Measuring aerosol UV absorption by combining use of shadowband and almucantar techniques

1. Measuring aerosol UV absorption by combining use of shadowband and almucantar techniques N.A. Krotkov, Goddard Earth Sciences and Technology Center /UMBC and NASA/GSFC P.K.Bhartia, J. Herman, NASA/GSFC, Jim Slusser , Gwen Scott, USDA UVB Monitoring network G. Labow, A.Vasilkov, SSAI T. Eck, O. Dubovik, GEST/UMBC and B. Holben, NAS A/GSFC

2. Why is aerosol UV absorption important ? Aerosol effects on UV trends may enhance reduce, or reverse effects of stratospheric O3 change TOMS overestimationof surface UV irradiance 22 21 + 10%-20% 23 3) Aerosol effects on photochemical smog production: aerosol scattering increases photolysis rates; while aerosol absorption decreases it: Change in BL ozone mixing ratios as a result of direct aerosol forcing: +20ppb ( =0.95) -24ppb ( =0.75)

3. Why is aerosol UV absorption important ? EP TOMS (1996-) AURA/OMI (2004 - ) 24 New TOMS/OMI aerosol UV absorption product needs validation

4. Possibility exist to derive column aerosol absorption from the ground: • (2) Diffuse sky radiation ~ aerosol scattering (**) • (1) Direct sun radiation ~ aerosol extinction () • Combining (1) and (2) measurements allows to derive aerosol single scattering albedo:  and absorption optical thickness: *(1-)

5. Practical implementation: (1) Diffuse-To-Direct Irradiance technique • First proposed byB.Herman, R.S.Browing and J. J. De Luisi [JAS,1975] andimplemented by J.J. De Luisi [1976] and M. King [JAS, 1979] in the VIS • was recently used byT.Eck et al [JGR 2003] in VIS • and by Wenny et al [1998], Petters et al [JGR, 2003], Wetzel et al [2003] and C.Goering, et al. [2004] in UV • All recent UV measurements were conducted with UV-MFRSR (YES) [L. Harrison and J. Michalsky ]

6. USDA UV-B Monitoring and Research Program operates US network of UV MultiFilter Rotating Shadowband Radiometers (UV-MFRSR) http://uvb.nrel.colostate.edu UVMFRSR was continuously operated at NASA/GSFC since October 2002 3 min measurements of total and diffuse irradiance measurements at 300, 305, 311, 117, 325, 332, 368nm

7. Mauna Loa solar calibration Daily Vo Calibration Transfer Diffuse/Total fraction and  Single scattering albedo,  RT model fitting Sphere radiometric calibration A-priori information  extinction by AERONET at 340nm –500nm AERONET also measures sky radiances enabling retrieval of size distribution and effective spectral refractive index at 440nm - http://aeronet.gsfc.nasa.gov

8. Error estimate • Error in absorption optical thickness: • abs (368nm) ~0.01 - 0.02 • ( limited by the measured accuracy of total voltage and calibration, V0) • Error in single scattering albedo : •  ~ abs / ~ 0.01/   ~ 0.02 (~0.5 ) • Error due to uncertainty in size distribution and real refractive index becomes comparable to the measured uncertainties only for large aerosol loadings (ext>0.5)

9. Siberian smoke plume on June 2, 2003 AERONET SSA UV-MFRSR AOT

10. Diurnal 368 dependence on June 24, 2003 AERONET SSA UV-MFRSR AOT

11. Diurnal 368 changes on August 25, 2003 AERONET SSA AOT UV-MFRSR

12. Comparison statistics (1): Extinction  (all clear sky cases ~10,000) •  368 < 0.02 (daily rms differences) for all clear sky days •   < 0.01 (daily rms differences) for  < 0.4 Daily aerosol extinction optical thickness rms differences between UV-MFRSR and AERONET CIMEL measurements at 368nm. 325nm 368nm

13. Comparison statistics (2): single scattering albedo (65 matchup cases in summer 2003) •  case average ( 65 matchup cases in Summer 2003) • <368>=0.93 +/-0.02 (1) at 368nm • <440> =0.95 +/-0.02 (1) at 440nm  mean difference comparable to retrieval uncertainty <440 - 368> ~0.02, rms difference: ~0.016

14. Comparison statistics (3): Imaginary part of refractive index (65 matchup cases in summer 2003) • Higher values for imaginary refractive index, k in UV: • <k368> ~0.01, k368~0.004 at 368nm • <k440> ~0.006, k440~0.003 at 440nm

15. Aerosol absorption optical thickness: Seasonal Dependence The absvalues show a pronounced seasonal dependence of ext with maximum values abs~0.05 at 368nm (~0.07 at 325nm) occurring in summer hazy conditions and <0.02 in winter-fall seasons, when aerosol loadings are small.

16. W results: AOT dependence The decrease of single scattering albedo with optical thickness suggests that the type of aerosol changes between summer and winter conditions.

17. Results: spectral dependence in UV-VIS ? ? SINGLE SCATTERING ALBEDO UV-MFRSR VIS- CIMEL UV MFRSR VISIBLE AERONET Wavelength

18. Summary • The shadowband method is complementary to the AERONET almucantar retrieval of , because • retrievals are more reliable at low solar zenith angles; • absolute sky radiances calibration is not required; • The variability in aerosol size distribution and real refractive index becomes comparable to the measured uncertainties only for large aerosol loadings (ext>0.5) • Combined use of both methods allows: • Deriving complete diurnal cycle of aerosol absorption • Considering days with low aerosol loadings, thus obtaining complete seasonal cycle of aerosol absorption • Extending spectral dependence of single scattering albedo into UV wavelengths

19. Future work • Continuing co-located measurements at GFSC location is important to improve the comparison statistics; • Extending UV-MFRSR spectral coverage to 440nm and CIMEL almucantar retrievals to 340 and 380nm to allow for spectral overlap between 2 types of aerosol absorption measurements; • Conducting simultaneous measurements at different sites with varying background aerosol conditions is desirable.

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