How does a Spectrophotometer Work?

One of the ways to measure the amount of ozone in the atmosphere is by spectroscopy. This is the technique used by the University of Alaska-Fairbanks (UAF) Dobson instrument. Spectroscopic instruments measure the intensity of light versus its wavelength. Light can be separated into its individual wavelengths by passing the light through a prism, as shown in the figure below. In the visible region, the wavelength corresponds to the color of the light. In the UV region, the human eye does not respond to this light, so we use a sensitive electronic eye to observe the light intensity. Each type of molecule absorbs certain wavelengths of light in a characteristic pattern, known as a spectrum. The spectrum of ozone peaks in the UV, thus ozone absorbs UV and protects us from its damaging effects. At wavelengths where ozone absorbs only part of the solar radiation, the amount of light that reaches the ground depends on the abundance of overhead ozone. Thus, measurement of the light intensity at a wavelength where light is partially absorbed by ozone allows us to quantify the abundance of overhead ozone.

How does the Dobson Spectrophotometer Work?

The Dobson spectrophotometer is a ground-based instrument that measures the amount of ozone present in the atmosphere. The Dobson spectrophotometer was designed by Gordon Dobson in the 1930's. The Dobson spectrophotometer measures ultraviolet light from the Sun at 2 to 6 different wavelengths from 305 to 345 nm. By measuring UV light at two different wavelengths, the amount of ozone can be calculated. One of the wavelengths used to measure ozone is absorbed strongly by ozone (305 nm), whereas the other wavelength is not absorbed by ozone (325 nm). Therefore the ratio between the two light intensities is a measure of the amount of ozone in the light path from the sun to the observing spectrophotometer.

The ratio between the two intensities are determined by an R-dial located on the top of the Dobson spectrophotometer. The R-dial controls a filter wedge that gradually blocks out the 325 nm light. As the R-dial rotates from 0 to 300 degrees, the filter wedge increasingly blocks out more light. At 0 degrees the wedge does not block out any light. At 300 degrees, the wedge nearly blocks all of the light. The filter wedge gradually blocks more and more light until the intensity of the 325 nm and 305 nm light are equal. The R-dial is calibrated with the filter wedge, so that the original intensity of the 325nm light can be determined from the R-dial reading. By taking the R-dial reading when the intensities of the two wavelengths are equal, their ratio is determined. The simple animation below demonstrates how the filter wedge works.

Aerosols present in the atmosphere scatter light at both wavelengths equally. Thus, aerosols have no affect on the ratio of light intensities between 305 and 325 nm.

The filter wedge is controlled by the R-dial. As the R-dial rotates from 0 to 300 degrees, more of the 325 nm light is blocked out. The filter wedge gradually blocks out more and more light until the light intensities between the two wavelengths are equal. By taking the R-dial reading when the light intensities are equal, the ratio of the two intensities can be determined.

Levels of ozone are reported in Dobson Units (DU). A Dobson unit is defined as the thickness of a column of ozone if all of the ozone from the Earth's surface to outer space were condensed to standard temperature and pressure (STP). An ozone measurement of 400 DU would be 4 mm thick at STP. 1mm ozone at STP = 100 DU.

For more information about the Dobson Spectrophotometer.