UPSC Prelims 2022 Q100: Which Gases Cause Acid Rain — Explained
Consider the following:
1. Carbon monoxide
2. Nitrogen oxide
3. Ozone
4. Sulphur dioxide
Excess of which of the above in the environment is/are cause(s) of acid rain?
- 1, 2 and 3
- 2 and 4 only
- 4 only
- 1, 3 and 4
Answer: (b) 2 and 4 only
Statement-wise explanation
- 1: incorrect — Carbon monoxide is a poisonous gas, but it does not form a strong acid with water. In the air it slowly oxidises to carbon dioxide. Carbon dioxide forms only weak carbonic acid, which gives unpolluted rain its natural pH of about 5.6. Rain counts as acid rain only when it falls below this level, so CO is not a cause.
- 2: correct — Nitrogen oxides (NO and NO2, together NOx) come from vehicle engines, power plants and other high-temperature combustion. In the atmosphere they react with water, oxygen and other chemicals to form nitric acid (HNO3). Nitric acid is one of the two main acids in acid rain.
- 3: incorrect — Ozone near the ground is a secondary pollutant. Sunlight forms it from NOx and volatile organic compounds, and it is a key part of photochemical smog. As an oxidant it can speed up the reactions that turn SO2 and NOx into acids, but it does not itself become an acid. It is therefore not counted as a cause of acid rain.
- 4: correct — Sulphur dioxide comes mainly from burning sulphur-containing coal and oil in thermal power plants, smelters and refineries. Volcanoes also release it. It oxidises and dissolves in water to form sulphuric acid (H2SO4), the largest single component of acid rain. The US EPA names SO2 and NOx as the primary causes of acid rain.
How to eliminate
Start from the fact almost every aspirant knows: sulphur dioxide is the textbook cause of acid rain (it is the gas behind 'marble cancer' on the Taj Mahal). That keeps the options containing 4. The next most familiar fact is that nitrogen oxides form nitric acid. Of the options that include 4, only one also includes 2, so that settles it. You can also work from the other end. Carbon monoxide is known for its toxicity, because it binds to haemoglobin, not for acidity. If you strike out 1, you are left with two options, and the choice turns on whether you remember NOx. The option '4 only' is the trap for candidates who remember only SO2. To remember: acid rain = sulphuric acid (from SO2) + nitric acid (from NOx). Ozone is linked with photochemical smog, and CO with poisoning.
Know more
Acid rain, or more broadly acid deposition, is precipitation more acidic than normal. Pure rain is already slightly acidic, at about pH 5.6, because CO2 dissolves to form weak carbonic acid. Acid rain is rain below that level, often in the range of 4.2–4.4.
How it forms
- SO2 + oxygen + water → sulphuric acid (H2SO4)
- NOx + oxygen + water → nitric acid (HNO3)
- The acids can travel hundreds of kilometres before they come down. They fall as wet deposition (rain, snow, fog) or dry deposition (acidic gases and particles settling on surfaces).
| Gas | Main sources | Role |
|---|---|---|
| SO2 | Coal-fired power plants, smelters, refineries, volcanoes | Main acid-rain precursor (sulphuric acid) |
| NOx | Vehicles, power plants, lightning | Acid-rain precursor (nitric acid); also forms ground-level ozone |
| O3 (ground-level) | Formed from NOx + VOCs in sunlight | Photochemical smog; oxidant, not an acid source |
| CO | Incomplete combustion, vehicles | Toxic (blocks oxygen transport in blood); not an acid source |
| CO2 | Combustion, respiration | Weak carbonic acid; sets natural rain pH ~5.6 |
Effects: Acid rain acidifies lakes and streams, harming fish. It leaches nutrients from soil and damages forests. It corrodes marble and limestone. The yellowing of the Taj Mahal, linked to SO2 from the Mathura refinery and nearby industry, is the standard Indian example.
Control in India: All four gases in this question are among the 12 pollutants in India's National Ambient Air Quality Standards (2009). To cut SO2 from coal-based thermal power plants, the government has required Flue Gas Desulphurisation (FGD) units. Moving from BS-IV to BS-VI fuel norms has reduced sulphur in fuel and NOx from vehicles.
NCERT Concepts to Revise
- Acid rain: Defines acid rain by its pH below about 5.6, the test every listed gas must pass.
- Air pollution: Separates primary from secondary air pollutants, which helps place each of the four listed gases.
- Smog: Classical smog and photochemical smog involve different gases, so knowing both helps sort the four options.