When fine particles are trapped in cold still air, they cause the brown haze that typically appears over cities in winter. How these pollutants are measured 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. Visibility is measured by shining a light through an air sample and determining the amount of light scattered by fine particles. The greater the concentration of particles, the greater the extent of light scattering and the lower the visibility. The measurement technique is called nephelometry.