The Ozone Layer

The United Nations’ International Day for the Preservation of the Ozone Layer is observed every year on September 16. Why has ozone become such an important issue? What is ozone? Why a day is observed every year for ozone? Read on……

Ozone Layer

Ozone was first discovered in 1840 by German chemist Christian schonbein. The word ozone is derived from the Greek word ‘ozein’, meaning scent. Ozone is made up of three oxygen atoms (O₃) while what we breathe – oxygen – has only two (O₂). 

Christian schonbein

Ozone is actually dangerous if inhaled. It is formed mainly by the action of ultraviolet (UV) rays from the sun on the oxygen molecules (O₂) in the atmosphere to split. In other words, it is formed by the photolysis of normal oxygen by ultraviolet radiation at wavelength below 242.5 nano meter. The splitting of free oxygen atoms reacts with other O₂ molecules to form O₃ (ozone). Interestingly, lightening may also form ozone during rainstorms. Most of the ozone in the atmosphere is concentrated in a thin layer some 10-40 kilometres above the ground. Ozone acts as a natural sunscreen for the planet, absorbing UV radiation from the sun. Up to 99% of the UV rays from the sun, which enters the earth atmosphere, do not go beyond this layer.  When the ozone molecule absorbs this radiation it is broken down, and later re-forms into new ozone molecules. It is important to register here that the amount of ozone in the stratosphere can vary with location, season and day-to-day weather condition. UV radiation can cause skin cancer and eye damage in humans, cause sunburn, cataract, reduce crop yields in agriculture, and harm marine and freshwater environments. Though ozone is a minor atmospheric constituent, with an average concentration of about 3 parts per million volume, the radiation properties of this gas make it a significant contributor to the radioactive energy balance of the atmosphere, and an important regulator of the ultraviolet solar radiation received at the earth surface. The geographical and vertical distributions of ozone in the atmosphere are determined by a complex interaction of atmospheric dynamics and photochemistry.

Who discovered ozone layer?

Henry Buisson

 Charles Fabry

French physicists Charles Fabry and Henry Buisson in 1913 discovered ozone layer. The first person to measure its thickness was a British Weather scientist G.M.B.Dobson. Dobson set up a network of stations to monitor ozone all over the world. The Dobson unit that measures the total amount of ozone in an atmospheric column is named after him.

How is ozone measured?

 Dobson spectrophotometer

Instruments that are commonly used to measure ozone are the Dobson Spectrophotometer and Light Detection and ranging (LIDAR). They may be placed on the ground, or carried aloft in balloons, aircrafts and satellites. Some measure locally by continuously drawing air samples into a small detection chamber. Others measure ozone remotely over long distances. The Dobson spectrometer was invented in 1924 by Gordan Dobson. The unit of measure for total ozone is called the Dobson unit.

G.M.B.Dobson

Ozone at the ground level:

Industrial Pollutant

Though stratospheric ozone protects us from the ultraviolet rays, ground level ozone is a pollutant and can be harmful. Ozone, which is found in the lowest layer of the atmosphere (troposphere), is usually a man-made phenomenon. It is a highly unstable and poisonous gas which is formed when oxides of nitrogen and hydrocarbons – emitted from vehicles and industries – combine with oxygen in the presence of sunlight. In other words, ground-level ozone is created through the interactions of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of heat and sunlight. Cars and gasoline-burning engines are large source of volatile organic compounds. VOCs are emitted from products such as paints, insecticides, cleaners, and industrial solvents. Nitrogen oxides are produced whenever fossil fuels are burned. NOx is primarily produced by motor vehicles and power plants. When the percentage of ozone at ground level reaching alarmingly high levels, ozone pollution occurs. This happens more often on a clear summer day when the air temperature is quite high. A higher percentage of ozone gives rise to health issues such as chest pain, congestion and coughing. Ozone can irritate your respiratory system, cause throat irritation, and suffocation. Ozone can aggravate asthma, and damage cells that line your lungs. It may also damage vegetation and ecosystems, leading to a substantial decline in the yield of food crops. Scientists fear that ozone pollution could pose a serious threat by 2050 in most cities around the world due to higher temperatures caused by global warming.

Ozone depletion:

Aerosol spray can

Styrofoam

Modern industries produce a lot of dangerous chemicals that do not exist in nature. Some of these chemicals are in the form of gasses, which make their way upward to the ozone layer. CFCs are one such family of gases cause ozone to break down. One CFC molecule can destroy up to a million ozone molecules before it is itself destroyed. CFCs can be found as the propellent in some spray cans and asthma inhalers, in the coolants used in air conditioners and fridges, and may also be released by the burning of materials such as Styrofoam (used to make disposable crockery and cutlery). Scientists feel that ozone layer may not recover until 2050. So, what can we do to help? Use spray products like deodorants that are marked ‘ozone friendly’ or ‘CFC-free’. When changing the coolant in your car or home air conditioner or refrigerator, go to a reputable dealer who supplies ozone-safe products.

A hole in the sky:

Ozone hole

Ozone hole does not mean there is a hole in the earth’s atmosphere. It, instead, refers to the depletion of ozone molecules in the stratosphere over Antarctica. The recurring springtime Antarctic ozone hole was first reported in a study in May 1985 by British Antarctic Survey scientist, Joseph Charles Farman, Brian Gardiner, and Jonathan Shanklin. Paul Crutzen, Mario Molina, and Sherwood Rowland are the scientists who explained how ozone is formed and decomposes through chemical processes in the atmosphere in the 1970s and won the Nobel Prize for Chemistry in 1995. They showed how sensitive the ozone layer is to the emissions of certain compounds which are man-made. Their paper showed that CFCs used in spray bottles destroy atmospheric ozone. The news sent shockwaves across the globe. It sparked debate about pollution, ultraviolet rays and our role in the depletion of ozone. It also brought together all nations to commit themselves to phase out ozone-depleting agents. It forced the world nations to sign up an important protocol.

The protocol called for the phasing out of the production and consumption of compounds that deplete ozone in the stratosphere – chlorofluorocarbons (CFCs), halons, and carbon tetrachloride by 2000, and methyl chloroform by 2005. Though a majority of countries have complied, some still haven’t.

The MONTREAL PROTOCOL:

The world nations acted swiftly to the danger of the ozone hole, soon after it was discovered in 1985. An international treaty was signed in September 16, 1987 to phase out the production of numerous substances responsible for ozone depletion, called the MONTREAL PROTOCOL ON SUBSTANCES THAT DEPLETE THE OZONE LAYER. The treaty was originally signed in 1987 but substantially amended in 1990 and 1992. In 1987 it was signed by 24 countries. Today 197 countries are signatories. The focus was especially on phasing out CFCs and halons.

Since the Montreal Protocol came into effect, the atmospheric concentrations of the most important chlorofluorocarbons and related chlorinated hydrocarbons have either levelled off or decreased, according to experts. Halon concentrations have continued to increase, as the halons are still being released by fire extinguishers, but their rate of increase has slowed down.

The amount of ozone in the atmosphere has decreased by 4 per cent every ten years since the late 1970s. sometimes there is a drastic ozone reduction over the poles. Ozone hole can be seen in satellite images. The Antarctic ozone hole in 2006 was the largest ever. In 1985, it was discovered that the depletion is at alarming rate over the polar regions, especially over Antarctica, during spring. This seasonal depletion is called the spring-time depletion. Every spring, the atmospheric ozone is rapidly destroyed by chemical processes over the Antarctic. The reduction is usually up to 70 per cent. First observed in 1985, the trend has been continuing.

What causes the depletion?

Ozone depletion occurs when chlorofluorocarbons (CFCs) and halons gases found in aerosol spray cans and refrigerants are released into the atmosphere. Other ozone depleting chemicals include carbon tetrachloride and trichloroethane. Anaesthetics, fire-fighting equipment and the manufacture of materials such as Styrofoam also release these substances. These chemicals can remain in the atmosphere for decades to over a century. When CFCs reach the atmosphere, they are first degraded by UV rays, degradation of CFC molecules releases free chlorine atoms. The basic cause of ozone layer depletion is that these chlorine atoms then break up ozone molecules. Ozone then disappears, the chlorine atoms are recreated in subsequent reactions. One chlorine atom can destroy 1,00,000 ozone molecules. Similarly, degradation of halons releases free bromine atoms, which destroy ozone in the same way as chlorine.

Depletion during spring:

Ozone is also created primarily in tropical latitudes, large scale air circulation patterns in the lower stratosphere move ozone toward the poles, where its circulation builds up. The extreme cold conditions in the polar regions produce polar stratospheric clouds. CFCs attach to ice particles in clouds. When the sun comes out again in the polar spring, the ice particles melt, releasing the ozone-depleting molecules. Once released, these ozone destroying molecules break up ozone molecules. Reductions of up to 70 percent in the ozone column were observed in spring over Antarctica and first reported in 1985.

Did you know?

Ozone therapy is helping people who have severe back pain because of slipped discs. Ozone is injected into the damaged disc and in no time, there is almost complete relief from pain.