Abstract
Abstract. In addition to the standard resolution product (10 km), the MODerate resolution Imaging Spectroradiometer (MODIS) Collection 6 (C006) data release included a higher resolution (3 km). Other than accommodations for the two different resolutions, the 10 and 3 km Dark Target (DT) algorithms are basically the same. In this study, we perform global validation of the higher-resolution aerosol optical depth (AOD) over global land by comparing against AErosol RObotic NETwork (AERONET) measurements. The MODIS–AERONET collocated data sets consist of 161 410 high-confidence AOD pairs from 2000 to 2015 for Terra MODIS and 2003 to 2015 for Aqua MODIS. We find that 62.5 and 68.4 % of AODs retrieved from Terra MODIS and Aqua MODIS, respectively, fall within previously published expected error bounds of ±(0.05 + 0.2 × AOD), with a high correlation (R= 0.87). The scatter is not random, but exhibits a mean positive bias of ∼ 0.06 for Terra and ∼ 0.03 for Aqua. These biases for the 3 km product are approximately 0.03 larger than the biases found in similar validations of the 10 km product. The validation results for the 3 km product did not have a relationship to aerosol loading (i.e., true AOD), but did exhibit dependence on quality flags, region, viewing geometry, and aerosol spatial variability. Time series of global MODIS–AERONET differences show that validation is not static, but has changed over the course of both sensors' lifetimes, with Terra MODIS showing more change over time. The likely cause of the change of validation over time is sensor degradation, but changes in the distribution of AERONET stations and differences in the global aerosol system itself could be contributing to the temporal variability of validation.
Highlights
The MODerate resolution Imaging Spectroradiometer (MODIS) sensors, on board the Earth Observing System (EOS) Terra and Aqua satellites, have been providing observations of Earth and the atmosphere for almost two decades (Salomonson et al, 1989)
The Dark Target (DT) products were designed with climate applications in mind and have been used to address a wide variety of geophysical science questions including the role of aerosols in climate-relevant processes (Kaufman et al, 2002; Christopher and Zhang, 2002; Yu et al, 2006), cloud and precipitation modifications (Koren et al, 2009, 2012; Yuan et al, 2011; Oreopoulos et al, 2016), and long-range transport of aerosols (Kaufman et al, 2005; Yu et al, 2012)
Those limited results suggested that expected error bounds should be broadened to the values seen in Eq (2) from those derived for the 10 km product (EE = ±[0.05 + 0.15 × AErosol RObotic NETwork (AERONET) aerosol optical depth (AOD)])
Summary
The MODerate resolution Imaging Spectroradiometer (MODIS) sensors, on board the Earth Observing System (EOS) Terra and Aqua satellites, have been providing observations of Earth and the atmosphere for almost two decades (Salomonson et al, 1989). Before becoming operational, Remer et al (2013) tested the algorithm by comparing 6 months of global 3 km retrievals from Aqua MODIS against available ground truth, while other independent studies (Munchak et al, 2013; Nichol and Bilal, 2016; He et al, 2017 and others) have done subsequent evaluation of the product regionally and locally. These limited comparisons suggested that the new AOD product would be sufficiently accurate to provide useful information and new perspective to the aerosol community. We present here the results of an analysis of a comparison of the global long-term MODIS 3 km product with collocated AErosol RObotic NETwork (AERONET) (Holben et al, 1998) observations
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