“…Zheng, Che, Xia, Wang, Yang, Chen, Wang, Zhao, Li, Zhang, Gui, Yang, Liang and Zhang [15] identified clean continent aerosols with AOD < 0.25 based on MODIS data. Chen, et al [21] used an AOD threshold value of 0.2 to identify clean aerosols over China based on Himawari-8 observations, and the clean aerosol is identical to the background aerosol type. Whether for ground or satellite aerosoltype classification, the background AOD threshold value varies substantially from 0.1 to 0.3 and has not yet been properly defined.…”