Slant Column Errors due to Assumption of Lambertian albedo

1. Slant Column Errors due to Assumption of Lambertian albedo • A. Rozanov, V. Rozanov, R. De Beek, and M. Weber • University of Bremen • Institute of Environmental Physics Snow BRDF, SZA=40°, 300-330 nm ocean glint BRDF, SZA=49.5°

2. 1 Selection of BRDF surface types • Very bright surface scenario: snow (Lambertian equivalent albedo= 0.79) • Bright surface scenario: desert in northern Africa LEA= 0.40) • Dark surface scenario: tropical forrest in central Africa (LEA= 0.085) • ocean glint (LEA=0.025) • Cox and Munk (1954) sun glint model, wind speed 5 m/sec, H2O complex refractivity at 350 nm • 2 Scenario selection and Simulations • April 55N trace gas scenario, SZA=49.5° • BRDF simulation and Lambertian albedo simulation • eight spectra, 240 LOS per full swath (10 LOS per ground pixel)

3. 3 Definition of Lambertian albedo (Popp, 1995): a Lambertian equivalent albedo  bidirectional reflectance  cosine of solar zenith angle ´ cosine of line-of-sight  solar azimuth angle ´ line-of-sight azimuth angle 4 CDI Simulation Results

4. Bright surface TOA radiance 550nm BRDF Lambertian

5. Bright surface TOA radiance (2) (Lambertian/BRDF-1)*100 VIS: <8% NO2: ~5% UV: <2%

6. Dark surface TOA radiance 550nm BRDF Lambertian

7. Dark surface TOA radiance (2) (Lambertian/BRDF-1)*100 VIS: <11% NO2: ~4% UV: <1%

8. Snow TOA radiance 550nm BRDF Lambertian

9. Snow TOA radiance (2) (Lambertian/BRDF-1)*100 VIS: <4% NO2: ~2% UV: <1%

10. Ocean glint TOA radiance 550nm BRDF Lambertian

11. Ocean glint TOA radiance (2) (Lambertian/BRDF-1)*100 VIS: <26% NO2: ~8% UV: <1.5%

12. 5 Slant column error •  maximum errors less than • 0.7% for all surface types •  larger errors seen with • bright surface types (snow, • desert) •  OClO column amounts too • small (April 55°N) for error • analysis

13. NO2 Ocean glint airmass factor 425-450 nm  NO2 AMF error is below 0.4%

14. Conclusion: • TOA radiance errors from Lambertian albedo are on the order of 1-2% in the UV, ~5% in the NO2 fitting region, and up to 25% at 550 nm • For mid-latitude scenario slant column errors are below 0.7% for all trace gases.  Small BRDF effect on differential structures • maximum slant column errors are observed with snow surface • NO2 AMF error for ocean glint (away from hot spot) is below 0.4%  Note: AMF error is proportional to slant column error! • Larger impact of BRDF neglicence may be in the visible O3 window  to be checked in Task 3 • Recommendation: • GOME2 slant column retrieval and airmass factor calculations should use effective reflectivities measured in absorption free regions, i.e. at 380 nm, as auxilliary geophysical parameter

15. Minimum Lambertian Equivalent Reflectivity (LER) • retrieved from GOME1 at 377nm

16. Postscriptum • What is the effect of direct observation of the ocean glint? • New CDI Simulation • April 55°N trace gas • 40° SZA • RAZ=0°/180° ocean glint BRDF, 40°SZA

17. Ocean glint TOA radiance (Lambertian/BRDF-1)*100 VIS: <50% NO2: <30% UV: <6%

18. Slant Column Error • Ocena glint (2) • < 0.5% for O3 and BrO • <2% für NO2 • correction possible using effective albedo in AMF calculation • LER formalism (Eck et al. 1995)