(a) What is the difference between fog and smog? What are the causes of smog and its effects on human health? Write short note on any one of the latest technologies to eliminate smog to avoid atmospheric pollution. (b) What is the role of oxygen-demanding wastes in water pollution? How it can be prevented? (c) What is the biosphere? Write a note on the 'Energy Resources' available in the biosphere. (d) What are the different layers of the atmosphere? On what basis these layers are classified? In which layer 'Auroras' are formed and where do satellites orbit?

(a) Fog and smog

Nature: fog is a natural cloud of tiny water droplets near the ground, while smog is a form of air pollution, combining smoke or chemical pollutants with fog or haze. Formation: fog forms when moist air cools, while smog results from emissions, often with sunlight or still air. Colour and smell: fog is white and odourless, while smog is often grey, yellow or brown, and may have a pungent smell. Health: fog itself is largely harmless, but smog is harmful.

Causes of smog: vehicle and industrial emissions; coal burning; brick kilns; crop-residue and waste burning; and dust. Sunlight converts nitrogen oxides and volatile organic compounds into ground-level ozone, and a temperature inversion traps the pollution near the ground, as occurs in Lahore each winter.

Effects on health: eye, nose and throat irritation; coughing and shortness of breath; worsening of asthma and bronchitis; greater risk of heart disease, stroke and lung cancer; and harm particularly to children, the elderly and pregnant women.

Latest technology, the smog tower: smog towers are large structures that act as giant air purifiers. Fans draw in polluted air, and filters, in some designs along with electrostatic charging, trap fine particles, and cleaner air is released. Examples have been built in Xi'an in China and in Delhi, and Lahore has also installed one. Their effect is limited to a local area, so they can only supplement cuts in emissions and cannot replace them.

(b) Oxygen-demanding wastes

Oxygen-demanding wastes are biodegradable organic materials, such as sewage, animal manure, food-processing waste and effluent from paper and sugar mills. In water, aerobic bacteria break them down and consume dissolved oxygen (DO) in the process. The amount of oxygen needed is measured as biochemical oxygen demand (BOD).

Role in water pollution: a heavy load lowers dissolved oxygen sharply, so fish and other aquatic life suffocate. Where oxygen runs out, anaerobic decomposition takes over, producing foul-smelling gases such as hydrogen sulphide and methane. The nutrients released may also encourage algal blooms, which further deplete oxygen when they die.

Prevention: the main steps are to (a) treat sewage and industrial effluent before discharge, using aeration and other biological processes; (b) reduce waste at source, and separate food waste; (c) manage animal manure, and keep it away from water; (d) set and enforce BOD discharge limits; (e) maintain aeration and adequate river flows, and construct wetlands to treat effluent.

(c) Biosphere and its energy resources

The biosphere is the part of the Earth where life exists. It includes the zones of land (lithosphere), water (hydrosphere) and air (atmosphere) that support living organisms, extending from deep ocean trenches to the lower atmosphere.

Energy resources: nearly all energy in the biosphere comes from the Sun. Plants capture it through photosynthesis, and it passes through food chains.

Solar energy: the original source, used directly through solar panels and thermal collectors. Biomass: wood, crop waste, manure and biofuels store solar energy in chemical form. Fossil fuels: coal, oil and natural gas, formed from ancient organisms, are non-renewable. Wind and hydropower: both are driven indirectly by the Sun, which heats the air and powers the water cycle. Other sources: geothermal energy from the Earth's interior, tidal energy from the Moon's gravity, and nuclear energy from uranium, none of which rely upon the Sun.

(d) Layers of the atmosphere

From the ground upward, the layers are the troposphere (up to about 8-15 km), stratosphere (to about 50 km), mesosphere (to about 85 km), thermosphere (to about 600 km) and exosphere (the outer limit).

Basis of classification: the layers are classified mainly by the change of temperature with altitude, which reverses at their boundaries. Temperature falls through the troposphere, rises in the stratosphere, falls in the mesosphere, and rises again in the thermosphere.

Auroras form mainly in the thermosphere, about 100 to 300 km up, when charged particles from the Sun strike atmospheric gases near the poles.

Satellites: low-Earth-orbit satellites, including the International Space Station at about 400 km, orbit within the thermosphere. Satellites at higher altitudes are in the exosphere or beyond, including geostationary satellites at about 35,786 km.