B1WinnieG.4

At low altitudes in the atmosphere, the concentration of ozone is usually about 0.01 ppm, but at 20-50 km above the surface, in the stratosphere, ozone is much more concentrated - about 1-10 ppm. This is called the ozone layer.

Ozone absorbs short wave radiation, especially ultra-violet. The amount of ultra violet radiation reaching the Earth's surface is greatly reduced by the ozone layer. Ultra-violet radiation has very damaging effects on living organisms.

-It increases mutation rates, by causing damage to DNA.
-It can cause cancers, especially of the skin.
-It causes severe sunburn and cataracts of the eye.
-It reduces photosynthesis rates in plants and algae and so affects food chains.

Measurements of ozone concentrations in the stratosphere have shown that there has been depletion throughout the world. Since the 1980s an ozone 'HOLE' has appeared over the Antarctic every year between September and October, which persists for several months.

CFCs - which are chloroflorocarbons

CFCs are chemical compounds manufactured by humans and released into the atmosphere. Ultra-violet light causes CFCs to dissociate and release atoms of chlorine. These chlorine atoms are highly reactive and cause complex reactions in which ozone is converted to oxygen. The reactions form a cycle, with the chlorine atoms being released again, so that they can go on to cause the destruction of more ozone.
*One chlorine atom can potentially cause the destruction of hundreds of thousands of ozone molecules.

CFCs were used very widely in the 1970s and 1980s:
-in refrigerators as refrigerants
-in aerosol cans as the propellant
-in gas-blown plastics used for fast-food packaging.
--> in order to reduce ozone depletion, refrigerants must be recycled, gas-blown plastics must be reduced, and CFC-free propellants must be used.

In 1987, after research had shown CFCs damage the ozone layer, an international treaty called the Montreal Protocol was signed. This treaty set targets for the replacement of CFCs with other chemicals that do not damage the ozone layer.

1. Bacteria consume the organic matter and proliferate.
2. The bacteria use oxygen in aerobic cell respiration. as the numbers of bacteria rise, they consume more and more oxygen, so the biochemical oxygen demand of the river water increases. The aeration of the water does not increase, so the water becomes deoxygenated. Fish and other animals that depend on dissolved oxygen are sometimes killed.
3. Digestion of organic matter by bacteria causes release of ammonia and phosphate. The ammonia is converted to nitrate by nitrifying bacteria. The increase in levels of nitrate, phosphate and other mineral nutrients is called eutrophication.
4. Eutrophication causes proliferation of algae and photosynthetic bacteria (formerly known as blue-green algae.) If the numbers of algae are high enough they cause a discolouration of water, called an algal bloom. the algae absorb nutrients from the water, reducing concentrations in the water.
5. The algae release oxygen by photosynthesis, so the water becomes reoxygenated.
6. Primary consumers feed on the algae, reducing their numbers. COnditions in the river are then similar to those before the sewage input--the river has recovered.

they are measured using biotic and abiotic variables at different distances below the sewage input.

1. Nitrate ions are very soluble and are leached from soils very easily if excessive amounts are applied to crops. If phosphate and other minerals also reach a high concentration, a river becomes eutrophic.
2. As with eutrophication caused by sewage pollution, algae proliferate and algal blooms develop. Nitrate from fertilizers sometimes cause such excessive growth of algae that some of the algae are deprived of light and die.
3. Bacteria decompose the dead algae. The bacteria create an increased biochemical oxygen demand and deoxygenation of the water.
4. Low oxygen levels kill fish and other aquatic animals.

Carbon dioxide dissolves in droplets of water in clouds and makes the precipitation that falls from the clouds slightly acidic. Sulfur dioxide and nitrogen oxides have the same effect, but can make the precipitation much more acidic - as low as pH3. Although there are some natural sources of these gases, human activities are the main source.
-Oxides of nitrogen and Sulfur dioxide that come from vehicles, power stations and industry create nitric acid and sulfuric acid in the clouds
-the clouds then release droplets of sulfuric acid and nitric dissolved in water in the cloud. Typically lakes at high altitudes become acidic quickly because they have shallow soils and are low in limestone. Acid falls in snow mist, fog or rain. Low-altitude lakes surrounded by deep soil and rich in limestone may be buffered against the effects of the rain.
-Acid substances are deposited as dust on trees and land.
-The acids are often deposited far away from their source.

Biomass can be used as a source of fuels such as methane and ethanol.

Biomass provides large amounts of fuel in the form of wood, crop residues and dried manure.

Methane is sometimes called marsh gas, because it is naturally produced in anaerobic conditions by methanogenic bacteria. These conditions are recreated in bioreactors used for methance generation. Any organic waste can be the raw material, but sewage and manure or slurry from farms are most commonly used. The raw material is loaded into the bioreactor where anaerobic conditions encourage the growth of three groups of naturally occurring bacteria.

-First group - convert organic matter into organic acids and alcohol
-Second group - convert organic acids and alcohol into carbon dioxide, hydrogen and acetate.
-Third group of bacteria are methanogens - they produce methane from carbon dioxide, hydrogen and acetate.

Carbon dioxide + hydrogen --> methane + water
(CO2 + 4H2 --> CH4 + 2H2O)

Acetate --> methane + carbon dioxide
(CH3OOH --> CH4 + CO2)

The gas that is produced in bioreactors is sometimes called biogas and is 40-70% methane. It can be used as fuel for cooking or lighting. If it is compressed in cylinders it can be used as a fuel in vehicles. It is renewable and non-polluting. Production of it helps dispose of potentially polluting wastes. the organic matter left over at the end of methane generation can be used as an organic fertilizer, rich in mineral nutrients.