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Aerosol Optical Depth
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AEROSOL OPTICAL DEPTH - Direct Sun Measurements

Sun photometer measurements of the direct (collimated) solar radiation provide information to calculate the columnar aerosol optical depth (AOD). AOD can be used to compute columnar water vapor (Precipitable Water) and estimate the aerosol size using the Angstrom parameter relationship. Two data versions (Versions 1 and 2) and three quality levels (Levels 1.0, 1.5, 2.0) exist for each product. While Levels 1.0 and 1.5 are provided in near real-time, the 12-month or longer delay (due to final calibration and manual inspection) ensures that the highest quality data can be found in Version 2, Level 2.0 data products.

A view of the sun photometer when measuring irradiance
+ AOD Levels
+ Explanation of AOD, PW and Angstrom parameter
+ Data Descriptions

Spectral De-Convolution Algorithm (SDA) Retrievals - Fine and Coarse Mode AOD and Fine Mode Fraction

The aerosol optical depth processing includes the spectral de-convolution algorithm (SDA) described in O'Neill et al. (2003). This algorithm yields fine (sub-micron) and coarse (super-micron) aerosol optical depths at a standard wavelength of 500 nm (from which FMF*, the fraction of fine mode to total aerosol optical depth can be computed). The algorithm fundamentally depends on the assumption that the coarse mode Angstrom exponent and its derivative are close to zero. Its advantage lies in the fact that it produces useful indicators of aerosol size discrimination at the frequency of extinction measurements.

The key output products of the SDA are all computed at a reference wavelength of λ = 500 nm. These products include the AOD (τa, using the nomenclature of O'Neill et al., 2003), the spectral derivative of the AOD (α) and the spectral derivative of a (α'). These three "generic" products are employed in the SDA to compute the principal SDA output products of  fine mode aerosol optical depth ( τf), the spectral derivative of the fine mode aerosol optical depth (αf), the spectral derivative of αf ( α'f), the fine mode fraction ( η =  τfa) and the coarse mode optical depth ( τc  =   τa -  τf ). Note that spectral derivatives can be evaluated at 500 nm (in a differential calculus sense) because we fit the input AERONET AODs with a second order spectral polynomial.

The spectral derivatives α and αf represent, respectively, (i) a generic Angstrom exponent containing information on the combined size contributions of fine and coarse mode aerosols (very strongly related to the typical multi-wavelength-regression Angstrom exponent while not being spectrally diluted in terms of allowing the extraction of higher orders of spectral curvature information) and (ii) size information specific to the fine mode size distribution. The theoretical underpinning of the SDA technique is that the aerosol world is assumed to be bimodal; not always true, but it is a hypothesis of surprisingly universal applicability (when its not true it clearly produces an optically equivalent bimodal representation of the real aerosol world; not unlike the more primitive but indisputably relevant regression Angstrom exponent reduces the aerosol world to straight lines in log AOD versus log λ space).

O'Neill, N. T.,T.  F., Eck, A. Smirnov, B. N.Holben, S. Thulasiraman, Spectral discrimination of coarse and fine mode optical depth,  J. Geophys. Res.,  Vol.. 108, No. D17, 4559-4573, 10.1029/2002JD002975, 2003.

* Fine Mode Fraction; the algorithm divides aerosols into fine and coarse mode fractions in an optical sense. This parameter differs moderately from the SMF (Sub-Micron Fraction) which divides according to a simple microphysical (radius) cutoff.


In July 2005, a number of improvements were made to the direct sun measurement procedures and processing algorithm. Improvements include satellite retrievals of the total column O3 and NO2 concentrations, pressure using reanalysis data, improved absorption coefficients for trace gases, and updated algorithms. All of the Version 2 improvements can be found in the Version 2 AERONET Direct Sun Algorithm and Spectral Corrections/Components tables.

The prior algorithm and processing (Version 1) will be available through at least 2006.

Quality Levels

AERONET Quality Levels

AERONET quality levels are provided in three levels: 1.0, 1.5 and 2.0. Level 1.0 data use the pre-field deployment sun calibration. Level 1.5 data use Level 1.0 data and apply a cloud-screening procedure. Data are raised to Level 2.0 after applying the final post-field deployment sun calibration to Level 1.5 data and manually inspecting these data.

Data Quality
+ Calibration
+ Cloud-screening Summary and Full Description (PDF)
+ Quality Control Checklist for Manual Inspection (PDF)

SDA Retrieval Quality Levels

The SDA retrievals utilize the AERONET AOD (Level 1.0, 1.5, and 2.0) products. Level 1.0 SDA products do not apply any restrictions. However, Levels 1.5 and 2.0 SDA impose additional criteria depending on the number of wavelengths, spectral range and air mass dependence of AOD for each wavelength as well as logic to remove outliers.

Criteria for Level 1.5 SDA retrievals:

1. At least three wavelength combinations must include 440 and 870nm with either 490, 500 or 675nm.

2. The AOD for each channel must be greater than or equal to 0.02/m, where m is the optical air mass.

3. Outliers are removed according to the following criterion:


Criteria for Level 2.0 SDA retrievals:

1. At least four wavelengths must be included for input. The spectral range must be bounded by 380 and 870nm with at least two additional wavelengths between the bounds (e.g., 440, 500, 675nm). The three channel combination of 380, 500, and 870nm will also be accepted.

2. The AOD for each channel must be greater than or equal to 0.02/m, where m is the optical air mass.

3. Outliers are removed according to the following criterion:


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NASA Official: Brent N. Holben
Last Updated: February 19, 2014