Global evaluation of the Collection 5 MODIS dark-target aerosol products over land

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 AERONETobserved 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 Angström 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.

Improving retrieval accuracy for aerosol optical depth by fusion of MODIS and CALIOP data

“…This rule shows MODIS underestimates AODs in light aerosol loadings and permits negative values. It coincides with Levy's analysis [15].…”

“…It is a common standard used in AOD validation as shown in many studies [11,13,15]. Furthermore, an accurate retrieval is defined by an instrument AOD retrieval that falls within the EE envelope,…”

“…Levy et al compared MODIS C005 AODs over dark land with AERONET collocations at 550 nm wavelength, the result shows RMSE=0.116, R 2 =0.778 (corresponding to correlation coefficient R=0.882) [15]. But their collocation points only cover dark targets.…”

“…Under other conditions, however, they show systematic deviation. For example, under the condition of low aerosol loading, the retrieve of AOD is more accurate in the eastern United States, Western Europe and Southern Africa sites (because their spectrum is closer to the middle "dark" and moderate "green" regimes); whereas the AOD retrieval is often overestimated by MODIS in western United States and Central Asia's sites -because of its relatively bright surface and less greenness, MODIS overestimates the AOD [15]. In another example, Jethva et al found that the errors in the fine-dominated AOD over the Indian region were large for most AOD retrievals in the C005 data [16].…”

Retrieval of aerosol optical depth over land based on a time series technique using MSG/SEVIRI data

“…It relies primarily on the use of low reflectance pixels or dark targets with low reflectance (Kaufman, 1993;Kaufman et al, 1990;Kaufman and Sendra, 1988) and requires prior knowledge of accurate ground surface reflectance (King et al, 1992). One major limitation of the MODIS DDV approach is that when the surface reflectance at 2.1 µm is above 0.15, no retrievals are performed and the assumption of transparency in this channel is not considered valid (Kaufman et al, 1997;Hsu et al, 2004;Remer et al, 2005;Levy et al, 2010). For the retrieval over bright surfaces such as arid, semi-arid and urban areas, Hsu et al (2004) developed the Deep Blue algorithm which uses the fact that the surface is much darker in the blue spectral region than that at longer wavelengths.…”