Ozone (O3): Reactions, Depletion, and Importance

Ozone (O₃) is a highly reactive gas composed of three oxygen atoms. It is a man-made product in the troposphere but occurs naturally in the stratosphere. It has an impact on living organisms on earth in different ways depending on its location in the atmosphere.

Types of Ozone (O₃)

According to the location of its production, ozone is typically divided into two main categories:

1. Stratospheric ozone (Good ozone).
2. Tropospheric ozone (Bad ozone).

Stratospheric ozone

Stratospheric ozone (good ozone) is naturally formed by the interaction between ultraviolet radiations and molecular oxygen (O₂). This ozone is in the form of a layer, known as the ozone layer. It is located about 30 miles above the earth’s surface. This layer reduces the amount of harmful ultraviolet radiation moving toward the earth.

Tropospheric ozone

Ground level or tropospheric/bad ozone is a secondary pollutant, produced by a photochemical reaction between two major categories of primary air pollutants, such as volatile organic compounds (VOC) and nitrogen oxides (NO_x). Such reactions depend on heat and sunlight which is one of the reasons for a higher concentration of tropospheric ozone on summer days. It is usually present 6 miles above the earth and serves as a secondary pollutant. Primary and secondary pollutants constitute overall environmental hazardous compounds, nowadays.

There are many sources of VOCs and NO_x that help to increase the ozone level in the troposphere. Chemical plants, gasoline pumps, oil paints, printing shops, and NO_x from combustion, power plants, vehicles, industrial furnaces, etc are a few of those sources. Tropospheric ozone also leads to the production of photochemical smog.

Structure of Ozone

Ozone is a polar molecule having bent (V-shaped) molecular geometry. Its structure is similar to a water molecule, but with resonance. The distance between two oxygen atoms is 127.2 pm and their angle is 116.78^o.

The central oxygen atom in ozone is sp² hybridized that contains also contains one lone pair. This overall molecule is a resonance hybrid of two contributing structures, one with a single bond on one side and the other with a double bond on another side. Hence, the bond order is 1.5 for both sides.

Reactions of Ozone

Ozone is a more powerful oxidizing agent as compared to simple oxygen molecules. At higher concentrations, it becomes unstable and decays into ordinary oxygen. Its half-life depends on different atmospheric conditions such as temperature, humidity, the movement of air, etc.

For example, its half-life time (HLT) is ∼25 hours (∼1500 mins) in air at room temperature, with zero humidity and zero air changes per hour (ACH). In an air changing environment, where air changes per hour from 5 to 8 ACH, ozone has a half-life of ∼30 minutes.

Reaction with metals

Ozone reacts with metals and oxidizes them to metal oxides. There are a few exceptions as some metals do not react with ozone such as gold, platinum, and iridium.

Reaction with NO_x and carbon compounds

Ozone also oxidizes the nitrous oxide to nitrogen dioxide which is further oxidized to nitrate radicals.

When ozone reacts with carbon, it gives carbon dioxide.

Reaction with sulfur compounds

Ozone oxidizes sulfides to sulfates. For example, it can oxidize lead (ll) sulfide to lead (ll) sulfate.

When it reacts with water and sulfur or sulfur dioxide, it gives sulfuric acid.

Reaction with alkene and alkynes (ozonolysis)

The process of ozonolysis is carried out in a solution that contains dichloromethane at a temperature of -78 ^oC. Organic ozonide is formed as a result of the cleavage and rearrangement of reactants. If the reaction is carried out with a reductive catalyst (such as zinc in acetic acid or dimethyl sulfide), ketones and aldehydes are formed. However, reaction with oxidative catalysts (aqueous or alcoholic H₂O₂), gives carboxylic acids.

The reaction of ozone with alkenes is called ozonolysis (breakdown). The results can be alcohols, aldehydes, ketones, and carboxylic acids depending on the type of the second step.

When ozone reacts with alkynes, the products are acid anhydride or diketone. Similarly, if the reaction is carried out in the presence of water, the product anhydride hydrolyzes to give two carboxylic acids.

Reaction with iron (Fe) and manganese (Mn)

The reaction of ozone with iron and manganese in water as a medium precipitates out hydroxides.

Reaction with cyanides (CN)

Cyanates (CNO^–) are produced when ozone reacts with cyanides (CN^–).

Decomposition of urea with ozone

Ozone decomposes urea into simple components like nitrogen gas, carbon dioxide, and water.

Ozone Layer Depletion

In the 1980s, it was found that the ozone layer is depleting. The stratospheric ozone layer is known as the earth’s sunscreen (a protective layer from UV rays). It has an important role in the upper atmosphere as the vibrational energy of the bonds is in the UV region. It absorbs this energy and filters the unwanted UV radiation from the sunlight from entering into earth’s atmosphere.

The depletion of the ozone layer increases the incoming ultraviolet radiation toward the earth. This increases the chances of overexposure of everything to ultraviolet rays which leads to health-related diseases such as skin cancer, cataracts, and immune suppression.

Causes of Ozone depletion

Compounds that contain halogens such as methyl chloroform, halons, and chlorofluorocarbons (CFCs) are very stable (inert). They are long-lasting compounds and are easily transported by winds into the upper atmosphere (stratosphere) where they break down and release chlorine and bromine free radicals which are very reactive intermediate species.

When these halogen-free radicals, such as chlorine and bromine make contact with ozone in the upper atmosphere, they destroy these molecules. One atom of bromine or chlorine can destroy over 1 million ozone molecules. This is a primary reason, why ozone is decreasing rapidly in the stratosphere.

Some natural processes such as volcanic eruptions also affect this layer indirectly. For example, volcanic eruptions produce a large number of tiny particles known as aerosols. These aerosols help to destroy ozone by increasing chlorine’s effectiveness. They also make a surface in the stratosphere where chlorofluorocarbon-based chlorine destroys it easily.

Ozone Hole: A Point To Ponder

Recently, ozone levels dropped in the Antarctic stratosphere known as the Antarctic ozone hole. This is possible due to the Antarctic spring. The Antarctic springs are the strong westerly winds that circulate and make an atmospheric container. During antarctic spring, the polar vortex (area of cold and rotating air that encircles both Earth’s polar regions) which contains over 50% of the ozone in the stratosphere is destroyed.

The causes of the ozone hole are CFCs, chlorine, bromine free radicals, etc. The chlorine catalyzed ozone hole or depletion has a great role in its destruction. It is also enhanced in the presence of polar stratospheric clouds (PSCs).

Stratospheric clouds

These polar stratospheric clouds are formed during the winter season when there is no sunlight. This decreases the temperature, resulting in polar vertex trapping the air and chilling it, eventually, making cloud particles. There are three types of polar stratospheric clouds:

1. Nitric acid trihydrate clouds
2. Slowly cooling water-ice clouds
3. Fast cooling water-ice clouds

These clouds provide a greater surface area for chlorine or bromine-based chemicals reaction. The products of these reactions destroy the ozone in spring (Antarctic spring).

In the stratosphere, most of the chlorine resides in compounds such as chlorine nitrate (ClONO₂). During antarctic winter and spring, reactions take place on the PSCs that convert the chlorine nitrate into reactive free radicals such as chloride-free radical (Cl^•) and chlorine peroxide fee radical (ClO^•). Clouds also remove NO₂ from the upper atmosphere by converting it into nitric acid. This helps to prevent newly formed ClO from being converted back into ClONO₂.

During winter, there is no sunlight is available to carry the chemical reactions on PSCs. However, in spring sunlight returns and provides energy for the initiation of photochemical reactions. This melts the polar stratospheric clouds and releases the ClO^• that drives the ozone hole mechanism. When the temperature is increased, ozone and NO₂ rich air start to flow from lower latitudes. This results in the destruction of polar stratospheric clouds (PSCs) and decreases the its depletion process and closes the ‘ozone hole’.

Ozone-Depleting Substances (ODS)

There are two classes of ozone-depleting substances. A few examples of these classes are listed below:

Class 1 ODS

• Trichlorofluoromethane (CCl₃F)
• Bromochlorodifluoromethane ( CF₂ClBr)
• Chlorotrifluoromethane (CF₃Cl)
• Carbon tetrachloride (CCl₄)
• Methyl chloroform (C₂H₃Cl₃)
• Methyl Bromide (CH₃Br)
• Chlorobromomethane (CH₂BrCl)

Class 2 ODS

• Dichlorofluoromethane (CHFCl₂)
• Chlorodifluoromethane (CHF₂Cl)
• Chlorofluoromethane (CH₂FCl)
• Tetrachlorofluoroethane (C₂HFCl₄)
• Hexachlorofluoropropane (C₃HFCl₆)

Health and Environment Effects of Ozone Depletion

Effects on human beings

The harmful ultraviolet radiation will not be filtered out if the ozone is depleted. This would cause diseases such as non-melanoma skin cancer and cataracts problems, etc.

Effects on Flora (plants)

Harmful radiations coming from the sun affect developmental processes in plants such as plant growth. There are also indirect changes in plants by the effect of these radiations. There are also changes in nutrient distribution within the plant, the timing of the developmental process, and the secondary metabolism of plants. These changes affect the plant’s competitive balance, diseases, herbivory, biogeochemical cycles, etc.

Effects on Marine Life

Solar ultraviolet radiation has a great effect on organisms living in the water, such as orientation and motility in phytoplanktons, developmental stages of fish, crabs, amphibians, and other marine animals. Their reproductive capacity and larval development are also affected by ultraviolet radiation.

Effects on synthetic building Materials

Ultraviolet radiation affects polymers, biopolymers, and other materials. High-intensity UV radiation can even break down the materials.

Production of Photochemical smog

Ozone present in the troposphere produces smog which has several biological effects on human beings and other animals, such as:

• Coughing
• Irritation in throat
• Chest pain
• Shortness of breath
• Eye irritation
• Heart and lungs diseases, etc

Applications of Ozone

Among many other disadvantages and dangers, ozone is also applicable to an enormous number of fields. Some applications are explained below:

• Catalytic ozonation

Ozone is used with a dissolved or solid catalyst to produce free radicals that are capable of oxidation of compounds. In catalytic ozonation, activated carbon or metal species in water dissolve as ions that are dispersed or fixed on specific materials. They initiate a quantitatively enhanced production of highly reactive radicals.

• Soil ozonation

The ozonation of contaminated soil can be carried out by direct method or under controlled conditions in the reactor. This increases the biodegradability of residual and nonvolatile organic compounds such as polycyclic aromatic hydrocarbons (PAH) in soil.

• Cooling towers

Ozone is used in cooling tower water treatment. It has great advantages such as being safe and easy to use, low costs, being produced on-site, no requirement of disinfectants, low corrosion rate, effective in wide pH ranges, etc over other oxidizing agents.

• Swimming pools

It is used in swimming pools as a disinfection agent and a replacement for chlorine and bromine. It is also a coagulation agent that enhances the performance of sand and carbon filters.

• Industrial laundries

Ozone is used for laundry because it has great cleaning efficiency. It saves hot water consumption. The use of ozone-containing detergents is more effective because it allows the more penetrating and cleaning effect of the detergents.

• Used as disinfectant and odor controller

Ozone is a powerful disinfectant. It can destroy any kind of pathogen when it comes into the vicinity. It is a biocide that can control odor, and kill viruses and bacteria, etc.

• Storage of Fruits and Vegetables

Ozone provides good storage for fruits and vegetables at a concentration of about 0.01 ml/L.

• Greenhouses and Horticulture

Its disinfection and ozonization properties of it are used in the horticulture industry to treat water and the environment.

• Strengthening of the Immune system

Due to the property of stimulating white blood cells, it can effectively boost the immune system in several cases.

• Bleaching agent

Ozone can act as a bleaching agent for ivory, oils, wax, starch, and delicate fabrics i.e. silk, etc.

Concepts Berg

What is the ozone layer and how is it being damaged?

Ozone is a protective layer around the earth that helps to protect it from incoming ultraviolet radiations. It is damaged due to several natural and anthropogenic activities, such as the release of CFCs, and NO_x that can destroy ozone.

What are the five uses of ozone?

• It protects humans and marine life from ultraviolet rays.
• It is used in soil and catalytic ozonation.
• Being a disinfectant, it can be used to purify water and air.
• It is used in laundry products and bleaching purposes.
• In industry, it is used as a disinfectant and storage environment.

Can we make ozone artificially?

There is a variety of methods to produce ozone. The most common method to prepare ozone is the catalytic oxidation of oxygen gas in a UV spark.

Why is ozone pollution or bad ozone harmful?

ozone is good for protecting the earth from UV radiation. However, in the troposphere, ozone (also called bad ozone) acts as a greenhouse gas and contributes to increasing the earth’s temperature.

What are the biological effects of ozone depletion?

There are different biological effects of ozone such as akin cancer, mutation of genes, damage to plants, etc.

References

• Ozone Bioindicators and Forest Health: A Guide to the Evaluation, Analysis, and Interpretation of the Ozone Injury Data in the Forest Inventory and Analysis program By Gretchen Cole Smith (University of Massachusetts), John W. Coulston (U.S Forest Services), and Barbara M. O’Connell (U.S Forest Services)
• What is ozone?  (epa.gov)
• Ozone (Wikipedia)
• Half-life time of ozone as a function of air conditions and movement (ojs.openagrar.de)
• Ozone-depleting substances (epa.gov)