(a) Briefly describe the various segments of atmosphere. How these segments are maintaining the Earth Radiation Balance? (b) How the organic particulate matter enters in the atmosphere. Also describe the sources? (c) What is natural radioactivity? How it is different from the artificial radioactivity? (d) What are the Fossils? Discuss the importance of paleontology.

(a) Segments of the atmosphere

Troposphere (surface to about 8-15 km): it contains most of the air and water vapour, and it is where weather occurs. Temperature falls with height. Stratosphere (up to about 50 km): it contains the ozone layer, and temperature rises with height as ozone absorbs ultraviolet light. Mesosphere (up to about 85 km): temperature falls again, and most meteors burn up here. Thermosphere (up to about 600 km): the thin air here absorbs high-energy solar radiation and becomes very hot, and auroras occur here. The ionosphere, within it and the mesosphere, reflects some radio waves. Exosphere (the outermost layer): it gradually fades into space.

Earth's radiation balance: for a stable climate, the energy absorbed from the Sun must equal the energy radiated to space. The atmosphere's contribution is as follows.

Reflection: about 30% of incoming sunlight is sent back to space by clouds, particles and the surface. Absorption of short waves: the upper layers and the ozone in the stratosphere absorb high-energy radiation and protect life, and the troposphere absorbs part of the rest. Greenhouse effect: the warmed surface emits infrared radiation. Greenhouse gases, mainly in the troposphere, absorb it and re-emit part of it downward, keeping the surface about 33°C warmer than it otherwise would be. Emission to space: the atmosphere eventually emits heat to space, balancing incoming energy. Rising greenhouse gases are now disturbing this balance, and this causes global warming.

(b) Organic particulate matter

Organic particulate matter consists of tiny carbon-containing solid or liquid particles suspended in the air. It enters the atmosphere by two routes:

Primary emission: the particles are released directly, through combustion or from mechanical and biological processes. Secondary formation: volatile organic compounds (VOCs) from vehicles, industry or plants react in the atmosphere, often with sunlight and ozone. The products have lower volatility, so they condense into new particles.

Sources

Natural sources: wildfires; VOCs released by forests, such as terpenes; pollen, spores and plant debris; and organic matter in sea spray. Human sources: vehicle exhaust; burning of coal, wood and other biofuels; crop-residue and waste burning; cooking; and industrial processes and solvents.

These particles reduce visibility, damage lungs and affect climate. They are a major part of the smog in cities such as Lahore.

(c) Natural and artificial radioactivity

Natural radioactivity is the spontaneous emission of radiation by unstable atomic nuclei that occur in nature, such as uranium-238, thorium-232, radium and potassium-40. It was discovered by Henri Becquerel in 1896.

Artificial (induced) radioactivity is produced when stable nuclei are made unstable by bombarding them with particles such as neutrons or alpha particles in reactors or accelerators. It was first produced by Irène and Frédéric Joliot-Curie in 1934.

Origin: natural radioisotopes occur in rocks, soil, air and living things, whereas artificial ones are made by human activity. Control: natural radioactivity can only be measured, while artificial radioactivity can be produced in chosen isotopes for chosen purposes. Examples and uses: artificial isotopes include cobalt-60, used in cancer treatment, and technetium-99m, used in medical imaging. Decay itself follows the same physical laws in both cases.

(d) Fossils and palaeontology

Fossils are the preserved remains or traces of ancient organisms found in rocks. They include bones, shells and leaf impressions, as well as trace fossils such as footprints. They usually form when an organism is buried rapidly in sediment, which later hardens into rock.

Palaeontology is the scientific study of fossils. Its importance:

(i) it provides evidence of evolution and the history of life; (ii) it helps to date rocks, through index fossils and comparison of layers; (iii) it reveals past climates and environments, which helps in understanding climate change; (iv) it helps in finding oil, gas and coal, using microfossils; (v) it shows how mass extinctions occurred and how life recovered; (vi) it supports the theory of continental drift, as identical fossils appear on distant continents. In Pakistan, for example, whale fossils such as Pakicetus show how whales evolved from land mammals.