1. TOTAL OZONE MONITORING BY GROUNDBASED INSTRUMENTS AS PART OF GAW J. Staehelin 1, R.Stübi 2, U. Köhler 3 and A Redondas 4 1 Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology Zürich, Switzerland 2 Federal Office of Meteorology and Climatology, MeteoSwiss, CH-1530 Payerne, Switzerland 3 Meteorological Observatory Hohenpeissenberg, Regional Dobson Calibration Centre RA VI, Albin-Schwaiger-Weg 10, D-82383 Hohenpeissenberg, Germany 4 Regional Brewer Calibration Center – Europe, Izaña Atmospheric Research Center, AEMET- Meteorological State Agency, Spain, C/ La Marina 20, 6 Planta, 38071 Santa Cruz de Tenerife Spain

2. 1. Introduction CIMO guide Chapter 17

3. Challenge: Total ozone series of Arosa long-term changes: small

4. Outline 2. Instruments and method 3. Dobson network 4. Brewer network 5. Experience with networks 6. Future 7. Conclusions

5. 2. Instruments and method operated by MeteoSwiss at Arosa: 2 Dobson instruments 3 Brewer instruments (2 single and 1 double Brewer) Dobson instrument Earlier version: 1920 th method presently used: IGY: 1958 Brewer spectrophotometer commerc. available: middle of 1980 th completely automated, SO 2 column, UV-B

6. Basic measurement principle (most precise measurements: direct sun) Sun photometry, wavelength region: 305-340 nm I (λ) = I o (λ) exp (-α(λ) X μ - β (λ) (p s /p o ) m R - δ (λ) m a ) X: total ozone amount (in Dobson units (DU) I (λ): solar irradiance at the wavelength λ measured at the Earth’s surface I o (λ): intensity outside the Earth’s atmosphere α(λ) is the monochromatic ozone absorption coefficient μ: relative slant path through ozone (air mass factor) β(λ): Rayleigh scattering coefficient p s : station pressure; p o : mean sea level pressure at 1013.25 hPa; m R : relative optical air mass corresp. to Rayleigh scattering (extinction) δ(λ): aerosol optical depth m a : relative optical air mass corresp. to aerosol scattering (extinction)

7. Wavelengths used in Dobson (wavelength pairs (AD)) and Brewer instruments (other Russian filter instruments and SAOZ (absorption in visible)

8. Calibration of instruments I o (λ): intensity outside the Earth’s atmosphere Extrapolation for  = 0) Langley plot method Requirement: clean atmosphere and stable ozone for one half day Difficult to ensure in extratropics Langley plot calibration for standard instruments in Mauna Loa observatory (Hawaii)

9. 3. Dobson network Calibration of station instruments: Dobson intercomparisons (every 4 years) (Ulf Köhler): Responsibility of Regional Calibration Centers Central Calibration Laboratory: NOAA, Boulder, USA (R. Evans)

10. 4. Brewer network Central Calibration Laboratory: Environment Canada, Toronto (T. McElroy) : Triad: 3 Brewer instruments, regular Langley plot calibration of one instrument at Mauna Loa Observatory European regional Brewer calibration center: Izaña (Tenerife) (A. Redondas): triad of Brewer spectrophotometers, regularly calibrated by the Langley plot method (redundancy in calibration scale) Calibration of station instruments (every 2 years): Canadian instruments: EC for European instr.: European Brewer calibration center Private companies (needs financing by stations) - IOS (International Ozone Service) - Kipp and Zonen (company producing Brewer instrument)

11. 5. Experience with networks Dobson Stability of calibration of World primary Dobson instrument (D083): Percent difference in calculated total ozone based on corrections to D083 A and D tables from Langley plot calibrations at Mauna Loa Observatory, Hawaii (Evans et al., 2004 and Komhyr et al., 1989).

12. Relative differences between standrad Dobson instruments and compared station instruments during initial calibrations of the intercomparisons since 1969 (from Köhler et al., 2004).

13. Experience with network: Brewer Brewer instruments: More advanced technology (fully automated, more flexible) Different institutions involved in calibration of station instruments (private companies, need funding from stations) Biannual Brewer workshop: information of new developments, training of station personnel, exchange in experience

14. Comparison with satellite measurements Absolute and relative (in percent from the total number) number of sites with ‘‘no issues’’ (see text) in the record in 5 bins for Dobson, Brewer, and filter instrument sites located between 60 o S and 60 o N (from Fioletov et al., 2008).

15. 6. Future

16. Ozone anomalies from model CMAM and measurements; Cly (Shepherd, 2008) more ozone at lower stratosphere: greenhouse gas

17. 7. Conclusions For column ozone: two independent networks operated under the auspices of GAW/WMO Networks: high quality measurements (except in polar regions at low solar elevation): suitable for long-term trend analysis and validation of satellite measurements Satellite instruments: limited life times, merged satellite series required for long-term trend analysis. High quality ground based networks need continuation !

18. Conclusions, cont. Ozone layer will recover from Ozone Depleting Substances (ODS, e.g. CFCs) (Montreal Protocol, 1987) Beneficial effect of Montreal Protocol on ozone layer by data analysis: controversial Demand for continuation of stratospheric ozone monitoring in future: Prediction of “super” recovery (more O 3 in extratropical tropopause region): climate change

19. Acknowledgement: SAG-Ozone Geir Braathen (WMO) Johanna Tamminen (IGACO-O 3 /UV) Frank Baier, Jack Fishman, Sophie Godin-Beekmann, Robert Evans 1, Ulf Koehler 12, Takeshi Koide 12, Ed Hare, Tom McElroy 2, Alberto Redondas 2, Herman Smit, Rene Stübi 12, Johannes Staehelin (chair), Richard Stolarski, Ronald van der A, Karel Vanicek 12, Mark Weber 1 : involved in Dobson measurements 2 : involved in Brewer measurements