Large contribution of meteorological factors to inter-decadal changes in regional aerosol optical depth
Authors: Che Huizheng , Gui Ke , Xia Xiangao , Wang Yaqiang , Holben Brent N. , Goloub Philippe , Cuevas-Agulló Emilio , Wang Hong , Zheng Yu , Zhao Hujia , Zhang Xiaoye 
Published: August 2019

Abstract. Aerosol optical depth (AOD) has become a crucial metric for assessing global
climate change. Although global and regional AOD trends have been studied
extensively, it remains unclear what factors are driving the inter-decadal
variations in regional AOD and how to quantify the relative contribution of
each dominant factor. This study used a long-term (1980–2016) aerosol
dataset from the Modern-Era Retrospective Analysis for Research and
Applications, version 2 (MERRA-2) reanalysis, along with two satellite-based
AOD datasets (MODIS/Terra and MISR) from 2001 to 2016, to investigate the
long-term trends in global and regional aerosol loading. Statistical models
based on emission factors and meteorological parameters were developed to
identify the main factors driving the inter-decadal changes of regional AOD
and to quantify their contribution. Evaluation of the MERRA-2 AOD with the
ground-based measurements of AERONET indicated significant spatial agreement
on the global scale (r= 0.85, root-mean-square error = 0.12, mean fractional error = 38.7 %, fractional gross error = 9.86 % and index of agreement = 0.94). However, when AOD observations from the China
Aerosol Remote Sensing Network (CARSNET) were employed for independent
verification, the results showed that MERRA-2 AODs generally underestimated
CARSNET AODs in China (relative mean
bias = 0.72 and fractional gross error =-34.3 %). In general,
MERRA-2 was able to quantitatively reproduce the annual and seasonal AOD
trends on both regional and global scales, as observed by MODIS/Terra,
although some differences were found when compared to MISR. Over the 37-year
period in this study, significant decreasing trends were observed over
Europe and the eastern United States. In contrast, eastern China and southern
Asia showed AOD increases, but the increasing trend of the former reversed
sharply in the most recent decade. The statistical analyses suggested that
the meteorological parameters explained a larger proportion of the AOD
variability (20.4 %–72.8 %) over almost all regions of interest (ROIs) during 1980–2014 when compared with emission factors (0 %–56 %). Further analysis also showed that SO2 was the dominant emission factor, explaining 12.7 %–32.6 % of the variation in AOD over anthropogenic-aerosol-dominant regions, while black carbon or organic carbon was the leading factor over the biomass-burning-dominant (BBD) regions, contributing 24.0 %–27.7 % of the variation. Additionally, wind speed was found to be the leading meteorological parameter, explaining 11.8 %–30.3 % of the variance over the mineral-dust-dominant regions, while ambient humidity (including soil moisture and relative humidity) was the top meteorological parameter over the BBD regions, accounting for 11.7 %–35.5 % of the variation. The results of this study indicate that the variation in meteorological parameters is a key factor in determining the inter-decadal change in regional AOD.