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Lecture 5a: Formation of Ozone Lecture 5a: Formation of Ozone HoleHole

Growth of the Antarctic ozone hole over 20 years, as observed by the satellite

Darkest blue areas represent regions of maximum ozone depletion.

延伸閱讀

• 揭開紫外線的奧秘 台灣地球日出版社• Stratospheric Ozone ： An Electronic

Textbook （ NASA ） http://see.gsfc.nasa.gov/edu/SEES/strat/class/S_class.htm

Brief History of Ozone Hole Discovery Brief History of Ozone Hole Discovery Ozone Production and DestructionOzone Production and Destruction Catalytic Cycle Catalytic Cycle Polar Stratospheric CloudsPolar Stratospheric Clouds CFC and Ozone DepletionCFC and Ozone Depletion

Source: “Atmosphere, Climate, and Change” by Graedel and Crutzen

臭氧濃度垂直分佈 氣溫垂直分佈

The History behind the Ozone Hole Dramatic loss of ozone in the lower stratosphere over Antarctica was first noticed in the 1970s by a research group from the British Antarctic Survey (BAS) who were monitoring the atmosphere above Antarctica from a research station much like the picture to the lower-right corner. Folklore has it that when the first measurements were taken in 1985, the drop in ozone levels in the stratosphere was so dramatic that at first the scientists thought their instruments were faulty. Replacement instruments were built and flown out, and it wasn't until they confirmed the earlier measurements, several months later, that the ozone depletion observed was accepted as genuine. Another story goes that the TOMS satellite data didn't show the dramatic loss of ozone because the software processing the raw ozone data from the satellite was programmed to treat very low values of ozone as bad readings! Later analysis of the raw data when the results from the BAS team were published, confirmed their results and showed that the loss was rapid and large-scale; over most of the Antarctica continent.

What Is Ozone And How Is It Formed? The existence of ozone (O3 : 3 oxygen atoms) is a natural process in the atmosphere. The earth's atmosphere is composed of several layers. We live in the "Troposphere" where most of the weather occurs; such as rain, snow and clouds.

Above the troposphere is the "Stratosphere"; an important region in which effects such as the Ozone Hole and Global Warming originate. Supersonic jet airliners such as Concorde fly in the lower stratosphere whereas subsonic commercial airliners are usually in the troposphere. The narrow region between these two parts of the atmosphere is called the “Tropopause”. Ozone forms a layer in the stratosphere, thinnest in the tropics (around the equator) and denser towards the poles.

The amount of ozone above a point on the earth's surface is measured in Dobson units (DU) - typically ~260 DU near the tropics and higher elsewhere, though there are large seasonal fluctuations. It is created when ultraviolet radiation (sunlight) strikes the stratosphere, dissociating (or "splitting") oxygen molecules (O2) to atomic oxygen (O). The atomic oxygen then quickly combines with oxygen molecules to form ozone.

It's ironic that at ground level, ozone is a health hazard - it is a major constituent of photochemical smog. However, in the stratosphere we could not survive without it. Up in the stratosphere it absorbs some of the potentially harmful ultra-violet (UV) radiation from the sun (at wavelengths between 240 and 320 nm, 1nm = 10-9m) which can cause skin cancer and damage vegetation, among other things.

Vertical Structure of Our Atmosphere

Troposphere

Stratosphere

Mes

osph

ere

The

rmos

pher

e

Exosphere

To compare, Mount Everest, the highest mountain, is about 8.8 km high. Concorde cruises at about 16km.

Over 90% of the earth's air mass lies below 10km.

To compare,

Radius of Earth = 6,377 km

Ozone Production and Ozone Production and DestructionDestruction

Chapman MechanismChapman Mechanism

Photodissociation(or photolysis)

visible light

(from The Earth System)

destroy O3 permanently

The reaction (2) becomes slower with increasing altitude while reaction (3) becomes faster. The concentration of ozone is a balance between these competing reactions. In the upper atmosphere, atomic oxygen dominates where UV levels are high. Moving down through the stratosphere, the air gets denser, UV absorption increases and ozone levels peak at roughly 20-25 km (i.e., lower stratosphere). As we move closer to the ground, UV levels decrease and ozone levels decrease. The layer of ozone formed in the stratosphere by these reactions is sometimes called the “Chapman layer”.

Source: WMO report 2003

總臭氧量的單位

300 Dobson Unit

總臭氧量南北分佈的季節變化

總臭氧量空間分佈的逐月變化

空氣的年齡：至流在大氣中的時間

年輕的空氣

老的空氣

The Ozone Hole often gets confused in the popular press and by the general public with the problem of global warming. Whilst there is a connection because ozone contributes to the greenhouse effect, the Ozone Hole is a separate issue. However it is another stark reminder of the effect of man's activities on the environment. Over Antarctica (and recently over the Arctic), stratospheric ozone has been depleted over the last 15 years at certain times of the year. This is mainly due to the release of man-

What Is the Ozone Hole?

made chemicals containing chlorine such as CFC's (ChloroFluoroCarbons), but also compounds containing bromine, other related halogen compounds and also nitrogen oxides (NOx). CFC's are a common industrial product, used in refrigeration systems, air conditioners, aerosols, solvents and in the production of some types of packaging. Nitrogen oxides are a by-product of combustion processes, e.g. aircraft emissions. The current levels of depletion have served to highlight a surprising degree of instability of the atmosphere, and the amount of ozone loss is still increasing.

Source: Halley Bay Station

It is important to appreciate that the atmosphere behaves differently from year to year. Even though the same processes that lead to ozone depletion occur every year, the effect they have on the ozone is altered by the meteorology of the atmosphere above Antarctica. This is known as the 'variability' of the atmosphere. This variability leads to changes in the amount of ozone depleted and the dates when the depletion starts and finishes. To illustrate this, the monthly averages for October, from 1980 to 1991, are shown above.

南極總臭氧量大量減少

1970 年代

1990 年代

瑞士測站的總臭氧量也同步減少

What Is Being Done? The first global agreement to restrict CFCs came with the signing of the Montreal Protocol in 1987 ultimately aiming to reduce them by half by the year 2000. Two revisions of this agreement have been made in the light of advances in scientific understanding, the latest being in 1992. Agreement has been reached on the control of industrial production of many halocarbons until the year 2030. The main CFCs will not be produced by any of the signatories after the end of 1995, except for a limited amount for essential uses, such as for medical sprays. The countries of the European Community have adopted even stricter measures than are required under the Montreal Protocol agreements. Recognizing their responsibility to the global environment they have agreed to halt production of the main CFCs from the beginning of 1995. Tighter deadlines for use of the other ozone-depleting compounds are also being adopted. It was anticipated that these limitations would lead to a recovery of the ozone layer within 50 years of 2000; the World Meteorological Organization estimated 2045 (WMO reports #25, #37), but recent investigations suggest the problem is perhaps on a much larger scale than anticipated.

臭氧洞（ <220DU ）的面積逐年增加

北美面積

南極大陸面積

南北極的比較

南極臭氧在春天迅速減少

But there was a problem with the Chapman But there was a problem with the Chapman theory. In the 1960s it was realized that the theory. In the 1960s it was realized that the loss of ozone given by reaction loss of ozone given by reaction (4)(4) was too was too slow. It could not remove enough ozone to slow. It could not remove enough ozone to give the values seen in the real atmosphere. give the values seen in the real atmosphere. There had to be other reactions, faster There had to be other reactions, faster reactions that were controlling the ozone reactions that were controlling the ozone concentrations in the stratosphere.concentrations in the stratosphere.

Missing Reactions

自然脫氮過程

氮肥

氟氯碳化物

火山活動

超音速飛機

宇宙射線

核彈試爆

Natural and anthropogenic ozone-destroyers

Other Ozone Destruction Other Ozone Destruction ProcessesProcesses

Other atmospheric trace constituents, such Other atmospheric trace constituents, such as nitrous oxide (N2O), water vapor, and freas nitrous oxide (N2O), water vapor, and freons, can also be photolyzed. They produce ons, can also be photolyzed. They produce highly reactive radicals that keep ozone abhighly reactive radicals that keep ozone abundances lower than they would otherwise undances lower than they would otherwise be.be.

These radicals include: nitric oxide (NO), atThese radicals include: nitric oxide (NO), atomic chlorine (Cl), bromine (Br) radicals, anomic chlorine (Cl), bromine (Br) radicals, and hydroxyl (OH) radicals.d hydroxyl (OH) radicals.

These radicals can destroy stratospheric ozThese radicals can destroy stratospheric ozone through one through “catalytic cycles”.“catalytic cycles”.

The Chlorine Catalytic CycleThe Chlorine Catalytic Cycle

The Catalyst

The Chlorine Catalytic CycleThe Chlorine Catalytic Cycle

(from WMO Report 2003)

The Nitrogen Catalytic CycleThe Nitrogen Catalytic Cycle

The Catalyst

(from The Earth System)

Chlorine SourcesChlorine Sources

(from WMO Report 2003)

Man-Made Sources for Man-Made Sources for CFCsCFCs

There are two kinds of CFCs: freon-There are two kinds of CFCs: freon-11 (CCl3F) and freon-12 (CCl2F2).11 (CCl3F) and freon-12 (CCl2F2).

Freon-11 has been used:Freon-11 has been used:

(1) as a propellant in spray cans(1) as a propellant in spray cans

(2) as a blowing agent for (2) as a blowing agent for producing foamsproducing foams

(3) to clean semiconductor chips.(3) to clean semiconductor chips.

Freon-12 has been used as Freon-12 has been used as

(1) a refrigerant(1) a refrigerant

(2) working fluid in most car air (2) working fluid in most car air conditioners.conditioners.

(from The Earth System)

Special Features of Polar Meteorology During the winter polar night, sunlight

does not reach the south pole. A strong circumpolar wind develops in the middle to lower stratosphere. These strong winds are known as the ”polar vortex”. This has the effect of isolating the air over the polar region.

Since there is no sunlight, the air within the polar vortex can get very cold. So cold that special clouds can form once the air temperature gets to below about -80C. These clouds are called Polar Stratospheric Clouds (see slide 20) but they are not the clouds that you are used to seeing in the sky which are composed of water droplets. PSCs first form as nitric acid trihydrate. As the temperature gets colder however, larger droplets of water-ice with nitric acid dissolved in them can form. These PSCs are crucial for ozone loss to occur. So, we have the first few ingredients for our 'ozone loss recipe'. We must have: Polar winter leading to the formation of the polar vortex which isolates the air within it. Cold temperatures; cold enough for the formation of Polar Stratospheric Clouds. As the vortex air is isolated, the cold temperatures persist.

The Polar VortexThe Polar Vortex

The wintertime The wintertime circulation over the circulation over the South Pole is South Pole is characterized by a characterized by a gigantic whirlpool of cold gigantic whirlpool of cold and dense air, called the and dense air, called the polar vortex.polar vortex.

The cold and dense cold The cold and dense cold air in the middle of the air in the middle of the vortex is subsiding.vortex is subsiding.

The sinking air carries The sinking air carries cloud particles along cloud particles along with it.with it.

Remove odd nitrogen Remove odd nitrogen from the stratosphere.from the stratosphere.

Very little ozone and odd Very little ozone and odd nitrogen can be brought nitrogen can be brought into the south pole.into the south pole.

Polar Stratospheric Clouds (PSCs)Polar Stratospheric Clouds (PSCs)

In winter the polar stratosphere is so cold (-80°C or below) that certain traIn winter the polar stratosphere is so cold (-80°C or below) that certain trace atmospheric constituents can condense.ce atmospheric constituents can condense.

These clouds are called “polar stratospheric clouds” (PSCs).These clouds are called “polar stratospheric clouds” (PSCs).

(Sweden, January 2000; from NASA website)

How PSCs Affect Ozone HoleHow PSCs Affect Ozone Hole The ice crystals in the polar stratospheric The ice crystals in the polar stratospheric

clouds provide surface for the ozone depletion clouds provide surface for the ozone depletion surface to occur more easily.surface to occur more easily.

On these cloud surfaces, certain forms of On these cloud surfaces, certain forms of chlorine that do not react with ozone are chlorine that do not react with ozone are converted into forms that do.converted into forms that do.

Polar stratospheric clouds set up the stage for Polar stratospheric clouds set up the stage for massive destruction of ozone to happen when massive destruction of ozone to happen when sunlight returns in the spring.sunlight returns in the spring.

Recipe for the Ozone Hole FormationRecipe for the Ozone Hole Formation The polar winter leads to the formation of the polar vortex The polar winter leads to the formation of the polar vortex

which isolates the air within it. which isolates the air within it. Cold temperatures form inside the vortex; cold enough for tCold temperatures form inside the vortex; cold enough for t

he formation of Polar Stratospheric Clouds (PSCs). As the vhe formation of Polar Stratospheric Clouds (PSCs). As the vortex air is isolated, the cold temperatures and the PSCs peortex air is isolated, the cold temperatures and the PSCs persist. rsist.

Once the PSCs form, heterogeneous reactions take place aOnce the PSCs form, heterogeneous reactions take place and convert the inactive chlorine and bromine reservoirs to nd convert the inactive chlorine and bromine reservoirs to more active forms of chlorine and bromine. more active forms of chlorine and bromine.

No ozone loss occurs until sunlight returns to the air inside No ozone loss occurs until sunlight returns to the air inside the polar vortex and allows the production of active chlorinthe polar vortex and allows the production of active chlorine and initiates the catalytic ozone destruction cycles. e and initiates the catalytic ozone destruction cycles.

Ozone loss is rapid. The ozone hole currently covers a geogOzone loss is rapid. The ozone hole currently covers a geographic region a little bigger than Antarctica and extends neraphic region a little bigger than Antarctica and extends nearly 10km in altitude in the lower stratosphere. arly 10km in altitude in the lower stratosphere.

The 1997 Ozone HoleThe 1997 Ozone Hole

Why No Ozone Hole in Artic?Why No Ozone Hole in Artic?

(from WMO Report 2003)

Thinning of Ozone Layer in Arctic Region

Lecture 5b: Acid RainLecture 5b: Acid Rain

酸雨正常雨水： pH 5.6

CO2( 氣 ) + H2O → CO2 H2O ‧ （溶解）CO2 H2O (‧ 溶解） H+ + HCO3

-

SO2 + H2O H+ + HSO3-

HSO3- + H2O2 HSO4

- + H2OHSO3

- + O3 HSO4- + O2

HSO4- H+ + SO4

2-

NO2 + HO + M → HNO3 + MHNO3 ( 氣 ) HNO3 （溶解）HNO3 ( 溶解 ) → H+ + NO3

-

水

Source: “Atmosphere, Climate, and Change” by Graedel and Crutzen

From: “Atmosphere, Climate, and Change” by Graedel and Crutzen

酸雨對建築物的影響

酸雨對植物、土壤的影響

酸雨對湖泊的影響湖泊酸化

酸雨對湖泊的影響=> 重金屬含量增加

酸雨對魚類的影響

鱸魚

軟體動物

負鼠蝦

鮭魚

鱒魚

刺蛄

鰷魚

梭子魚

二氧化硫的排放趨勢

高煙囪政策污染物的跨國傳送：國際問題

多少二氧化硫來自其他國家？

境外傳

入

Dirty

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