1 Ground-based Remote Sensing of Aerosols Pawan K Bhartia Laboratory for Atmospheres NASA Goddard Space Flight Center Maryland, USA

ISSAOS Linkages Laboratory Measurements In situ field Measurements Ground-based remote sensing Satellite remote sensing  Direct-sun  Sky-radiance  Hem. Irradiance  Lidar  Solar occultation  Solar backscattered  Lidar  Aethalometer  Nephalometer  Particle Counters  Chemical prop  Optical prop  Particle shape A Priori information

ISSAOS Outline Basic Concept Basic Concept Multi-spectral Solar extinctionMulti-spectral Solar extinction Multi-angle sky radianceMulti-angle sky radiance Polarization of sky radiancePolarization of sky radiance Hemispherical IrradianceHemispherical Irradiance Instruments/Network Instruments/Network Cimel/AERONETCimel/AERONET Shadow-band radiometersShadow-band radiometers

ISSAOS Terms, Symbols & Definitions I0I0 I Spectral Irradiance (watt/m 2 /nm): I=I 0 exp(-  ext ) Aerosol Optical Thickness AOT  ext =  scat +  abs Single Scattering Albedo (SSA):  0 =  scat /  ext Refractive Index:  =n( )+ik( ) Note: For internally mixed aerosols, AOT depends largely on REAL(  )=n( ), while  0 depends on IMG(  )=k( ) Scattered radiance Usually written simply as 

ISSAOS Solar Extinction Technique 00 I0I0 I   Rayl can be estimated with ±0.005 accuracy if surface press is known.  ozone is the most important gaseous absorber for 0.34  m it can be estimated with ±0.005 accuracy using available satellite data.  To estimate AOT to ±0.01 accuracy, I/I 0 should be known to the 1-2% level  aer =aerosol extinction optical thickness (AOT)

ISSAOS Micro-physical Meaning of AOT n(r)=particle size distribution Q ext =aerosol ext coefficient, varies with, and refr index (n)  ext =ext cross-section of particles N= particle number density  ext is total effective area of particles, varies with n=1.55 n=1.40

ISSAOS dependence of  or  -dependence of  or   larger implies that mean particle size is smaller, varies from 2-4 in earth’s atmosphere.   is called the Angstrom coeff. Varies between 0-2. Power-law (Junge) size distribution For typical bimodal size distribution Power law =3.8,  =1.8

ISSAOS Estimation of size distribution from -dependence of  or  wt fns 22 0.34   is sensitive to a limited range of particle volumes  As W moves to right with increase in  it  samples larger particles = issue: W is very sensitive to REAL(  ), which varies significantly.

ISSAOS Sky-Radiance Method  I0I0 L L=path radiance (watt/m 2 /nm/steradian) P(  )=Scattering Phase function P(  ) contains information about REAL(  ), particle size distribution, and particle shape 

ISSAOS Scattering Phase Function Phase fn Phase fn Ratio P(n=1.55)/P(n=1.43) P(r eff =0.49  )/P(r eff =0.25  ) scatt is minimum for  =120˚ sens to n is max at  =120˚ sens to size is max at 0˚ and 160˚ Inaccessible from ground

ISSAOS Almucantar Technique =0=0 Reundant meas allow quality control, and minimizes cloud contamination. Inversion of almucantar radiances provides size dist & refractive index Redundant meas

ISSAOS Estimation of SSA from Almucantar measurements Given size distr and refractive index derived from the Almucantar technique, L can be calculated at all scattering angles. Given  aer from the direct sun technique, SSA can be estimated. Conceptual Basis

ISSAOS SSA from the Total Hemispheric Irradiance Method  A detector with flat plate diffuser measures T.  R can be estimated from the asymmetric factor (related to asymmetry in scattering phase function in backward and forward hemispheres).  A is related to  abs =  ext (1-   ). Reflected Diffuse Absorbed Direct Flat plate diffuser I0I0 00

14 Instruments & Networks

ISSAOS Cimel Sunphotometer

ISSAOS Aerosol Robotic Network (AERONET) Established by NASA/GSFC & LOA-PHOTONS (CNRS) Many collaborating agencies & institutes Key Products  multi-spectral AOT  Size Distribution  Refractive Index  SSA

ISSAOS Calibration slope=ln(v 0 -v std )  -  std =ln(v 0 -v std )/m  -  std 1/m=cos  0 lnv 0 ln(I/I 0 )=ln(v/v 0 )=-m  lnv m=sec  0 Langley plot Method clean site  approx const during the day Std instrument Method Any site

ISSAOS Further Reading

ISSAOS Shadow Band Radiometer Multi-filter Rotating Shadow-Band Radiometer (MFRSR) Shadow-band  Measures total and diffuse irradiance on a flat plate.  direct=total-diffuse  Direct/total is not affected by instrument calibration, and I 0 cancels out.  Since direct can be calibrated using standard methods, the calibration can be transferred to total.  However, since direct=I 0 cos  0 e -m  one needs to know the “cosine response” of the diffuser accurately. Provides multi-spectral AOT and SSA

ISSAOS Shadow-band vs Almucantar for measuring SSA Shadow-band Shadow-band + simpler, faster, and less noisy -less accurate (needs asymmetry factor) -requires cloudless sky Almucantar Almucantar + more accurate - slower, noisier, and more complex - possible in presence of broken clouds

ISSAOS UV-MFRSR vs AERONET SSA GMT (hr) 440 nm 332 nm 368 nm AERONET (440 nm) MFRSR Santa Cruz, Bolivia Sept 27, 2007

ISSAOS UV-MFRSR/AERONET AAOT Comparison AAOT: Aerosol Abs Optical thickness (  abs )  abs =  ext (1-  0 )  abs =  0 ( /  ) -k k=1 for black carbon =3 for desert dust >>1 for organic carbon

ISSAOS UV-Rotating Shadow-band Spectroradiometer (UV-RSS) RSS 1024 Spectroraiometer Ref: Yankee Env. Systems Ref: Michalski & Kiedron, ACP, 2008

ISSAOS Summary Diect-sun irradiance measurements from sun photometers provide -dep AOT, which provide some aerosol size info. Diect-sun irradiance measurements from sun photometers provide -dep AOT, which provide some aerosol size info. Almucantar measurements from AERONET provide particle size, shape, and refractive index. Almucantar measurements from AERONET provide particle size, shape, and refractive index. SSA can be obtained from almucantar, but hemispherical irradiance provides greater sampling and precision, and go into the UV. SSA can be obtained from almucantar, but hemispherical irradiance provides greater sampling and precision, and go into the UV.