Ground-based Remote Sensing of Aerosols

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

2. Linkages A Priori information Ground-based remote sensing Satellite remote sensing Laboratory Measurements In situ field Measurements • Aethalometer • Nephalometer • Particle Counters • • • • • • • • • Direct-sun • Sky-radiance • Hem. Irradiance • Lidar • Solar occultation • Solar backscattered • Lidar • Chemical prop • Optical prop • Particle shape ISSAOS 2008

3. Outline • Basic Concept • Multi-spectral Solar extinction • Multi-angle sky radiance • Polarization of sky radiance • Hemispherical Irradiance • Instruments/Network • Cimel/AERONET • Shadow-band radiometers ISSAOS 2008

4. Terms, Symbols & Definitions Scattered radiance I0 I Usually written simply as t Spectral Irradiance (watt/m2/nm): I=I0exp(-text) Aerosol Optical Thickness AOT: text=tscat+tabs Single Scattering Albedo (SSA): w0=tscat/text Refractive Index: m(l)=n(l)+ik(l) Note: For internally mixed aerosols, AOT depends largely on REAL(m)=n(l), while w0 depends on IMG(m)=k(l) ISSAOS 2008

5. Solar Extinction Technique I0 taer =aerosol extinction optical thickness (AOT) • tRayl can be estimated with ±0.005 accuracy if surface press is known. • ozone is the most important gaseous absorber for l<1 mm. At l>0.34 mm it can be estimated with ±0.005 accuracy using available satellite data. • To estimate AOT to ±0.01 accuracy, I/I0 should be known to the 1-2% level q0 I ISSAOS 2008

6. Micro-physical Meaning of AOT n=1.40 n(r)=particle size distribution Qext=aerosol ext coefficient, varies with l, and refr index (n) sext=ext cross-section of particles n=1.55 N= particle number density text is total effective area of particles, varies with l ISSAOS 2008

7. l-dependence of s or t For typical bimodal size distribution Power-law (Junge) size distribution • larger n implies that mean particle size is smaller, varies from 2-4 in earth’s atmosphere. • a=n-2 is called the Angstrom coeff. Varies between 0-2. Power law n=3.8, a=1.8 0.3 0.5 2 ISSAOS 2008

8. Estimation of size distribution from l-dependence of s or t wt fns 2m l= 0.34 • s is sensitive to a limited range of particle volumes • As W moves to right with increase in l, itsamples larger particles issue: W is very sensitive to REAL(m), which varies significantly. ISSAOS 2008

9. Sky-Radiance Method I0 L=path radiance (watt/m2/nm/steradian) P(Q)=Scattering Phase function Q P(Q) contains information about REAL(m), particle size distribution, and particle shape q L ISSAOS 2008

10. Scattering Phase Function Phase fn Phase fn Ratio Inaccessible from ground P(n=1.55)/P(n=1.43) P(reff=0.49m)/P(reff=0.25m) sens to n is max at Q=120˚ sens to size is max at 0˚ and 160˚ scatt is minimum for Q=120˚ ISSAOS 2008

11. Almucantar Technique q=q0 Redundant meas Reundant meas allow quality control, and minimizes cloud contamination. Inversion of almucantar radiances provides size dist & refractive index ISSAOS 2008

12. Estimation of SSA from Almucantar measurements Conceptual Basis Given size distr and refractive index derived from the Almucantar technique, L can be calculated at all scattering angles. Given taer from the direct sun technique, SSA can be estimated. ISSAOS 2008

13. SSA from the Total Hemispheric Irradiance Method I0 Reflected Absorbed q0 Direct Diffuse Flat plate diffuser • A detector with flat plate diffuser measuresT. • R can be estimated from the asymmetric factor (related to asymmetry in scattering phase function in backward and forward hemispheres). • A is related to tabs=text(1-w0). ISSAOS 2008

14. Instruments & Networks

15. Cimel Sunphotometer http://www.cimel.fr/index_us.html ISSAOS 2008

16. Aerosol Robotic Network (AERONET) http://aeronet.gsfc.nasa.gov/ • Key Products • multi-spectral AOT • Size Distribution • Refractive Index • SSA Established by NASA/GSFC & LOA-PHOTONS (CNRS) Many collaborating agencies & institutes ISSAOS 2008

17. Calibration ln(I/I0)=ln(v/v0)=-mt t-tstd=ln(v0-vstd)/m Langley plot Method clean site t approx const during the day Std instrument Method Any site • slope=ln(v0-vstd) lnv • • t-tstd • • • • • • • lnv0 • • m=secq0 1/m=cosq0 ISSAOS 2008

18. Further Reading ISSAOS 2008

19. Shadow Band Radiometer Multi-filter Rotating Shadow-Band Radiometer (MFRSR) • Measures total and diffuse irradiance on a flat plate. • direct=total-diffuse • Direct/total is not affected by instrument calibration, and I0 cancels out. • Since direct can be calibrated using standard methods, the calibration can be transferred to total. • However, since direct=I0cosq0e-mt one needs to know the “cosine response” of the diffuser accurately. Shadow-band Provides multi-spectral AOT and SSA ISSAOS 2008

20. Shadow-band vs Almucantar for measuring SSA • Shadow-band + simpler, faster, and less noisy • less accurate (needs asymmetry factor) • requires cloudless sky • Almucantar + more accurate - slower, noisier, and more complex - possible in presence of broken clouds ISSAOS 2008

21. Santa Cruz, Bolivia Sept 27, 2007 MFRSR AERONET (440 nm) 440 nm 368 nm SSA 332 nm GMT (hr) UV-MFRSR vs AERONET ISSAOS 2008

22. UV-MFRSR/AERONET AAOT Comparison AAOT: Aerosol Abs Optical thickness (tabs) tabs=text(1-w0) tabs=t0(l/l0)-k k=1 for black carbon =3 for desert dust >>1 for organic carbon ISSAOS 2008

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

24. Summary • Diect-sun irradiance measurements from sun photometers provide l-dep AOT, which provide some aerosol size info. • 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. ISSAOS 2008