Sulfur Dioxide, Nitrogen Oxides and Acid Rain

Introduction

Clean rain is naturally slightly acidic, but in the twentieth century rain over industrial regions of Europe and North America became far more acidic than it should have been. Stone statues lost their faces, forests turned brown and fish vanished from lakes. The culprits were two groups of gases released by burning fuels: sulfur dioxide and nitrogen oxides . This page follows these gases from chimney and exhaust pipe to the rain that falls hundreds of kilometres away.

Core explanation

Natural rain. Rain dissolves a little carbon dioxide from the air, forming weak carbonic acid, so unpolluted rain has a pH of about 5.6. Acid rain is rain with a pH lower than this, sometimes as low as 4 or even below.

Sulfur dioxide. Coal, and to a lesser extent crude oil fractions, contain sulfur as an impurity. When the fuel burns, the sulfur reacts with oxygen:

sulfur + oxygen → sulfur dioxide S + O₂ → SO₂

Power stations burning coal were historically the biggest source. Volcanoes also release sulfur dioxide naturally.

Nitrogen oxides. Air is mostly nitrogen, which is normally very unreactive. But inside a car engine or a power station furnace the temperature is so high that nitrogen and oxygen from the air react together:

N₂ + O₂ → 2NO

Nitrogen monoxide (NO) then reacts with more oxygen to form nitrogen dioxide (NO₂), a brown, choking gas. Together they are called NOₓ . Note that the nitrogen comes from the air, not from the fuel. Lightning produces some NOₓ naturally.

From gas to acid. In the atmosphere these gases react with oxygen and dissolve in water droplets in clouds. Sulfur dioxide eventually forms sulfuric acid (H₂SO₄) and nitrogen dioxide forms nitric acid (HNO₃). These strong acids fall as acid rain, snow, fog or dry deposits. Winds can carry them across national borders, so one country's pollution can harm another's forests.

Effects of acid rain.

Target Effect --- --- Limestone and marble buildings Calcium carbonate reacts with acid and slowly dissolves; carvings lose detail Metal structures Iron and steel corrode faster Lakes and rivers Water becomes acidic; aluminium ions leach from soil; fish and insect larvae die Trees and soil Nutrients such as calcium and magnesium are washed out of soil; leaves are damaged; trees weaken

The reaction with limestone is:

calcium carbonate + sulfuric acid → calcium sulfate + water + carbon dioxide CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Reducing acid rain. Sulfur can be removed from fuels before burning; power stations can use flue gas desulfurisation to capture SO₂; catalytic converters on cars turn NOₓ back into nitrogen; and switching from coal to renewable energy removes the source. Lakes can be treated with powdered limestone to neutralise acidity, but this treats the symptom rather than the cause.

Step-by-step reasoning

To trace acid rain from source to damage:

1. Identify the source: sulfur in fuel, or nitrogen from air in a hot engine. 2. Name the gas: SO₂ or NOₓ. 3. The gas reacts with oxygen and water in clouds. 4. Sulfuric or nitric acid forms and falls as rain. 5. The acid reacts with limestone, metals, soil or lake water.

Visual explanation

Imagine a diagram with a power station chimney on the left releasing grey SO₂ and a road releasing brown NO₂. Arrows carry the gases up into a cloud, where drops turn from blue to orange to show acid. Rain then falls on the right onto a crumbling statue, a lake with dead fish and a forest of bare trees.

Real-world analogy

Acid rain acting on a limestone statue is like a sugar cube left under a slowly dripping tap. Each drop removes only a tiny amount, but over years the sharp corners round off and the shape is lost.

Real-world example

Many old limestone churches and cathedrals in British cities have gargoyles and carved faces that have been worn almost smooth. Much of this damage happened when coal smoke filled city air. Since coal burning in cities was reduced and sulfur emissions were cut, the rate of damage has fallen sharply.

Why?

Why do car engines produce nitrogen oxides when petrol contains no nitrogen? The engine draws in air, which is about 78% nitrogen. The burning fuel creates temperatures high enough to break the strong triple bond in N₂, so nitrogen and oxygen from the air combine.

Common misconception

"Acid rain is caused by carbon dioxide." Carbon dioxide makes rain only very weakly acidic (pH about 5.6), and this is natural. The strongly acidic rain that damages lakes and buildings comes mainly from sulfur dioxide and nitrogen oxides.

Worked example

Question: A power station burns coal containing sulfur. Explain how this could harm fish in a lake 300 km away.

Reasoning: Sulfur burns to form SO₂, which is carried by wind. In clouds it forms sulfuric acid, which falls as acid rain into the lake and its catchment. The lake's pH falls and toxic aluminium is released from the soil.

Answer: SO₂ from the coal forms sulfuric acid in clouds; acid rain lowers the lake's pH and releases aluminium ions, which kill fish.

Quick check

1. Which acid forms in clouds from nitrogen dioxide? Answer: Nitric acid (HNO₃).

Exam focus

Examiners often ask where the nitrogen in NOₓ comes from: answer "from the air, reacting with oxygen at the high temperatures in engines". For sulfur dioxide, say "from sulfur impurities in the fuel". Give a named effect of acid rain and a method of reducing each gas.

Advanced insight

Sulfur dioxide emissions in Europe and North America have fallen by more than 80% since around 1980, one of the great successes of environmental chemistry. Some lakes have recovered, but soils lose calcium and magnesium slowly and regain them even more slowly, so forests can take decades to return to health.

Summary

Sulfur dioxide forms when sulfur impurities in fuels burn; nitrogen oxides form when nitrogen and oxygen from air combine at high temperatures in engines and furnaces. In clouds these gases become sulfuric and nitric acids, which fall as acid rain with a pH below 5.6. Acid rain erodes limestone, corrodes metals, acidifies lakes and damages trees. Removing sulfur, scrubbing flue gases and catalytic converters all help.

Practice questions

1. Write a word equation for the formation of sulfur dioxide from sulfur in a fuel. Answer: Sulfur + oxygen → sulfur dioxide. 2. Explain why nitrogen oxides are produced in car engines. Answer: The high temperature makes nitrogen and oxygen from the air react together. 3. Give two effects of acid rain on the environment. Answer: Any two: it dissolves limestone buildings, corrodes metals, kills fish in lakes, damages trees and leaches nutrients from soil. 4. Why can acid rain damage forests in a country that burns very little coal? Answer: Winds carry SO₂ and NOₓ long distances across borders before they fall as acid rain. 5. Explain why adding powdered limestone to an acidic lake raises its pH. Answer: Calcium carbonate is a base that reacts with and neutralises the acid.