Atmospheric Pollutants

Complete combustion of a hydrocarbon fuel happens when there's PLENTY of oxygen:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

The carbon ends up fully oxidised to CO₂. CO₂ isn't directly toxic but it's the main driver of greenhouse warming.

Incomplete combustion happens when oxygen is LIMITED:

$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$

Carbon monoxide (CO) is the dangerous product:

• Colourless, odourless — can't be detected by your senses.
• Binds to haemoglobin ~200× more tightly than O₂. Blocks oxygen transport in blood.
• Symptoms: headache, drowsiness, then unconsciousness, then death.
• Sources: faulty gas boilers, blocked chimneys, car exhausts in enclosed spaces.

This is why homes need CO alarms — the gas itself is invisible. Engines with catalytic converters convert CO to CO₂ before it leaves the exhaust.

Sulfur dioxide (SO₂) comes from burning fuels containing sulfur impurities (especially coal and some heavy fuel oils):

$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$

Nitrogen oxides (NOₓ) form in HIGH-TEMPERATURE combustion (car engines, power stations): N₂ and O₂ in the air react to make NO, which oxidises further to NO₂:

$\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$

Both gases dissolve in atmospheric water vapour:

• SO₂ → H₂SO₃ (sulfurous acid) → eventually H₂SO₄ (sulfuric acid).
• NO₂ → HNO₃ (nitric acid).

The result: acid rain (rain with pH typically 4-5, much more acidic than the natural ~5.6 of CO₂-saturated rain).

Effects:

• Damages forests (leaf injury, soil acidification, leaching of nutrients).
• Damages buildings made of limestone or marble (CaCO₃) — visible in old monuments.
• Acidifies lakes — kills fish and amphibians (low pH disrupts gills, egg development).

Mitigation:

• Low-sulfur fuels and flue-gas desulfurisation ('scrubbers') at power stations.
• Catalytic converters in cars reduce NOₓ.
• Switch to renewables (no SO₂ or NOₓ from solar, wind).

Particulate matter (PM) is tiny solid particles released by incomplete combustion (diesel exhaust, coal, wood burning, brake/tyre wear).

• PM10: particles up to 10 micrometres — reach the throat and upper airways.
• PM2.5: under 2.5 micrometres — much smaller, can reach deep into lungs and even cross into the bloodstream.

Effects: asthma, lung damage, heart disease. Air pollution from PM is estimated to cause millions of premature deaths globally each year.

Reduction:

• Particulate filters on diesel exhausts.
• Cleaner combustion (higher efficiency engines, better fuel quality).
• Switch from solid fuels in homes (wood, coal) to cleaner alternatives.

Methane (CH₄) is a much more powerful greenhouse gas than CO₂ over short timescales (~28× more warming per molecule over 100 years). Sources:

• Livestock (cattle digestion, manure).
• Landfill (organic waste breaking down).
• Natural gas leaks from pipelines and wells.
• Wetlands and rice paddies.

Modest reductions (capturing landfill gas, plugging leaks, less meat consumption) could quickly reduce warming because methane's atmospheric lifetime is only ~12 years.