Observationally-constrained estimates of global fine-mode AOD
Small aerosols are mostly anthropogenic, and the fine-mode aerosol optical depth (fAOD) can be used to infer anthropogenic aerosol amounts. We estimate AOD and fAOD globally on a monthly time scale from 2001 to 2010 by integrating monthly satellite-based (MODIS and MISR) and ground-based (AERONET) observations. For fAOD, three integration methods are developed to utilize global coverage by satellite data and maximize the influence of AERONET data. We evaluate each method by applying the method without a few randomly-chosen AERONET data and comparing its output with the few AERONET data. The best method of the three is based on integrating the Ångström exponent (AE) data from MODIS, MISR and AERONET, and its output is closer to AERONET fAOD accuracy than MODIS or MISR fAOD over both ocean and land.
Using our integrated data, we find that the global 2001–2010 average of 500 nm AOD and fAOD is 0.17 and 0.089, respectively. Eastern China as a region shows the largest decadal-mean fAOD. The linear trend of global AOD or global fAOD from 2001 to 2010 is found to be slightly negative. This decreasing trend is particularly pronounced over the West (Western Europe and US/Canada combined) where AOD and fAOD reductions are about −20%. By contrast, fAOD in India and eastern China combined increased slightly. These results reflect the overall anthropogenic aerosol emission reduction in the West, and stagnating conditions in Asia. Moreover, our results in the West are consistent with the so-called surface brightening phenomenon in the recent decades.