200 likes | 332 Views
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
E N D
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
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)
Why is aerosol UV absorption important ? EP TOMS (1996-) AURA/OMI (2004 - ) 24 New TOMS/OMI aerosol UV absorption product needs validation
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-)
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 ]
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
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
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)
Siberian smoke plume on June 2, 2003 AERONET SSA UV-MFRSR AOT
Diurnal 368 dependence on June 24, 2003 AERONET SSA UV-MFRSR AOT
Diurnal 368 changes on August 25, 2003 AERONET SSA AOT UV-MFRSR
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
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
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
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.
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.
Results: spectral dependence in UV-VIS ? ? SINGLE SCATTERING ALBEDO UV-MFRSR VIS- CIMEL UV MFRSR VISIBLE AERONET Wavelength
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
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.