Abstract
Abstract. NASA's MODIS sensors have been observing the Earth from polar orbit, from Terra since early 2000 and from Aqua since mid 2002. We have applied a consistent retrieval and processing algorithm to both sensors to derive the Collection 5 (C005) dark-target aerosol products over land. Here, we validate the MODIS along-orbit Level 2 products by comparing to quality assured Level 2 AERONET sunphotometer measurements at over 300 sites. From 85 463 collocations, representing mutually cloud-free conditions, we find that >66% (one standard deviation) of MODIS-retrieved aerosol optical depth (AOD) values compare to AERONET-observed values within an expected error (EE) envelope of ±(0.05 + 15%), with high correlation (R = 0.9). Thus, the MODIS AOD product is validated and quantitative. However, even though we can define EEs for MODIS-reported Ångström exponent and fine AOD over land, these products do not have similar physical validity. Although validated globally, MODIS-retrieved AOD does not fall within the EE envelope everywhere. We characterize some of the residual biases that are related to specific aerosol conditions, observation geometry, and/or surface properties, and relate them to situations where particular MODIS algorithm assumptions are violated. Both Terra's and Aqua's–retrieved AOD are similarly comparable to AERONET, however, Terra's global AOD bias changes with time, overestimating (by ~0.005) before 2004, and underestimating by similar magnitude after. This suggests how small calibration uncertainties of <2% can lead to spurious conclusions about long-term aerosol trends.
Highlights
As aerosols are major components in Earth’s global climate system, their global distribution must be characterized in order to understand their impacts
We see that the quality of the MODIS/AERONET comparison is strongly dependent on QA Confidence (QAC)
The relatively bright surface conditions would lead to overestimation of aerosol optical depth (AOD). We find it interesting that the AOD at Bonanza Creek (64◦ N, 148◦ W) is severely overestimated in polluted conditions, which suggests that our assumed aerosol model (SSA ∼ 0.91) is too absorbing to represent the dense smoke (SSA ∼ 0.97) observed at the site (Eck et al, 2009), possibly due in part to significant burning of peat fuels in the region
Summary
As aerosols are major components in Earth’s global climate system, their global distribution must be characterized in order to understand their impacts. Derived products include the fine-model AOD (τf or fAOD) and Angstrom exponent (α) defined by using AOD values at 0.47 and 0.65 μm The union of these AOD and size parameters, plus diagnostic parameters and retrieval Quality Assurance (QA), comprise the set of Level 2 (L2) aerosol products. The C004 MODIS-derived aerosol products were compared to global sunphotometer data, and 68% fell within an EE envelope of ±(0.05 + 20%) on a global scale (Remer et al, 2005) This and other studies (e.g., Levy et al, 2005) demonstrated that there were locations and conditions where the C004 errors were systematically larger. We compare the MODIS-derived aerosol products with measurements by ground-based sunphotometers, for spectral AOD, and for aerosol size parameters (including Angstrom exponent and fine AOD) in Sect.
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