The quality of the aerosol optical thickness (AOT) data retrieved operationally from MODIS
sensors aboard the Terra and Aqua satellites, over land and ocean, from 2000 to 2003 (Aqua only
from June 2002), were evaluated thoroughly, and the utility and synergisms of the two sensors in
retrieving aerosols for global, regional, and local applications were examined. Because of
periodic updates of the MODIS aerosol algorithm, a series of data versions have been produced
and distributed, and those currently in wide circulation are Terra-MODIS versions 3 and 4 data
(T003 and T004) and Aqua-MODIS version 3 data (A003); though the algorithm version used to
retrieve T004 and A003 was almost the same. These three data sets (T003, T004, and A003)
were evaluated independently and comparatively with collocated AOT from ground-based
AERONET sun photometers. The analysis shows that at 550 nm wavelength, 67.3%, 55.0%, and
55.5% of AOT from T003, T004, and A003, respectively, meet the pre-specified accuracy
conditions of ±(0.05 + 0.2aot) over land, while about 63.3%, 59.4%, and 62.2%, respectively,
fall within the more stringent range of ±(0.03 + 0.05aot) over ocean. However, when based on
equal standards of comparison and regression analysis, aerosol retrievals are much more accurate
over ocean than over land. Both MODIS and AERONET AOTs at 550 nm collocated over
AERONET stations were grouped into three aerosol size modes based on AERONET Angstrom
exponent value ranges, and time series of their monthly averages at Terra overpass times show
that there is a net increase in the monthly average loading of the large size mode aerosols from
2000 to 2003, especially over ocean. Note that this trend was based only on data over
AERONET sites, and does not represent the full global statistics. Analysis of MODIS full
regional AOT averages from 12 land and 6 oceanic regions, shows that aerosol loading exhibits
an annual cycle in almost every region, with the exception of very remote oceanic regions such
as the Central Pacific. On the basis of regional monthly averages, West Africa, China, and India
show the highest peak monthly mean AOT value of ~0.7 at 550 nm, while the highest over-ocean
aerosol loading occurs over the Mediterranean and Mid-Atlantic oceans, with a regional monthly
peak of ~0.35, which is half of the peak over land. The magnitude of day-to-day variation
between morning (Terra) and afternoon (Aqua) AOT varies from region to region, and increases
with aerosol loading for any given region. However, none of the regions examined shows any
consistent regional trend in morning-to-afternoon aerosol loading; all showing almost equal
likelihoods of increase or decrease from morning to afternoon.