Abstract
Abstract. Retrieving aerosol optical depth (AOD) from top-of-atmosphere (TOA) satellite-measured radiance requires separating the aerosol signal from the total observed signal. Total TOA radiance includes signal from the underlying surface and from atmospheric constituents such as aerosols, clouds and gases. Multispectral retrieval algorithms, such as the dark-target (DT) algorithm that operates upon the Moderate Resolution Imaging Spectroradiometer (MODIS, on board Terra and Aqua satellites) and Visible Infrared Imaging Radiometer Suite (VIIRS, on board Suomi-NPP) sensors, use wavelength bands in “window” regions. However, while small, the gas absorptions in these bands are non-negligible and require correction. In this paper, we use the High-resolution TRANsmission (HITRAN) database and Line-By-Line Radiative Transfer Model (LBLRTM) to derive consistent gas corrections for both MODIS and VIIRS wavelength bands. Absorptions from H2O, CO2 and O3 are considered, as well as other trace gases. Even though MODIS and VIIRS bands are “similar”, they are different enough that applying MODIS-specific gas corrections to VIIRS observations results in an underestimate of global mean AOD (by 0.01), but with much larger regional AOD biases of up to 0.07. As recent studies have been attempting to create a long-term data record by joining multiple satellite data sets, including MODIS and VIIRS, the consistency of gas correction has become even more crucial.
Highlights
Aerosols are fine particles in the atmosphere that scatter and/or absorb incoming solar radiation, and because of this they are active players in Earth’s energy budget (IPCC, 2013)
We introduce the relationship of gas abundance to its transmittance spectra, which is the theoretical basis for gas corrections in DT aerosol optical depth (AOD) retrievals
Performing aerosol optical depth retrieval, from satellite measurements, requires extracting the aerosol signal from the total radiance measured by the sensor at the top of the atmosphere
Summary
Aerosols are fine particles in the atmosphere that scatter and/or absorb incoming solar radiation (insolation), and because of this they are active players in Earth’s energy budget (IPCC, 2013). Some of the interactions requiring removal continue to receive considerable attention as new sensors are deployed and new aerosol remote-sensing algorithms are derived These include characterizing the contribution from the surface and masking clouds (Hutchison et al, 2008; Shi et al, 2014). For the US 1976 Standard Atmosphere (US76, 1976), with total column ozone of 344 Dobson units (DU), the gas absorption optical depth (τ GAS) is about 0.03 in this band This is of similar magnitude to pristine AOD (∼ 0.05) and is equal to the required measurement accuracy (GCOS, 2011; GCOS-IP, 2016). Other trace gases, including carbon dioxide and methane, absorb shortwave radiation in wavelength-specific regions While these gases are more evenly distributed (well mixed) across the globe, failing to correct for their absorption would lead to errors in aerosol retrieval.
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