carbon dioxideA colourless gas with a faint tingling smell and taste. Atmospheric carbon dioxide is the source of carbon for plants. As carbon dioxide is heavier than air and does not support combustion, it is used in fire extinguishers. It is a normal constituent of the atmosphere, relatively innocuous in itself but playing an important role in the greenhouse effect. It is produced during the combustion of fossil fuels when the carbon content of the fuels reacts with the oxygen during combustion. It is also produced when living organisms respire. It is essential for plant nutrition and in the ocean phytoplankton is capable of absorbing and releasing large quantities of the gas, CO2
carbon dioxide tax Compulsory charges levied on fuels to reduce the output of carbon dioxide (CO2), which is a colourless and odourless gas substance that is incombustible
nitrogen dioxideA reddish-brown gas; it exists in varying degrees of concentration in equilibrium with other nitrogen oxides; used to produce nitric acid, NO2
sulphur dioxideEmissions of the gas given off during the burning of fossil fuels in power stations and other boilers. Sulphur dioxide is created because sulphur is an impurity in most coal and oils. When the fuel is burned the hot sulphur reacts with oxygen in the atmosphere to form sulphur dioxide, SO2
titanium dioxideA white, water-insoluble powder that melts at 1560°C, and which is produced commercially from the titanium dioxide minerals ilmenite and rutile; used in paints and cosmetics