This document discusses algorithms and processing methods used by AERONET to calibrate direct sun and diffuse sky radiance measurements from its global network of sun photometers and derive aerosol optical depth, precipitable water, and other aerosol properties. It describes current algorithms and plans for refinements, including updating extraterrestrial solar flux calculations, Rayleigh scattering calculations, ozone and water vapor modeling, and implementing corrections for water vapor and gases in retrieving aerosol optical depth at 1020nm and 1640nm. It also discusses recalibrating the water vapor channel of AERONET instruments and quality assuring the updated precipitable water database through comparisons with GPS and microwave radiometer measurements.

1. AERONET Processing
Algorithms Refinement
AERONET Workshop
May 10 - 14, 2004, El Arenosillo, Spain

2. AERONET Data Flows
http://aeronet.gsfc.nasa.gov
Flux measurements
Direct - l=340, 380, 440, 500, 670, 870, 940, 1020 nm
Diffuse - l=440, 670, 870, 1020 nm (alm, pp, pol)
Calibration and processing information
Aerosol optical depth and
precipitable water computations
Holben et al.
RSE, 1998
Holben et al.
JGR, 2001
Smirnov et al.
RSE, 2000
Dubovik and King
JGR, 2000
Dubovik et al.
JGR, 2000
Cloud screening and quality control
Eck et al.
JGR, 1999
Inversion products
Volume size distribution (0.05<R<15 mm),
refractive index, single scattering albedo
(l=440, 670, 870, 1020 nm)

3. AERONET Data Flows
Current and future additions
Flux measurements
Sun - l=340, 380, 440, 500, 670, 870, 940, 1020 nm +1640 nm (412, 532, 555 nm)
Sky - l=440, 670, 870, 1020 nm + 500, 1640 nm + ? 340, 380 nm
Calibration and processing information H2O, CO2, CH4
Aerosol optical depth and precipitable water
computations (1020, 1640, 940 nm) + extra ta(1020 nm)
Cloud screening and quality control
Inversion products
Almucantar retrievals - spherical and spheroid models (4 wavelengths), level 2
Almucantar retrievals - 6 wavelengths + ? 340, 380 nm
Principal plane retrievals - 4 wavelengths, level 2; ? 6 wavelengths
? Combined retrievals (almucantar and principal plane)

4. Current Processing Algorithm
 Extraterrestrial Solar Flux - Neckel and Labs
1981; and Frohlich and Wehrli 1981
 Rayleigh optical depth - Edlen 1966
 Ozone amount - LUT (5ºx 5º Lat Long) -
London et al. 1976
 Water vapor content - Bruegge et al. 1992;
Reagan et al. 1992
 Water vapor correction for AOD (1020 nm) -
none
 CO2, CH4, WV for AOD (1640 nm) - N/A

5. Processing Algorithm Refinement
 Extraterrestrial Solar Flux - Woods et al. 1996
 Rayleigh optical depth - Bodhaine et al. 1999
 Ozone amount - LUT (1ºx 1º Lat Long) -
TOMS data 1979-2002
 Water vapor content - Michalsky et al. 1995;
Schmid et al. 1996
 Water vapor correction for AOD (1020 nm) -
LBLRTM
 CO2, CH4, WV for AOD (1640 nm) - LBLRTM

6. Extraterrestrial Solar Flux
www.astm.org

7. Courtesy Dr. Jay Herman,
code 916, NASA/GSFC

8. Rayleigh Optical Depth

9. Rayleigh Optical Depth

10. Tw = exp(-A(mu)B)
Ln V(l) + m[ta(l)+tr(l)]=
= Ln [V0(l)d-2] - A(mu)B
V(l) = V0(l)d-2 exp{-m[ta(l)+tr(l)]}Tw(l)
Tw(l) = ∫ E0(l)S(l)exp[-mtw(l)] dl
/ E0(l)S(l)dl

11. Parameters A and B for various filter batches

12. Optical Depth Correction at 1020 nm

13. Optical Depth Correction at 1640 nm

15. AERONET Precipitable Water Comparison

16. AERONET-GPS Precipitable Water Comparison

17. AERONET-GPS Precipitable Water Comparison

18. What to do next
 Recalibrate Master instruments (water
vapor channel only)
 Recalculate calibration constants (water
vapor channel only) for all instruments in
the network
 Update database on water vapor content
 Quality assure PW (WVC) database
 Compare QC/QA PW with GPS and MWR
retrievals