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DATA - Descriptions - Version 1
 

 

TIME FORMAT
  • All times are provided as Greenwich Mean Time (GMT) for the solar day.
  • All averages are calculated using the GMT solar day.
  • The GMT solar day may overlap two calendar days for some AERONET sites.
  • "Day" is defined as the GMT solar day.
DATA FORMAT
Data Format Description
All Points These data are available for each measurement or retrieval moment.
Daily Average These data are calculated from all points for each day when three (3) or more points are available.
Monthly Average of Daily Averages

These data are calculated using the daily average for each day to compute the monthly average.

Equation for the Monthly Average of Daily Averages

Monthly Average of Weighted Daily Averages

These data are calculated using the weighted daily average for each day to compute the monthly average.

Equation for the Monthly Average of Weighted Daily Averages


DATA TYPES

*See references below table for more detailed information.

Data Type Description
Aerosol Optical Depth (AOD)  
Level 1.0 (Raw) Unscreened and may not have final calibration applied
Level 1.5 (Cloud Screened) Automatically cloud cleared but may not have final calibration applied. These data are NOT quality assured.
Level 2.0 (Quality Assured) Pre- and post-field calibration applied, automatically cloud cleared and manually inspected
Precipitable Water The total water vapor in the column derived from the 935nm channel.
Percent Triplet Variability Each AOD measurement is comprised of a triplet measurement: These measurements are taken every 30 seconds for one minute. The variability of this measurement can provide insight on the quality of the data.
Angstrom Parameter The angstrom parameter is calculated for all available wavelengths within the Angstrom parameter range. For example, the Angstrom 870-440nm includes the 870, 670, 500 and 440 nm AOD data. A special case for a polarized instrument is Angstrom 870-440nm which only includes 870, 670 and 440 nm AOD data (polarized instruments do not have 500nm channel).
Instrument Information  
Exact Wavelength The exact filter wavelength is used to determine AOD and other parameters.
Raw Data  
Polar Principal Planes Calibration applied to polarized principal planes scenario data (See Page 31 of the Cimel Sun Photometer User Manual for more information on almucantars)
Bidirectional Reflectance Distribution Function (BRDF) Calibration applied to BRDF scenario data
Almucantars Calibration applied to almucantar scenario data (See Page 30 of the Cimel Sun Photometer User Manual for more information on almucantars)
Principal Planes Calibration applied to principal planes scenario data (See Page (See Page 31 of the Cimel Sun Photometer User Manual for more information on almucantars)

Nakajima Almucantar Retrievals

 
SKYRAD.PAK Derived phase functions, aerosol size distribution and sky optical depth
Version 1 Almucantar Retrievals  
Size Distribution Volume particle size distribution is retrieved logarithmically equidistant bins from 0.05um to 15um.
Refractive Index The real refractive index (n from 1.33 to 1.6) and the imaginary part (k from 0.0005 to 0.5) is retrieved for wavelengths corresponding to sky radiance measurements.
SSA Single scattering albedo (SSA) is retrieved for wavelengths corresponding to sky radiance measurements. Fine and coarse modes are not provided due to the assumption of the retrieval to use the same complex index of refraction for all particle sizes.
AOD Coincident Calculated by averaging the level 1.5 or 2.0 AOD data values (Level 2.0 has priority) +/- 16 minutes of the almucantar retrieval time (typically uses three to five AOD points for the coincident average)
Volume Parameters Volume concentration, volume median radius, effective radius, standard deviation (total, fine and coarse modes)
AOD Extinction Derived values for AOD from almucantar retrieval (total, fine and coarse modes). The total mode is determined by the sum of the fine and coarse modes.
Phase Functions Phase function is retrieved at 83 scattering angles at wavelengths corresponding to sky radiance measurements (total, fine and coarse modes).
Asymmetry Factor Integrated value for phase functions (total, fine and coarse modes) where positive values indicate scattering in the forward direction.
AOD Absorption Equation: (1-SSA)*AOD (where AOD is AOD Extinction)- The single scattering albedo is used for each incidence of an almucantar retrieval. The AOD is calculated by adding the derived almucantar AOD fine and coarse modes. AOD absorption is calculated for the following channels: 1020,870,670 and 440 nm.(total, fine and coarse modes)
Combined Retrievals Combination of all Dubovik Almucantar Retrievals except phase functions.
Version 1 Almucantar Retrieval Settings  
Level 1.5 (Spherical and Spheroid)

Determined using Level 1.5 (2.0 if available) AOD data

No additional screening

Level 2.0 - Spherical and Spheroid (Recommended default options)

Determined using Level 2.0 AOD; solar zenith angle greater than 25 degrees; number of angles is greater than 20;

Spherical: sky error < 5%

Spheroid: sky error <10%

Level 2.0 (User-defined Options) Determined using Level 2.0 AOD; activates Advanced Level 2.0 Retrieval Parameters
Spherical Level 2.0

All of the products provided except when the following conditions are met:

  1. Solar zenith angle is smaller than 45 degrees, or
  2. Coincident AOD tau440 is less than 0.4
Condition 1 (Above) Condition 2 (Above)

Not Provided:

Single Scattering Albedo; Real and imaginary parts of refractive index; and AOD Absorption

Not Provided:

Single Scattering Albedo, Real and imaginary parts of refractive index

Non-spherical Level 2.0
  1. Same as Spherical Level 2.0
  2. Real parts of refractive index for 670 and 440nm are not provided
  3. The minimum on the size distribution curve on the interval 0.05-1 micron that is closest to 1 micron
    3a. Size distribution values are not provided for sizes smaller than where the minimum appears
    3b. Fine mode values are not provided for modal products
Spheroid Level 2.0 All of the products are provided except when the Angstrom parameter (870-440nm) is greater than 0.6.
Combined Spherical and Spheroid Level 2.0

All of the products are provided for these retrievals. The selection of spherical and spheroid retrievals follows these rules:

If the spherical model is level 2.0, then the spherical is used; otherwise, if the spheroid model is level 2.0, then the spheroid is used as the recommended level 2.0 retrieval.

Version 1 Almucantar Retrieval Filters  
Angles (No.) Number of symmetric angles available from the almucantar
Solar Zenith Angles (Degrees) Minimum and maximum solar zenith angle range in degrees
Additional Solar Zenith Angle Limit If calculated solar zenith angle is less than solar zenith angle limit, then single scattering albedo and real and imaginary parts of the refractive index are not provided in the download.
AOD @ 440 nm If the observed AOD at 440 is less than the AOD @ 440nm limit, then single scattering albedo and real and imaginary parts of the refractive index are not provided
Angstrom Parameter (440-870nm) Limit When non-spherical (spherical model) data type is selected and the sky radiance error is between 5 and 15%, the almucantar will be passed to level 2.0 if the calculated angstrom parameter is smaller than angstrom parameter limit.
Sky Error (%) (Spherical and Spheroid) Sky radiance error in percent

Please read these papers for a thorough explanation of the data types:

  • Version 1 - Almucantar Inversion Products Description (PDF)
  • Version 1 - Almucantars Level 2.0 Procedure Flow Chart (PDF)
  • Version 1 - Almucantar Retrieval Download Tool Information (PDF)
  • Cimel Sun Photometer Manual (PDF)
  • Angstrom, A., On the atmospheric transmission of Sun radiation and on dust in the air, Geogr. Ann., 12, 130-159, 1929.
  • Dubovik, O., B.N.Holben,T. Lapyonok, A.Sinyuk, M. I. Mishchenko, P. Yang, and I.Slutsker, 2002: Non-spherical aerosol retrieval method employing light scattering by spheriods,Geophys. Res. Lett., 29, 54-1 - 54-4.
  • Dubovik, O. and M. D. King, 2000: A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements, J. Geophys. Res., 105, 20 673-20 696.
  • Dubovik, O., A. Smirnov, B. N. Holben, M. D. King, Y.J. Kaufman, T. F. Eck, and I. Slutsker, 2000: Accuracy assessments of aerosol optical properties retrieved from AERONET sun and sky-radiance measurements, J. Geophys. Res., 105, 9791-9806.
  • Holben B.N., T.F.Eck, I.Slutsker, D.Tanre, J.P.Buis, A.Setzer, E.Vermote, J.A.Reagan, Y.Kaufman, T.Nakajima, F.Lavenu, I.Jankowiak, and A.Smirnov, 1998: AERONET - A federated instrument network and data archive for aerosol characterization, Rem. Sens. Environ., 66, 1-16.
 
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Last Updated: February 10, 2013