AEROCOM/AEROSAT AAOT & SSA study, part I: evaluation and intercomparison of satellite measurements

Abstract

Abstract. Global measurements of absorptive aerosol optical depth (AAOD) are scarce and mostly provided by the ground network AERONET (AErosol RObotic NETwork). In recent years, several satellite products of AAOD have appeared. This study's primary aim is to establish the usefulness of these datasets for AEROCOM (AEROsol Comparisons between Observations and Models) model evaluation with a focus on the years 2006, 2008 and 2010. The satellite products are super-observations consisting of 1° × 1° × 30min aggregated retrievals. This study consist of two parts: 1) an assessment of satellite datasets; 2) their application to the evaluation of AEROCOM models. The current paper describes the first part and details an evaluation with AERONET observations from the sparse AERONET network as well as a global intercomparison of satellite datasets, with a focus on how minimum AOD (Aerosol Optical Depth) thresholds and temporal averaging may improve agreement. All satellite datasets are shown to have reasonable skill for AAOD (3 out of 4 datasets show correlations with AERONET in excess of 0.6) but less skill for SSA (Single Scattering Albedo; only 1 out of 4 datasets shows correlations with AERONET in excess of 0.6). In comparison, satellite AOD shows correlations from 0.72 to 0.88 against the same AERONET dataset. We do show that performance vs. AERONET and satellite agreements for SSA significantly improve at higher AOD. Temporal averaging also improves agreements between satellite datasets. Nevertheless multi-annual averages still show systematic differences, even at high AOD. In particular, we show that two POLDER products appear to have a systematic SSA difference over land of about 0.04, independent of AOD. Identifying the cause of this bias offers the possibility of substantially improving current datasets. We also provide evidence that suggests that evaluation with AERONET observations leads to an underestimate of true biases in satellite SSA. In the second part of this study we show that, notwithstanding these biases in satellite AAOD and SSA, the datasets allow meaningful evaluation of AEROCOM models.

Reducing Aerosol Forcing Uncertainty by Combining Models With Satellite and Within‐The‐Atmosphere Observations: A Three‐Way Street

“…Polarized, multi-spectral multi-angular measurements improve retrieval sensitivity to the real part of the particle refractive index and to the width of the particle size distribution, as well as broadening the range of conditions under which meaningful particletype constraints can be obtained (e.g., Mishchenko & Travis, 1997;Dubovik et al, 2011;Hasekamp et al, 2011). Despite this, systematic differences in aerosol absorption derived from different algorithms applied to the same polarimetric sensor still exceed the requirements for adequate constraints on aerosol forcing (Schutgens et al, 2021). Although the deployment of new polarimeters with increased polarimetric accuracy (e.g., Werdell et al, 2019) should reduce these discrepancies to some degree, algorithmic assumptions will still be required (e.g., when layers containing different aerosol types occur in the atmospheric column); as with all new measurements, the quality of the results remains to be demonstrated.…”