  In this study, we evaluated the performance of the MODIS C6.1 AOD products
and compared them with the C6 products over regions in China with multifarious
underlying surfaces during 2001 to 2016. The AOD retrievals were validated
against 20 AERONET sites, and the results show that the R for DT retrievals in
C6.1 is 0.946, while the fraction within the EE can be considered relatively
low at only 54.03%. DB retrievals in C6.1 have a slightly lower R value, but
the other criteria are superior to DT. Comparing the results over urban and
vegetation areas in C6.1, the overall quality of the DB retrievals is better
than the DT retrievals in urban areas. The performance of DT is significantly
superior to DB in the LEV areas. For the HEV areas, DB performs synthetically
better than DT. In the spatial distribution aspect in C6.1, most of the DB AOD
values are less than those of DT, and the relationship between DT and DB varies
with the different land cover types. For the AOD coverage in C6.1, DT
retrievals with high coverage mainly distribute in east-central China. However,
the effects of high surface reflectance lead to low AOD coverage in the
southwest. In contrast, the AOD coverage of DB tends to be high in areas where
the main land cover type is bare soil and tends to be low in areas affected by
snow. In terms of the comparison between C6.1 and C6, the overestimation of DT
over urban areas in C6 is effectively mitigated in C6.1. However, a nearly
systematic decline in DT is discovered in C6.1 as well. With respect to DB,
consistent AOD coverage distribution is observed, with only subtle distinction.
The AOD coverage of DB in C6.1 appears higher than that in C6 in the middle,
south, and northeast of China. The quality of the DB retrievals in C6.1
increases slightly compared to C6, and the most remarkable improvement is
observed for the coarse aerosol particles.
