Dobson spectrophotometer

Dobson spectrophotometer spect A photoelectric spectrophotometer used in the determination of the ozone content of the atmosphere compares the solar energy at two wavelengths in the absorption band of ozone by permitting the radiation of each to fall alternately upon a photocell. dab-san Spek-tro-fa tam-ad-ar docosane org chem C22H46 A paraffin hydrocarbon, especially the normal isomer CH3(Cf2l2)2oCH3. dak-a,san ... 

Komhyr, W. D R. D. Grass, R. D. Evans, R. K. Leonard, D. M. Quincy, D. J. Hofmann, and G. L. Koenig, Unprecedented 1993 Ozone Decrease over the United States from Dobson Spectrophotometer Observations, Geophys. Res. Lett., 21, 201-204 (1994). 

The measuring device is Robertson-Berger broadband filter UV radiometer (RB-biometer). The time series of 10 min sums obtained during August 1, 1996 to June 25, 1997 was supplemented by (i) daily averages of total ozone concentration measured by Dobson spectrophotometer in HK [17, 18], (ii) hourly totals of global solar radiation measured by a Kipp-Zonen CP-11 in HK, and hourly fractions of sunshine... 

X >> TOTAL OZONE LONG-TERM AVERAGE MEASURED WITH THE DOBSON SPECTROPHOTOMETER... 

Several parameters are observed total amount of ozone (Dobson spectrophotometer), absorption in the 9.6-micron band, and emission from the ozone at several different zenith distances. The value of each of these observations may be expressed as the sum of contributions from the different layers of the ozone. Total amount of ozone is, of course, merely the sum of the amounts in each layer the infrared absorption, however, is pressure-dependent and a pressure-dependence term must be introduced. The sums are somewhat more complicated for the emission, but still are easily calculated. The problem is to find a distribution of ozone which will give rise to the... 

Some modern versions of the Dobson spectrophotometer exist and continue to provide data, e. g. the Brewer spectrophotometer in the WMO-GAW ozone observing system comprises more than 100 stations worldwide that measure total column O3 and O3 profiles in the troposphere and stratosphere. Fig. 2.82 shows the total ozone record from Antarctica, showing two main features a) the drastic drop in ozone after 1975 and b) the stagnation after 1993, likely as the result of the Montreal Protocol. 

Experience showed that the performance of Brewer-Mast sondes critically depends on the prelaunch procedure of the sondes, whereas the ECC sondes need less sophisticated control before launch. The integrated ozone amount measured by the sonde can be compared with the total ozone amount measured by sun photometry (usually performed by Dobson spectrophotometers). This requires the extrapolation of the O3 profile above the burst level of the balloons (residual ozone). This comparison can be used to check the quality of the ozone profile of the sonde. The factor obtained by the sum of the ozone measured by the sonde plus the residual ozone divided... 

The model was initially compared with LOWTRAN 7 (5) results and with Brewer spectrophotometer measurements carried out at Lisbon in 1990. LOWTRAN-MESTRad results show a reasonable good agreement over 290-400 nm range with a mean difference of about 10%. Figure 4 shows de Model/LOWTRAN ratios for direct, diffuse and global spectral irradiances computed for the same conditions of ozone (332 Dobson Units), solar zenith angle (45.2°) and aerosol optical depth (0.0). 

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. 

Dobson, Gordon Miller Bourne (1889-1976) British physicist and meteorologist He built the first ozone spectrophotometer and studied the results over many years (now called Dobson spectrometer or Dobsonmeter giving the Dobson unit). 

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