Seasonal variability and trends in global type-segregated aerosol optical depth as revealed by MISR satellite observations.

This study utilized a long-term (2001-2018) aerosol optical component dataset retrieved from the Multiangle Imaging Spectroradiometer (MISR), Version 23, to perform comprehensive analyses of the global climatology of seasonal AODs, partitioned by aerosol types (including small-size, medium-size, large-size, spherical, and non-spherical). By dividing eight different AOD bins and performing trend analysis, the seasonal variability and trends in these type-segregated AODs, as well as in the frequency occurrences (FOs) for different AOD bins, globally and over 12 regions of interest, were also investigated. In terms of particle size, small-size aerosol particles (diameter < 0.7 μm) contribute the largest to global extinction in all three seasons except winter. A similar globally dominant role is exhibited by spherical aerosols, which contribute 68.5%, 73.3%, 71.6% and 70.2% to the global total AOD (TAOD) in spring, summer, autumn and winter, respectively, on a global average scale. FOs with different aerosol loading levels suggested that the seasonal FOs tend to decrease progressively with increasing aerosol loading, except for Level 1 (TAOD< 0.05). Examination of the seasonal distribution of FOs revealed that the FO at Level 1 (Level 2, 0.05 < TAOD< 0.15) is much larger in summer/winter (winter/autumn) than in spring/autumn (spring/summer) over most areas of the world. However, the FOs for Level 3 (0.15 < TAOD< 0.25) to Level 8 (TAOD> 1.0) generally exhibit greater intensity in spring/summer than in autumn/winter. Temporal trend analyses showed that the seasonal TAOD experiences a significant decline during 2001-2018 in most regions globally, except in South Asia, the Middle East, and North Africa. Opposite seasonal trends in the above regions are closely related to the increase in FOs in the range of 0.4 < TAOD< 1.0. The global average TAOD shows the most pronounced decline in spring, falling by -10.4% (P < 0.05). Examination of the trends in type-segregated AODs further revealed that the decreases in size-segregated (shape-segregated) AODs all contribute to the decrease in seasonal TAOD, with small-size AOD (spherical AOD) contributing most significantly.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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