Variation of the Aerosol Optical Properties and Validation of MODIS AOD Products over the Eastern Edge of the Tibetan Plateau Based on Ground-Based Remote Sensing in 2017

The aerosol change over the Tibetan Plateau (TP) is poorly understood due to the scarcity of aerosol measurements over the TP. The aerosol optical properties were measured with a CE-318 sun photometer from January to December in 2017 at the Litang station (30.00 degrees N, 100.16 degrees E, 3950.5 m a.s.l.) over the eastern edge of the TP to characterize the variations in the aerosol optical properties and assess the applicability of MODIS AOD products at the eastern edge of TP. The analyses show that the annual mean aerosol optical depth at 440 nm (AOD(440nm)) is 0.08 +/- 0.03. The mean annual Angstrom exponent between 440 and 870 can (AE(440-870nm)) is 0.72 +/- 0.23. The monthly-averaged AOD(440nm) exhibits a maximum (0.11 +/- 0.04) in July and minimum (0.05 +/- 0.02) in January, while the monthly mean AE(440-870nm) shows a maximum (1.08 +/- 0.19) in July and minimum (0.48 0.32) in February. The findings based on measured AOD(440nm) and AE(440-870nm) indicate low AOD, dominated by coarse particles in winter and fine particles in summer over the eastern TP edge. Three datasets of MODIS AOD products with 3 km spatial resolution as well as with 10 km spatial resolution based on Dark Target (DT) and Deep Blue (DB) algorithms are validated using the ground -based measurements over the eastern edge of the TP. Although the validations show a low accuracy of MODIS AOD products of the three datasets over this TP region, the MODIS AOD products based on the DB algorithm with 10 km resolution could be used in the atmospheric environment over the eastern edge of the TP based on the best match between AOD data and ground-based measurements, the lowest retrieval error, and the largest percentage falling within the expected error envelope.