The impact of aerosol vertical distribution on aerosol optical depth retrieval using CALIPSO and MODIS data: case study over dust and smoke regions

Global quantitative aerosol information has been derived from MODerate Resolution
Imaging SpectroRadiometer (MODIS) observations for decades since early 2000 and widely used for
air quality and climate change research. However, the operational MODIS Aerosol Optical Depth (AOD)
products Collection 6 (C6) can still be biased, because of uncertainty in assumed aerosol optical properties
and aerosol vertical distribution. This study investigates the impact of aerosol vertical distribution on
the AOD retrieval. We developed a new algorithm by considering dynamic vertical profiles, which is an
adaptation of MODIS C6 Dark Target (C6_DT) algorithm over land. The new algorithm makes use of the
aerosol vertical profile extracted from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation
(CALIPSO) measurements to generate an accurate top of the atmosphere (TOA) reflectance for the AOD
retrieval, where the profile is assumed to be a single layer and represented as a Gaussian function with the
mean height as single variable. To test the impact, a comparison was made between MODIS DT and Aerosol
Robotic Network (AERONET) AOD, over dust and smoke regions. The results show that the aerosol vertical
distribution has a strong impact on the AOD retrieval. The assumed aerosol layers close to the ground can
negatively bias the retrievals in C6_DT. Regarding the evaluated smoke and dust layers, the new algorithm
can improve the retrieval by reducing the negative biases by 3–5%.