Multiangle Imaging Spectroradiometer (MISR) global aerosol optical depth validation based on 2 years of coincident Aerosol Robotic Network (AERONET) observations

TLDR

The study evaluates the MISR early post‑launch aerosol optical thickness retrieval algorithm over land and ocean by quantitatively comparing it with a two‑year AERONET Sun photometer record, aiming to refine the algorithm through uncertainty reporting, trend analysis, and outlier investigation. The authors stratify coincident observations by season and aerosol type, compare MISR AOT values with AERONET measurements, and identify trends and outliers to guide algorithm improvements. The comparison shows that about two‑thirds of MISR AOT values fall within 0.05 or 20 % of AERONET, with higher correlations for maritime sites (~0.9) than dusty sites (~0.7), spectral slopes agreeing for biomass burning and continental aerosols, and the authors propose adding absorbing spherical particles, realistic dust analogs, richer multimodal mixtures, and improved low‑light calibration to achieve unprecedented spaceborne AOT accuracy.

Abstract

Performance of the Multiangle Imaging Spectroradiometer (MISR) early postlaunch aerosol optical thickness (AOT) retrieval algorithm is assessed quantitatively over land and ocean by comparison with a 2‐year measurement record of globally distributed AERONET Sun photometers. There are sufficient coincident observations to stratify the data set by season and expected aerosol type. In addition to reporting uncertainty envelopes, we identify trends and outliers, and investigate their likely causes, with the aim of refining algorithm performance. Overall, about 2/3 of the MISR‐retrieved AOT values fall within [0.05 or 20% × AOT] of Aerosol Robotic Network (AERONET). More than a third are within [0.03 or 10% × AOT]. Correlation coefficients are highest for maritime stations (∼0.9), and lowest for dusty sites (more than ∼0.7). Retrieved spectral slopes closely match Sun photometer values for Biomass burning and continental aerosol types. Detailed comparisons suggest that adding to the algorithm climatology more absorbing spherical particles, more realistic dust analogs, and a richer selection of multimodal aerosol mixtures would reduce the remaining discrepancies for MISR retrievals over land; in addition, refining instrument low‐light‐level calibration could reduce or eliminate a small but systematic offset in maritime AOT values. On the basis of cases for which current particle models are representative, a second‐generation MISR aerosol retrieval algorithm incorporating these improvements could provide AOT accuracy unprecedented for a spaceborne technique.