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Level 2 Product Contents

All DT product have separate and distinct retrieval algorithms for data collected over land or over ocean however values derived from both algorithms are reported out in each level 2 file.  For a general overview of the algorithm please look through the appropriate pages of the Aerosol Overview section of our website. For algorithm details please see our ATBD section of the website.

SDS Naming Conventions

SDS names will indicate if the parameter is coming from an ocean or land retrieval or if data from both are combined into a single SDS.

The example below illustrates the naming conventions for the SDS parameters in a 10 Km MODIS Terra Level 2 file.  The overall parameter description comes first followed by the “Land”, “Ocean”, “Land_Ocean”, “Deep Blue” or “Deep_Blue_Combined” designation.  “Deep_Blue_Combined” refers to the merged DT_DB product. DB parameters are only available over land and only in the 10 KM product since DB does not have a 3Km product. If unspecified all SDS refer to a DT only product. Example List of SDS Names

When attempting to call these parameters for computational analysis names must be exact and include underscores.

SDS Contents

Shown below is an explanation of the information content provided within each MODIS SDS using the variable “Corrected_Optical_Depth_Land” as an example.  Dimensions for this MODIS SDS are 203 x 135 x 3 see description below.

Sample of SDS content information

short – variable specification

Solution_3_Land=3 number of values for parameters with multiple value output at each pixel (the z dimension of the sds)

Cell_Along_Swath and Cell_Across_Swath - the dimension of this parameter as returned by the algorithm in the x and y directions. In this case 203 and 135.

valid_range – the minimum and maximum raw values returned by the algorithm

_FillValue – the value returned when there is no algorithm retrieval

long_name – a brief text descriptor of the variable. When multiple wavelengths are mentioned in the long name there will be a third dimension for this SDS parameter. In this case 3, one for each wavelength.

units – unit descriptor or name applied to this variable

scale_factor – the scaling value applied to the raw numbers returned by the algorithm

add_offset – the offset value added to the raw numbers returned by the algorithm

Parameter_Type – Input or Output

Cell_Along_Swath_Sampling and Cell_Across_Swath_Sampling– interval number of rows or columns sampled, full number of values sampled in this dimension, number of values averaged to obtain output parameter. This refers to sampling from the level 1B raw calibrated data file.

You can use this link to the file spec page of the MODIS-Atmos site which gives a complete list of these outputs for all of the MOD04 MYD04 SDS parameters.  Use this link for the 3 Km products


Aerosol Product SDS’s Explained

The most important and frequently used parameters from the aerosol products relate to aerosol optical depth (AOD), and indicators of particle size such as fine mode fraction, and angstrom exponent. The background information for each grouping is the same for MODIS and VIIRS.  We begin with a detailed explanation of the MODIS parameters and will then provide the equivalent information for VIIRS.   The bottom of the page has a table which lists all of the SDSs for both MODIS and VIIRS.


Aerosol Optical Depth (AOD)

The aerosol optical depth (AOD or τ; also called aerosol optical thickness or AOT) refers to the optical loading of the aerosols in the atmospheric column.  AOD is a unitless value. It is most closely related to the total surface area of the aerosol.  AOD is spectrally dependent, meaning it varies by wavelength.  In general most remote sensing studies looking at AOD are using values at 0.55 microns.

Ground data from the AErosol RObotic NETwork (AERONET) is used to validate MODIS , VIIRS and many other aerosol products. A full discussion of AERONET is beyond the scope of this guide but users should be aware that AERONET measurements are made at 0.50 microns and interpolated to 0.55 when compared to MODIS or VIIRS.

The most commonly used SDS’s pertaining to AOD are listed here and explained below:












The DT product for both MODIS and VIIRS includes small negative Aerosol Optical Depth retrieval values in order to avoid an arbitrary negative bias at the low AOD end in long term statistics. This is because neither instrument has enough sensitivity over land to retrieve aerosol to better than +/-0.05. The consequence is that in very clean conditions the algorithm cannot truly distinguish between AOD values in the range of -0.05 to 0.05.  If we eliminate all the negative numbers and keep only the positive numbers, we would introduce an artificial bias to the long term statistics, therefore we allow negative retrievals down to -0.05. For end users: If you are calculating long-term statistics simply include the negatives in your analysis. If you are looking at individual retrievals then count negative retrievals as 'very clean'. You could force them to be AOD = 0, for example. It really depends on the application. However, these small negative AOD values are valid retrievals and do contain useful information.

Retrieval Product Quality

All of the AOD products come with an indicator of the quality of the retrieval referred to as the QA flag, with a value at each retrieval pixel location.  QA flag values are found in an SDS that is separate from the product value SDSs.  QA flag values range from the lowest quality of 0 to the highest quality of 3.  For Land based products we suggest using only QA 3.  For Ocean based products we suggest using only QA 2 and 3.  Note that several other remote sensing products come with a QA flag where 0 is considered the highest quality.

Corrected_Optical_Depth_Land – optical depth reported from the land retrieval at 0.47, 0.55, and 0.66 microns.  Minimum Value is -0.05 and Maximum Value is 5.00.

MODIS has a separate SDS of Corrected_Optical_Depth_Land_wav2p1 for AOD at 2.1 microns

While negative values do not indicate an actual value occurring in nature they are allowed in our retrieval product to produce better overall aggregate retrieval statistics.  Only QA value 3 is recommended for use. “Corrected” in the SDS name is a legacy from earlier products.  There is no “Uncorrected” version of this SDS.

The VIIRS DT version reports the land retrieval in a single SDS at 0.48, 0.55, 0.67, and 2.2 microns.


Effective_Optical_Depth_Average_Ocean - optical depth reported from the ocean retrieval at 0.47, 0.55, 0.66, 0.86, 1.24, 1.63, and 2.13 microns. Minimum Value is -0.10 and Maximum Value is 5.00.  While negative values do not indicate an actual value occurring in nature they are allowed in our retrieval product to produce better overall aggregate retrieval statistics.  Only QA values of 2 and 3 are recommended for use. This SDS averages all retrievals within the algorithm’s acceptable error limits.  There is another ocean SDS, Effective_Optical_Depth_Best_Ocean, that returns the retrieval with the lowest fit error.  The Average result SDS is preferred for use.  VIIRS does not include the Effective_Optical_Depth_Best_Ocean sds.

Optical_Depth_Land_And_Ocean – optical depth retrievals for land and ocean reported only at 0.55 microns and only for high quality retrievals, QA 3 for Land QA 2-3 for Ocean.  Minimum Value is -0.10 and Maximum Value is 5.00.  This SDS is suggested for use when the user only wishes to look at our recommended quality data and only at 0.55 microns.

Image_Optical_Depth_Land_And_Ocean - optical depth retrievals for land and ocean reported for all qualities of data (0 – 3) at 0.55 microns.  This SDS is suggested for use only to provide the greatest coverage when producing images and not for quantitative studies.

Land_Ocean_Quality_Flag – Quality assurance (QA) flags for the Ocean and Land aerosol retrieval.

Range of values is 0 – 3 where zero is lowest quality.  QA values are subjectively assigned by the algorithm team based on numerical standards such as number of input pixels used for the retrieval, proximity to bright land or ocean glint and error fitting values.

Deep_Blue_Aerosol_Optical_Depth_550_Land – optical depth retrievals using the DB algorithm at 0.55 microns for all quality of data.  QA flags are 1,2 or 3 where 1 is low. AOD values range from 0 to 5.

Deep_Blue_Aerosol_Optical_Depth_550_Land_Best_Estimate - optical depth retrievals using the DB algorithm at 0.55 microns for higher quality data for DB (QA 2 or 3). AOD values range from 0 to 5.

Deep_Blue_Aerosol_Optical_Depth_550_Land_QA_Flag - Quality assurance (QA) flags for DB Land aerosol retrieval. Range of values is 1 – 3 where one is lowest quality

AOD_550_Dark_Target_Deep_Blue_Combined – AOD at 0.55 micron for land and ocean.  DT values are used over ocean.  Over land the algorithm will select the product, either DT or DB, with the higher QA value.  When QA values are equal the DT-DB values are averaged.

AOD_550_Dark_Target_Deep_Blue_Combined_Algorithm_Flag – Flag indicating which algorithm was selected for the value of the land retrieval 0 – Dark Target, 1 – Deep Blue, 2 – Mixed.

AOD_550_Dark_Target_Deep_Blue_Combined_QA_Flag – Flag indicating the QA value of the AOD retrieval in the merged product.  Range is 0 – 3 where zero is low.


Parameters Related to Particle Size

Fine and Coarse Mode

In general atmospheric aerosols occur in a bimodal distribution and the dark-target algorithm operates under this assumption. The smaller particles are referred to as the fine mode or accumulation mode aerosols. These particles have radii between 0.1 and 0.25 microns. The larger particles comprise the coarse mode. These particles generally have radii between 1.0 and 2.5 microns.  The aerosol fine mode fraction is the proportion of fine mode aerosols to the total. This is an optical measurement of the proportion by volume. The proportion of aerosol attributed to fine or coarse mode can then be multiplied by the total AOD to determine the fine or coarse mode AOD.  The proportional AOD is only reported for the ocean product.  We feel that the algorithms’ ability to distinguish fine or coarse mode over land is not accurate enough to permit us to make a fine mode AOD calculation.  Note that there is no QA value for the following parameters.

Optical_Depth_Ratio_Small_Land- Fraction of AOD contributed by the fine dominated model for land at 0.55 microns. Valid Range: 0 to 1

Optical_Depth_Ratio_Small_Ocean - Ratio of Optical Depth of Small Mode vs Effective Optical Depth at 0.55 microns for best (1) and average (2) solutions.  Valid Range: 0 to 1

Description: AOD for Small Mode of Average Solution at 7 bands 0.47, 0.55, 0.66, 0.86, 1.24, 1.63, and 2.13 µm.  Valid Range: -0.05 to 5.0

Description: Aerosol Optical Thickness for Small Mode of Best Solution at 7 bands 0.47, 0.55, 0.66, 0.86, 1.24, 1.63, and 2.13 µm. Valid Range: -0.05 to 5.0


Ångström Exponent

The Ångström Exponent is often used as a qualitative indicator of mean particle size.  As a rough guideline Ångström Exponent values in the range of 2 indicate small particles which might be associated with pollution or biomass burning. Values in the range of 1 or less indicate the presence of large particles such as sea salt or dust. For the MODIS algorithm Ångström Exponent is not a true measurement but is a derived value.

Description: Angstrom Exponent for 0.55 and 0.86 µm
Dimensions: (Solution_Ocean, Cell_Along_Swath, Cell_Across_Swath)
Valid Range: -1.0 to 5.0

In Collection 6, the preliminary estimated error for angstrom_exponent_1 is 0.45; pixels with an AOD > 0.2 are expected to have a more accurate angstrom exponent.

Description: Angstrom Exponent for 0.865 and 2.130 µm
Dimensions: (Solution_Ocean, Cell_Along_Swath, Cell_Across_Swath)
Valid Range: -1.0 to 5.0

Description: Angstrom Exponent at 0.47 and 0.67 µm
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: -1.0 to 5.0

Other useful parameters

Description: This is the aerosol type used in the retrieval.  These are defined by location and season.

The quantitative properties associated with each model can be found here:

Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: 1 to 5  (1 = Continental, 2 = Moderate Absorption Fine, 3 = Strong Absorption Fine,4 = Weak Absorption Fine, 5 = Dust Coarse)

Description: Solution number index (1 through 4) for small particles. Indices of ocean models 1 through 4 correspond to accumulation (small) mode models with effective radii 0.10, 0.15, 0.20, 0.25 µm, respectively.
Dimensions: (Solution_Ocean, Cell_Along_Swath, Cell_Across_Swath)
Valid Range: 1 to 4

Description: Solution number index (5 through 9) for large particles. Indices of ocean models 5 through 7 correspond to coarse (large) mode models of marine (sea salt) particles with effective radii 1.0, 1.5, 2.0 µm, respectively. Indices of ocean models 8 and 9 correspond to coarse (large) mode models of mineral dust particles with effective radii 1.5 and 2.5 µm, respectively.

Geolocation and Time Parameters

Description: Geodetic Longitude
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: -180 to +180 degrees east

Description: Geodetic Latitude
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: -90 to +90 degrees north

Description: International Atomic Time at Start of Scan replicated across the Swath
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: 0.0 to 3.1558E+9 seconds since 1 January 1993 00:00:00

Solar and Viewing Geometry Parameters

Description: Solar Zenith Angle, Cell to Sun
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: 0 to +180 degrees

Description: Solar Azimuth Angle, Cell to Sun
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: -180 to +180 degrees

Description: Sensor Zenith Angle, Cell to Sensor
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: 0 to 180 degrees

Description: Sensor Azimuth Angle, Cell to Sensor
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: -180 to 180 degrees

Description: Scattering Angle
Dimensions: (Cell_Along_Swath, Cell_Across_Swath)
Valid Range: 0 to 180 degrees


MODIS and VIIRS SDSs compared:

The table below shows the equivalent SDS names for the MODIS and VIIRS DT products. Not all SDSs exist for both products.  Note the differences in capitalization. Also note that VIIRS SDSs are grouped into geolocation_data or geophysical_data folders which must be specified when calling the data.  See the “Aerosol Products File Format Basics” section on page 5 for more information about accessing the SDS data.

Table Comparing MODIS and VIIRS Scientific Data Sets (SDS)
   group: geolocation_data
Longitude longitude
Latitude latitude
Solar_Zenith solar_zenith_angle
Solar_Azimuth solar_azimuth_angle
Sensor_Zenith sensor_zenith_angle
Sensor_Azimuth sensor_azimuth_angle
Scattering_Angle Scattering_Angle
Glint_Angle Glint_Angle
  group: geophysical_data
Land_sea_Flag Land_Sea_Flag
Aerosol_Cldmask_Land_Ocean Aerosol_Cldmask_Land_Ocean
Cloud_Pixel_Distance_Land_Ocean Cloud_Pixel_Distance_Land_Ocean
Land_Ocean_Quality_Flag Land_Ocean_Quality_Flag
Optical_Depth_Land_And_Ocean Optical_Depth_Land_And_Ocean
Image_Optical_Depth_Land_And_Ocean Image_Optical_Depth_Land_And_Ocean
Average_Cloud_Pixel_Distance_Land_Ocean Average_Cloud_Pixel_Distance_Land_Ocean
Aerosol_Type_Land Aerosol_Type_Land
Fitting_Error_Land Fitting_Error_Land
Surface_Reflectance_Land Surface_Reflectance_Land
Corrected_Optical_Depth_Land Corrected_Optical_Depth_Land
Optical_Depth_Ratio_Small_Land Optical_Depth_Ratio_Small_Land
Number_Pixels_Used_Land Number_Pixels_Used_Land
Mean_Reflectance_Land Mean_Reflectance_Land
STD_Reflectance_Land STD_Reflectance_Land
Mass_Concentration_Land Mass_Concentration_Land
Aerosol_Cloud_Fraction_Land Aerosol_Cloud_Fraction_Land
Effective_Optical_Depth_Average_Ocean Effective_Optical_Depth_Average_Ocean
Optical_Depth_Small_Average_Ocean Optical_Depth_Small_Average_Ocean
Optical_Depth_Large_Average_Ocean Optical_Depth_Large_Average_Ocean
Mass_Concentration_Ocean Mass_Concentration_Ocean
Aerosol_Cloud_Fraction_Ocean Aerosol_Cloud_Fraction_Ocean
Effective_Radius_Ocean Effective_Radius_Ocean
PSML003_Ocean PSML003_Ocean
Asymmetry_Factor_Average_Ocean Asymmetry_Factor_Average_Ocean
Backscattering_Ratio_Average_Ocean Backscattering_Ratio_Average_Ocean
Angstrom_Exponent_1_Ocean Angstrom_Exponent_1_Ocean
Angstrom_Exponent_2_Ocean Angstrom_Exponent_2_Ocean
Least_Squares_Error_Ocean Least_Squares_Error_Ocean
Optical_Depth_Ratio_Small_Ocean_0.55micron Optical_Depth_Ratio_Small_Ocean_0.55micron
Optical_Depth_by_models_ocean Optical_Depth_By_Models_Ocean
Number_Pixels_Used_Ocean Number_Pixels_Used_Ocean
Mean_Reflectance_Ocean Mean_Reflectance_Ocean
STD_Reflectance_Ocean STD_Reflectance_Ocean
Wind_Speed_Ncep_Ocean Wind_Speed_Ncep_Ocean
Topographic_Altitude_Land Topographic_Altitude_Land