The measurement technique is called nephelometry. Ozone is measured by shining ultraviolet light through an air sample and determining the amount of light absorbed by the ozone molecules. The more light is absorbed, the higher the concentration of ozone. While nitrogen dioxide cannot be measured directly, levels of this pollutant can be determined by measuring nitric oxide. The measurement technique is called chemiluminescence.
Light is emitted when nitric oxide reacts with an oxidant.
The amount of light emitted is a measure of the concentration of the nitric oxide. The nitrogen dioxide concentration is the difference between these two measurements. Carbon monoxide is measured by drawing sample air into a cell where a beam of infrared light is passed through it to a photodetector. Some of the light is absorbed by carbon monoxide in the sample, the amount being proportional to the number of molecules present. By comparing the light intensity received by the photodetector through the sample cell with that received through a similar cell containing reference gas, the concentration of carbon monoxide may be determined.