hÓa hỌc mÔi trƯỜng khÔng khÍ gv: ths. nguyỄn thanh hẢi bài giảng cho lớp mt k30

HÓA HỌC MÔI TRƯỜNGKHÔNG KHÍ

GV: ThS. NGUYỄN THANH HẢI

Bài giảng cho lớp MT K30

ThS. Nguyễn Thanh Hải

TẦNG OZONE

Cơ chế tạo thành ozone

O2 + h2O

Sự phân hủy ozone tự nhiênO3 + hO + O2

O3 + O 2O2

2O3 + h 3O2

O2 + O + M O3 + M

3O2 +h2O3

Các tia UV

• UVA: 320 – 400 nm

• UVB: 290 – 320 nm

• UVC: <290 nm

TẦNG OZONE

a) O2+ hv (<242nm) -> 2O

b) O+O2+M -> O3+M

c) O3 + hv (<320nm) O +O2

d) O + O32O2

M là một phân tử khí trong khí quyển (O2 hoặc N2)

Phổ hấp thu của DNA

Tia UV làm các phân tử DNA tham gia các phản ứng quang hóa

TẦNG OZONE• Hydroxyl (

.OH + O3 = HO2

. + O2

. + O =

.OH + O2

O + O3 = 2 O2

• Chlorine và bromine (Cl và Br)

Cl. + O3 = ClO

. + O2

ClO. + O = Cl

. + O2

O + O3 = 2 O2

• Nitric oxide (NO)NO + O3 = NO2 + O2

NO2 + O = NO + O2

O + O3 = 2 O2

TẦNG OZONE

• Vùng UV-B mở rộng tới vùng UV-C khiến ozone không thể loại bỏ hết các tia UV

Ảnh hưởng của sự suy giảm ozone

• The UV-B region will expand as there will be insufficient ozone to remove all the UV light

Effect of Ozone Depletion

• Currently, one in five Americans will develop skin cancer in their lifetime

Basal cell carcinomaBasal cell carcinoma

• 1-2% increase in skin cancer for each 1% decrease in ozone

• Naturally, ozone is continually formed and destroyed by light

Causes of Ozone Depletion

Formation 3O2 +h2O3

Removal 2O3 + h 3O2

• This results in a steady state concentration of ozone in the stratosphere

• There are minor depletion reactions caused by other stratospheric gases including water vapor, nitrogen oxides, methane, and methyl bromide (produced from biological activity on sea-salt)

• Human activity increases the amount of naturally occurring methane and nitrogen oxides in the stratosphere

Anthropogenic Ozone Depletion

• The major causes are photochemical decomposition of chlorinated and brominated hydrocarbons

CFClCFCl33 (CFC-11) (CFC-11)

• Ultraviolet light causes photochemical breakdown, releasing Cl or Br free radicals

• The mechanism of the reaction is a typical free radical reaction

CFClCFCl33 + + hhCFClCFCl22· + Cl·· + Cl·

• A free radical can only be destroyed by reaction with another free radical• Steps 2 and 3 in the mechanism form a cycle, so one atom of Cl produced in step 1 can

destroy many thousands of O3 molecules

ClO· + ·O ClO· + ·O OO22 + Cl· + Cl·

Cl· + OCl· + O33 ClO· + OClO· + O22

• The ultimate fate of the Cl atom may be to react with CH4

Cl· + CHCl· + CH44CHCH33· + HCl· + HCl

• The CFCl2· probably continues to be photolysed to produce more free chlorine

CFClCFCl22· + · + hhCFCl + Cl·CFCl + Cl·

• HCl eventually washes out into the troposphere, but it acts as a reservoir of Cl atoms that is the cause of the polar ozone holes

Polar Ozone Depletion• The polar ozone holes

are caused by a different mechanism, in which polar stratospheric clouds of nitric acid trihydrate act as a catalytic surface for the reaction of chlorine carriers (HCl and ClONO2)

Lifetimes of CFC’s• One of the primary problems with CFC’s is that

they do not react in the troposphere, so can diffuse into the stratosphere for a very long time

CFC-11CFC-11TrichlorofluoromethaneTrichlorofluoromethane (45 years)(45 years)

CFC-115CFC-115MonoMonochloropentafluoroethanechloropentafluoroethane (1700 years)(1700 years)

• Adding hydrogen to the molecule dramatically speeds up its decomposition in the troposphere

HCFC-21HCFC-21DichlorofluoromethaneDichlorofluoromethane (2 years)(2 years)

The Montreal Protocol• Originally ratified in 1987, it has been amended

5 times (1990, 1992, 1995, 1997, 1999) Only 12 countries have not ratified this treatyOnly 12 countries have not ratified this treaty

•Equatorial Guinea•Eritrea•Guinea-Bissau

•Afghanistan•Bhutan•Cook Islands

•Iraq•Niue•East Timor

•Andorra•Holy See•San Marino

• Limitations on the use of CFC’s in aerosol containers began voluntarily in the 1970’s

• CFC’s have not been used in aerosols in Canada since mid 1980’s (exception some medical inhalers until 2005)

The Montreal Protocol• Phaseout of production

of Class I ozone depleting substances by 1995

chlorofluorocarbons halons carbon tetrachloride methyl chloroform hydrobromofluorocarbons methyl bromide (2005)

• Phaseout of production of Class II HCFC’s by 2030 by developed nations

The Canadian Picture We have dramatically reduced our consumption of We have dramatically reduced our consumption of ozone depleting substances.ozone depleting substances.

The Future• Although no longer allowed, there are still large

amounts of CFC’s in already produced goods.

The Future• Although no longer allowed, there are still large

amounts of CFC’s in already produced goods.

It is estimated that the It is estimated that the ozone layer will not return ozone layer will not return to its pre-1980 level until at to its pre-1980 level until at least 2050.least 2050.

The Greenhouse Effect

• A misnomer, since the way the atmosphere retains heat has little to do with the way a greenhouse actually works.

• Greenhouses prevent convection. The atmospheric greenhouse effect works by absorption and re-radiation.

Historical Background

• Joseph Fourier (1768–1830) studied the role of solar radiation

• John Tyndall (1820–1886) in 1859 emphasized the role of water vapor

• Svante Arrhenius (1859 - 1927) emphasized the role played by carbon dioxide

How the Greenhouse Effect Actually Works

• The sun is a blackbody radiator with a temperature of about 5700 K

• Gases in the earth’s atmosphere absorb energy where they have allowed quantum transitions

• Direct absorption of solar radiation contributes very little to atmospheric heating

• There is a steady state between the amount of solar radiation the earth receives, and what it re-radiates back into space

• About 70% of the sun’s irradiation reaches the earth’s surface (surface albedo)

• The average temperature of the earth is about 15 ºC (288 K)

• The earth re-radiates as a black body radiator at a much lower temperature than the sun

• A very large part of this energy is absorbed by water vapor in the earth’s atmosphere

• A much smaller amount of the earth’s radiant energy is absorbed by CO2

• CO2 is important as it absorbs strongly in a region where water does not

• Other gases also contribute to make up the total absorption spectrum of the earth’s atmosphere

• There is a large atmospheric window in the visible, and a much smaller one in the infrared

• The net result is that the earth’s atmosphere is warmer because it absorbs the infrared energy radiated by the earth

• A 235 W/m2 radiating black body would have a temperature of -18 ºC

• Since the earth’s actual average temperature is about 15 ºC, the greenhouse effect contributes approximately 33 º of heating

Greenhouse Gases

• Water vapor is by far the most important greenhouse gas

Greenhouse Gases

• Animal respiration and oceanic evaporation release 150 billion T of CO2 per year

• Anthropogenic sources– Carbon dioxide is the most

important anthropogenic greenhouse gas (7 billion T per year)

– Primary source is from combustion

C8H18 + 25/2 O2 8 CO2 + 9 H2O

Greenhouse Gases

• Methane (CH4) is naturally present in the atmosphere from the decomposition of organic material

• Anthropogenic sources– Production and transport of

coal, natural gas, and oil

• Huge quantities of methane are trapped in permafrost

– Decomposition of organic substances in municipal solid waste landfills, rice paddies

– Raising of livestock

Greenhouse Gases

• Nitrous oxide (N2O) forms in tropical soils and is also released from the ocean

• Anthropogenic sources– Agricultural fertilizers

– Biomass burning

– Industrial manufacturing (nylon, nitric acid)

Greenhouse Gases

• There are no natural sources of CFC’s, HCFC’s, or SF6

• Anthropogenic sources– Aerosols, foam manufacture, air conditioning

and refrigeration

– HCFC’s are much less damaging to the ozone layer, but equally or even more potent greenhouse gases

Greenhouse Gases• The global warming potential (GWP) of a

greenhouse gas depends on– its ability to absorb infrared light

– its lifetime in the troposphere

Greenhouse Gases

• The main greenhouse gases have vastly different concentrations in the atmosphere

Greenhouse Gases

• The net effect for a greenhouse gas is found by multiplying its atmospheric concentration times its GWP

Greenhouse Gas Feedback

• One of the reasons greenhouse gases are predicted to have such a significant effect, even though they are present in small quantities, is because of the synergistic effect on water.

• Increasing global temperature will raise the atmospheric concentration of H2O. Water vapor is the most important greenhouse gas.

• Increasing water vapor will also cause more cloud formation, and the difficulty in estimating this effect is one of the major limitations on climate prediction models.

CO2 and Temperature

• There is a strong historical correlation between atmospheric CO2 concentration and temperature

CO2 and Temperature

• There has been a rapid rise in CO2 concentration since the industrial revolution, together with a large temperature increase

Correlation Does Not Equal Causation

• There is a theoretical reason why increasing atmospheric CO2 concentration should cause a temperature rise

The Ozone Depletion - Global Warming Link

• In general the public incorrectly links ozone depletion and global warming, often considering them the same thing.

• Many of the HCFC replacements for CFC’s are much stronger infrared absorbers than either CO2 or CFC’s.

• As these compounds replace the CFC’s, there is potential for new and important greenhouse gases.

Is Ethanol a Better Fuel?• Burning ethanol does not cause a significant reduction in CO2

production

CH3CH2OH (g) + 3O2 (g) 2CO2 (g) + 3H2O (l)

Hc = -1277 kJ

C8H18 (g) + 25/2O2 (g) 8CO2 (g) + 9H2O (l)

Hc = -5116 kJ

• You have to burn more moles of ethanol to get the same amount of energy as from gasoline

Is Ethanol a Better Fuel?• Comparing the amount of carbon dioxide produced per unit of energy

produced shows that ethanol and gasoline are virtually the same

C8H18 1.56 x 10-3 mol CO2/kJ

CH3CH2OH 1.57 x 10-3 mol CO2/kJ

Is Ethanol a Better Fuel?

Is Ethanol a Better Fuel?• The CO2 produced by burning the ethanol

was just removed from the atmosphere by photosynthesis in the plant

• However, enormous quantities of CO2 will also be produced by agricultural practice, transportation, production and distillation of the ethanol

Is Ethanol a Better Fuel?• Ethanol production for fuel is about equal to the

combustion of fossil fuels

hÓa hỌc mÔi trƯỜng khÔng khÍ gv: ths. nguyỄn thanh hẢi bài giảng cho lớp mt k30

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