# Spatiotemporal Variation in Absorption Aerosol Optical Depth over China
## 4 Conclusions  
The detailed characteristics of the atmospheric OMI AAOD as well as the AAE in China over 2005 to 2018 were investigated based on the OMI dataset. The OMI AAOD\({}_{500}\) generally portrayed a good correlation (\(\sim\)0.55) with ground-based AERONET AAOD\({}_{500}\) at  
Figure 12: The annual mean and variances of AAE\({}_{388-500}\) on episode and non-episode days in each city in six regions during 2017 (**a**), and 2018 (**b**). The error bar represents standard deviation.  
four typical stations around China. During the study period, the annual mean OMI AAOD bespeaks multifarious spatial distributions across China with a mean value of 0.046 for AAOD\({}_{388}\) and 0.022 for AAOD\({}_{500}\) and it shows a slight fluctuation during 2005-2015, and a relatively fast increase after that. The high-value center of the OMI AAOD is predominantly scattered over the northwestern TB region, the BTH region and the surrounding northern part of Henan province, and the SB in China.  
The annual average AAOD\({}_{500}\) during 2005-2018 follows the order of the TB (0.041) > the YRB (0.023) > BT (0.026) > the SB (0.023) > NS (0.021) > the PRD (0.017). Our results indicated that the AAOD\({}_{388 nm}\) and AAOD\({}_{500 nm}\) increases by a factor of approximately 1.5, especially in the northwest by 2 times. The situation of absorptive aerosol pollution became severe in the recent years of 2016-2018. The AAE\({}_{388-500}\) depicts the opposite trend except for the TB with a high value of 3.058. The winter or spring seasons have a higher mean AAOD, followed by autumn, whilst the summer displays a relatively weaker value. The monthly average AAOD\({}_{388}\) in the TB, SB, YRB, BT, PRD, and NS regions shows the smallest values of 0.072, 0.024, 0.026, 0.027 in July, 0.024 in June, and 0.025 in September, respectively, whereas the peak events of AAOD\({}_{388}\) occur in January for NS (0.055), the YRB (0.077) and BT (0.067), April for the TB (0.123), February for the SB (0.073), and March for the PRD (0.075), respectively. The monthly average AAEs exhibit the smallest value in December and January and largest in June. The daily AAE for episode days is generally lower than those in non-episode days. Sandstorm weather is more frequent in March, April, and May, causing high concentrations of dust aerosols and AAOD over the TB in spring. In addition, the meteorological conditions in winter and early spring associated with more energy consumption are conducive to the accumulation of high BC aerosol pollution, causing high alert levels for the AAOD from November to the following March. Future research will be conducted to further gain a more detailed understanding of the relationship between chemical composition, sources of aerosol absorption and AAOD values, as well as the need for more data to analyze the relationship.  
Conceptualization, M.M.; methodology, H.J.; software, H.J.; validation, H.J.; formal analysis, H.J.; investigation, H.J.; resources, H.J.; data curation, H.J.; writing--original draft preparation, M.M.; writing--review and editing, M.M.; visualization, X.Z.; supervision, X.Z.; project administration, X.Z.; funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
Analyses and visualizations used in this paper were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. We also acknowledge the mission scientists and Principal Investigators who provided the data used in this research effort.  
The authors declare no conflicts of interest.