Large contribution of meteorological factors to inter-decadal changes in regional aerosol optical depth

Abstract

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.

Validation of MODIS C6.1 and MERRA-2 AOD Using AERONET Observations: A Comparative Study over Turkey

“…Figure 2 shows also that MODIS and MERRA-2 somewhat overestimate and underestimate AOD, respectively, with respect to AERONET AOD when the full period is taken into consideration. The underestimation in AOD by MEERA-2 is in line with previous studies that investigated AOD globally (Che et al [2]; Shi et al [47]; Buchard et al [48]). However, with a focus on storm episodes of mineral dust over four sunphotometer ground-based sites in Australia, Mukkavilli et al [49] found overestimation in MEERA-2 AOD.…”

“…Anthropogenic aerosols influence the Earth's radiation budget and climate directly by scattering solar light in the cloud-free atmosphere and indirectly by changing the cloud properties [1,2]. Anthropogenic aerosols on the whole cool the climate, while black carbon and mineral dust absorb solar radiation.…”

Long‐Term Trends of High Aerosol Pollution Events and Their Climatic Impacts in North America Using Multiple Satellite Retrievals and Modern‐Era Retrospective Analysis for Research and Applications version 2

“…Overall, the spatial patterns and seasonal variations of AOD trends in this study are consistent with previous studies of AOD evolution (e.g., Che et al, 2019) and suggest a relatively high degree of consistency in AOD trends between the three data sets. The results thus provide confidence in use of MERRA-2 to examine causes and consequences of these trends.…”

Can MERRA-2 Reanalysis Data Reproduce the Three-Dimensional Evolution Characteristics of a Typical Dust Process in East Asia? A Case Study of the Dust Event in May 2017

“…The time series of Multi-angle Imaging SpectroRadiometer (MISR) images shows that the Gobi Desert accounts for >75% of the dust emission events in east Asia [13]. As a result of the long-distance transport of dust particles in the upper atmosphere, dust particles can change the atmospheric conditions at a regional or even global scale [14][15][16]. Dust aerosols from the Taklimakan and Gobi deserts can travel thousands of miles, affecting large areas of China [9,[17][18][19], Korea and Japan [20], and even the north Pacific and North America [21,22].…”