(i) Agradual and continuous rise in the temperature of the earth surface is called as global warming. High level of green house gases in the atmosphere and rise in atmospheric temperature are the major causes of global warming.
(ii) Global warming can be reduced by
(a) Reducing deforestation and planting trees
(b) Lesser use of fossil fuel.
(c) Stopping the emission of green house gases specially CFCs.
(iii) Values exhibited by Preeti
(a) Inquisitiveness.
(b) Environment concern.

A healthy ecosystem is stable. It has a functional balance amongst the different population present in the ecosystem. The important advantages of a healthy ecosystem are:
(i) Purification of air and release of oxygen:The healthy ecosystem helps in purifying air by intercepting the particulate matter by the vegetation and making them to settle down. The air is further purified by removal of CO₂ and release of oxygen  during photosynthethetic activity.
(ii) Water: A greater amount of rain water is held over the soilby plant litter which slowly percolates down and becomes the source of springs and rivers.
(iii) Floods do not occur because of little run off.
(iv) Protection of soil: Healthy ecosystem helps in checking soil erosion because the soil particles remain bound together by plant roots. land slides also are rare.
(v) Biodiversity: The natural and healthy ecosystems help to maintain the biodiversity with <:1 variety of genes, gene pool, species and habitates.

Effects of man-made chemicals on ozone layer.

Industrial effluents released into water contain toxic substances, such as cadmium, lead, zinc, copper, mercury and cyanides, besides some salts, acids and alkalies. All these materials can prove harmful for health. They may reach human body.
Tertiary Treatment:
 It involved removal of nitrates and phosphates.
The water, after above treatment, is then released. It can be reused.
The sewage treatment is quite expensive, and only the first two steps are followed in many developing countries.
Minamata is disease caused by heavy metal contaminated water.

Ozone layer as protective layer:
The ozone layer in the stratosphere is very useful to human beings because it absorbs the major part of harmful ultraviolet radiation coming from the sun. Therefore, it is called protective layer. However, it has been observed that the ozone layer is getting depleted. One of the reasons for depletion of ozone layer is action of aerosols spray propellants. These are the chemicals such as fluorocarbons and chloro-fluorocarbons. These compounds react with ozone gas in the atmosphere thereby depleting it. Scientists all over the world are worried at the destruction of ozone layer. If the ozone layer in the atmosphere is significantly decreased, these harmful radiations would reach the earth and would cause many damages such as skin cancer, genetic disorders in man and other living forms. Efforts are being made to find substitutes of these chemicals which do not react with ozone.

Air pollution. Air pollution refers to the release into the atmosphere of materials that are harmful to man, other animals, plants and buildings or other objects.
Preventive measures for air pollution
To prevent and control air pollution two types of measures can be adopted.
1. Instead of releasing poisonous gases containing various pollutants into the atmosphere they could be destroyed or used by some other measures.
2. Converting harmful pollutants to harmless products and then releasing them into the atmosphere.

Atmospheric pollutants and their sources:

FOAM: Friends of Arcata Marsh
1. It is a citizens group responsible for the upkeep and safeguarding of a project of integrated waste water treatment in the town of Arcata in the northern coast of California.
2. Collaborating with the biologists of Humboldt State University, the people of the town created an integrated waste water treatment process within a natural system.
The cleaning occurs in two stages:
(i) The conventional sedimentation, filtration and chlorination are done; after this, the dangerous pollutants like dissolved heavy metals remain.
(ii) The biologists developed a series of six connected marshes over 60 hectares of marshland; and seeded into them appropriate plants, algae, fungi and bacteria, which neutralise, absorb and assimilate the pollutants.
3. As water flows through the marshes, it gets purified naturally.
4. The marshes also constitute a sanctuary with a high level of biodiversity including fishes, birds and other animals which reside there now.

Electrostatic precipitator
1. It is the most widely used method for removal of particulate matter; about 90% of particulate matter is removed from the exhaust of thermal power plants.
2. It has electrode wires and a stage of collecting plates.
3. The electrode wires are maintained at several thousand volts, which produce a corona that releases electrons.
4. These electrons get attached to the (dust) particles and give them a net negative charge within a very small fraction of a second
5. The collecting plates are grounded and hence attract the charged particles.
6. The velocity of air between the plates must be low enough to allow the particles to fall on them.