Evaluation and Comparison of MODIS Collection 6.1 and Collection 6 Dark Target Aerosol Optical Depth over Mainland China Under Various Conditions Including Spatiotemporal Distribution, Haze Effects, and Underlying Surface.

Modifications such as degrading the retrieval quality of mixed pixels in the coastline area and revising surface characterization scheme have been made to the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6.1 (C6.1) Dark Target (DT) aerosol optical depth (AOD) product to address inaccuracy over urban areas. In this study, comprehensive evaluations of modifications to the MODIS C6.1 DT AOD product, in comparison with Collection 6 (C6), are conducted over mainland China under different spatial distributions, seasons, and air quality for the period 2010–2017, combined with validation against Aerosol Robotic Network (AERONET) AOD measurements. The preliminary result showed that the C6.1 DT AOD product in China displayed an overall good performance with high R2 (0.87) and low root‐mean‐square error (0.23) against ground measurements. Moreover, the C6.1 DT AOD product was an overall improvement over C6, with greater correlation and lower uncertainty against ground measurements, especially for the North China Plain and Central China, although this was not the case for Western China. The improvement was also seasonal, being distinct in spring but less pronounced in winter, and negatively correlated with the level of air pollution. Furthermore, the analysis of DT AOD retrievals in different urbanized areas illustrated that the updated DT algorithm worked well in completely urbanized areas, where 97% of C6.1 DT AOD retrievals were an improvement over C6, while approximately 10% of C6.1 DT AOD retrievals were deteriorated in semiurbanized areas. Additionally, the DT AOD retrievals in areas with relatively low enhanced vegetation index and high surface reflectance were significantly improved, mitigating problems associated with the DT algorithm, further improving the reliability of MODIS DT AOD products. Plain Language Summary: Modifications have been made to the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) product to address inaccuracy over urban areas. This study evaluates the MODIS AOD product in comparison with the old version. Specifically, we evaluated products' performance over mainland China under different spatial distributions, seasons, and air quality for the period 2010–2017. It was demonstrated that the updated AOD retrieval algorithm show a visible improvement in highly urbanized areas, as well as areas with relatively low enhanced vegetation index and high surface reflectance. However, because the surface reflectance scheme was trained over the continental United States, it could not effectively describe the complex surfaces in Eastern and Western China, and therefore, further improvement is required in this regard. Key Points: C6.1 DT AOD product is more suitable than C6 for most of seasons and haze period in ChinaThe modified products exhibited differences in regard to the spatial distribution, season variations, and air quality levelThe improvement from C6 to C6.1 is varied along with vegetation index and surface reflectance in urbanized areas [ABSTRACT FROM AUTHOR]