Can MODIS Detect Trends in Aerosol Optical Depth over Land

The Moderate Resolution Imaging Spectroradiometer(MODIS) sensor onboardNASA’s Aqua satellite has been collecting valuable data about the Earth systemfor more than 14 years, and one of the benefits of this is that it has made it possible to detect the long-term variation in aerosol loading across the globe. However, the long-term aerosol optical depth (AOD) trends derived from MODIS need careful validation and assessment, especially over land. Using AOD products with at least 70 months’ worth of measurements collected during 2002–15 at 53 Aerosol Robotic Network ( AERONET) sites over land, Mann–Kendall (MK) trends in AOD were derived and taken as the ground truth data for evaluating the corresponding results from MODIS onboardAqua. The results showed that the AERONETAOD trends over all sites in Europe and North America, as well as most sites in Africa and Asia, can be reproduced by MODIS/Aqua. However, disagreement in AOD trends between MODIS and AERONETwas found at a few sites in Australia and South America. The AOD trends calculated from AERONETinstantaneous data at the MODIS overpass times were consistent with those from AERONETdaily data, which suggests that the AOD trends derived from satellite measurements of 1–2 overpasses may be representative of those from daily measurements.