# Spatiotemporal Variation in Absorption Aerosol Optical Depth over China
## 3 Results and Discussion
### Spatial and Annual Variation in OMI AAOD  
Figure 4 presents the spatial distributions of the annual averaged OMI AAOD\({}_{388}\) and AAOD\({}_{500}\) in China during 2005-2018. [PERSON] reported that the AAOD over East Asia is predominantly contributed to by dust and BC, whilst other aerosols make a small fraction of the contributions to the AAOD [54]. According to the measurement results of East Asian tropospheric aerosol motion research, [PERSON] et al. estimated the mass absorption efficiencies of dust and BC at 550 nm to be 0.03 and 9.5 m\({}^{2}\) g\({}^{-1}\), respectively [55]. The highest AAOD generally occurs in the vast but sparsely populated northwest region with AAOD\({}_{388}\) above 0.85. Apparently, compared with other locations in China, aerosol absorption in northwest China has a greater contribution towards the total aerosol presence. From the inter-annual changes, the AAOD values in the economically developed and densely populated Beijing-Tianjin-Hebei (BTH) region, northern Henan, and the Sichuan Basin (SB) are also significantly higher, which may be due to regional emission sources and specific local features mixed with meteorological influence. Figure 4: Distribution of AAOD\({}_{388}\) and AAOD\({}_{500}\) over China from 2005 to 2018. To explore the variations in the absorption of aerosols over China on a broader temporal scale, Figure 5 shows the annual variation in area-average BC (a) and dust (b) surface mass concentration, and BC (c) and dust (d) column mass density, over the study area in China, the Tarim Basin (TB), and Beijing and Tianjin (BT) over 2005-2018 dependent on MERRA-2 data [35]. Generally speaking, the BC surface mass concentration and column mass density began to rapidly increase after 2005 and reached their maximum in 2007, and then slowly decreased (Figure 5a,c). The phenomenon may be primarily associated with anthropogenic emissions. With the rapid industrial development and urbanization, China's economy is accelerating in growth, and energy consumption such as coal and oil is also increasing. Subsequently, the government has adopted positive measures to control and prevent air pollution. The BC concentrations display a certain downward trend, but held firm at a relatively high level between 2008 and 2018, indicating the need for long-term implementation of BC emission-reduction policies. BC was mostly concentrated in the industrial and agricultural regions as a result of incomplete combustion of fossil fuels and biomass burning. As shown in Figure 5, BT is located in northern China with relatively high BC concentrations, and the TB is located in remote north-western area of China with low BC concentrations. As for dust surface mass concentration and column mass density in China (Figure 5b,d), that in the TB is much higher than that in BT and the national average. The peaks appeared in 2018 with annual average values of 373 \(\upmu\)g/m\({}^{3}\) and 6.2 \(\times\) 10\({}^{5}\)\(\upmu\)g/m\({}^{2}\), respectively, revealing an extensively analogous tendency to the AAOD. The AAODs achieved in the TB are larger than those over BT, with dust stemming from the Taklimakan Desert contributing the most to the total AAOD. As shown in Figure 6, the annual distribution provides an overview of the OMI AAOD over China. The averaged AAOD\({}_{388}\) and AAOD\({}_{500}\) in China are 0.046 \(\pm\) 0.018 and 0.022 \(\pm\) 0.009 during 2005-2018, respectively. The highest annual-averaged AAOD\({}_{388}\) and AAOD\({}_{500}\) was observed to be 0.055 and 0.026 in 2018. AAOD pollution was the lowest over China in 2008 with 0.043 for AAOD\({}_{388}\) and 0.021 for AAOD\({}_{500}\). The absorptive aerosol  
Figure 5: Annual variation in area-average BC surface mass concentration (**a**), dust surface mass concentration (**b**), BC column mass density (**c**), and dust column mass density (**d**) over the study area in China, the TB, and BT from 2005 to 2018.  
pollution became severe in the recent years of 2016-2018.