Impact of MODIS-derived cloud cover on aerosol optical depth measured with a sun photometer: a case study from Nanjing, China

Sun photometers have been used increasingly to monitor the atmospheric environment by measuring indicators such as aerosol optical depth AOD. However, ground-measured AOD results are subject to the presence of clouds in the air. When cloud cover is not extensive, it is still possible to use sun photometry to determine AOD, even though accuracy is reduced by cloud contamination. This research aims to detect cloud cover from Moderate Resolution Imaging Spectroradiometer MODIS data and then assess its impact on in situ-measured AOD. Normalized difference cloud index NDCI and linear spectral unmixing LSU were used to detect cloud cover from MODIS data. AOD at the time of acquisition of MODIS data was measured on the ground by sun photometry within 20 min of satellite overpasses 10 min before and 10 min after. Correlation analysis of NDCI-and LSU-derived cloud cover with in situ-measured AOD data demonstrates that LSU has a higher correlation coefficient with AOD than with NDCI. At 550 nm, a unit of cloud cover e.g. 1% raises ground-observed AOD by 0.0157. The findings of this study can be used to modify ground-derived AOD results to improve their reliability.