Popular summary.Aerosols, tiny solid or liquid particles suspended in the atmosphere, were once only a side note in the Atmospheric Sciences. Today we realize the importance of aerosols in instigating or mitigating climate change, in modifling clouds and large-scale precipitation patterns and in affecting human health. Unlike greenhouse gases, which are well-mixed and long-lasting in the atmosphere, aerosols are temporally and spatially variable with lifetimes of a few days to a few weeks. Their transient natures make aerosols difficult to characterize and their effects on climate, hydrology and health difficult to model. Satellites provide the best means to observe the global aerosol system and narrow the uncertainties associated with aerosol characterization, but the satellite observations must be sufficiently accurate to be useful. The MODerate resolution Imaging Spectroradiometer (MODIS) aboard both NASA's Terra and Aqua satellites provides a unique tool to discern the global impact of aerosols. The products derived from MODIS data include aerosol optical thickness, which is a measure of aerosol amount, as well as products that describe the size of the aerosol particles. The MODIS aerosol retrievals are continuously evaluated against ground-truth of an existing global network of highly accurate instruments (AERONET). The results show an accuracy for the MODIS aerosol products that will sufficiently narrow the uncertainty of global aerosol characterization. Furthermore, the MODIS derivation of aerosol particle size aids in discriminating between man-made aerosol and naturally produced aerosols. This is a major step forward in narrowing the uncertainties associated with estimating the total anthropogenic effect on climate.

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.

[1] The recently released Collection 5 Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol products provide a consistent record of the Earth's aerosol system. Comparing with ground-based AERONET observations of aerosol optical depth (AOD) we find that Collection 5 MODIS aerosol products estimate AOD to within expected accuracy more than 60% of the time over ocean and more than 72% of the time over land. This is similar to previous results for ocean and better than the previous results for land. However, the new collection introduces a 0.015 offset between the Terra and Aqua global mean AOD over ocean, where none existed previously. Aqua conforms to previous values and expectations while Terra is higher than what had been expected. The cause of the offset is unknown, but changes to calibration are a possible explanation. Even though Terra's higher ocean AOD is unexpected and unexplained, we present climatological analyses of data from both sensors. We find that the multiannual global mean AOD at 550 nm over oceans is 0.13 for Aqua and 0.14 for Terra, and over land it is 0.19 in both Aqua and Terra. AOD in situations with 80% cloud fraction are twice the global mean values, although such situations occur only 2% of the time over ocean and less than 1% of the time over land. Aerosol particle size associated with these very cloudy situations does not show a drastic change over ocean, but does over land. Regionally, aerosol amounts vary from polluted areas such as east Asia and India, to the cleanest regions such as Australia and the northern continents. As AOD increases over maritime background conditions, fine mode aerosol dominates over dust over all oceans, except over the tropical Atlantic downwind of the Sahara and during some months over the Arabian Sea.