More on Ozone

Ozone is generated naturally by short-wave solar ultraviolet radiation, and appears in our upper atmosphere (ozonosphere) in the form of a gas. Ozone also may be produced naturally by passing an electrical discharge - such as lightning - through oxygen molecules. Lightning is a perfect example of making an abundance of O3 to purify the earth's atmosphere Nature's way. Most of us have noticed the clean, fresh smell in the outdoor air after a thunderstorm, or the way clothing smells after it's been dried outside on a clothesline in the sun.

Oxygen Molecules

Oxygen, as we know, has two atoms. High voltage, as from lightning, breaks these two atoms apart. Quickly, these atoms hop back together in threes {O3}. Confused, these atoms do not like this arrangement andwant desperately to dissolve this uncomfortable trio. So as this O3 molecule floats in the air, when one of the atoms spots a contaminant molecule to attach itself to, it breaks away from the other two atoms. To its surprise, this attachment is actually an attack on the contaminant and creates a microscopic explosion. Both the contaminant and the atom are destroyed. This leaves the other two atoms behind as pure oxygen {O2} without the presence of the contaminant. The  explosion changes the contaminant into carbon dioxide and hydrogen, which we can breathe.

Ozone Molecules (O3) converted from oxygen (O2) as a result of an electrical charge

 Should the O3 molecule not find a contaminant in its environment, it will attack itself  to change its configuration of O3 back to O2 (normaloxygen) in 20 to 30 minutes at room temperature and normal humidity.



 

One oxygen atom splits off to oxidize contaminant, leaving behind breathable oxygen and purified air.

Ozone is highly reactive, so it interacts with most contaminates and allergens it encounters. 

 

The "lightning method" of ozone production has been duplicated commercially by many manufacturers of ozonating air purifiers and is known as corona discharge. In this method, 5,000 to 10,000 volts of electricity is used to split the O2 atoms to produce ozone. However, in addition to safety concerns and high operating costs, air purifiers utilizing the corona discharge method are plagued by unpredictable levels of ozone production - ozone "blasts" - and also produce undesirable and unhealthful byproducts such as nitric oxides. These oxides actually irritate the respiratory system - not what you want in an air purifier. So in spite of claims that these units give you a "thunderstorm in a box," they have clear drawbacks.

How Else Does Ozone Purify?

Ozone is also biocidal - which means it kills harmful biological and bacterial contaminants. This biocidal action results from its reaction with the double bonds of fatty acids in bacterial cell walls, membranes and the protein capsid of viruses. In bacteria, the oxidation results in a change in cell permeability and leakage of cell contents into solution. Ozone attacks these cell walls, breaking down membranes and ultrastructural components of the organism. In more simple terms, the unstable electrons of ozone blast holes through the membranes. This occurs by cell lysing or rupturing the cell wall of viruses, bacteria, yeast, and abnormal tissue cells, thereby destroying them by inactivation of the microorganism's enzymes. In viruses, alteration of the protein capsid prevents the virus from being taken up by susceptible cells.

Ozone displays an "all or nothing" effort in terms of destroying bacteria. It is such a strong germicide that only a few micrograms per liter are required to demonstrate germicidal action. Factors like humidity, temperature, pH, ozone concentration levels, type of organism and time, determine the kill rate for pathogens. The action of ozone gas in water is instantaneous. After oxidation, ozone returns to its original form of oxygen, with out leaving any toxic by-products or residues.

Ozone oxidizes natural organic compounds like acetic and oxalic acids, as well as synthetic substances like nitro- and chloro-benzic compounds, detergents, herbicides and composite pesticides. Ozone oxidizes inorganics such as iron, manganese, heavy metals, cyanide, sulfides, and nitrates in water.  Ozone retards the ripening of fruits and vegetables by destroying ethylene gas and bad odors, which are produced by aging and decay.

The Chemistry of The Ozone Layer

A look at the life cycle of ozone in the atmosphere and the effects of pollution

  The Ozone Cycle in the Stratosphere

Atmospheric ozone is found in varying concentrations from the Earth’s surface to a height of about 60 km. The maximum amount of ozone occurs between 20 and 30 km in the region of the stratosphere characterized by increasing temperatures-the ozone layer. The ozone layer is like "Earth’s natural sunscreen" because it filters out harmful ultraviolet (UV) rays from sunlight before they reach the planet’s surface. Ozone is seen in the troposphere, however, where it is actually a harmful component of smog. Ozone is an air pollutant is found to cause respiratory problems to those exposed to it during smog events.

Above the stratosphere most oxygen exists in atomic form having been dissociated from O2 molecules by UV-c photons from sunlight. Chemists represent this form of oxygen with an asterisk (O* ) to emphasize the fact that the atom is in a particularly reactive, excited state when it really wants to react. In the stratosphere itself the intensity of UV-C light is less and the air is denser therefore the molecular oxygen concentration is much higher.

Figure1: The types of photons being released by the sun are related to the amount of ozone found in the stratosphere. The ozone is most concentrated in the region of the stratosphere penetrated by UV-b and UV-a light. This is the type of UV-b light which is particularly harmful to humans is the type of UV light that the ozone layer protects us from. ( http://see.gsfc.nasa.gov//edu/SEES/strat/class/S_class.htm)

 

When an oxygen molecule absorbs a UV-C photon the process of photodissociation takes place:

Equation 1:

O2 + UVphoton ( <240nm) 2 O*

* Represents electronically "excited" state of the atom with excess energy

The most likely fate of oxygen atoms in the stratosphere created by the photochemical decomposition of O2 is subsequent collision with intact diatomic molecules of oxygen (O3) resulting in ozone (O3) production:

O* + O2 O3*

The excess energy represented by the asterisk must be transferred away from the O3 molecule or the reverse decomposition will occur. The excess energy can be released by colliding with another atom or molecule represented by M (usually N2 or O2) and releasing some of the excess energy:

Equation 2:

O* + O2  O3*

O3* + M O3 + M*

_____________________

O + O2 + M O3 + M*

Figure 2: This is a summarizes the process described in equation 1 and equation 2 above. The photochemical decomposition of O2 is diagrammed on the left side of the figure and the formation of O3 is shown on the equations on the right side of the figure. (http://see.gsfc.nasa.gov//edu/SEES/strat/class/S_class.htm)


 

 

At this point the ozone starts to do its job by absorbing UV light with wavelengths shorter than 320 nm providing a shield against high-energy radiation penetrating the Earth’s surface. The photodecomposition of ozone reverses the reaction leading to its formation:

 

Equation 3:

O3 + hv ( <320nm) O2 + O* (photochemical decomposition)

O* + O3 2O2

O*+ O* + M O2 + M* (spontaneous reaction)

 

 Processes of Ozone Depletion

Fluorinated chlorocarbons were developed in 1930 by the General Motors Research Laboratories in a search for a non-toxic non-flammable refrigerant to replace sulphur dioxide and ammonia then in use. CFC-12, pure CF2Cl2, is produced by reacting carbon tetrachloride with gaseous hydrogen flouride. It was widely used as a liquid coolant and was later discovered to be useful as good thermal insulator.

 CFC-11, CFCl3, was also used for foam products used to insulate refrigerators, freezers, and buildings. Both CFC-11 and CFC-12 were used extensively in aerosol spray cans.

The other CFC of environmental concern is CF2Cl-CFCl2, CFC-113. It was used as a cleaning compound for electronic circuit boards in the 1970s. The uses of CFCs all ultimately lead to atmospheric release, since even ‘hermetically sealed’ refrigerators and closed-cell foams finally leak to the air. About 90 percent of all the CFC-11 and CFC-12 produced is thought to have been released.

CFCs are virtually inert near the earth’s surface because they lack hydrogen-carbon bonds that tend to break apart and release toxic chlorine radicals into the lower atmosphere. CFCs have no tropospheric sink (they are not consumed in the troposphere like many released chemicals) and therefore all molecules eventually rise to the stratosphere.

Sherwood Rowland and Mario Molina, chemists at the University of California, Irvine, recognized in 1974 that chlorine from CFCs may be damaging the ozone layer and published a ground-breaking article in Nature that year. This was published just after Crutzen’s discovery in 1970 that naturally occurring nitrogen oxides catalytically destroy ozone. At this point in 1974 nearly 970 million kg or 2 billion pounds of CFCs were being produced each year. In 1995 the Nobel Prize for Chemistry was awarded to these three scientists for their studies in ozone depletion.

 Figure 3: A look at the amount of CFC use and release in 1974. (http://see.gsfc.nasa.gov//edu/SEES/strat/class/S_class.htm)

 

 

 

The lack of reactivity that makes CFCs so useful commercially is also what allows them to survive in the atmosphere and diffuse into the stratosphere where they are exposed to high-energy radiation causing photodissociation. Because vertical motion in the stratosphere is slow, the atmospheric lifetime of CFC-11 molecules is around 60 years and that of CFC-12 is around 105 years. The C-Cl bonds in CFCs is weak enough that free chlorine radicals are readily formed in the presence of light with wavelengths in range 190 to 225 nm. For example the reactions of C-12 are:

Equation 4:

CF2Cl2 + hv CF2Cl + Cl

Cl + O3 ClO + O2

ClO + O Cl + O2

_________________

O + O3 O2 + O2

Another important class of ozone-depleting chemicals are halons: bromine-containing, hydrogen free substances such as CF3Br and CF2BrCl. Halons were developed by the U.S. Army Corps of Engineers after WWII for extinguishing fires in tanks and aircraft. Total global production of the three halons (Halon-1211,-1300, and –2400) was about 17M lbs in 1985. Like CFCs, halons have no tropospheric sink. They are of special concern because their ozone depleting potential per molecule is estimated to ten times that of CFCs.

Like chlorine, bromine radicals released by photochemical decomposition can destroy ozone:

Equation 5:

Br + O3 BrO + O2

BrO + O Br + O2

_________________

O3 + O 2O2

Figure 4: This figure diagrams in the left hand corner the formation of an O3 molecule which can then be destroyed by the free chlorine radical released by CFCs in the stratosphere like in equation 3. Bromine containing compounds (BrOx)destroy ozone in a similar reaction as in equation 4.

   

The Antarctic Ozone Hole

In 1985, Josesph Farman, Brian Gardiner, and Jonathan Shanklin in the British Antartic Survey discovered that ozone was disappearing over Antarctica during this southern hemisphere spring period in amounts much less than even the naturally occurring low amounts over Antarctica during this season.

The ozone hole occurs because of special polar winter weather conditions in the lower stratosphere that temporarily convert all the Cl that is stored in the inactive forms HCl and ClONO2 into the active Cl and ClO . As the stratosphere cools to very cold temperatures over the Antarctic during the southern winter, polar stratospheric clouds (PSC’s) form. When temperatures drop a few degrees below –80oC a special type of crystal forms. These inactive forms of Cl can react on the thin aqueous layers on the surfaces of these PSC crystals and produce Cl products that can catalytically destroy ozone.

Overall, there is an ozone destruction rate of about 2% per day occurring each September due to the combined effects of the various catalytic reaction sequences. By early October almost all the ozone is wiped out between altitudes of 15 and 20 km.

The key ingredients for polar ozone losses are: high chlorine and bromine levels, cold temperatures during the late winter, and relative isolation of the polar region from the midlatitudes. Increasing levels of CFCs and bromine compounds over the past few decades has caused the Antarctic ozone hole since temperatures are always cold in the late winter.

Figure 5: The amount of ozone over Antarctica in October when the concentration is the lowest is steadily declining. The amount of ozone is represented in Dobson units -one DU is equivalent to a 0.01 mm thickness of pure ozone if it were at ground level temperature and pressure.

 

 Biological Effects of Ozone Depletion

Most biological effects of sunlight arise because UV-B can be absorbed by DNA molecules. Most skin cancers in humans are due to overexposure to UV-B in sunlight. The incidence of malignant skin cancer affecting one in 100 Americans is thought to be associated with high UV exposure especially at a young age.

It is predicted that there will be a 1-2% increase in malignant skin cancer for each 1% decrease in ozone. People living at latitudes around 45o N are at an increased risk. It is predicted that there will be an eventual 22% increase in malignant cancer due to a 6.6% decrease in ozone between 1979 and 1992 over those regions.

It is also speculated that ozone depletion increasing UV-B exposure will interfere with the photosynthesis process and plants will produce less leaf, seed, and fruit. It is also predicted that production of phytoplankton near the surface of sea water will be at risk thus affecting the marine food chain.

 Policy and Ozone Depletion

The usage of aerosol spray cans containing CFCs was essentially eliminated in the 1970s by the US, Norway, Canada, and Sweden although not in other parts of the world. A national agreement wasn’t formerly made until 1987 at the conference in Montreal, Canada. The Montreal Protocol was initiated at this conference and amended in 1990, 1992, 1995, and again in 1997. It was finally decided that "all ozone-depleting chemicals" are destined for phaseout all over the world. All legal production of CFCs was ended in 1995 in developed nations. Developing nations have been given until 2010 to curb production.

Developed countries have also agreed to end production of HCFCs by 2030 and developing countries by 2040. Halon production was banned in 1994 but the use of stock piles continues. Methyl bromide will be phased out by developed countries by 2005 and developing countries by 2015.

Figure 6: This is a look at CFC use in 1991 as compared to 1974 (Figure 3). The overall amount of CFC use is much smaller in 1991 and the percentage released by propellants is much lower due, in part, to aerosol bans in the 1970s. (http://see.gsfc.nasa.gov//edu/SEES/strat/class/S_class.htm)

  

The EPA also enacted a market-based system to phaseout ozone depleting substance production. In 1988 the EPA assigned annual permits called "allowances" to five CFC producers, three halon producers, fourteen CFC importers, and six halon importers. Allowances permitted a company to make or import a certain quantity of CFCs and halons for the U.S. market and also let companies produce an additional 10% for export to developing nations.

Companies could trade their allowances within their company and to other firms as well as outside the U.S. In 1990 the system was amended so that each ozone depleting substance(ODS) was given a separate allowance taking into account its ozone depleting potential instead of all ODS being treated equally. The amended system also included a 1% tax to "pay" the environment on every allowance trade.

 The Future of the Ozone Layer?

Only in the last two decades have scientists been able to semiquantitative predictions about the causes of ozone loss and the resulting consequences. There is now solid evidence, however, that man-made chemicals like CFC’s and halons greatly contributing on ozone depletion and the banning of these chemicals is necessary for future improvement. Plans to "fix" the ozone holes have been unsuccessful so far with schemes like injecting massive amounts of ethane into the stratosphere in hopes it would react with the free chlorine radicals and convert it back into inactive hydrogen chloride. This particular scheme was unsuccessful because one of the products of this reaction is also an ozone depleting substance. The best idea is to stop the release of all ozone depleting chemicals and ban all further production with constant research for any other ozone depleting chemicals that we might be emitting. As far as "fixing the ozone layer," that may be an impossible task, but further research will persist especially as biological effects worsen.

 

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