Ozone Layer Depletion

Ozone Layer Depletion

Introduction 

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Ozone is a bluish gas located in the stratosphere which protects the earth by absorbing UV-B and prevents this harmful radiation from reaching the earth. Research has shown that the ozone is slowly being depleted. We will discuss:  The causes of ozone depletion  The impacts ozone depletion has on the environment  The current status of the ozone  Solutions to the problem

What is ozone? Ozone is a stable molecule composed of three oxygen atoms. While stable, it is highly reactive. The Greek word ozein means “to smell” and O3 has a strong pungent odor. Electric discharges in air often produce significant quantities of O3 and you may have smelled O3 near these sources.

Ozone in the atmosphere

The ozone layer

Ultraviolet protection by ozone

Ozone absorbs UV light in the solar irradiation that is harmful to life

Ultraviolet protection by ozone

The overlap of ground level radiation with the sunburn sensitivity curve would be much greater without the filtering effects of the ozone layer.

Ozone formation and destruction in the stratosphere

Chapman Theory • O2+ hv (<242nm) -> 2O b) O+O2+M -> O3+M c) O3 + hv (<320nm)O +O2 d) O + O32O2

formatio n Destructio n

Where M is a random air molecule (O2 or N2)

Steady-state O3 concentration

k a kb [M ] 1/ 2 [O3] =( ) [O 2] kc kd

Chapman theory describes how sunlight converts the various forms of oxygen from one to another, explains why the highest content of ozone occur in the layer between 15 and 50 km,

Prediction by Chapman theory vs. Observation

Using Chapman theory

There must be other O3 destruction pathways Catalytic ozone destruction X + O3 = XO + O2 XO + O = X + O2 Net reaction O + O3 = 2 O2 X is a regenerated in the process – act as a catalyst. The chain reaction continues until X is removed by some side reaction.

The important catalysts for stratospheric O3 destruction 

Hydroxy radical (OH) .

OH + O3 = HO2. + O2 HO2. + O = .OH + O2 Net: O + O3 = 2 O2

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Chlorine and bromine (Cl and Br) .

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Cl + O3 = ClO + O2 ClO. + O = Cl. + O2 Net: O + O3 = 2 O2 

Nitric oxide (NO) NO + O3 = NO2 + O2 NO2 + O = NO + O2 Net: O + O3 = 2 O2

HOx cycle

ClOx cycle

NOx cycle

The two-sided effect of NOx

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NOx provides a catalytic chain mechanism for O3 destruction. NOx inhibit the HOx and ClOx cycles for O3 destruction by removing radical species in the two cycles. The relative magnitude of the two effects is altitude dependent.  

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>25 km, the net effect is to destruct O3. (NOx accounts for >50% of total ozone destruction in the middle and upper troposphere.) In the lower stratosphere, the net effect is to protect O3 from destruction.

The catalytic destruction reactions described so far, together with the Chapman cycle, account for the observed average levels of stratospheric ozone, they are unable to account for the ozone hole over Antarctica. The ozone depletion in the Antarctica is limited both regionally and seasonally. The depletion is too great and too sudden. These observations can not be explained by catalytic O3 destruction by ClOx alone.

Causes of Depletion 

According to the Environmental Protection Agency, the discovery of an “ozone hole” over Antarctica in 1985 focused attention on the idea that humans can have a significant impact on the global environment. There are also a number of natural causes of ozone depletion.

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When the following substances reach the stratosphere, they break down under intense ultraviolet light, and release chlorine or bromine atoms, which degrade the ozone.

Chlorofluorocarbons (CFCs) 

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CFCs is the abbreviated form of ChloroFluoroCarbons, a collective name given to a series of compounds containing chlorine, fluorine and carbon atoms. Examples: CFCl3, CF2Cl2, and CF2ClCFCl2. Related names  HCFCs: Hydrochloroflorocarbons, halocarbons containing hydrogen atoms in addition to chlorine, fluorine and carbon atoms.  HFCs: hydroflorocarbons, halocarbons containing atoms of hydrogen in addition to fluorine and carbon atoms.  Perhalocarbons: halocarbons in which every available carbon bond contains a haloatoms.  Halons: bromine-containing halocarbons,

Chlorine atom Sources: Photolysis of Cl-containing compounds in the stratosphere. CFCl3 + hv (185-210nm) CFCl2. + Cl. CF2Cl2 + hv (185-210nm) CF2Cl. + Cl. Subsequent reactions of CFCl2 and CF2Cl more Cl atoms The principal Cl-containing species are: CF2Cl2, CFCl3, CFCl2, CF2Cl, CCl4, CH3CCl3, Sources for Cl-containing compounds (need to be CF2HCl,inCH3Cl long-lived the troposphere) •Man-made: e.g. CFCs •Natural: e.g. methyl chloride from biomass burning.

Chlorine atom (Continued) Termination reactions for Cl Cl. + CH4  CH3. + HCl

Stable in the stratosphere Removed from air by precipitation when it migrates to the troposphere

ClO. + NO2 + M ClONO2 + M Reservoir species

Relatively unreactive but can regenerate reactive species upon suitable conditions ClONO2 + hv ClO + NO2

Nitric oxide 

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NO is produced abundantly in the troposphere, but all of it is converted into NO2 HNO3 (removed through precipitation) NO in the stratosphere produced from nitrous oxide (N2O), which is much less reactive than NO. N2O + hv N2 + O (90%) N2O + O 2 NO (~10%)

Removal processes: Inhibit the NO2 + .OH HNO3 ClO. + NO2 ClONO2

HOx and ClOx cycles

Hydroxy radical 

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Accounts for nearly one-half of the total ozone destruction in the lower stratosphere (16-20 km). Sources O3 + hv (<325nm) = O2 + O1D (2%) = O2 + O3P (98%) O1D + H2O = 2 .OH (major) O1D + CH4 = .OH +CH3. (minor)

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Termination reaction .

OH + NO2 HNO3

How Humans Cause Depletion 

CFC’s (chlorofluorocarbons)     

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Coolants for refrigerators Aerosol propellants Cleaning solvents Electric equipment Blowing agents to produce plastic foam and insulation

Halon 

Fire Extinguishing agent (only until 1994)

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Carbon Tetra Chloride   

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Fire Extinguishers Aerosol Spray Propellants Dry Cleaning

Methyl Chloroform 

Industrial Solvents

Natural Causes of Ozone Depletion 

Aerosols emitted from:    

Volcanic Eruptions The Ocean Cow Farts Burning Fossil Fuels

Ozone Depletion Potential (ODP) The ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11. Ozone Depletion Potentials (ODP) 7

ODP (averages)

6 5 4 3 2 1 0

CFC's

Halons

Carbon Tetra Chloride

Substances

Methyl Chloroform

Environmental Impacts 

Increase in UV-B reaching the earth’s surface, which causes harm to :    

Humans Animals Plants and Agriculture The Ocean and Aquatic Ecosystems

Impact on Humans and Animals 

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Damaging health effects primarily with skin, eyes, and immune system Reduced air quality Human exposure to UV-B depends on    

Individual’s location Duration and timing of outdoor activities Precautionary behavior Skin color and age

Plants and Agriculture      

Reduction of air quality reduces crop yields Decrease in photosynthetic activity Susceptibility to disease Changes in plant structure and pigmentation Retardation of growth Field Study: Soybean Harvests

Ocean and Aquatic Ecosystems  

Diminishes productivity of the oceans Decreases species such as fish and shrimp 

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Humans and other consumers are dependent on these higher species Populations outside the local ecosystem are potentially at risk

Status of Ozone Depletion 

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Ban of production and consumption of compounds that deplete the ozone layer. Air Quality Improvements Statistically New Technology

Solutions  

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Many substitute products have been made Increased public knowledge of ozone depletion New Technology Policy and Regulations

Policy 

1987, The Montreal Protocol was signed  

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1990 Clean Air Act Amendments 

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Ban of CFC production More than 160 countries have signed the treaty Established U.S. regulatory program to protect the stratospheric ozone layer

Individual and Corporate Responsibility