Stratospheric ozone depletion

Ozone (Greek ozein, “to smell”) is a pale blue, highly poisonous gas with a strong odor. It is made up of  three oxygen atoms and is an allotrope of oxygen. Its molecular formula is O³. Ozone is formed from normal oxygen in ultraviolet light. It can also be broken down in the presence of ultraviolet light. Ozone is a powerful oxidizing agent. Ozone is the third most important greenhouse gas after carbon dioxide and methane.
 Earth's atmosphere consists of a number of different layers. The tropospheric ozone, the so- called bad ozone is considered a pollutant at ground level (lower atmosphere). The stratospheric ozone, the so-called good ozone naturally occurs in the earth’s upper atmosphere (15-50 km from the earth’s surface). About 90% of atmospheric ozone resides in the stratosphere while the remaining 10% is present in the troposphere extending up to 10 km. Stratospheric ozone  is a natural sun screen which protects life by absorbing harmful sun’s short- wavelength ultraviolet radiation. Hence it is referred as ‘the ozone shield/umbrella.’  Ozone is essential to the health of the atmosphere.
 Stratospheric ozone affects climate by absorbing incoming heat radiation from the earth’s surface thereby cools the earth. In the troposphere near the ground level, ozone is formed when pollutants emitted by cars, power plants, refineries and chemical industries. Tropospheric ozone is a component of photochemical smog and it poses a serious health problem in many cities. Increasing concentrations of tropospheric ozone could be attributed to raising global temperature. The residence time of tropospheric ozone is short and measured in days. It is irregularly distributed by time of the day, geographic location and altitude.

Ozone production

Stratospheric ozone – The stratosphere is in a constant cycle with oxygen molecules and their interaction with UV rays. This process is called Chapman cycle. UV light can split up ordinary molecular oxygen into two atomic oxygen atoms.

O₂+UV photon---à O+O

The atomic oxygen can quickly combine with molecular oxygen (in the presence of a third body) to yield ozone (O³).

O+O₂+third body---àO₃+third body

Tropospheric ozone – ozone is produced in the troposphere in the presence of sun light as a result of the photolysis of nitrogen dioxide (NO²). When nitrogen oxide (NO) and nitrogen dioxide (NO²) are present in sun light, NO² is destroyed by the reaction.

NO₂ + sun light -----àNO+O (atomic oxygen)

The atomic oxygen will react with the available oxygen molecules in the presence of other molecules, M as

O + O₂ + M--à O₃ + M

Ozone depletion

Ozone is lost through the following pair of reactions.

O₃ + UV photon ----àO₂ + O; O + O₃ ---à2O₂

The first of two reactions serves to regenerate atomic oxygen for the second reaction which converts the ozone back to molecular oxygen.

Ozone depleting substances (ODS)

The most important ozone depleting compounds are chloro fluorocarbons (CFC-11 and CFC-12), carbon tetrachloride, methyl chloroform (solvents), methane, nitrous oxide,freons and halons (bromine containing gases). These compounds are chemically inert in the lower atmosphere, but in the stratosphere they are decomposed by UV radiation to release chlorine or bromine, which acts as a catalyst in the ozone destruction process.Bromine and chlorine are the major players that initiate a series of chemical events that destroy ozone. Ozone thinning allows more UV rays to penetrate the stratospheric ozone layer.

 Ozone hole in Antarctica

There is considerable decrease in the total column of ozone in the lower stratosphere (about 40%) 15 – 20 km above Antarctica, which is called ozone hole. The area of greatest decline is centered over Spitzberger, halfway between Scandinavia and the North Pole. The Antarctic ozone hole was discovered by Joseph C. Farman a British atmospheric scientist of Antarctic survey in 1985. The reason for the severe loss of Arctic ozone is due to the presence of clouds in the stratosphere called polar stratospheric clouds. These clouds provide a surface on which the chemical reactions that result in the destruction of ozone can take place. Winds also play a key role in ozone destruction. The cold air over Antarctica in winter creates a huge ‘whirlpool’ of fast – moving air circling ‘Antarctic vortex.’ This vortex effectively insulates Antarctica from the rest of the atmosphere from warmer air. This vortex still lowers the Antarctic temperature and cause the formation of more ice-crystal clouds and the destruction of even more ozone.

Photochemical Smog

The term smog was derived from the words ’fog’ and ‘smoke’. The term smog was first used in 1905 by H.A. Des Voeux.  The photochemical smog is a condition that develops, when primary air pollutants (oxides of nitrogen and volatile organic compounds ) interact  under the influence of sunlight  to produce a mixture of hundreds of different hazardous chemicals known as secondary pollutants like ozone, organic acids  and PAN (peroxy acetyl nitrate).

Smoke + fog +sun light----à smog

 The most famous London smog occurred in the winter during December 1952 which claimed about 6, 500 human lives. This industrial smog was caused by burning of coal for energy generation. Another photochemical smog occurred in Los Angeles during summer in July 1973.  The burning of fossil fuels from automobiles created this smog which claimed 3000 human lives. In 1963, New York City fog incident killed 400 people.

Adverse effects of ozone

 Ozone distorts plant growth and reduces crop yield.  Ozone damages rubber, nylon, plastics, dyes and paints. Breathing O₃ affects both the respiratory and nervous systems, resulting in respiratory distress, headache, and exhaustion. Ozone exposure weakens human immune system. Ozone thinning greatly increases the risk of sun burn, skin cancer and cataracts.For every 1% of ozone depleted, 2% of UV-B is able to reach the earth. Each 1% drop in ozone is thought to increase human skin cancer rates by 4-6%.

Montreal protocol

Montreal protocol, the protocol on substances that deplete the ozone layer, an international treaty was adopted in Montreal, Canada on Sept.16, 1987 by 25 nations. The Montreal protocol was the first step to protect the stratospheric ozone. The protocol set limits on the production of chloroflurocarbons, halons, carbon tetrachloride, hydrochlorofluorocarbons, methyl chloroform, methyl bromide and other ozone depleting substances that release chlorine or bromine gases into the ozone layer of the atmosphere. The Montreal protocol has been repeatedly strengthened by other amendments.

Quote for self-reflection

"Scientists have found evidence that human activities are
  disrupting the ozone balance."