GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 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°, nmocean glint BRDF, SZA=39° April 55N scan April 5N scan, rel. Az-=0

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/ Selection of BRDF surface types m Very bright surface scenario: snow (Lambertian equivalent albedo= 0.79) m Bright surface scenario: desert in northern Africa LEA= 0.40) m Dark surface scenario: tropical forrest in central Africa (LEA= 0.085) m 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 m April 55N trace gas scenario, SZA=49.5° m April 5N tracegas scenario, SZA=39° (ocean glint, rel. Az.=0°) m BRDF simulation and Lambertian albedo simulation  eight spectra, 240 LOS per full swath (10 LOS per ground pixel)

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/ Definition of Lambertian albedo (Popp, 1995): aLambertian equivalent albedo  bidirectional reflectance  cosine of solar zenith angle  ´cosine of line-of-sight  solar azimuth angle  ´line-of-sight azimuth angle

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Bright surface TOA radiance (2) (Lambertian/BRDF-1)*100 VIS: <8% NO2: ~5% UV: <2% 4 CDI Simulation Results

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Snow TOA radiance (2) (Lambertian/BRDF-1)*100 VIS: <4% NO2: ~2% UV: <1%

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Ocean glint TOA radiance (2) (Lambertian/BRDF-1)*100 VIS: <26% NO2: ~8% UV: <1.5%

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/ 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

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 NO2 Ocean glint airmass factor nm  NO2 AMF error is below 0.4%

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Conclusion: m 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 m For mid-latitude scenario slant column errors are below 0.7% for all trace gases.  Small BRDF effect on differential structures m maximum slant column errors are observed with snow surface m NO2 AMF error for ocean glint (away from hot spot) is below 0.4%  Note: AMF error is proportional to slant column error! m Larger impact of BRDF neglicence may be in the visible O3 window  to be checked in Task 3 Recommendation: m 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

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Minimum Lambertian Equivalent Reflectivity (LER) retrieved from GOME1 at 377nm

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Postscriptum m What is the effect of direct observation of the ocean glint? New CDI Simulation l April 55°N trace gas l 40° SZA l RAZ=0°/180° ocean glint BRDF, 40°SZA

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Ocean glint TOA radiance (Lambertian/BRDF-1)*100 VIS: <50% NO2: <30% UV: <6%

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Slant Column Error Ocena glint (2) l < 0.5% for O3 and BrO l <2% für NO2 l correction possible using effective albedo in AMF calculation m LER formalism (Eck et al. 1995)

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Bright surface TOA radiance 550nm BRDF Lambertian

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Dark surface TOA radiance (2) (Lambertian/BRDF-1)*100 VIS: <11% NO2: ~4% UV: <1%

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Dark surface TOA radiance 550nm BRDF Lambertian

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Snow TOA radiance 550nm BRDF Lambertian

GOME2 Error Study WP 260: Slant Column Errors due to neglicence of BRDF PM3 02/2002 Ocean glint TOA radiance 550nm BRDF Lambertian