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

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

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

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  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. W results: spectral dependence 368nm

  16. 368nm – 332nm

  17. 368nm –332nm- 325nm

  18. W spectral dependence

  19. 368nm

  20. 368nm – 332nm

  21. 368nm – 332nm –325nm

  22. W spectral dependence

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

  24. 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.

  25. W results: Seasonal Dependence No clear W seasonal dependence

  26. 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.

  27. 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

  28. 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.

  29. Backup

  30. Practical implementation: (2) sun and sky radiance technique AERONET/CIMEL Sun photometers provide global extinction measurements by direct sun technique from 340nm to 1020nm - http://aeronet.gsfc.nasa.gov

  31. CIMEL sky radiance almucantar measurements/inversions: AERONET measures sky radiances enabling retrieval of column size distribution and effective spectral refractive index m=n() – ik() in VIS Measurements : t(l) and I(l,Q) l = 0.44, 0.67, 0.87, 1.02 mm 2o ≤ Q≤ 150o (up to 30 angles) > 440nm measurements

  32. SSA retrieval method: • Fitting measured diffuse fraction with RT model by iterating on imaginary part of refractive index separately at each wavelength; • Using MFRSR extinction optical thickness and AERONET inverted particle size distribution and real part of refractive index at 440nm as input to RT model • Fixed surface albedo 0.02 and actual pressure scaling of Rayleigh optical thickness • Measured Total ozone is used for calculation of ozone optical thickness • Using Mie code to calculate single scattering albedo

  33. SENSITIVITY OF UV-MFRSR MEASUREMENTS TO AEROSOL ABSORPTION Relationship between Rayleigh normalized total transmittance, TR and abs at 368nm, assuming fixed ext=0.167 (red) and 0.2 (purple) and o=33o,70o. Linear regression model (1) is fitted to al data points assuming variability due to size distribution as random errors

  34. Aerosol UV absorption experiment (2002-04): 1. UV-MFRSR calibration and performance at GSFC

  35.  absorption in UV Mauna Loa solar calibration Daily Vo Calibration Transfer Global measurements of direct sun aerosol extinction  by AERONET at 340nm and 380nm USDA UV-B Monitoring and Research Program - http://aeronet.gsfc.nasa.gov http://uvb.nrel.colostate.edu

  36. UV-MFRSR on-site calibration (V0) UV-MFRSR cosine corrected direct-normal voltage UV-MFRSR spectral band model Direct pressure measurements Interpolated CIMEL a corrected for pressure Brewer/TOMS total ozone measurements Airmass factor

  37. MFRSR: cosine corrected voltages We derive individual V0 by numerically integrating high resolution spectral transmittance TR() within each filter bandpass:

  38. UV-MFRSR spectral band model MFRSR spectral band model takes into account actual UV-MFRSR spectral response functions (SRF) as well as spectral variation of the solar flux and atmospheric extinction within each filter bandpass of the instrument.

  39. Extrapolating AERONET AOT AERONET direct sun aerosol extinction optical thickness at 340nm, 380nm, 440nm, and 500nm normalized by (380). Extrapolation to longer UV-MFRSR channels using quadratic least squares fit of ln() versus ln() and linear extrapolation from 340nm and 380nm. The difference in ext between quadratic and linear interpolation methods is typically less than 0.005 at 368nm.

  40. Daily <ln(V0)> AOT comparisons

  41. AOT comparisons

  42. Vo as diagnostic tool: diffuser cleaning after cleaning: +4% before cleaning

  43. Vo as diagnostic tool: incomplete shadowing Morning meas. OK Shadowing problem

  44. Unexplained V0 results Diurnal V0 changes

  45. Comparison statistics: (1) Extinction  (all clear sky cases ~10,000) •   < 0.02 (daily rms differences) for all clear sky days •   < 0.01 (daily rms differences) for  < 0.5 Daily aerosol extinction optical thickness rms differences between UV-MFRSR and AERONET CIMEL measurements at 368nm.

  46. AOT comparisons

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