Dobson instruments

An alternate technique uses the back-scattered solar ultraviolet (BUV) radiation reflected into space and measured outside the atmosphere from an orbiting satellite. The vertical distribution of ozone within this total column can be measured with the ground-based Dobson instrument through the umkehr technique, which depends upon the variation in ultraviolet penetration versus solar zenith angle over a period of several hours. The vertical distribution of ozone can also be determined from the wavelength dependence of the BUV signal. 

The results of limited ozone measurements by balloon ascents allow a far more detailed analysis of the factors influencing ozone distribution than the measurements of the total ozone amount made over many years at various locations by the Dobson instrument. 

Ozone data includes measurements from 380 quality controlled Vaisala ECC-ozone soundings. Ozone profiles from soundings have been inspected visually and by comparing the profile based total column ozone to the spectrometric column ozone measured preferably by Dobson spectroradiometer in Marambio or by satellite based TOMS-instrument. No normalisation factor was used to correct the profiles dubious spikes were nevertheless corrected. Soundings were made twice a month from January until July and twice a week from August until January. Occasional interruptions of soundings have existed. 

The total transmittance of the filter to the incident solar radiation changes according to the amount of ozone and the optical depth of the air mass. The instrumental calibration for determining the total amount of ozone was made through simultaneous observations with Dobson s spectrophotometer. The results of these calibrations are shown in Figure 2, in which the amount of ozone is expressed in 10 cm. 

In the 1920s, G.M.B. Dobson began measuring the amount of ozone in the atmosphere. Although ozone is formed in the higher regions of the stratosphere, most of it is stored in the lower stratosphere, where it can be measured by instruments on the ground or in balloons, satellites, and rockets. Dobson measured levels of stratospheric ozone of more than 300 Dobson units (DU). His measurements serve as a basis for comparison with recent measurements. 

FIGURE 20.29 This false color image shows total stratospheric ozone amounts over the southern hemisphere for September 24, 2006, as recorded by the Ozone Monitoring Instrument (OMI) mounted on the Aura spacecraft. The dramatic depletion of the ozone layer over Antarctica is revealed with the help of the false color scale at the bottom of the figure. Ozone amounts are commonly expressed in Dobson units 300 Dobson units is a typical global average over the course of a year. The size of the Antarctic ozone hole was near a record high and the levels of ozone near a record low on this date. 

Dobson unit The standard way to express ozone amounts in the atmosphere. One DU is 2.7 x 1016 ozone molecules per square centimeter. One Dobson unit refers to a layer of ozone that would be 0.001 cm thick under conditions of standard temperature (0°C) and pressure (the average pressure at the surface of the Earth). For example, 300 Dobson units of ozone brought down to the surface of the Earth at 0°C would occupy a layer only 0.3 cm thick in a column. Dobson was a researcher at Oxford University who, in the 1920s, built the first instrument (now called the Dobson meter) to measure total ozone from the ground. 

Plate 8. Global view of the ozone column abundance (Dobson) on 30 November, 1999 as measured by the GOME instrument - Values have been assimilated by the Royal Meteorological Institute of the Netherlands. Note the very low ozone column abundance over Antarctica. Courtesy of Hennie Kelder, KNMI, Netherlands. 

Ground-based measurements of total ozone use the Dobson spectrometer. This instrument measures the column density of ozone in the atmosphere by optical absorption, with the sun or the moon as background source. The data are expressed as an equivalent column height at standard pressure and... 

The Dobson Unit Imagine that all the ozone in a vertical column of air reaching from the Earth s surface to the top of the atmosphere were concentrated in a single layer of pure 03 at the surface of the Earth, at 273 K and 1.013 x 105 Pa. The thickness of that layer, measured in hundredths of a millimeter, is the column abundance of 03 expressed in Dobson units (DU). The unit is named after G. M. B. Dobson, who, in 1923, produced the first ozone spectrometer, the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity of solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which arc not. 

Scientists, who had no inkling of what they were about to discover, began measuring stratospheric ozone concentrations over 80 years ago using relatively unsophisticated instrumentation. Eventually, as a credit to G. M. B. Dobson, one of the scientists who invented an early instrument used for these measurements, the unit of ozone concentration in the atmosphere became... 

Dobson, R.L.M., A.P. D Silva, SJ. Weeks, and V.A. Fassel Multidimensional Laser-based Instrument for Characterisation of Environmental Samples for Polycyclic Aromatic Compounds. Anal. Chem. 58, 2129 (1986). 

I would like to express my gratitude to all contributors who have kindly provided the materials for the CD-ROM Richard Moore (University of California, San Diego), Robert Thompson (Georgia State University), Kenny McAlpine (University of Glasgow), Stephan Schmitt and Jake Mandell (Native Instruments), Nicolas Fournel (Synoptic), Karnataka Group, Dave Smith (Seer Systems), Aluizio Arcela (University of Brasilia), Xavier Rodet and Adrien Lefevre (Ircam), Trevor Wishart, Archer Endrich, Richard Dobson and Robert Fraser (CDP), Jacques Chareyron (University of Milan), Arun Chandra (University of Illinois),... 

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