Abstract
When retrieving Aerosol Optical Depth (AOD) from passive satellite sensors, the vertical distribution of aerosols usually needs to be assumed, potentially causing uncertainties in the retrievals. In this study, we use the Moderate Resolution Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS) sensors as examples to investigate the impact of aerosol vertical distribution on AOD retrievals. A series of sensitivity experiments was conducted using radiative transfer models with different aerosol profiles and surface conditions. Assuming a 0.2 AOD, we found that the AOD retrieval error is the most sensitive to the vertical distribution of absorbing aerosols; a −1 km error in aerosol scale height can lead to a ~30% AOD retrieval error. Moreover, for this aerosol type, ignoring the existence of the boundary layer can further result in a ~10% AOD retrieval error. The differences in the vertical distribution of scattering and absorbing aerosols within the same column may also cause −15% (scattering aerosols above absorbing aerosols) to 15% (scattering aerosols below absorbing aerosols) errors. Surface reflectance also plays an important role in affecting the AOD retrieval error, with higher errors over brighter surfaces in general. The physical mechanism associated with the AOD retrieval errors is also discussed. Finally, by replacing the default exponential profile with the observed aerosol vertical profile by a micro-pulse lidar at the Beijing-PKU site in the VIIRS retrieval algorithm, the retrieved AOD shows a much better agreement with surface observations, with the correlation coefficient increased from 0.63 to 0.83 and bias decreased from 0.15 to 0.03. Our study highlights the importance of aerosol vertical profile assumption in satellite AOD retrievals, and indicates that considering more realistic profiles can help reduce the uncertainties.
Highlights
Aerosol observations are essential in climate and environmental studies such as quantifying Earth’s energy budget and characterizing surface air quality
We focus on the Beijing-PKU site, because complete aerosol optical and vertical measurements by the sun photometer and micro-pulse lidar (MPL) lidar are available here
We discuss the impact of aerosol vertical distribution assumption on satellite Aerosol Optical Depth (AOD) retrievals through model sensitivity studies and retrieval experiments
Summary
Aerosol observations are essential in climate and environmental studies such as quantifying Earth’s energy budget and characterizing surface air quality. Modern sensors like the Moderate Resolution Spectroradiometer [2,3], launched on board Aqua in May 2002 and Terra in December 1999, provide aerosol information at up to seven spectral bands, ranging from visible to near-IR wavelengths, and cover the entire globe every 1 to 2 days Another advanced satellite sensor —the Visible Infrared Imaging Radiometer Suites [4]—was launched onboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite in October 2011. It collects images and measurements of land and atmosphere in 22 spectral bands, covering wavelengths from 412 to 12,050 nm, and provides routine retrievals of land, aerosol and cloud properties. The AOD errors can be attributed to several main factors, including cloud screening [3], surface reflectance parameterization [7], aerosol model assumptions [7,8,9,10], and aerosol vertical profile assumptions [10,11]
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