Introduction

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Ranging in size from nanometers (nm) to tens of micrometers (μm), atmospheric particulate matter known as aerosols impact the Earth’s radiative budget, climate change, hydrological processes, and the global carbon, nitrogen and sulfur cycles.  To understand the wide-ranging effects of aerosol, it is necessary to measure the aerosol characteristics globally with high spatial and temporal resolution. The polar-orbiting MODerate resolution Imaging Spectrometer (MODIS-Salmonson et al 1989) on board NASA Terra (1999 - ) and Aqua (2002 - ) sensor was developed, in-part, for the capability of observing global aerosol.  The family of Dark-Target algorithms, discussed here, was originally developed to take advantage of MODIS’s ability to observe aerosols.  In fact, because MODIS has been flying for so long, the aerosol record is now over 20 years. However, with the imminent de-orbiting of both Terra and Aqua missions before 2025, means that such a global data record could be in danger of ending.  Fortunately, DT is versatile, and can be run on any sensor with appropriate coverage and sampling in spectral, spatial, and temporal domains. Hence it has been ported to VIIRS (on Suomi-NPP and now the JPSS series of polar orbiting satellites) in low earth orbit (LEO), as well as to ABI (on GOES-R), AHI (on Himawari), and other sensors now in geostationary (GEO) orbit. It has also been ported to run on higher-spatial resolution sensors (e.g., eMAS), to study aerosol processes near sources or near clouds.

The DT aerosol algorithm is actually two independent algorithms, one for deriving aerosols over land (DT-L) and the second for aerosols over ocean (DT-O).  The general science and methodology of both flavors were conceived and developed before the Terra launch and described in depth in Kaufman, et al. (1997b), Tanré, et al. (1997) and ATBD-96. The theoretical basis of DT-O is largely the same as pre-launch.  DT-L has had more significant changes, first described by Levy et al. (2007a,b) and ATBD-09.

The basic aerosol products from DT include total spectral ‘aerosol optical depth’ (AOD or t) and ‘Fine aerosol-Model Weighting’ (FMW or h). In the literature, the concept of t is sometimes referred as ‘aerosol optical thickness’ (AOT), but AOD is preferred for this document. The concept of FMW is also variously defined, however, here h refers to the fractional contribution of fine (small sized particles) to the total t, and is reported at a particular wavelength (0.55 mm). Each retrieval of AOD + FMW product is also associated with Quality Assurance/Confidence (QAC) and additional products representing diagnostic and derived quantities. 

Instruments

MODIS instrument

The MODerate resolution Imaging Spectrometer (MODIS) instrument flies on the Earth Observation System’s (EOS) Terra and Aqua satellites. Both satellites are polar-orbiting (LEO), with Terra on a descending orbit (southward) over the equator about 10:30 local sun time (LST), and Aqua on an ascending orbit (northward) over the equator about 13:30 LST. From a vantage 705 km above the surface and a ±55° view scan, each MODIS instrument views the earth with a swath about 2330 km. With ~16 full orbits/day, VIIRS observes nearly the entire globe (small gaps at equator) each day. Orbit patterns repeat every 16 days. MODIS performs measurements in 36 spectral channels (or bands) that cover the solar to thermal infrared spectrum region between 0.41 to 14.2 μm (Salomonson et al., 1989). Nominal pixel resolutions (at nadir) are 0.25 km (for 2 bands), 0.5 km (for 5 bands) and 1 km (for 29 bands).  Detailed specifications and components can be found at http://modis.gsfc.nasa.gov.  For the DT aerosol retrieval, we rely on window (small gas absorption) Bands 1 through 7 (B1-B7), with “centroid” wavelengths calculated as in Table 3b.  B1 and B2 (red and NIR) are observed at 0.25 km resolution, with B3-B7 (blue, green, SWIR1, SWIR2 and SWIR3) at 0.5 km resolution.  For cloud masking and other purposes, we also use 1km resolution B26 (cirrus band in the SWIR) as well as BXX and BXX in the Thermal infrared (TIR). The copies of MODIS on Terra and Aqua are similar enough that we can use the same centroid wavelengths (within 1 nm) to describe both instruments. (see appendix XXX for details of calculations).

MODIS is not a “camera”, rather it makes use of a continually rotating scan mirror. Each scan of the mirror images 10 lines of 1 km pixels, (20/40 lines of 0.5/0.25 km pixels). Because of the 55° swath convolved with Earth’s curvature, 1 km pixels grow to approximately 4.8 by 2.0 km at swath edge. This gives rise to the geolocational oddity known as the panoramic “bow-tie” effect that means the scans are partially overlapping towards swath edge. For simplification, pixel size will refer to nadir pixel size, unless stated otherwise.

VIIRS

The Visible Near Infrared Radiometer Suite (VIIRS) was launched aboard Suomi-NPP in 2011, on NOAA-20 in 2017, on NOAA-21 in 2022, and on future NOAA Joint Polar Satellite System (JPSS-3 and 4) satellites. All VIIRS copies are at elevation of ~825 km with ascending equator crossings around 13:30 LST. NOAA-20 is placed a half-orbit behind SNPP (50 minutes), so crosses the equator at the same LST but to the west. With a higher altitude and slightly wider view ±56°, VIIRS observes a 3040 km wide swath which entirely covers the globe (no gaps) with ~14 orbits per day. Like MODIS, VIIRS is multi-spectral, but with only 22 bands (covering 0.41 to 12.3 mm).  Nominal spatial resolution for the 5 “Imagery bands” (I-bands; I1-I5) are at 0.375 km, with the remaining “Moderate bands” (M-bands; M1-M16) and the DayNightBand (DNB) at 0.75 km. The DT retrieval (in daylight) makes direct use of wavelengths described in Table 3b, which are generally similar as those to MODIS. These include window bands (M3, M4, M5, M7, M9, M10 and M11), cirrus reflective band (M8), and TIR bands (MXX and MXX).  Note that DT also is now making use of the I2 (red wavelength I-band) to help with cloud masking (described in XXX). Note that unlike MODIS on Terra and Aqua, the copies of VIIRS on SNPP and NOAA-20 differ slightly in their wavelength spectral response, leading to differences of 3-5 nm in some bands. VIIRS on NOAA-21 is nearly identical to that on NOAA-20.

VIIRS is similar in technology to MODIS in that it uses a scanning mirror, and is hampered by bow-tie effect and pixel overlap. However, VIIRS onboard processing is such that it partially compensates for the bow-tie effect, by deleting bow-tie influenced scans through a “pixel trim”  (https://www.star.nesdis.noaa.gov/jpss/documents/AMM_All/VIIRS_SDR/Provi…).  For SNPP these data cannot be recovered, but they can be estimated via interpolation. 

ABI

Advanced Baseline Imagers (ABI) were launched upon GOES-R (became GOES-16 on orbit) in 2017, GOES-S (GOES-17) in 2019, and GOES-T (GOES-18) in 2022. GOES-16 became NOAA’s operational GOES-East (over equator at 75.0°W), whereas GOES-17 became GOES-West (137.0°W). After a period of overlap at GOES-West position between June 2022 and December 2022, GOES-18 became the operational GOES-West, with GOES-17 moving into “storage” near 105°W.  All three GOES satellites operated at a “checkout” position (near 87°W) for some time before moving into operational positions.  From an altitude of ~36,000 km, each ABI carves out a scan pattern that includes mesoscale, continental, and full-disk (FD) defined areas. Prior to April 2019, the scan pattern took 15 minutes, but is now 10 minutes (https://www.goes-r.gov/users/abiScanModeInfo.html).  Each ABI makes 144 FD scans per day, observing hemisphere with radius approximately 82° in longitude/latitude (local zenith angle). Currently, DT runs on FD,

ABI observes at 16 wavelength bands, covering a range of VIS though TIR. Table 3b lists the wavelengths used for DT. Analogous to MODIS or VIIRS, GOES-ABI has blue, NIR and SWIR2 bands (G1, G3 and G5) at 1 km resolution, red band (G2) at 0.5 km resolution, and the remaining bands at 2 km resolution. There is no Green or SWIR1 bands, so the DT algorithm must include compensations.

Each FD image is approximately 10884 x 10884 pixels (for the 1 km resolution).  Pixel-overlap (bow tie) is not a problem for a GEO imager, however, pixel sizes toward FD limbs increase greatly. 

AHI

Advanced Himawari Imagers (AHI) were launched on Japan’s Himawari-8 (H08) satellite in 2015 and on Himawari-9 (H09) in 2016.  Both are located near 140.7°E.  H08 was operational until December 2022 when it was replaced by H09.  H08 is currently in storage, standby orbit. Like ABI, AHI carves out a scan pattern that includes FD imagery every 10 minutes.

Like ABI, AHI also observes 16 wavelength bands, however instead of the 1.37 mm “cirrus” band AHI included a 0.51 mm Green band.  All other bands are similar to ABI.  Thus DT retrieval uses H1, H2, H4 and H5 at 1 km resolution, H3 (Red) at 0.5 km resolution, and the remainder at 2 km resolution. The FD is slightly larger, covering 11000 x 11000 pixels (for the 1km resolution). 

Generalization of the algorithm

As the DT algorithm has expanded to encompass several instruments we have attempted to generalize the algorithm which we refer to as the "DT-Package".  Rather than include references to specific bands or wavelengths we will use the name in the "Characteristics" column of the following table. The name will refer the the wavelengths listed in the table below for the specified instrument.

ATBD Table 1

* From MODIS, DT-Package uses the half-kilometer (HKM) aggregation of bands 1 and 2 for the aerosol retrieval, plus the original QKM data of band 1 for cloud masking. 

From VIIRS, DT-Package uses the M-band resolution of the red band M5 for the retrieval and the I-band resolution (I1) for cloud masking.  

For GEO, DT uses red band interpolated to 1.0 for the retrieval, and original 0.5 km red band for cloud masking.