Ozone Layer

The Earth’s ozone layer protects all life from the sun’s harmful radiation, but human activities have damaged this shield. Less protection from ultraviolet light will, over time, lead to higher skin cancer and cataract rates and crop damage. The ozone layer is one layer of the stratosphere, the second layer of the Earth’s atmosphere. The stratosphere is the mass of protective gases clinging to our planet. Ozone is only a trace gas in the atmosphere only about 3 molecules for every 10 million molecules of air. But it does a very important job. Like a sponge, the ozone layer absorbs bits of radiation hitting Earth from the sun. Even though we need some of the sun’s radiation to live, too much of it can damage living things.

The ozone layer acts as a shield for life on Earth. Ozone is good at trapping a type of radiation called ultraviolet radiation, or UV light, which can penetrate organisms’ protective layers, like skin, damaging DNA molecules in plants and animals. There are two major types of UV light: UVB and UVA. UVB is the cause of skin conditions like sunburns, and cancers like basal cell carcinoma and squamous cell carcinoma. In the last thirty years, it has been discovered that stratospheric ozone is depleting as a result of anthropogenic pollutants. There are a number of chemical reactions that can deplete stratospheric ozone; however, some of the most significant depletion comes from the catalytic destruction of ozone by freed halogen radicals like chlorine and bromine. Overview of Ozone Depletion

Ozone depletion is term used to describe occurrences regarding the Earth’s ozone layer. One of these observable trends is the slow and constant deterioration of the ozone in the atmosphere of about three percent per decade. The other is a great, though seasonal, reduction in the amount of ozone in the atmosphere over the polar regions, commonly described as an ozone hole. Although, the concentration of ozone in the stratosphere naturally increases and decreases with things like altitude, temperature, and weather, the considerable level of ozone reduction is not due to only natural factors (7).

Synthetic chemicals and gases play a huge role in ozone depletion. Aerosols and chlorofluorocarbons, or CFCs, have been found to be largely responsible for the depletion of the ozone layer. The reduction of the ozone layer presents a large risk for many chemical and biological processes on the Earth’s surface. Exposure to radiation that is usually shielded by the ozone layer has a variety of damaging effects on living organisms.

The Chapman Mechanism
In 1930, Sydney Chapman presented a sequence of reaction mechanisms to explain the formation, depletion, and maintenance of the ozone layer. The Chapman mechanism (or cycle) is made up of four reaction mechanisms. The first step in the creation of ozone occurs when short-wavelength UV light from the sun hits a molecule of oxygen gas. The light (or photon) has so much energy that it breaks the oxygen bond holding the atoms together, thus creating two oxygen atoms. Through this process, the oxygen essentially absorbs the short-wavelength UV light, but this still leaves a significant amount of UV light with longer wavelengths, which is where ozone comes in. In the second process, each of the two remaining oxygen atoms will then attach to two oxygen gas molecules, creating two separate ozone molecules.

Short-wavelength UV light has enough energy to break apart ozone molecules (which are easier to separate than oxygen molecules). Thus, in the third part of the cycle, the ozone gas then breaks into one oxygen gas molecule and an oxygen atom, hence absorbing much of the remaining UV light. Otherwise, the O3 molecule can combine with another oxygen atom producing two molecules of oxygen gas, producing the forth step. These steps determine the overall amount of ozone in the stratosphere. However, Chapman’s cycle didn’t account for the destruction of ozone by free radical and catalytic processes, and ozone harsh chemicals. The Chapman Mechanism

Free Radical and Catalytic Processes of Ozone Depletion
There are a number of catalytic processes of ozone depletion that occur above the ozone layer of the stratosphere where ozone molecules are less abundant. A catalytic process occurs when a molecule, which will act as the catalyst, reacts with an ozone molecule to remove one of the oxygen atoms to generate oxygen gas and the oxygenated form of the molecular catalyst. Next, the oxygenated molecule reforms by reacting with an oxygen atom to create another molecule of O2. Many of the catalysts are free radical. Free radicals are described as atoms or molecules that have an odd number of electrons, resulting extremely reactive substance. Overview of Catalytic Process of Ozone Depletion

Nitric oxide (NO) is a free radical catalyst molecule responsible for a small portion of the depletion of ozone (4). It occurs in the stratosphere when molecules of nitrous oxide (N2O) ascend from the troposphere layer where they sometimes react with light energized oxygen atoms to produce mostly N2 and O2 molecules, and infrequently nitric oxide (Scheme 4). Nitric oxide destroys ozone by the catalytic process described above. Creation of Nitric Oxide from a Nitrous Oxide Molecule

Catalytic Process of Ozone by Nitric Oxide

Ozone Depletion in Ozone Scarce Regions
Destruction of ozone also happens in the oxygen atom scarce region on the lower ozone layer. When there is no free oxygen to complete the catalytic cycle that the Chapman mechanism describes, two catalysts break down two ozone molecules creating two oxygenated molecules and two molecules of O2 gas. The oxygenated molecules return back into their original forms producing yet another molecule of O2 gas. Therefore, in this catalytic process, two ozone molecules react with two catalysts to produce three molecules of diatomic oxygen. Ozone Depletion without Atomic Oxygen

Synthetic Chemical Processes of Ozone Depletion and CFCs
Example of a Chorofluorocarbon

The most well-known contributors to ozone depletion are chlorofluorocarbons (CFCs). CFCs are hydrocarbons where either some or all of the hydrogen atoms have been substituted by fluorine and chlorine atoms. These chemicals used frequently as refrigerants and, until lately, as propellants in aerosols. When chlorofluorocarbons are released into the air, they rise into the atmosphere where they react with ultraviolet light. In the reaction, the CFC molecules break down into atoms and smaller molecule fragments that act as ozone depleting catalysts. CFCs significantly increase ozone depleting processes in the Earth’s atmosphere.

The molecule fragments containing chlorine atoms go through a sequence of decomposition reactions that increase the amount of stratospheric chlorine. Chlorine atoms, as mentioned before, are highly reactive. These atoms undergo many forms of reactions that deplete ozone in the process. Methyl chloride gas (CH3Cl) decomposes when it reacts with ultraviolet light, similarly to CFCs, or can react with an OH radical. Chlorine atoms are extremely efficient ozone depleting catalysts; single atomic chlorine is responsible for the destruction of tens of thousands of ozone molecules. Chlorine Acting as a Catalyst

Chemistry of Ozone Depletion
CFC molecules are made up of chlorine, fluorine and carbon atoms and are extremely stable. This extreme stability allows CFC’s to slowly make there way into the stratosphere (most molecules are not around long enough to cross into the stratosphere from the troposphere). This prolonged life in the atmosphere allows them to reach great altitudes when photons are more energetic. When the CFCs come into contact with these high energy photons their individual components are freed from the whole. The following reaction displays how Cl atoms have an ozone destroying cycle: Chemical equation

CFCl3 + UV Light ==> CFCl2 + Cl
Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2

The free chlorine atom is then free to attack another ozone molecule

Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2

and again…

Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2

and again.

Chlorine is able to destroy so much of the ozone because it is a catalyst. Chlorine initiates the breakdown of ozone and combines with a freed oxygen to create two oxygen molecules. After each reaction, chlorine is able to begin the destructive cycle again with another ozone molecule. One chlorine atom can thereby destroy thousands of ozone molecules. Because ozone molecules are being broken down they are unable to absorb any ultraviolet light so we experience more intense UV radiation at the earth’s surface.

POSSIBLE EFFECTS OF OZONE DEPLETION

As ozone depletes in the stratosphere, it forms a ‘hole’ in the layer. This hole enables harmful ultraviolet rays to enter the Earth’s atmosphere. Ultraviolet rays of the Sun are associated with a number of health-related, and environmental issues. Let us take a look at how ozone depletion affects different life forms.

Impact on Humans
Skin cancer: Exposure to ultraviolet rays poses an increased risk of developing several types of skin cancers, including malignant melanoma, basal and squamous cell carcinoma. Eye damage: Direct exposure to UV radiations can result in photokeratitis (snow blindness), and cataracts. Immune system damage: Effects of UV rays include impairment of the immune system. Increased exposure to UV rays weakens the response of the immune system. Accelerated aging of skin: Constant exposure to UV radiation can cause photo allergy, which results in the outbreak of rash in fair-skinned people. Other effects: Ozone chemicals can cause difficulty in breathing, chest pain, throat irritation, and hamper lung functioning.

Effects on Amphibians
Ozone depletion is listed as one of the causes for the declining numbers of amphibian species. Ozone depletion affects many species of amphibians at every stage of their life cycle. Some of the effects are mentioned below. Hampers growth and development in larvae

Changes behavior and habits
Causes deformities in some species
Decreases immunity. Some species have become more vulnerable to diseases and death Retinal damage and blindness in some species

Effects on Marine Ecosystems
In particular, plankton (phytoplankton and bacterioplankton) are threatened by increased UV radiation. Marine phytoplankton play a fundamental role in both the food chain as well as the oceanic carbon cycle. Plankton play an important role in converting atmospheric carbon dioxide into oxygen. Ultraviolet rays can influence the survival rates of these microscopic organisms, by affecting their orientation and mobility. This eventually disturbs and affects the entire ecosystem.

Impact on Plants
In some species of plants, UV radiation can alter the time of flowering, as well as the number of flowers. Plant growth can be directly affected by UV-B radiation. Despite mechanisms to reduce or repair these effects, physiological and developmental processes of plants are affected.

Another observation is an increase in the ozone present in the lower atmosphere due to the decrease in the ozone in the stratosphere. Ozone present in the lower atmosphere is mainly regarded as a pollutant and a greenhouse gas, that can contribute to global warming and climate change. However, studies have pointed out that the lifespan of lower atmospheric ozone is quite less, compared to stratospheric ozone. At the same time, increase in the level of ozone in the lower atmosphere can enhance the ability of sunlight to synthesize vitamin D, which can be regarded as an important beneficial effect of ozone layer depletion.

Growing concern for ozone depletion led to the adoption of the Montreal Protocol in 1987, in order to reduce and control industrial emission of chlorofluorocarbons (CFCs). Such international agreements have succeeded to a great extent in reducing the emission of these compounds. However, more cooperation and understanding among all the countries of the world is required to mitigate the problem. You too can do your bit to save the ozone. Use/buy more recycled products, save energy, take public transport, and, most importantly, spread awareness. Our individual efforts can go a long way in saving the Earth’s blanket.

Reflection:
The ozone layer is a concentration of ozone molecules in the stratosphere. About 90% of the planet’s ozone is in the ozone layer. The layer of the Earth’s atmosphere that surrounds us is called the troposphere. The stratosphere, the next higher layer, extends about 10-50 kilometers above the Earth’s surface. Stratospheric ozone is a naturally-occurring gas that filters the sun’s ultraviolet (UV) radiation. A diminished ozone layer allows more radiation to reach the Earth’s surface. For people, overexposure to UV rays can lead to skin cancer, cataracts, and weakened immune systems. Increased UV can also lead to reduced crop yield and disruptions in the marine food chain. UV also has other harmful effects.

It is caused by the release of chlorofluorocarbons (CFCs), hydrofluorocarbons (HCFCs), and other ozone-depleting substances (ODS), which were used widely as refrigerants, insulating foams, and solvents. The discussion below focuses on CFCs, but is relevant to all ODS. Although CFCs are heavier than air, they are eventually carried into the stratosphere in a process that can take as long as 2 to 5 years. Measurements of CFCs in the stratosphere are made from balloons, aircraft, and satellites. When CFCs and HCFCs reach the stratosphere, the ultraviolet radiation from the sun causes them to break apart and release chlorine atoms which react with ozone, starting chemical cycles of ozone destruction that deplete the ozone layer.

One chlorine atom can break apart more than 100,000 ozone molecules. Other chemicals that damage the ozone layer include methyl bromide (used as a pesticide), halons (used in fire extinguishers), and methyl chloroform (used as a solvent in industrial processes for essential applications). As methyl bromide and halons are broken apart, they release bromine atoms, which are 60 times more destructive to ozone molecules than chlorine atoms.

REFERENCES

1.The Ozone Holes. In Environmental Chemistry. W. H. Freeman and Company 2.The Reaction of Ozone with Ammonia. In Photochemical Smog and Ozone Reactions. American Chemical Society 3.Rowland, S.F. Stratospheric Ozone Depletion. Philosophical Transactions: Biological Sciences. JSTOR 4.Stratospheric Chemistry: The Ozone Layer. In Environmental Chemistry. W. H. Freeman and Company 5.Ozone Science: The Facts Behind the Phaseout, 2010. United States Environmental Protection Agency. 6.Stones in a Glass House: CFCs and Ozone Depletion; Investor Responsibility Research Center Inc. 7.Formation of Aerosols by Reaction of Ozone with Selected Hydrocarbons. In Photochemical Smog and Ozone Reactions. American Chemical Society 8. The Process of Ozone Depletion, 2010. United States Environmental Protection Agency 9.National Geographic Society