Handheld sun photometer (Microtops II) spectral measurements of the direct solar radiation provide information to calculate the columnar aerosol optical depth (AOD), water vapor and Angstrom parameter. Microtops II instruments currently in the network have five channels but they may have one of two configurations: 340, 440, 675, 936nm or 440, 500, 675, 870, and 936nm. The Version 2 AERONET direct sun algorithm is employed to compute these atmospheric properties.
+ Data Descriptions  AERONET data product descriptions
+ Data Column Descriptions  Column header and data units for data products
+ Data Usage Policy
+ GPS Instructions, Measurement Protocols, and Data Download Instructions
+ Data Download Instructions for Linux
+ Video (AVI, MOV)  Instructions on using the Microtops attached to a GPS (Transcript)
Data Processing
Measurement points are grouped temporally into series. If the interval between two points in a measurement sequence is more than two minutes, then these points are placed into different series (see Figure). A series can have one or more measurement points (typically five or more). A series is considered a single data point (an average of the measurement points in the group); and a sequence of series in a day may be used to compute the daily average.
Definitions:
Point: Instantaneous measurement
Series: Average of points within the identified group
Daily average: Average of series for the day
Data Quality
The Level 1.0 (unscreened) measurement series are formed from Level 1.0 points. The cloud and pointing error screening is applied to Level 1.0 data to produce a Level 1.5 data set. The Level 1.5 data series are raised to Level 2.0 (qualityassured) series after final calibration values are applied and manual data inspection is completed.
Level 1.5  Cloud and pointing error screening criteria:
Within a series, the minimum aerosol optical depth for each point is identified at each wavelength (τ_{ai}). The following criteria are examined for cloud and pointing errors:
 If the difference ( τ_{ai}  τ_{ai} min) for each spectral channel is less than the maximum of { τ_{ai} min *0.05, 0.02}, then the point within a series is considered cloud and pointing error free.
If the above screening removes all but one point from a series, then an additional criterion below is applied to the spectral channels:
 If the Angstrom parameter computed using all available channels between 440 and 870 nm is greater than 0.1, then the point is considered cloud and pointing error free.
Level 2.0  Quality Assurance Criteria:
 Final postdeployment calibration values are applied to the data set.
 Spectral channels are evaluated for filter degradation, other possible instrumental problems, or data anomalies.
 Aerosol optical depth data are inspected for possible cloud contaminated outliers.
Spectral DeConvolution Algorithm (SDA) Retrievals  Fine and Coarse Mode AOD and Fine Mode Fraction
The aerosol optical depth processing includes the spectral deconvolution algorithm (SDA) described in O'Neill et al. (2003). This algorithm yields fine (submicron) and coarse (supermicron) 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 (τ_{a}, 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 ( τ_{f}), the spectral derivative of the fine mode aerosol optical depth (α_{f}), the spectral derivative of α_{f} ( α'_{f}), the fine mode fraction ( η = τ_{f} /τ_{a}) and the coarse mode optical depth ( τ_{c} = τ_{a}  τ_{f} ). 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 α_{f} 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 multiwavelengthregression 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, 45594573, 10.1029/2002JD002975, 2003.
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:
Abs(AOD_{500nm}AOD_{SDA500nm})>(0.02+AOD_{500nm}*0.005)
4. If within the series any tau_fine or tau_coarse is beyond tau_mean plus/minus 2*stdev for the series we eliminate this measurement.
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 wavlengths 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:
Abs(AOD_{500nm}AOD_{SDA500nm})>(0.01+AOD_{500nm}*0.005)
4. If within the series any tau_fine or tau_coarse is beyond tau_mean plus/minus 2*stdev for the series we eliminate this measurement.
More Information:
+ Explanation of AOD, PW, and Angstrom parameter
+ AERONET Version 2 Direct Sun Algorithm Improvements
