Ozone in the Troposphereand Stratosphere

5.6 and 5.7

Layers of the Atmosphere:

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

Photochemical Smog:

• Frequency and severity of smog depends upon:

1. Local topography (landscape)2. Climate- Precipitation and winds3. Population Density4. 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

Ozone in the Stratosphere

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:

1. Ozone consists of oxygen atoms. 2. Ozone is poisonous. 3. Methane destroys the ozone layer. 4. Chlorofluorocarbons are greenhouse gases. 5. Ozone concentrations are highest in the

troposphere. 6. 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

Ozone Hole:

Causes of Ozone Depletion: Human Activities

• Chlorofluorocarbons (CFCs)• CFCs have been used in 1. Aerosol cans (Hairspray)2. Coolants in air conditioners and

refrigerators (Freon)3. As foam blowing agents for insulation

and packaging (Styrofoam)

Additional causes:

• Due to compounds such as:1. Halons-in fire retardants2. Methyl Bromide–in pesticides3. Methyl chloroform-in industrial solvents4. Carbon tetrachloride- in pesticides &

dyes5. 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 down ozone 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