1. Validation of OMI UV products: results of two ground UV measurement campaigns in Austria P. Weihs 1, M. Blumthaler 2, H. E. Rieder 1,3,*, A. Kreuter 2, S. Simic 1, W. Laube 1, A. W. Schmalwieser 5, J. Wagner 1, A. Tanskanen 4, A. Webb 6, R. Kift 6, G.P. Gobbi 7, and A. Arola 4 [1] {Institute for Meteorology, Department Water - Atmosphere - Environment, University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria} [2] { Division for Biomedical Physics, Innsbruck Medical University, Innsbruck, Austria} [3] {Institute for Geography and Regional Research, University of Vienna, Vienna, Austria} [4] {Finnish Meteorological Institute, Helsinki, Finland} [5] {Institute of Medical Physics and Biostatistics, University of Veterinary Medicine, Vienna, Austria} [6] {Univ. Manchester UK} [7] {ISAC-CNR, Rome, Italy} June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

2. Ground UV-measurement campaigns Vienna and surroundings: 15. May –30. July 2007 Innsbruck and surroundings 25. August – 18. September 2007 15. February – 8. March 2008 Sonnblick and surroundings 22. April – 9. May 2008 June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

3. Aims of the study - Study representativness of OMI UV for one pixel -Homogeneity of UV within one pixel -Influence of aerosols -Influence of ground pollution -Influence of topography -Altitude -Shading effects -Influence of ground albedo -Influence of cloudiness

4. BOKU university (256 m) 16°19‘ E, 48° 14‘ N Measurement campaign Vienna (May – July 2007) Vienna Veterinary Univ. (166 m) 16°25‘ E, 48° 14‘ N TGM (178 m) 16°22‘ E, 48°13‘ N

5. BOKU university (256 m) 16°19‘ E, 48° 14‘ N Measurement campaign Vienna (May – July 2007) Strebersdorf (175 m) 16°23‘ E, 48°18‘ N Grossenzersdorf (156 m) 16°33‘ E, 48°12‘ N Vienna Veterinary Univ. (166 m) 16°25‘ E, 48° 14‘ N Bad Vöslau (286 m) 16°12‘ E, 48°57‘ N TGM (178 m) 16°22‘ E, 48°13‘ N

6. Methods during the measurement campaigns 1.Calibration and intercomparison of all instruments at one site 2.Installation of instruments at the sites 3.Regular service and calibration of instruments 4.Analysis of data June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

7. Vienna campaign: instrumentation At each station UV biometers (erythemal UV) Site BOKU: Sunphotometer (aerosol opt.depth) Fish eye camera (cloudiness) TGM and BOKU station Portable NOLL sun photometer (aerosol optical depth) Meteorological data from Austrian Weather Service and from BOKU station: Global and diffuse radiation, Cloudiness, Sunshine duration

8. Partly cloudy Results: Sub-pixel variation Comparison of UV index at 6 stations Cloudy (sunshine duration = 0) Clear sky (sun shine duration = 100%)

9. => No pollution effect may be seen

10. Clear sky Cloudy conditions Sub-pixel variation: Variability of erythemal UV as a function of distance and averaging time V(stat x) = Σ abs(Ri – Rmean) *100 N R=UV(x)/UV(1)

11. Ratio of OMI UV to ground measurements Clear sky ratio: 1-1.5 Partly cloudy ratio: 0.5-4 Overcast ratio:0.6-4.5

12. Influence aerosol optical depth on UV June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

13. Results: Ratio of OMI to ground based measurements as a function of aerosol optical depth

14. Average differences between stations - Clear sky : < ±5% (no pollution effect) - Cloudy conditions: up to 60% Sub-Pixel fluctuations - Clear sky : < ±5% within 5 km (1 h average) - Cloudy conditions: up to 200% and even larger - Sub-pixel variation decreases with increasing time averaging: 3 h average: variation <20% within a 10 km distance Conclusion Vienna Campaign (1)

15. Conclusion Vienna Campaign (2) Comparison with OMI UV: - Clear sky: OMI/ground data: 1-1.5 - Cloudy: OMI/ground data: 0.5-4.5 - Ratio OMI UV/ground UV measurements strongly depends on aerosol optical depth June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology Still to be done: Study of influence of tropospheric ozone Work published in ACP disc., 8, 3693-3720, 2008.

16. Aims of Innsbruck campaign - Compare ground measured UV with OMI UV - Study sub-pixel variation - Effect of topography - Altitude - Shading effects - Effect of ground albedo - Effect of cloudiness June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

17. Instrumentation: Innsbruck campaign 2(3) Bentham spectrometer (UV-irradiance and actinic flux) 2 UV biometers (erythemal UV) Photodiode array, ocean optics spectrometer (UV-irradiance) Velis LIDAR (Aerosol optical depth, aerosol profile, aerosol characteristics) Sun photometer (Aerosol optical depth) Fisch eye camera (cloud fraction, cloud type)

18. Measurement campaign in Innsbruck: step 1: calibration and intercomparison of instruments June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

19. Innsbruck 616m Measurement campaign Innsbruck: step 2 Installation of instruments Hafelekar 2280m Lans 833m June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

20. Measurement campaign Innsbruck: step 3, Measurements, regular service and calibration of instruments

21. First preliminary results June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

22. First preliminary results

25. NEXT STEPS - Analysis of data of Innsbruck campaign - Include also data of Innsbruck September campaign and of Sonnblick campaign - Investigation of effects of - Altitude - Ground albedo - Combine 1-D and 3-D radiative transfer calculations with data analysis June 2008 | Department of Water, Atmosphere and Environment | Institute of Meteorology

26. Acknowledgement Thanks to -OMI International Science Team and NASA Aura Validation Data Center (AVDC) for the satellite data used in this study. - Institute of Meteorology and Geophysics, University of Innsbruck, Austria, for kindly providing the PFR-sunphotometer. This work was performed in the framework of the International ESA/KNMI/NIVR OMI "Announcement of Opportunity for Calibration and Validation of the Ozone Monitoring Instrument“. The work was funded by FFG, Austria within the scope of the project OMI-ASAP “Validation of Ozone Monitoring Instrument (OMI) Ground UV Products” and by FWF, Austria, within the scope of the project “Investigation of UV actinic flux in mountainous region”.