Validation and uncertainty estimates for MODIS Collection 6 “Deep Blue” aerosol data

TLDR

The Deep Blue algorithm, first released in MODIS Collection 5, extends AOD retrieval to bright arid land, and the upcoming Collection 6 will further expand coverage to all cloud‑free, snow‑free land surfaces. The study aims to validate the Collection 6 Deep Blue AOD retrieval and quantify its uncertainties. The authors validated the Collection 6 Deep Blue AOD at 550 nm by comparing MODIS Aqua retrievals with AERONET measurements from 60 sites to quantify uncertainties. The highest‑quality data exhibit an absolute uncertainty of roughly (0.086 + 0.56 τ M)/AMF, which for a typical AMF of 2.8 translates to about 0.03 + 0.20 τ M, comparable to other satellite AOD products, with regional variability and comparison to Collection 5 also reported.

Abstract

The “Deep Blue” aerosol optical depth (AOD) retrieval algorithm was introduced in Collection 5 of the Moderate Resolution Imaging Spectroradiometer (MODIS) product suite, and complemented the existing “Dark Target” land and ocean algorithms by retrieving AOD over bright arid land surfaces, such as deserts. The forthcoming Collection 6 of MODIS products will include a “second generation” Deep Blue algorithm, expanding coverage to all cloud‐free and snow‐free land surfaces. The Deep Blue dataset will also provide an estimate of the absolute uncertainty on AOD at 550 nm for each retrieval. This study describes the validation of Deep Blue Collection 6 AOD at 550 nm ( τ M ) from MODIS Aqua against Aerosol Robotic Network (AERONET) data from 60 sites to quantify these uncertainties. The highest quality (denoted quality assurance flag value 3) data are shown to have an absolute uncertainty of approximately (0.086+0.56 τ M )/AMF, where AMF is the geometric air mass factor. For a typical AMF of 2.8, this is approximately 0.03+0.20 τ M , comparable in quality to other satellite AOD datasets. Regional variability of retrieval performance and comparisons against Collection 5 results are also discussed.