Ozone in the Troposphere and Stratosphere. 5.6 and 5.7. Layers of the Atmosphere:. Ozone:. It is a form of oxygen (O 3 ) In the troposphere -It is a human made pollutant
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Ozone in the Troposphereand Stratosphere 5.6 and 5.7
Ozone: • It is a form of oxygen (O3) • In the troposphere-It is a human made pollutant • It forms when sunlight catalyzes reactions between nitrogen oxides and volatile hydrocarbons (from the burning of fossil fuels)
The chemistry of Ozone (O3): • Nitrogen oxide (a pollutant) reacts with oxygen to from nitrogen dioxide, which is a brown gas- 2NO(g) + O2(g) → 2NO2(g) • NO2 absorbs sunlight and breaks up to release oxygen atoms which in turn reacts with atmospheric oxygen to form ozone • NO2(g) + sunlight → NO(g) + O(g) • O(g) + O2(g) → O3(g)
Ozone as a Pollutant: • It is a toxic gas- Irritates eyes, causes breathing difficulties and can increase susceptibility to infection • It acts as an oxidizing agent- Damages crops and forests • It is highly reactive- Attacks fabrics and rubber materials
The Problem with Tropospheric Ozone: • It is the main and most harmful component of photochemical smog • Photochemical smog: It is a mixture of ozone and about a 100 primary and secondary pollutants formed under the influence of sunlight
Photochemical Smog: • Frequency and severity of smog depends upon: • Local topography (landscape) • Climate- Precipitation and winds • Population Density • Fossil fuel use
Ozone in the smog: • It reduces air visibility • It causes health problems • It stresses plants and reduces their vigor • It lowers crop yields and declines forests • It is a secondary greenhouse gas
Ozone has a dual role! • In the Troposphere- It is harmful • In the Stratosphere- It is beneficial
Ozone in the Stratosphere: • In the Stratosphere (10 to 45 km above Earth’s surface), Ozone is naturally produced and is beneficial! • Oxygen reacts with harmful Ultra-Violet rays (UV) from the Sun to form Ozone • Ozone prevents the solar UV from penetrating the Earth’s surface- It acts a shield
Why is UV harmful? • UV radiation can cause skin cancer • It can cause eye cataracts • It can weaken immunity • It can mutate living tissue in aquatic life • It effects photosynthetic organisms-crops (and especially phytoplankton, the base of the food web and their consumers-zooplankton)
The chemistry of Ozone in the Stratosphere: • Ozone is created by photolysis of oxygen: • O2→O+O • O + O2→O3 • Ozone is destroyed by photolysis: • O3→O2 + O • Both processes aid in the absorption of UV
Class Activity: Decide whether these statements are true or false: • Ozone consists of oxygen atoms. • Ozone is poisonous. • Methane destroys the ozone layer. • Chlorofluorocarbons are greenhouse gases. • Ozone concentrations are highest in the troposphere. • The greatest loss of stratospheric ozone has occurred over the equator.
Problem of the 21st century: • Ozone Depletion -The Ozone layer is thinning over Antarctica • This is also known as the “Ozone Hole” • The Ozone Hole is 27.5 million km2 • A slight thinning of the Ozone layer has also been observed over the Arctic
Causes of Ozone Depletion: Human Activities • Chlorofluorocarbons (CFCs) • CFCs have been used in • Aerosol cans (Hairspray) • Coolants in air conditioners and refrigerators (Freon) • As foam blowing agents for insulation and packaging (Styrofoam)
Additional causes: • Due to compounds such as: • Halons-in fire retardants • Methyl Bromide–in pesticides • Methyl chloroform-in industrial solvents • Carbon tetrachloride- in pesticides & dyes • Nitrous Oxide-in burning of coal & breakdown of fertilizers in soil
The chemistry behind it: • CFCs and other compounds slowly drift up to the stratosphere where UV breaks them down into chlorine, fluorine, bromine etc. • Sunlight catalyzes chemical reactions in which chlorine, fluorine, bromine etc. breaks downozone molecules, which in turn becomes oxygen molecules
Furthermore: • Chlorine, bromine etc. remain untouched and are able to break down thousands of ozone molecules • On the other hand: Halogenated gases release halogens when exposed to UV • These atoms slow the rate of ozone reformation
How can we reduce ozone depletion? • Recycling old refrigerants and air conditioners • Use alternative products to gas/foam blown products-water blown products • Alternative propellants-hydrocarbons • Alternatives to methyl bromide-natural pesticides and fertilizers
Global efforts to recover the Ozone Layer • Montreal Protocol: In 1987, an agreement was made between many countries to reduce CFC production by 50% in the next one year • In the 1990s, ozone depletion worsened and so stricter standards were set • Since then, 183 countries have ratified this agreement
Phasing out the CFCs • United States and other highly developed countries have phased out use of CFCs • Developing countries are to phase out CFCs and methyl bromide by 2005 • Instead they use hydroflourocarbons (HFCs)—but it’s a green house gas! • They also use hydrochloroflourocarbons (HCFCs)—they do not attack ozone as much!
Another culprit for Ozone Depletion- Halon-1211 • The U.N. Environment Program is working with China and Korea to accelerate the phasing out of halon-1211 by 2006 to 2010 • An international fund-Montreal Multilateral Fund is available to help developing countries to go towards safer alternatives
Life of CFCs • CFCs are extremely stable and will continue to deplete the ozone layer for another 50 years • Full recovery is expected to take place by 2050 if all nations and people comply