Abstract
Abstract. Global measurements of absorbing 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 been developed. 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∘×30 min aggregated retrievals. This study consists of two papers, the current one that deals with the assessment of satellite observations and a second paper (Schutgens et al., 2021) that deals with the evaluation of models using those satellite data. In particular, the current paper 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 between satellite observations. All satellite datasets are shown to have reasonable skill for AAOD (three out of four datasets show correlations with AERONET in excess of 0.6) but less skill for SSA (single-scattering albedo; only one out of four 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. However, we show that performance vs. AERONET and inter-satellite agreements for SSA improve significantly 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 (Polarization and Directionality of the Earth's Reflectances) products appear to have a systematic SSA difference over land of ∼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.
Highlights
Aerosol is an important component of the Earth’s atmosphere that affects the planet’s climate, the biosphere and human health
We suggest that it is rather unlikely that three different satellite products have an SSA bias at low AERONET DirectSun (AOD) that is similar to AERONET and that this low bias in POLDERGRASP-M analysis is real
We evaluate several remote sensing datasets of absorbing aerosol optical depth (AAOD) and SSA from a variety of sensors (CALIOP on CALIPSO, Ozone Monitoring Instrument (OMI) on Aura, POLDER on PARASOL), in preparation of an AEROCOM model evaluation
Summary
Aerosol is an important component of the Earth’s atmosphere that affects the planet’s climate, the biosphere and human health. AERONET (AErosol RObotic NETwork, Holben et al, 1998) is a global but spatially sparse network of Sun photometers that includes two scanning protocols (almucantar and hybrid) that allow the inversion of measured radiances into particle size distributions and refractive indices (Dubovik and King, 2000). From this inversion, columnar AAOD (absorbing aerosol optical depth) can be derived. This yields spatially sparse in situ observations of absorbing aerosol While these measurement networks have proven to be very important to our understanding of absorbing aerosol, a satellite-derived AAOD would contribute greatly by adding spatial context in regions with ground-based instruments and measurements in regions without such instruments.
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