Air pollution is caused by substances that are not normally part of the atmosphere’s composition. Important air pollutants are sulphur dioxide, nitrogen oxides and carbon monoxide usually emitted in automobile exhausts and power-house smoke. Pollution detectors work by making use of the chemical or physical properties of the pollutants. For example, the detection of nitrogen oxides is based on the emission of light as a result of a chemical reaction. This phenomenon is called chemiluminescence. If nitrogen oxides are present light is emitted which can be detected by a photo detector. Sulphur dioxide is detected by introducing it into a flame and then analyzing the colour produced by an instrument called flame photometer.
Carbon monoxide is detected by emission of infrared radiation of a particular frequency when excited by an arc or a spark. An infrared spectrometer is used to detect the presence of this gas in air.
The quantitative measurement of the gases is based on the intensity of light or spectra they emit.