Observationally-constrained estimates of global small-mode AOD

Abstract. Small aerosols are mostly anthropogenic, and an area average of the small-mode aerosol optical depth (sAOD) is a powerful and independent measure of anthropogenic aerosol emission. We estimate AOD and sAOD globally on a monthly time scale from 2001 to 2010 by integrating satellite-based (MODIS and MISR) and ground-based (AERONET) observations. For sAOD, three integration methods were developed to maximize the influence of AERONET data and ensure consistency between MODIS, MISR and AERONET sAOD data. We evaluated each method by applying the technique with fewer AERONET data and comparing its output with the unused AERONET data. The best performing method gives an overall error of 13 ± 2%, compared with an overall error of 62% in simply using MISR sAOD, and this method takes advantage of an empirical relationship between the Ångström exponent (AE) and fine mode fraction (FMF). This relationship is obtained by analyzing AERONET data. Using our integrated data, we find that the global 2001–2010 average of 500 nm AOD and sAOD is 0.17 and 0.094, respectively. sAOD over eastern China is several times as large as the global average. The linear trend from 2001 to 2010 is found to be slightly negative in global AOD or global sAOD. In India and eastern China combined, however, sAOD increased by more than 4% against a backdrop of decreasing AOD and large-mode AOD. On the contrary to India and China, the west (Western Europe and US/Canada combined) is found to have a sAOD reduction of −20%. These results quantify the overall anthropogenic aerosol emission reduction in the west, and rapidly deteriorating conditions in Asia. Moreover, our results in the west are consistent with the so-called surface brightening phenomenon in the recent decades.