Sources and effects of SO2, NOx and CO

Sulphur dioxide ($\text{SO}_2$)

Sources: burning of coal and fuel oil containing sulphur ($\text{S} + \text{O}_2 \rightarrow \text{SO}_2$), smelting of sulphide ores, oil refineries, and volcanoes.

Effects:

• A colourless gas with a sharp smell that irritates the eyes and respiratory tract and aggravates asthma and bronchitis.
• Oxidises to $\text{SO}_3$ and then to sulphuric acid, causing acid rain that damages lakes, forests and buildings.
• Bleaches and damages leaves (chlorosis); corrodes metals and attacks limestone and marble.

Oxides of nitrogen ($\text{NO}_x$ = NO + $\text{NO}_2$)

Sources: high-temperature combustion in vehicle engines and power plants, where atmospheric $\text{N}_2$ and $\text{O}_2$ combine ($\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$); also fertiliser plants and lightning.

Effects:

• $\text{NO}_2$ is a reddish-brown toxic gas that damages lung tissue and lowers resistance to infection.
• Takes part in photochemical smog and ozone formation.
• Forms nitric acid, which adds to acid rain.
• Damages plants and fades fabrics.

Carbon monoxide (CO)

Sources: incomplete combustion of carbon fuels, mainly petrol vehicles, also biomass burning, forest fires and industries.

Effects:

• Colourless and odourless. It combines with haemoglobin about 200-250 times more strongly than oxygen, forming carboxyhaemoglobin (COHb) and reducing oxygen transport.
• Causes headache, dizziness, drowsiness, and at high levels unconsciousness and death.
• It is especially dangerous in closed garages and poorly ventilated kitchens. In the open atmosphere it is slowly oxidised to $\text{CO}_2$.