Vice President

NOAA - Climate Analysis Centre
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CNRS - Service d'Aeronomie
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GENEVA, 18 December 1991

Annex: 1

Dear Colleagues,

      You may recall the extensive information on the Bass and Paur (1985) ozone absorption coefficients presented at both the Halkidiki and Gottingen Ozone Symposia, and that the Commission had charged Dr. Bob Hudson, in collaboration with Dr. Carl Mateer, to study and make recommendations on their application in the WMO Global Ozone Observing System (GO3OS),

      I am glad to inform you that, after very elaborate studies carried out by Dr. Hudson who also assessed the other recent measurements of ozone absorption coefficients, a set for use in the standard Dobson wavelengths was recommended. Further calculations carried out by Drs. Mateer and Komhyr were necessary and consultations with other colleagues resulted in the Guidance attached to this letter.

      Since the main recommendation to start calculating and reporting to the WMO-WO3DC total ozone amount using the new (Bass and Paur) ozone absorption coefficients from 1 January 1992 was also endorsed by the WMO Executive Council Panel on Environmental Pollution and Atmospheric Chemistry, this recommendation is being communicated formally by the Secretary-General of WMO, Professor G.O.P. Obasi, to the Permanent Representatives of the countries having total ozone measurements for implementation.

      The present letter and attached Guidance are forwarded to you in order to keep you informed and to facilitate timely transition to the new 1992 scale.

      Finally, I would like to convey the appreciation of both the Commission and WMO to our colleagues, Drs Hudson, Mateer and Komhyr for their dedicated efforts towards successful 'completion of this important task.

      With best wishes for the New Year.
  Yours sincerely,

(Dr. Rumen D. Bojkov)
International Ozone Commission

To : Members of the Ozone Commission

1 JANUARY 1992*

1.  Background

      Early observations with Dobson spectrophotometers, first employing photocells and later photomultipliers, were processed using ozone absorption coefficients based on the laboratory data of Ny and Choong (1933). At the beginning of the International Geophysical Year (1 July, 1957), new absorption coefficients were adopted for use (Dobson, 1957), this time employing the laboratory values of Vigroux (1953). Pre-IGY Dobson spectrophotometer ozone values were converted to the Vigroux scale by multiplying them by 1.36.

      Because the Vigroux coefficients gave inconsistent ozone values from observations on different combinations of wavelengths (e.g. CD-wavelength observations yielded ozone values nearly 10% smaller than did AD-wavelength observations), the Vigroux AD-wavelength ozone absorption coefficients were adopted as standard. Based on a redetermination of the ozone absorption coefficients by Vigroux (1967) and an examination of the coefficients by G.M.B. Dobson in light of atmospheric observations made in Canada, the United Kingdom and elsewhere, a modified set of Vigroux ozone absorption coefficients was derived which gave highly consistent ozone amounts from observations made on different wavelengths. These modified coefficients, which preserved the original Vigroux AD-wavelength coefficient as standard were recommended by the International Association of Meteorology and Atmospheric Physics (IAMAP) for use in the WMO Global Ozone Observing System (GO3OS) beginning 1 January 1968. Rayleigh molecular scattering coefficients used with the ozone absorption coefficients in processing ozone data were derived from the Rayleigh Cabannes formula (Dobson 1957).

      During the mid 1980's, several research groups redetermined ozone absorption coefficients in the Hartley and Huggins bands. In 1988, the International Ozone Commission assigned an action item to R.D. Hudson and C.L. Mateer to derive and recommend for use improved Dobson instrument ozone absorption coefficients based on the new laboratory data and to set a date for their use.

      Hudson (1990) recommended use of the Bass-Paur (1985) ozone absorption coefficients, adjusted to account for the temperature dependence (as determined by Barnes and Mauersberger 1987) of the Hearn (1961) 253.7 nm wavelength ozone absorption coefficient. Subsequently, Mateer (1990) used a "dynamic" approach to compute effective ozone absorption coefficients for the Dobson instruments. The method not only weighs the Bass-Paur coefficients with the instrument slit function weights, at an ozone-weighted mean temperature determined from convolution of appropriate temperature and ozone profiles but also takes into account solar flux weighting at the instrument slits. Subsidiary data used by Mateer in obtaining results were the slit function data for world standard Dobson spectrophotometer 83 determined experimentally by Komhyr (1982); the extra-terrestrial solar spectrum compiled by Furukawa et al. (1966); the Rayleigh scattering cross-sections calculated by Bates (1984); the atmospheric temperature profiles in the US Standard Atmosphere, 1962 and the US Standard Atmosphere supplements, 1966: the standard ozone profiles compiled by Mateer, DeLuisi and Porco (1980); and the standard ozone profiles compiled by Bhartia et al. (1984) for use in re-evaluation of NIMBUS 7 SBUV ozone profile data.

      The Dobson instrument ozone absorption coefficients tentatively recommended for use by Mateer (1990) were examined by Komhyr early in 1990 for consistency in ozone amounts derived from quasi-simultaneous observations on different wavelengths. Observational data used were those obtained with Dobson Instrument 83 at Mauna Loa Observatory during summertimes since 1975 (Komhyr et al. 1989). Using the new coefficients, only small differences were observed in ozone amounts derived from observations made on different wavelengths compared with ozone amounts derived from observations on AD-wavelengths. The range of differences in ozone values derived from observations on A, BD, B, CD, D wavelengths was only 3.8%. Komhyr noted that the range of differences could be reduced to 0.9% if the coefficients recommended for use by Mateer, 1990 for B and D wavelengths were increased by 1.3% and 2.0% respectively. This modification of the B and D coefficients was agreed upon after consultations with Hudson, Mateer and Bojkov and a decision was made to recommend use of the new coefficients in processing Dobson and Brewer instrument ozone data beginning 1 January 1992. This was also endorsed by the WMO Executive Council Panel of Experts/CAS Working Group on Environmental Pollution and Atmospheric Chemistry for implementation in the WMO Global Ozone Observing System (GO3OS).

*: Prepared by R.D. Hudson, W.D. Komhyr, C.L. Mateer and R.D. Bojkov on behalf of the ozone Commission of IAMAP

2.  New Dobson Instrument Ozone Absorption Coefficients

      The effective ozone absorption coefficients listed in Table 1 are the Bass-Paur coefficients adjusted to account for the temperature dependence of the Hearn (1961) 253.7 nm wavelength ozone absorption coefficient and adapted for use with Dobson instruments by Mateer (1990) but with the Mateer B and D wavelength coefficients increased by 1.3% and 2.0% respectively in order to achieve agreement in ozone values to within about 1% from observations made on different wavelengths. Table 1 also lists the slit weighted average Rayleigh molecular scattering coefficients derived from the data of Bates (1984). The Bates coefficients are generally considered to be the best currently available.

TABLE 1 Ozone Absorption and Molecular Scattering coefficients for Use with Dobson ozone Spectrophotometers Beginning 1 January 1992 Wavelength ALFA BETA d(BETA)/d(ALFA) (nm) (atm-cm)-1 (atm)-1 (atm-cm/atm) 305.0 0.489 325.0 0.375 A 1.806 0.114 0.063 308.9 0.466 329.1 0.355 B 1.192 0.111 0.093 311.5 0.450 332.4 0.341 C 0.833 0.109 0.131 317.5 0.414 339.9 0.310 D 0.374 0.104 0.278 AD 1.432 0.010 0.007 BD 0.818 0.007 0.009 CD 0.459 0.005 0.011

(a)      The effective ozone absorption coefficients listed in Table 1 should be adopted as standard for total ozone measurements with Dobson ozone spectrophotometers;

(b)      The Rayleigh scattering coefficients listed in Table 1 should be adopted as standard for total ozone measurements with Dobson spectrophotometers;

(c)      The standard effective ozone absorption coefficients apply to the US Standard Atmosphere 1962 and the Bhartia et al. standard ozone profile for 45°N and 325 DU total ozone. The values mu=2 and ozone weighted mean temperature -46.3°C are also inherent in these coefficients;

(d)      Zenith Blue and Zenith Cloud total ozone observations should be evaluated using existing empirical cloud charts that were devised to yield total ozone amounts which agreed with ozone values determined quasi-simultaneously from direct sun. AD-wavelength observations [with Vigroux (1953) absorption coefficient 1.388]. Convert to the new scale by multiplying all values by 0.9143;

(e)      These recommendations should be put into effect 1 January 1992.

      Provisional ozone data obtained using the new absorption and scattering coefficients should continue to be sent routinely to the WMO world Ozone Data Center in Canada for archiving. Each station submitting their data should include in the line for comments a message "processed with 1992 absorption coefficients". These standardized data sets could be subject to additional improvements in special studies, e.g through application of seasonal mean temperature profile corrections, or corrections related to values of mu*X. 3.  Converting Past Data

      To adjust total ozone data obtained during 1 July 1957 - 31 December 1991 on the Vigroux (1953, 1967) AD-wavelength ozone absorption coefficient scale (with ozone absorption coefficient 1.388 to the 1992 absorption coefficients:


      Errors in the converted data will be less than 0.3% for 220<550 D.U., 1<3 and 680<1013 mbar. Note that because of the use of improved Rayleigh scattering coefficients, the correction factor of 0.9743 is slightly larger than the factor 1.388/1.432="0.9693" derived from the ratio of the old and new AD- wavelength ozone absorption coefficients.

      Since July 1957, a few stations have published total ozone data from C- and CD-wavelength direct sun observations using the Vigroux (1953) ozone absorption coefficients scale, utilizing absorption coefficient values of 0.865 cm-1 and 0.491 cm-1 respectively. To convert such data to the new 1992 scale (based on the modified Bass-Paur (1985):


4.  Instructions for Brewer

      Total ozone measurements made with the Brewer spectrophotometer have been reported to the WMO World Ozone Data Center (WO3DC) in Toronto using absorption coefficients adjusted to make the Brewer instrument measurements directly comparable with those made by the Dobson instrument. The Dobson measurements are based on the Vigroux scale of absorption coefficients adopted by WMO, IAMAP since 1 January 1968. Effective 1 January 1992, the total ozone measurements made with the Dobson instrument are to be reported to the WO3DC based on the ozone absorption coefficient measurements made by Bass and Paur (1985) as explained above.

      In coordination with the conversion of absorption coefficient scale for the Dobson instrument, the total ozone measurements made the Brewer instrument are also to be reported to the WO3DC on the Bass-Paur (BP) scale effective 1 January 1992.

      Reporting Brewer ozone data on the BP absorption coefficient scale is achieved by multiplying the daily average total ozone values by 0.973.

For direct sun measurements this can be done by dividing the absorption coefficients (both for ozone and SO2) by 0.973 and entering them into the instrument constant files.

For zenith sky measurements the daily summarized value is multiplied by 0.973.

      For data submission to the WO3DC the fact that the ozone measurements are on the BP scale should be confirmed by including the statement "BP SCALE" on line five of the monthly data submission record.


Barnes J., and K. Mauersberger, Temperature dependence of the ozone absorption cross section at the 253.7 nm mercury line, J. Geophys. Res., 92, pp. 14861-14864. 1987 Bass A.M., and R.J. Paur, The ultraviolet cross-sections of ozone: I. The measurements in Atmospheric ozone (Ed. C.S. Zerefos and A. Ghazi), Reidel, Dordrecht, Boston, Lancaster, pp. 606-610, 1985 Bates, D.R., Rayleigh scattering in air, Planet. Space Sci., 32(6), pp. 785-790, 1984 Bhartia, P.K., D. Silberstein, B. Monosmith. and A.J. Fleig, Standard profiles of ozone from ground to 60 km obtained by combining satellite and ground-based measurements, in Atmospheric ozone (Ed. C.S. Zerefos and A. Ghazi), Reidel, Dordrecht, Boston, Lancaster, pp. 243-247, 1984 Dobson, G.M.B, Observers' Handbook for the ozone spectrophotometer, Ann. Int. Geophys. Year, 5, Part I. 46-89, 1957 Furukawa, P.M., P.L. Haagenson, and M.J. Scharberg, A composite, high-resolution solar spectrum from 2080 to 3600 A, NCAR TN-26, 55 pp., 1967 Hearn, A.G., The absorption of ozone in the ultraviolet and visible regions, Proc. Phys. Soc. London, 78, p.932, 1961 Hudson, R.D., A recommended set of ultraviolet cross sections for aeronomic purposes, to be published, 1991 Komhyr, W.D., Operations handbook-ozone observations with a Dobson spectrophotometer, Global Ozone Res. and Monit. Proj. Rep. 6, 125 pp., World Meteorological Organization, Geneva, Switzerland, 1980 Komhyr, W.D., Slit functions for Dobson spectrophotometer 83, Unpublished manuscript, NOAA Air Resources Laboratory, Geophysical Monitoring for Climatic Change, Boulder, Colorado, 12 pp., 1982 Komhyr, W.D., R.D. Grass, and R.K. Leonard, Dobson Spectrophotometer 83: A standard for total ozone measurements, 1962-1987, J. Geophys. Res., 94(D7), 9847-9861, 1989 Mateer, C.L., J.J. DeLuisi, and C.C. Porco, The short Umkehr method, Part 1: Standard Ozone profiles for use with the estimation of ozone profiles by the inversion of Short Umkehr observations, NOAA Tech. Memo. ERL ARL-86, 20 pp., 1980 Mateer, C. L., Application of Bass-Paur 1984 ozone absorption coefficients to ozone measurements with Dobson spectrophotometers, Unpublished manuscript, 13 pp., 1990 Ny Tsi-Ze and Choong Shin-Piaw, Chin. J. Phys., 1, 38, 1933 US Standard Atmosphere, 1962, US Govt. Printing Office, Washington, D.C., 1962 US Standard Atmosphere Supplements, 1966. US Govt. Printing Office, Washington, D.C., 1966 Vigroux, E., Contribution áà l'étude expéérimentale de l'absorption de l'ozone, Ann. Phys. 8, 709-762, 1953