Aerosol Toolkit (ATK)

Suspended particles (aerosols) play an important role in air quality, human health and Earth's radiative balance. They are one of the main contributors of light scattering in the atmosphere radiative transfer, modifying the local and planetary albedo and absorbing the upward terrestrial thermal radiation. Understanding the radiometric effect of aerosol optical properties from measured at-sensor radiance is therefore necessary to properly characterize them and to include their radiative effects in Earth's energy budget. From a remote sensing perspective, characterizing aerosols improve the quality of atmospheric correction and further data processing.

The aerosol toolkit (ATK) permits defining new aerosol models for the implemented atmospheric RTM beyond the default models. In this section we will demonstrate how users can define their own aerosol models based on different options, which include: (1) aerosol optical properties, (2) particle size/volume distribution and (3) interface with OPAC aerosol database.

Tool overview

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The Aerosol Toolkit (ATK) can be open from the main ALG interface by pressing the ATK button () or, alternatively, through the menu bar by clicking on ToolsATK:

The ATK is composed of three main elements being:

  1. The RTM selection pop-up menu, where the user can select the atmospheric RTM for which desires to define a new aerosol model.
  2. The aerosol models listbox, where it will be displayed the names of the user-defined aerosol models for the selected RTM.
  3. The aerosol parameters table, where users can visualize and edit the parameters of a selected aerosol model. Users can will also use this table to instert the parameters of a new user-defined aerosol model.
    1. Notice that, in ALG, the definition of a new user-defined aerosol largely depends on the selected atmospheric RTM. Indeed, the parameterization of new aerosols might be different for different atmospheric RTMs (e.g., micro-physical properties, optical properties, aerosol mixtures). Please refer to the relevant section for each RTM.

      The new user-defined aeorols models defined with ATK will be stored in the aerdb.h5 stored in the UserData folder. Please check here for further description about the content of ATK's user aerosol database file.

6SV user-defined aerosols

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When selecting the 6SV (sixSV) atmospheric RTM from the RTM selection pop-up menu, the Aerosol model selection pop-up menu will be automatically displayed below the aerosol parameters table:

The current options are: (1) Optical properties, (2) User's components, (3) particle size distribution (Log-Normal, Gamma or Junge power-low), (3) OPAC-based. Notice that the Sun-Photometric Distribution is displayed but not yet implemented in ATK. Each of these aerosol model parameterization will use different input parameters that are to be read by 6SV in the input configuration files.

To introduce a new user-defined aerosol model, press the button and/or select one of the options in Aerosol model selection pop-up menu. The Aerosol name text box will be enabled to set the aerosol name and the aerosol parameters table will be also enabled to allow inputting the corresponding aerosol parameters. The following sections will describe each aerosol model options and their input parameters.

To remove an aerosol model from the database, simply select it in the aerosol models listbox and press the Remove button ().

Optical properties

    6SV allows users to input their own aerosol optical properties that where previously calculated and stored into a .mie file. With this option, users can define define their own extinction coefficient, single scattering albedo (0 to 1) (SSA) and asymmetry parameter (-1 to 1) so that they can be read and used in 6SV.

    In principle, 6SV allows to use a specific definition of the aerosol phase function, however this option is not yet implemented in ALG. Instead, ALG will internally use the asymmetry parameter together with the Henyey-Greenstein parametric approximation of the phase function according to the following equation:

    $$Φ(θλ)_{HG} = {1}/{4π}{1-g_λ^2}/{(1+g_λ^2-2g_λ\cosθ)^{3/2}}$$

    where Φ is the aerosol phase function as function of the scattering angle θ and gλ is the spectrally-dependent asymmetry parameter.

    Alternatively, users can import the aerosol optical properties as calculated by the Mie sub-routine in 6SV (.mie file), OPAC (.out file) or user-defined OPAC-based aerosols from ALG (.h5 file, read here). This option allows to re-use and store user-defined aerosols between different RTM models or from previous execution of the OPAC aerosol database. Notice that, in the current version of ALG, the Henyey-Greenstein approximation will still be used by 6SV to automatically generate the phase function.

    The option to import previously calculated aerosol optical properties is available by pressing the Import file(s) button. For more information about this option check the corresponding section in MODTRAN aerosols.

User's components

    The user's component option allows to define a new aerosol model using the volumetric percentage (0 to 100%) of four basic components (dust-like, water-soluble, oceanic and soot). The sum of these four components must be 100%.

    To save the introduced parameters in ATK's aerosol database, click on the Save button. If the user does not keep the requirement of 100% volumetric sum, each input will be scaled so that the sum is 100%.

    After saving the aerosol model into the database, an informative message window will pop-up and the new aerosol model will appear in the aerosol models listbox. Users can display and edit the aerosol parameter values by clicking on the aerosol model. Please notice that the parameters will only be updated after pressing the Save button. Users can edit the parameter values and save the aerosol model as a new model after changing the aerosol name in the corresponding text box.

Particle size distribution

    In 6SV, users can define their own aerosol model using a volumetric (size) distribution function. Three options are available being:

    • Multimodal Log-Normal distribution: this distribution is typically used for externally-mixed suspended particles and is the model of choice for most aerosol modeling applications. With the log-normal distribution, users can set up to 4 modes (particle types) defining their: (1) volumetric percentage CV,i (between 0 and 1), (2) mean radius rV,i and standard deviation σi, and (3) spectrally-dependent refractive index (nr and ni). In addition, the minimum/maximum radius of the size distribution (Rmin and Rmax) must be defined. The Log-Normal distribution is shown in the equation below:

      $${dV}/{dr} = ∑↙{i=1}↖N{C_{V,i}}/{√{2π}r\ln{σ_i}}\exp[-{1}/{2}({\ln{r} - \ln{r_{V,i}}}/{\ln{σ_i}})^2]$$
    • Modified Gamma distribution: this distribution typically applies to water clouds (e.g., stratus, cumulus, fog). In this case, only one particle type can be defined with the following input parameters: (1) shape parameters α, β and γ, (2) spectrally-dependent refractive index, and (3) minimum/maximum radius of the size distribution. The modified Gamma distribution is shown in the following equation:

      $${dV}/{dr} = N_0({r}/{r_{mod}})^α\exp[-β({r}/{r_{mod}})^γ]$$
    • Junge Power-Law distribution: together with the Log-Normal distribution, the Junge Power-Law distribution is also used to represent the distribution of aerosol particle size in the atmosphere. This distribution is particularly suitable to describe background aerosols of diameter greater than about 0.1 μm. In this case, a single parameter α and the spectrally-dependent refractive index should be given as input to define a new aerosol model. The Junge Power-Law distribution is shown in the following equation:

      $${dV}/{dr} = N_0{r^α}/{r}$$

    For any of these distributions, the aerosol parameters table will be preset as shown in the following figure:

    Each row will display the name of the variables corresponding to each aerosol. The first row will be dedicated to input the minimum/maximum size of the particle distribution. The second row will be dedicated to the parameters of the distribution. Rows 3 and 4 will finally be dedicated to input respectively the 20 values of the real and imaginary parts of the refractive index. Notice that these 20 values correspond to the following wavelengths (in μm), fixed internally by 6SV: 0.35, 0.4, 0.412, 0.443, 0.47, 0.488, 0.515, 0.55, 0.59, 0.633, 0.67, 0.694, 0.76, 0.86, 1.24, 1.536, 1.65, 1.95, 2.25 and 3.75. Table cells preset with a NaN value are not editable.

    In the case of a multimodal Log-Normal distribution, users can define more than one particle type by pressing the button o nthe left of the table. This will automatically expand the table size to allow users to input the parameters for the added particle type. To remove a particle, press the button. Notice that only the last particle on the table will be removed.

    Click on the Save button to save the new aerosol model into the aerosol database. In the case of the multimodal Log-Normal distribution, ATK will automatically rescale the volumetric percentages so that the sum 100%.

OPAC-based

    Unfortunately, in the current version v2.1 of 6SV, users cannot define new aerosol models based on their optical properties (i.e., extinction, absorption and phase function). Accordingly, interfacing with the OPAC aerosol database is not straightforward. However, OPAC defines aerosol particles using the multimodal Log-Normal distribution as also available in 6SV user-defined aerosols (read more here). Moreover, OPAC database stores the required parameters (i.e., mode radius, standard deviation and refractive indices) for a set of 10 aerosol components. Altogether, OPAC-based aerosols can be defined for 6SV. ATK automatically extracts the information from OPAC aerosol database and converts it into the required parameters of a multimodal Log-Normal distribution so that it can be read by 6SV.

    When selecting the OPAC-based aerosol model, the aerosol parameters table will be preset as shown in the following figure:

    Here, users can set the particle number percentage (0-100%) for up-to 4 components. The sum of these four components must be 100%. If it is not the case, after pressing the Save button, ATK will scale all percentages to sum 100%. Notice that the number percentage used here is not the same as volumetric percentage as used in the multimodal Log-Normal distribution. ATK will automatically convert the number percentage into volumetric percentage after automatically run the OPAC software.

    The definition of an OPAC-based aerosol will also require the user selection of a relative humidity value (in %) from the diplayed pop-up menu at the bottom-left of the aerosol parameters table. By default, the value of 70% will be used.

    Press the Save button to save the new aerosol in the ATK's aerosol database file.

MODTRAN user-defined aerosols

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When selecting any version of the MODTRAN atmospheric RTM from the RTM selection pop-up menu, the Aerosol model selection pop-up menu will be automatically displayed below the aerosol parameters table:

The current options are: (1) Optical properties, and (2) OPAC-based. In practice, both aerosol model parameterization are converted into aerosol optical properties so that can be read by MODTRAN in the input configuration files.

To introduce a new user-defined aerosol model, press the button and/or select one of the options in Aerosol model selection pop-up menu. The Aerosol name text box will be enabled to set the aerosol name and the aerosol parameters table will be also enabled to allow inputting the corresponding aerosol parameters. The following sections will describe each aerosol model options and their input parameters.

To remove an aerosol model from the database, simply select it in the aerosol models listbox and press the Remove button ().

Notice that, despite MODTRAN5 and MODTRAN6 have the same user-defined aerosol model options, the aerosol models defined in ATK will be only assigned to the model selected in the RTM selection pop-up menu. There is not current option to copy-paste the aerosol parameters from one MODTRAN version to another.

Optical properties

    In MODTRAN, users can define their own aerosol models by inputting the corresponding aerosol optical properties. This means, users can define the extinction coefficient, single scattering albedo (0 to 1) (SSA) and asymmetry parameter (-1 to 1) for a user-defined spectral grid.

    In principle, MODTRAN allows to use a specific definition of the aerosol phase function, however this option is not yet implemented in ALG. Instead, the asymmetry parameter is used together with the Henyey-Greenstein parametric approximation of the phase function according to the following equation:

    $$Φ(θλ)_{HG} = {1}/{4π}{1-g_λ^2}/{(1+g_λ^2-2g_λ\cosθ)^{3/2}}$$

    where Φ is the aerosol phase function as function of the scattering angle θ and gλ is the spectrally-dependent asymmetry parameter.

    Alternatively, users can import the aerosol optical properties as calculated by the Mie sub-routine in 6SV (.mie file), OPAC (.out file) or user-defined OPAC-based aerosols from ALG (.h5 file, read here). This option allows to re-use and store user-defined aerosols between different RTM models or from previous execution of the OPAC aerosol database.

    To use this functionality with 6SV aerosols in the .mie file, follow the next steps:

    1. Start by defining one (or more) new aerosol model for 6SV using the particle size distribution option and save it into the ATK's aerosol database.
    2. Create a new LUT for 6SV using ALG in which the only variable is the aerosol model (MAERO). The wavelength range is not important.
    3. After running 6SV with the selected aerosol model(s), a set of .mie file(s) will have been created in the selected output folder.
    4. Back into ATK, press on the Import file(s) button and select one or more .mie files.
    After these steps, the optical properties calculated in 6SV will be read and displayed in the aerosol parameters table, ready to be edited and saved into the ATK's aerosol database as a new MODTRAN aerosol model. Notice that, if more than one .mie file were selected, ATK will automatically save the aerosol optical properties into the aerosol database file without the option of editing them. You can always re-edit them clicking on them on the aerosol models listbox.

    For loading OPAC output files (.out), simply select one or more files and click on the Import file(s) button. Again, if more than one file is selected, ATK will automatically save all the aerosols in the aerosol database file.

    Finally, this functionallity permits loading a pre-calculated OPAC aerosol optical properties that are stored in ALG's _OPAC.h5 file. In this case, follow the next steps:

    1. Create a new LUT using ALG in which the only variable is the aerosol model. If the model allows it, generate a set of user-defined OPAC aerosols.
    2. On the spectral configuration step, notice that the wavelength range in this case is important since it will define the output wavelengths of the aerosol optical properties.
    3. After running the RTM with the selected aerosol model(s), a single *_OPAC.h5 file will have been created in the selected output folder. This file contains the aerosol optical properties that have been previously calculated running OPAC.
    4. Back into ATK, press on the Import file(s) button and select one or more *_OPAC.h5 files.

    Notice that in both OPAC options (.out file or the *_OPAC.h5 file), the aerosol optical properties from OPAC will be loaded and stored in the aerosol databse file only for the 70% relative humidity.

    Press the Save button to save the new aerosol in the ATK's aerosol database file. ATK will will automatically normalize the extinction coefficient to 1 at 550 nm as required by MODTRAN.

OPAC-based

    As an alternative to the definition of a new aerosol model by their optical properties, these can be simulated using the OPAC aerosol database. When selecting the OPAC-based aerosol model, the aerosol parameters table will be preset as shown in the following figure:

    Here, users can set the particle number percentage (0-100%) for up-to 4 components. The definition of an OPAC-based aerosol will also require the user selection of a relative humidity value (in %) from the diplayed pop-up menu at the bottom-left of the aerosol parameters table. If not selected any specific value, the default value of 70% will be used.

    When pressing the Save button ATK will automatically run OPAC and extract the calculated aerosol optical properties to be saved in the ATK's aerosol database file.

LibRadtran user-defined aerosols

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ATK is not yet compatible with the libRadtran RTM. However this is work in progress.

ATK's user aerosol database file

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The parameters of the user-defined aerosol models using ATK are stored in the aerdb.h5. In order to be used by ALG, this aerosol database file must be available in the UserData folder of the ALG's software package. The ATK's aerosol database file (aerdb.h5) is splitted in groups, each of them containing the aerosol models defined for an atmospheric RTM (e.g., /MODTRAN5). Within each of these groups, the parameters of the user-defined aerosol models (e.g., aerosol optical properties, particle size distribution parameters,...) are stored in several datasets, each of them corresponding to a new aerosol model (e.g., /MODTRAN5/aerosol_name). Each of these datasets come together with several attributes:

  • varnames: string with the list of the parameter names used in a given aerosol model. Each parameter name is separated by commas.
  • units: string with the units of each parameter in varnames. Each unit is separated by commas.
  • modeltype: ID number identifying the type of aerosol model (e.g., for 6SV: User's components, multimodal Log-Normal, OPAC-based...).
An additional attribute (opacdata) is assigned to OPAC-based aerosols in order to store the user-input particle number percentage.

When selecting an atmospheric RTM to generate a new LUT, ALG will detect if a mixing-layer aerosol model variable is defined. In such case, ALG will automatically read the ATK's aerdb.h5 file and extract the user-defined aerosols for the selected atmospheric RTM. These models will now appear in the Key Input Variables window as shown in the figure below:

When introduced in a LUT, the selected ATK's aerosol models will have an assigned ID with an integer value higher than 10.