VIIRS Aerosol Optical Depth
Frequently Asked Questions about the VIIRS Aerosol Optical Depth Product
1) What is this product?
This product outputs instantaneous optical depth, ranging from a unitless -0.5 to 5.0. This product is useful not only to identify the presence of aerosols in the atmosphere but also in identifying the thickness of that aerosol. This information can assist with forecasting dust storms and issuing dust storm warnings (specifically regarding visibility), identifying volcanic ash, recognizing smoke from fires that may not yet be large enough for the VIIRS Active Fires Product to detect, and being aware of decreased air quality.
2) How often do I receive this data?
The S-NPP satellite is part of the Afternoon Train (A-Train) of satellites. It crosses the equator at ~1:30am and ~1:30pm local time every day. However, this product is only produced during daytime hours.
3) How do I display this product in AWIPS-II?
4) How do I interpret the color maps associated with this product?
5) What should I use in conjunction with this product to produce a better forecast?
Geostationary satellite imagery, including the GOES-R Aerosol Detection product. Also the VIIRS Aerosol Detection Product.
6) How is this product created?
AOD (or aerosol optical thickness, τ) is a measure of extinction of radiation by aerosols due to absorption and scattering. It depends on the chemical composition and size of aerosol particles, and thus varies with the wavelengths of radiation. It also depends on the amount of aerosol in the atmosphere, and as such it is also a measure of aerosol loading. Mathematically, it is defined as the integrated extinction coefficient over a vertical column of unit cross section, and as such it is unitless. The primary product is AOD at 550 nm.1
The narrow band measurements of the VIIRS/ABI sensor in the 0.412 to 2.25 μm range are used to derive the aerosol parameters. Other bands are used to create the VIIRS Cloud Mask (VCM) which the aerosol algorithm uses as input and in internal tests to characterize surface conditions. The band names are those of the VIIRS Moderate Resolution Bands (M-bands). The aerosol algorithm from VIIRS uses the following bands: M1, M2, M3, M5 and M11 to derive AOD over land and M4, M5, M6, M7, M8, M10 and M11 to derive AOD over water.1
AOD is retrieved by comparing TOA reflectances observed in multiple bands with those computed for a range of discrete AOD values and aerosol models by coupling atmospheric and surface contributions. Separate algorithms are used for retrieval over land and ocean due to distinct surface properties and aerosol types. For efficient retrieval, look-up tables are used to store the re-calculated atmospheric contributions for a wide range of geometries and for a finite number of candidate (typical) aerosol models.1
Daytime pixels suitable for aerosol retrieval are selected using upstream cloud, sun-glint, fire and snow masks. Amounts of atmospheric gases, wind speed and direction are obtained from ancillary data. Land/water mask is used to select the appropriate (land or ocean) algorithm.1
In addition to the upstream masks, internal tests are used to screen out pixels not appropriate for aerosol retrieval. Results of these and other tests performed during the retrieval are applied to set quality flags (Sections 184.108.40.206 and 220.127.116.11). Due to the relatively weak aerosol signal and large uncertainties associated with the surface reflectance the algorithm does not make retrieval over sun-glint areas over water. Over land, a method that extends the retrieval to bright snow-free land surface is implemented.1
1NOAA/NESDIS/STAR. NOAA NESDIS Center for Satellite Applications and Research: EPS Aerosol Optical Depth (AOD) Algorithm Theoretical Basis Document. v1.0, November 20, 2015.