Chapter 2 - Protecting the Ozone Layer

Stratosphere: (2)

15 km - 30 km(20 mile) - contains ozone layer beneficial to us.

Outer (valence) electrons

electrons in the outermost shell of an atom.

Troposphere: (2)

0 - 15 km(10 mile) - contains ozone as pollutant harmful to us.

HCFCs and CFCs (2)

▪ HCFCs are alternatives to CFCs: - they decompose more readily in the troposphere so they will not accumulate to the same extent in the stratosphere.

Biological Effects of Ultraviolet Radiation (6)

▪ The consequences depend primarily on: 1. The energy associated with the radiation. 2. The length of time of the exposure. 3. The sensitivity of the organism to that radiation. ▪ The most deadly form of skin cancer, melanoma, is linked with the intensity of UV radiation and the latitude at which you live. - An Australian product uses "smart bottle" technology; bottle color changes from white to blue when exposed to UV light.

The Electromagnetic Spectrum

The various types of radiation seem different to our senses, yet they *differ* only in their respective λ and ν.

nm

nano meter = 10^-9 m = 0.0000004 inch

Octet rule:

tendency of an atom to attain the status of having eight electrons.

Ozone Layer

the stratospheric region of maximum ozone concentration that protects us from being exposed to UV light

Absorption of UV light (7)

▪ Absorption (filtering) of the UV light by the stratospheric ozone is very important in protecting living things from the damaging effects as illustrated in Figure 2.11. ▪ Unfortunately, the average stratospheric ozone concentration has dropped significantly in the last 20-30 years. ▪ Living things are now exposed to greater intensities of damaging radiation than in the past. For example, a 6 % decrease in stratospheric ozone could mean a 12 % rise in skin cancer. ▪ Wearing protective sunscreen is one way to reduce the risk of skin cancer. Such products contain compounds that absorb UV-B to some extent together with others for absorbing UV-A. ▪ However, because sunscreens allow you to be exposed for a longer time without burning, they may ultimately cause greater skin damage ▪ Not only skin damage but also retinal damage can take place. Cataracts, a clouding of the lens of the eye, can also be caused by excessive exposure to UV-B. Wearing optical-quality sunglasses capable of blocking at least 99 % of UV-A and UV-B is a sensible action for protecting the eyes. ▪ Other creatures than human beings are also affected by UV radiation; increase in UV radiation will bring harm to young marine life, such as floating fish eggs, fish larvae, phytoplankton, etc. In a global perspective, the effects of the increase in UV radiation on phytoplankton are particularly noteworthy because the effects are linked to global warming. We will see how the effects are linked to global warming in the last section of this chapter.

II. Introduction to ozone problem - Formation of ozone in the stratosphere: (4)

▪ As oxygen gas absorbs energy from lightning, it becomes ozone: - 3O2 + energy → 2O3 ▪ Ozone in the stratosphere filters ultraviolet light from the sun. - So ozone protects us from damaging solar radiation.

VIII. Replacements for CFCs (5)

▪ CFCs are in the process of phase out and alternative materials are being developed and used for refrigerants, etc. ▪ Factors to be considered when chemists develop alternative materials are toxicity, flammability, and stability. ▪ Toxicity: If a molecule contains chlorine, it is likely to be toxic. Thus, chlorine should be avoided or kept in minimum numbers. ▪ Flammability: If a molecule contains too many hydrogens, it is likely to be flammable (meaning dangerous). Thus, hydrogen atoms should be kept in minimum number. ▪ Stability: If a molecule is too much stable, then it is likely to stay longer in the stratosphere. This in turn would intensify global warming.

How does CFC destroy ozone?

▪ CFCs are known to destroy stratospheric ozone via several pathways: the free radical Cl∙ in Eq. 3 pulls away an oxygen atom away from the O3 molecule, froms chlorine monoxide, ClO∙, and leaves an O2 molecule, ▪ Then these free radical ClO∙ join themselves to form ClOOCl, ▪ Now ClOOCl decomposes in the two step sequence, ▪ Adding chemical equations (A), (B), (C), and (D) removes all species in red, leaving only the two species in the following net equation: ********************************* ▪ Remember free radicals are very reactive; this nature of free radicals make this sequential reactions possible. ▪ A free radical is a highly reactive chemical species with one or more unpaired electrons. - An unpaired electron is often indicated with a dot

CFCs (6)

▪ CFCs do not occur in nature, because there are no known natural sources of CFCs; - they are 100 % man-made chemicals and are used for propellants in spray cans, cooling systems, sterilizers for surgical instruments, etc ▪ Two of the most widely used CFCs are Freon 11 and Freon 12. ▪ Other contributors to the destruction of ozone are the free radicals of ∙OH and ∙NO. But they are formed in the atmosphere from both natural sources and human activities ▪ one on the left hand side is Freon 11, CFC-11, it's structure has one carbon, 3 chlorines, and one fluorines ▪ the one on the right hand side is Freon 12, CFC-12, which has one carbon, two chlorines, and two fluorines ******************************** ▪ here we see two examples of CFCs - we used to use these chemicals a lot for freezing purposes in our refrigerators, or automobiles, and even in hairspray - but now, at least in industrialized countries, we do not use CFCs but we use alternative chemicals - in underdeveloped countries they still use CFCs chemicals ▪ when these chemicals go up and eventually reach the stratosphere, they are causing a breakdown in the ozone molecules, causing the depletion of the ozone layer

Bonds of CFCs (5)

▪ CFCs have strong bonds, so the molecules can remain for long periods. - When they eventually reach stratosphere, the high-energy photons of UV-C can break the bonds of CFCs: ▪ when they get broken, that releases Cl radical (∙Cl) - now the radical that contains the dot, is a single electron that is very reactive - so that Cl dot (Cl radical) can move on to next chain reactions which can ultimately breakdown ozone molecules ******************************************* ▪ CFCs has strong bonds, so they can remain for a long period ▪ when they eventually reach the stratosphere, those high energy UV light rays can break the bonds of CFCs as you can see in the diagram ▪ Cl dot is what we call a chlorine radical - now the radical that contains the dot, is a single electron that is very reactive - so that Cl dot (Cl radical) can move on to next chain reactions which can ultimately breakdown ozone molecules

Concentrations of Ozone across the atmosphere (7)

▪ Here you see the concentrations of ozone across the atmosphere ▪ in the middle there is a high concentration in the stratosphere, and that prominent high concentration of ozone is what we call the ozone layer - that concentration is getting smaller and smaller due to human activity gradually destroying the ozone layer - this is causing a health problem to the people on the earth's surface ▪ If you go down to the bottom, (the troposphere) you see a small increase in ozone and that's BAD ozone - that ozone is from pollution ▪ in this chapter we're not looking at the tropospheric (bad) ozone, but we're looking at the stratospheric called the ozone layer

V. Effects of UV light on living resources (8)

▪ Highly energetic UV photons can excite electrons and break bonds in biological molecules. ▪ UV-B & UV-C is well filtered out by O3 in the stratosphere. - This is most fortunate, because radiation in these wavelengths is particularly damaging to living things. ▪ Biological sensitivity: a quantized result of experiments in which the damage to DNA is measured at various wavelengths. ▪ How to read Fig. 2.10: biological sensitivity at 320 nm is about 10^-5 (or 0.00001); biological sensitivity at 280 nm is 100 (or 1). - Thus radiation at 280 nm is 100,000 times more damaging than radiation at 320 nm. ▪ Reason for this is very simple: as the wavelengths become shorter, the energy becomes greater, causing more damage. ▪ In Fig. 2.11, UV-B domain shows greater biological sensitivity than UV-A domain, because UV-B has shorter wavelength than UV-A.

Protecting the Ozone Layer in the stratosphere - Why is the ozone layer getting smaller?

▪ In this chapter we're going to look at the depletion of the ozone layer in the stratosphere & we're going to ask questions like why is the ozone layer getting smaller - as you can see in the left hand side of the diagram especially in the southern polar region where you see the deep blue color - the deep blue color is the area where the ozone layer depletion is most distinct

IR (3)

▪ Infrared radiation of sunlight having wavelength 700 nm - 4000 nm ▪ IR warms earth, causing molecules to move, rotate, and vibrate. ▪ Photons in the IR light carry energies that are not strong enough to break bonds in chemical species.

Chapman Cycle (4)

▪ It shows the natural steady state processes in which ozone is both formed and destroyed: ▪ The natural production of ozone in the Chapman cycle is triggered when an O2 molecule absorbs a photon of the UV light. ▪ The natural destruction of ozone in the Chapman cycle takes place when an oxygen atom slowly collides with an ozone molecule. ▪ The steady state distribution of stratospheric ozone by this natural cycle (Chapman cycle) is severely disturbed by human activities.

See table 2.2 on page 70

▪ Ne - 8 outer electrons ▪ Cl - 7 outer electrons ▪ Si - 4 outer electrons ▪ Al - 3 outer electrons ▪ F - 7 outer electrons

IV. Production & destruction of O3 in the natural cycle (3)

▪ Overall concentration of ozone remains constant in the natural cycle. ▪ This means that the rate of ozone production and destruction in the natural cycle are the same. - This kind of state is called steady state.

Filtering of UV by oxygen: (3)

▪ Photons in UV-C (200 ~ 280 nm) have the strongest energy among the UV radiations. ▪ Thus they can break the O=O bond in O2 which is stronger than the bond in O3. ▪ The photons in the UV-C light breaking the O2 bond means that they are being absorbed onto O2, resulting in disappearance (filtering) of this specific UV radiation. ********************************************* ▪ Ozone in the stratosphere filters ultraviolet light from the sun. So ozone protects us from damaging solar radiation. ▪ how ozone in the stratosphere protects us from being exposed to UV light: - in this chemical reaction, the ozone in the stratosphere, O3, reacts with UV photons and converts that O3 to O2 with O ... so in that process, UV photons get filtered and that's how we see the ozone in the stratosphere removes UV light from the sun - as a result, that's how ozone protects us from damaging solar radiation from UV light

Examples of alternative materials (4) Halons (4)

▪ Some examples of developed material alternative to CFCs are HCFCs (hydrochlorofluorocarbons): two important examples of HCFCs are HCFC-22 and HCFC-141b. - Because HCFCs themselves have some adverse effects on the ozone layer, they are regarded only as an interim solution in the industrialized countries. ▪ Another examples of developed material alternative to CFCs are HFCs (Hydrofluorocarbons): two important examples of HFCs are HFC-125 and HFC-32. - HFCs would be a right choice for refrigerants in the long run, because they have no chlorine to interact with ozone, and their number of hydrogen atoms is small enough to facilitate decomposition in the lower atmosphere without being flammable under normal condition. ▪ The Montreal Protocol also calls for replacement of halons, in addition to CFCs, because halons are even more effective than CFCs in causing destruction to the ozone layer. - Halons are compounds similar to CFCs, in which bromine or fluorine atoms replace some or all of the chlorine atoms. - Halons proved to be very effective fire extinguishers and are used to protect property that would be vulnerable to water and other conventional fire-fighting chemicals. - Thus they have been used for fire-extinguisher for electronics, computer installations, and rare book rooms. ********************************* ▪ since we now know that the CFCs are the culprit for the depletion of ozone in the stratosphere, we have developed alternative materials to use instead of CFCs ▪ one of the alternatives is HFCs - the advantage is that this does not have cl in it's compound structure, so that it will not interact with ozone - HFCs would be the right choice for regerants in the long run

I. Essential background knowledge (5)

▪ Sunlight consists of particles called photons which propagate through the space like waves with various wavelengths. ▪ The Sun bombards earth with countless photons carrying varying amounts of energy as they have different wavelengths such as UV, Visible, Infrared, etc. ▪ The order of the wavelength of solar radiation: ▪ shorter < UV < Visible light < Infrared < longer ▪ The shorter the wavelength of solar radiation, the greater the energy it carries (thus more harmful to us). *************************************************** ▪ sunlight consists of particles called photons which propagate through space like waves with various wavelengths ▪ the sun bombards earth with countless photons carrying varying amounts of energy as they have different wavelengths, such as UV light, visible light, infrared light, and so on ▪ the order of the wavelengths of solar radiation goes from shortest, to second shortest visible light (UV), and then to the longer wavelength radiation ▪ the shorter the wavelength of solar radiation, the greater the energy it carries.. therefor it gets more harmful to us - that's why among UV light, visible light, and infrared light... UV is the most harmful solar radiation (bc it has the most energy) ▪ protecting us from UV light by the Ozone layer in the stratosphere is a good one.

VI. Man-made destruction of the stratospheric ozone (7)

▪ The average stratospheric ozone concentration has dropped significantly in the last 20-30 years. - The striking decrease in spring stratospheric ozone has been observed in Antarctica over the last 40 years (Fig. 2.14). ▪ The area having ozone less than 220 DU is defined as the "ozone hole." - Total ozone destruction in the hole occurred from an altitude of 15 to 20 km. ▪ The extent of these ozone depletions cannot be explained by O3 destruction in the natural cycle, because natural processes are insufficient to cause or explain the magnitude of the abnormally large decrease in ozone. ▪ Agents mainly responsible for the stratospheric ozone depletion are CFCs (chlorofluorocarbons) which are compounds composed of the elements chlorine, fluorine, and carbon. - CFCs is the chemical that mainly destroys the ozone layer

Net equation

▪ The net equation (E) clearly shows the conversion of ozone into oxygen gas, i.e., destruction of ozone. And CFCs is the initial chemical species (Eq. 3) that trigger the chain reactions, leading to the eventual destruction of ozone. ▪ Because of Eq. A there must be an inverse relation between O3 and ∙ClO. In other words, decrease in O3 must be accompanied by increase in ∙ClO. Compelling evidence for this inverse relation was obtained from Antarctic stratosphere (Fig. 2.16).

III. Filtering of UV radiation by O2 & O3 (6)

▪ The process by which ozone protects us from damaging solar radiation involves the interaction of matter and energy from the Sun. ▪ Solar UV radiation is greatly diminished by passing through oxygen and particularly through ozone in the stratosphere. ▪ Types and characteristics of UV radiation:

VII. Responses to a global concern (3)

▪ The signing of the Montreal Protocol in 1987 initiated regulatory measures to protect the ozone layer. Ensuing meetings in various parts of the world over the past twenty years have implemented more stringent policies. ▪ The key initial strategy for reducing chlorine in the stratosphere was to stop production of CFCs. ▪ A total of 189 countries have now ratified the Montreal Protocol. They have reaffirmed that the production of CFCs and other fully halogenated CFCs is to be eliminated by 2010 by all parties, no matter the basic domestic economic needs.

Wavelength vs. Frequency (4)

▪ Wavelength (λ) = distance traveled between successive peaks (nm). ▪ Frequency (ν) = number of waves passing a fixed point in one second (waves/s or 1/s or s-1 or Hz). - longer wavelength = lower frequency - shorter wavelength = higher frequency

Free radical: (4)

▪ a chemical species having unpaired electrons; ▪ this status of having unpaired electrons makes free radical unstable; ▪ to avoid this unstable status, free radical strives to achieve a status of having paired electrons by reacting with other chemical species; ▪ thus free radical is very reactive.

Chlorine radical catalyst reactions

▪ here we see the Cl dot (chlorine radical) produced from CFCs when CFCs reacts with the UV light ▪ so the CFCs gets decomposed, this Cl in the chain reaction reacts with ozone molecule, so ozone gets destroyed and O3 becomes O2, and then Cl radical becomes Cl O radical (ClO dot) ▪ this Cl O dot, since it has a radical it's very reactive again and it combines with another Cl O dot so then it becomes ClOOCl ▪ this ClOOCl continues to react with the UV light and decomposes to ClOO dot and Cl radical - now here you see another Cl radical which goes back to the top - so you see Cl radical continues to break down ozone molecule ▪ so if you combine all of this process together then the net reaction mediated by the Cl radical is that two molecules of O3 becoming 3 molecules of O2 ▪ So what it means is that ozone gets continually broken down to O2 molecule

Protecting the Ozone Layer in the stratosphere - Isn't ozone hazardous to human health? (2)

▪ isn't ozone hazardous to human health ?? ▪ *yes, BUT only when it's in the lower atmosphere, (the troposphere)* ▪ we're not looking at that ozone in that level in this chapter ▪ *in this chapter we're looking at ozone in the higher level of the atmosphere, (the stratosphere) at that level it is beneficial to us* ▪ we're not dealing with the ozone being hazardous in the troposphere in this chapter ▪ we're going to look at the beneficial ozone in the stratosphere (higher level)

Ozone layer (5)

▪ the stratospheric region of maximum ozone concentration. ▪ The total amount of ozone in the layer is surprisingly small because at this altitude the air is very thin. ▪ Though having small concentration, however, the ozone layer still plays a critical role in keeping the UV light from reaching the earth surface. ▪ Status quo with ozone: There is a significant depletion of ozone in the stratosphere & As a consequence, we are more exposed to dangerous UV radiation which can cause skin cancer. ▪ Key questions to ask: What has caused the stratospheric ozone depletion? Why is this serious? What has been done to slow down or correct the problem? ************************************************************************** - Don't take the word "layer" literally as thick, fluffy blanket. - there is a significant depletion of ozone in the stratosphere, and as a result, we are more exposed to dangerous UV light which can cause skin cancer bc it is not filtered by ozone as much as it used to be

UV radiation & molecular bonds (2)

▪ this cartoon shows how strong UV radiation is because it has sufficient energy to break down the bond within the molecule ▪ so if we are exposed to UV light, then those molecules in our skin can be broken down and that can eventually cause skin cancer

Experimental analyses of ClO∙ concentrations

▪ this diagram shows the distributions of the concentrations of stratospheric ozone and stratospheric chlorine measured in the stratosphere above the earth's surface of the southern polar region ranging from the latitude of 63 degrees south to 72 degrees south ▪ as you can see, when you see the stratospheric ozone concentration decrease drastically, in the line blue, that corresponds with the drastic increase in chlorine concentration in red - so there's a mirror image between ozone reduction and chlorine increase, indicating that the chlorine is the cause of ozone depletion - this chlorine is originating from CFCs ▪ so we call ozone hole in the southern polar region , which is confirmed by this concentration distribution .. the mirror image between ozone depletion and chlorine increase ▪ so we have to regulate this CFCs which has produced chlorine,and that Cl ate up this ozone

UV (4)

▪ ultra violet radiation of sunlight having wavelength 200 nm - 400 nm ▪ Photons in the UV light are sufficiently energetic to displace electrons within neutral molecules, converting them into positively charged species. ▪ Even UV photons of shorter wavelength break bonds, causing molecules to come apart. - In living things, such changes disrupt cells and create cancer.

Visible light (2)

▪ visible sunlight having wavelength 400 nm - 700 nm ▪ Photons in the visible light hit the cells of our retinas, excite electrons in the molecules in our body, which in turn trigger a series of complex chemical reactions that ultimately lead to "seeing."