STAR / SMCD / EMB Aerosol Remote Sensing

NESDIS Aerosol Products: GOES

The advantage of monitoring aerosol from a geostationary platform is that frequent observations of the same area available only from geostationary orbits permit tracking rapid movement of pollution, which is important for air quality monitoring.

Aerosol optical depth (AOD) has been routinely retrieved at NESDIS/STAR for the contiguous U.S. (CONUS) from the Geostationary Operational Environmental Satellite (GOES)-East and West satellites. The product is known as the GOES Aerosol and Smoke Product (GASP). It includes retrievals for nominal 4-km pixels every 30 minutes approximately between 12 UTC and 21 UTC at 15 and 45 minutes after the hour during daytime. The technique has been described in detail by Knapp (2002) and by Knapp et al. (2002); only the main features of the algorithm are highlighted here.

GASP - AOD is derived for the effective wavelength of 550 nm from cloud-free radiances in the GOES imager visible band (0.52 - 0.72 µm) over sufficiently dark surfaces. Cloud- free scenes are identified using a cloud mask based on spatial and spectral tests similar to those developed for AVHRR (Pavolonis et al., 2005). The retrieval process starts out with establishing a composite clear scene from which the surface albedo is estimated. The composite is created by selecting the second darkest pixel from the visible images of the most recent 14-28 days, and it is updated every time the retrieval runs. Next, the surface albedo is retrieved from the composite reflectance assuming a minimum aerosol optical thickness of 0.02 at 550 nm. AOD the is retrieved by comparing the instantaneous satellite- observed reflectance with calculated top-of-atmosphere reflectances. In the surface albedo and AOD retrievals a look-up table (LUT) is used for efficiency. The LUT is based on a single (continental) aerosol model, constant amounts of atmospheric gases, and assumes a Lambertian surface.

The availability of only a single channel and the lack of on-orbit calibration limits the quality of aerosol retrievals from the current GOES satellites. Nevertheless, comparisons of AOD with ground observations at 36 Aerosol Robotic Network (AERONET, Holben et al., 1998) sites show the retrievals to be within ±0.13 of the AERONET values (Knapp et al., 2005).

Images and access to GASP binary data are at http://www.ssd.noaa.gov/PS/FIRE/GASP/gasp.html

The table below summarizes the main features of the product.

Characteristics of the GASP Aerosol Product

Satellite:	GOES
Instrument(s):	Imager
Instrument/algorithm PI:	Shobha Kondragunta/Kenneth Knapp, NOAA
Contact details:	Shobha Kondragunta (Shobha.Kondragunta at noaa.gov)
Parameter(s):	aerosol optical thickness
Aerosol algorithm:	GOES Aerosol and Smoke Product (GASP)
Cloud screening:	based on CLAVR
Aerosol model:	continental
Retrieval assumptions:	an increase in AOD makes a proportional increase in satellite-observed radiance; minimum AOD is 0.02 at 550 nm
Retrieval limitations:	over dark surface only
Spatial, temporal coverage:	Contiguous US
Spatial, temporal resolution:	4 km/every 30 min
Operations status:	operational
Validation status:	AERONET & other satellite inter-comparison studies
Quality control:
Last algorithm version:	January 7, 2009
Product format description:	http://www.ssd.noaa.gov/PS/FIRE/GASP/gasp.html
Algorithm description:	http://www.ssd.noaa.gov/PS/FIRE/GASP/20090107_GASP_Algorithm_Updates.doc

References: