(A) What is 'Acid Rain'. Describe its causes and how it can be prevented? (B) What is 'Water Pollution'. Discuss its causes and measurement methods. Name the countries with the highest and lowest percentage of it. (C) Write a comprehensive note on 'Smog'. (D) Describe different causes of 'Tsunami'. Is there any difference between a Tsunami and a tidal wave? Name the worst Tsunami ever recorded.

(A) Acid rain

Acid rain is precipitation more acidic than normal. Clean rain has a pH of about 5.6 owing to dissolved CO₂, so acid rain generally has a pH below this.

Causes: the main pollutants are sulphur dioxide (SO₂) and nitrogen oxides (NOₓ), from coal and oil burning in power plants and factories, from smelting and from vehicles. Volcanoes and lightning add smaller natural amounts. These gases react in the air with water and oxygen to form sulphuric acid and nitric acid.

Prevention: the main measures are to (i) burn low-sulphur fuels; (ii) fit scrubbers to chimneys, and catalytic converters to vehicles; (iii) shift to renewable energy and use energy efficiently; (iv) improve public transport; (v) enforce emission limits, including agreements across borders, because the pollutants can travel far. Adding lime can treat acidified lakes and soils, but it does not address the cause.

(B) Water pollution

Water pollution is the contamination of rivers, lakes, seas or groundwater by substances or microbes, to the point that the water becomes harmful to people or ecosystems or unfit for use.

Causes: untreated sewage; industrial effluent with metals and chemicals; pesticide and fertiliser runoff; solid waste and plastics; oil spills; mining; and naturally occurring arsenic in groundwater, which is a concern in parts of Pakistan and Bangladesh.

Measurement methods

Biochemical oxygen demand (BOD): oxygen used by microbes to break down organic matter. Chemical oxygen demand (COD): oxygen needed to oxidise pollutants chemically. Dissolved oxygen (DO): low levels indicate poor water quality. Physical and chemical tests: pH, turbidity, temperature, total dissolved solids, nitrates and heavy metals. Biological tests: coliform bacteria such as E. coli indicate faecal contamination, and the presence of sensitive aquatic species indicates river health.

Highest and lowest countries: there is no single official percentage ranking of water pollution, as results depend on what is measured. One indicator is the 2024 Environmental Performance Index category on sanitation and drinking water, which ranks countries by protection from unsafe water, not pollution as such. Singapore ranked first, among the safest, followed by countries such as Italy and the UK. The heaviest burdens from unsafe water generally fall on low-income countries in Sub-Saharan Africa and South Asia, including Pakistan, where the challenge is widely reported.

(C) Smog

Smog, from smoke and fog, is a form of severe air pollution that reduces visibility and harms health. There are two main types. Sulphurous (London-type) smog comes from burning coal in cold, damp conditions; the Great Smog of London in 1952 killed thousands. Photochemical smog forms when sunlight acts upon nitrogen oxides and volatile organic compounds from vehicles and industry, producing ground-level ozone and other irritants, as historically in Los Angeles.

Causes: vehicle exhaust, industry, brick kilns, coal burning, crop-residue and waste burning, and construction dust. A temperature inversion, in which warm air sits over cool air, traps pollutants close to the ground.

Effects: coughing, eye irritation, asthma and heart and lung disease; reduced visibility and road accidents; and crop damage. Lahore and other Punjab cities now suffer severe smog each winter, leading to school closures and health emergencies.

Control: cleaner fuels and vehicle standards, zigzag technology for kilns, alternatives to crop burning, better public transport, monitoring and enforcement, and regional cooperation with India, since pollution crosses the border.

(D) Tsunami

Causes: a tsunami results when a large body of water is suddenly displaced by (i) an undersea earthquake, the most common cause; (ii) a submarine or coastal landslide; (iii) a volcanic eruption; or (iv) rarely, a meteorite impact.

Tsunami and tidal wave: yes, they differ. True tides are regular rises and falls of sea level caused by the gravity of the Moon and Sun. A tsunami has nothing to do with tides, and is caused by sudden displacement of water. Though the term tidal wave is often used for a tsunami, scientists avoid it as misleading.

Worst tsunami: the Indian Ocean tsunami of 26 December 2004, triggered by a magnitude 9.1 earthquake off Sumatra, was the deadliest on record, killing about 230,000 people in 14 countries. The tallest recorded wave was at Lituya Bay, Alaska, in 1958, reaching about 524 metres, but its death toll was small.