Lecture 10 Topic: Stratosphere Ozone Depletion. Acid Rains.

Section objectives:

1. The nature of ozone and mechanism of ozone layer work

2. Ozone depletion: history, sources and the effects of ozone layer destruction

3. Air pollution: wet and dry smog, indoor air pollution.

4. The nature, sources & the effects of acid rain

5. Sustainable strategies on ozone depletion & acid rain problems. International cooperation

1. The nature of ozone and mechanism of ozone layer work

troposphere ~ 8-15 km

stratosphere ~ 50-60 km

mesosphere ~ 95-120 km

thermosphere ~ 600 km

Ozone gas (O₃) - is a form of oxygen, which is present in the atmosphere.

Ozone exists at two levels. At ground level, it is poisonous to life (Tropospheric ozone). In the stratosphere, it exists as a layer 15 to 55 km above the earth, where it forms an important barrier against harmful ultra-violet (UV) rays from the sun (Stratospheric ozone).

The ozone layer is a renewable form of protection that converts harmful UV radiation into heat. UV includes: UV-A (320 – 400 nm); UV-B (280 – 320 nm; UV-C (220 – 280 nm). Ozone layer filters out all the sun’s ultraviolet in the range of 220 nm – 290 nmOzone also helps to control the temperature of the outer stratosphere.

How to measure Ozone? The atmospheric ozone columnar density above a point on the earth's surface is measured in Dobson units (DU) One Dobson unit refers to a layer of ozone that would be 10 µm thick under standard temperature and pressure. For example, 300 DU of ozone brought down to the surface of the Earth at 0 °C would occupy a layer only 3 mm thick. One DU is 2.69×10²⁰ ozone molecules per square meter.

A baseline value of 220 DU is chosen as the starting point for an ozone hole since total ozone values of less than 220 Dobson Units were not found in the historic observations over Antarctica prior to 1979. The Dobson unit is named after Gordon Dobson, who in the 1920s built the first instrument to measure total ozone from the ground.

2. Ozone depletion: history, sources & the effects of ozone layer destruction

In the upper atmosphere ozone is regularly created and destroyed naturally. The activity of the sun causes variations in the Ozone layer throughout the year.

The Chapman Reactions

O2 + hv		O + O	(1)
O + O2		O3	(2)
O3 + hv		O2 + O	(3)
O + O3		O2 + O2	(4)

Reaction (2) becomes slower with increasing altitude while reaction (3) becomes faster. The concentration of ozone is a balance between these competing reactions. In the upper atmosphere, atomic oxygen dominates where UV levels are high. Moving down through the stratosphere, the air gets denser, UV absorption increases and ozone levels peak at roughly 20km. As we move closer to the ground, UV levels decrease and ozone levels decrease. The layer of ozone formed in the stratosphere by these reactions is sometimes called the 'Chapman layer'.

However, certain artificial chemical gases also destroy ozone. Although these gases are harmless to humans, they drift up to the stratosphere, where they form the compounds that destroy ozone.

CFCs (Chlorofluorocarbons) are most important of these gases, in term of quantity and impact. There is no natural way to remove them once they are in the atmosphere. Not only do they contribute to ozone destruction, but they are also one of the main greenhouse gases (See the topic: "Greenhouse effect").

History: Scientists have been measuring the concentration of ozone over the Antarctic since 1957. They have noticed it thinning significantl)- in the last 10 to 15. years. Dramatic loss of ozone in the lower stratosphere over Antarctica was first noticed in the 1970s by a research group from the British Antarctic Survey (BAS) who were monitoring the atmosphere above Antarctica from a research station.

In 1982 they discovered an unexpectedly large fall in concentration, and an expedition in 1987 found an ozone "hole" the size of the USA and deeper than the height of Mount Everest. A similar hole has now been detected in the Arctic. In 1991 it was found that ozone was being destroyed tuice as fast as had been expected.

Loss of ozone has serious environmental effects. Sunlight supplies the energy for life on earth, but it also contains UV radiation (especially UV-B), which causes living tissue to decompose. Ozone absorbs UV light, so destruction of the ozone layer allows much more radiation from the sun to reach the earth's surface than normal.

Two primary threats on ozone layer:

• release of CFC

• jet travel in the stratosphere (nitric oxide, NO)

Industries can contribute to ozone depletion if:

They manufacture chemicals that damage ozone: CFCs and HCFCs, methyl chloroform, carbon tetrachloride or halons.

They use CFCs or HCFCs:

- in freezers, air conditioners and cooling systems. These chemicals are released only after these products have been disposed of.

- as solvents to clean and degrease products such as electronic circuit boards.

- as propellants in aerosol cans such as spray-on glue.

- to make synthetic foam and packaging.

They use methyl chloroform for metal cleaning, in adhesives or for electronic cleaning.

They use carbon tetrachloride in any chemical process.

They use fire extinguishers that contain halons.

The effects of ozone depletion

The sun’s radiation that passes through the ozone layer’s protective barrier can cause early aging, wrinkling of the skin, and even skin cancer, eye damage (cloudy vision, cataracts) in people. One estimate is that for every 1% loss of ozone there are 4% extra cases of skin cancer in humans. It also harms plants and animals (phytoplankton, young fish, and therefore food chains). Economic impact – crop damage and material damage. Social impact – urban pollution and malnutrition