticstratosphere before the Spring.

Schematic to illus-trate the time evo-lution of the mainprocesses associatedwith the develop-ment of the Antarc-tic ozone hole.(a) The polar vortex.(b) Chlorine species.(c) Ozone.

Spring of 2002

Spring of 2002The sharp decrease in the area covered by the Antarcticozone hole in 2002, and the decrease in the severity of thehole in that year is due to a series of unusual stratosphericwarmings that occurred during the winter in the southernhemisphere in 2002.

This warmed the polar vortex collar region, which cut offthe formation of PSCs, and turned off O3 loss in September2002, and transported O3-rich air over Antarctica.

Spring of 2002The sharp decrease in the area covered by the Antarcticozone hole in 2002, and the decrease in the severity of thehole in that year is due to a series of unusual stratosphericwarmings that occurred during the winter in the southernhemisphere in 2002.

This warmed the polar vortex collar region, which cut offthe formation of PSCs, and turned off O3 loss in September2002, and transported O3-rich air over Antarctica.

In 2003, which was a cold year, the ozone hole returned toits earlier extent of ∼25 million square kilometers.

What about the Arctic?

What about the Arctic?Does an ozone hole, similar to that in the Antarctic, developin the Arctic stratosphere during winter in the northernhemisphere?

Although a vortex can develop over the Arctic in the north-ern winter, it is generally not as well developed as theAntarctic vortex.

Also, temperatures in the Arctic stratosphere are not as lowas those in the Antarctic stratosphere.

Although the conditions that produce severe loss of ozonein the Antarctic are not as common in the Arctic, the to-tal ozone column in the Arctic spring has, on average, de-creased over the past two decades.

However, prior to the winter of 1995–96 there was not astratospheric ozone hole in the Arctic comparable to thatin the Antarctic.

During the northern winter of 1996–97, the longest lastingpolar vortex on record developed over the Arctic, and inMarch 1997 the average ozone column over the Arctic (354DU) was the lowest in 20 prior years of observations.

During 1998 a major stratospheric warming event erodedthe polar vortex, raised temperatures above that requiredfor the formation of PSCs, and transported ozone into thelower Arctic stratosphere. Consequently, in March 1998the ozone column over the Arctic returned to its pre-1990svalue.

During the northern winter of 1996–97, the longest lastingpolar vortex on record developed over the Arctic, and inMarch 1997 the average ozone column over the Arctic (354DU) was the lowest in 20 prior years of observations.

During 1998 a major stratospheric warming event erodedthe polar vortex, raised temperatures above that requiredfor the formation of PSCs, and transported ozone into thelower Arctic stratosphere. Consequently, in March 1998the ozone column over the Arctic returned to its pre-1990svalue.

In the winter of 1999–2000 the average total ozone columnin March poleward of 63◦N was lower by ∼75 DU than theaverage climatological value, and these low values continuedthrough 2003.

It appears therefore that while concentrations of CFCs re-main high in the atmosphere, the Arctic stratosphere hasthe potential to cause the same dramatic losses in O3 as theAntarctic stratosphere.

However, the combination of chemical and meteorologicalconditions that lead to such reductions are not as commonin the Arctic as they are in the Antarctic.

Global mean total column ozone for the period 1997–2001was ∼3% below the 1964–1980 average value.

No trends in total column ozone were observed in the tropics(25◦N–25◦S) during the period 1980–2000.

Average ozone columns between latitudes 63◦ − 90◦ for the northern

hemisphere in March and the southern hemisphere in October.

The International Response

The International ResponseConcerns about the health and environmental hazards ofincreased UV radiation at the Earth’s surface led to inter-national agreements to eliminate the production and use ofcompounds known to deplete stratospheric O3 by the year2000.

Consequently, CFCs in the lower atmosphere are no longerincreasing. An analysis (in 2003) of 20 years of satellitedata showed a slowing in the reduction of O3 at an altitudeof ∼33 km starting in 1997, and simultaneous slowing in thebuildup of harmful Cl.

However, due to the long lifetimes of CFCs, the concentra-tions of Cl in the stratosphere are expected to continue torise for some time.

Therefore, it is predicted that the O3 layer will not com-pletely recover until about the middle of the 21st century.

The Montreal Protocol

The Montreal ProtocolThe Montreal Protocol on Substances that Deplete the OzoneLayer was a landmark international agreement originallysigned in 1987, with substantial subsequent amendments.

Since it takes several decades for all of the air in the tro-posphere to cycle through the upper stratosphere, whereCFCs are broken down, it will take a similar period of timeto remove all of the CFCs from the atmosphere from thetime their production is brought completely to a halt.

The Montreal ProtocolThe Montreal Protocol on Substances that Deplete the OzoneLayer was a landmark international agreement originallysigned in 1987, with substantial subsequent amendments.

Since it takes several decades for all of the air in the tro-posphere to cycle through the upper stratosphere, whereCFCs are broken down, it will take a similar period of timeto remove all of the CFCs from the atmosphere from thetime their production is brought completely to a halt.

The Copenhagen Amendment (1992) called for the com-plete elimination of CFCs by 1996, and the Vienna Amend-ment (1995) called for the complete elimination of HCFCsby 2020.

An Interesting Historical Review

For an interesting review of the emergence of evidence onthe ozone hole in the Antarctic, see the article:

Data Collection and the Ozone Hole:Too much of a Good Thing?

Maureen Christie, Proc. ICHM, 1.1 (2004).On course website.

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