Long-term distribution and evolution trends of absorption aerosol optical depth with different chemical components in global and typical regions

Different types of atmospheric aerosols have different climatic effects. In this study, MERRA-2 reanalysis data of absorption aerosol optical depth (AAOD) products at 550 nm from 1980 to 2018 were used to analyse the long-term distribution characteristics and evolution trends of the AAOD of different chemical components globally and in 12 typical study areas. We also analysed the seasonal and interannual monthly variations of the different chemical components of AAOD. In the 40-year study period from 1980 to 2018, the maximum value of total AAOD (TAAOD) appears in the southern regions of SD (Sahara Desert), CSA (Central Southern Africa), NC (Northern China), SC (Southern China), and SEA (Southeastern Asia) (> 0.040). The highest value of dust AAOD (DUAAOD) is in SD (0.030-0.040), and the contribution rate reaches 80 %; while in SC, SEA, and AMZ, black carbon AAOD (BCAAOD) contributes 80 %-90 %. The high-value area of DUAAOD in SD-ME-NWC expands in spring, and the dust belt formed in summer results in a larger DUAAOD (> 0.050). The proportion of BCAAOD in autumn and winter is larger in the dust belt, which is another major contributor to AAOD in this region. The monthly distributions of TAAOD in SEA, CSA, NC, and AMZ are mainly affected by biomass combustion, while the DU in ME (Middle East), NWC (Northwestern China), and SD has a greater effect on AAOD, and the TAAOD in NEA (Northeastern Asia), WEU (Western Europe), EUS (Eastern United States), SC, SA (Southern Asia), and other regions is mainly affected by both DU and BC + OC (in which OC refers to organic carbon). The interannual variations of BCAAOD and OCAAOD tend to be flat before 2000, and then show an increasing trend. BCAAOD has the largest relative contribution (at about 60 %), followed by DUAAOD (at about 30 %), and then OCAAOD has the smallest contribution (at less than 10 %). From a global perspective, AAOD shows different increasing trends during 1980-2018, 1980-1992, and 1993-2005, and decreases or even completely reverses during 2006-2018. This paper provides the distribution characteristics and evolutionary trends of different chemical components of AAOD, which can improve scientific understanding of global- and regional-scale aerosols and their climatic effects.