Dobson
spectrophotometers can be used to measure both total column ozone and profiles of ozone in the
atmosphere. Ozone is tri-atomic oxygen, O3; ozone molecules absorb harmful
UV light in the atmosphere before it reaches the surface of the earth. No UVC radiation penetrates to the ground as it is absorbed in the
ozone-oxygen cycle. However some longer-wave and less harmful
UVB and most of the
UVA are not absorbed as ozone is less opaque to these frequencies, so they penetrate to the ground level of Earth in higher quantities. The sources of
light used may vary. Beside the direct
sun light, the light from the clear sky,
moon or
stars may be used. The Dobson spectrometer measures the total ozone by measuring the relative intensity of the UVB radiation that reaches the Earth and comparing it to that of UVA radiation at ground level. If all of the ozone were removed from the atmosphere, the amount of UVB radiation would equal the amount of UVA radiation on the ground. As ozone does exist in the atmosphere, the Dobson Spectrometer can use the ratio between UVA and UVB radiation on the ground to determine how much ozone is present in the
upper atmosphere to absorb the UVC radiation. The ratio is determined by turning the R-dial, which can be rotated a full 300°, on the instrument. The spectrometer compares two different wavelength intensities, UVB (305 nm) and UVA (325 nm), in order to calculate the amount of ozone. When turned, the R-dial filters and blocks out the light of the UVA wavelength until the intensity of the two wavelengths of light are equal. The ratio of the two wavelengths at incidence can be calculated once the filtered intensities are the same. The results are measured in
Dobson Units, equal to 10 μm thickness of ozone compressed to
Standard conditions for temperature and pressure (STP) in the column. If all of the ozone in the atmospheric column one was measuring were compressed to STP, the thickness of the compressed atmosphere in mm would equal an answer in Dobson Units divided by 100. The vertical distribution of ozone is derived using the
Umkehr method. This method relies on the intensities of reflected, rather than direct,
UV light. Ozone distribution is derived from the change in the ratio of the same UV-pair frequencies with time as the sun sets. An "Umkehr" measurement takes about three hours, and provides data up to an altitude of 48 km, with the most accurate information for altitudes above 30 km. The Dobson method has its drawbacks. It is strongly affected by
aerosols and
pollutants in the atmosphere, because they also absorb some of the light at the same wavelength. Measurements are made over a small area in the direction of the sun. Today this method is often used to calibrate data obtained by other methods, including satellites. ==Instruments and manufacturers==