# Spatiotemporal Variation in Absorption Aerosol Optical Depth over China
## 3 Results and Discussion
### OMI AAOD over Typical Areas of China
As shown in Figure 9a,b, the AAOD value over the TB is the largest among the six regions, and reaches its peak in 2018 with 0.140 and 0.065 for AAOD values at 388 and 500 nm, respectively. Additionally, NS, the YRB and BT all reach the yearly mean maximum in 2017 with 0.062, 0.077, and 0.066 for AAOD\({}_{388}\), and 0.031, 0.040 and 0.033 for AAOD\({}_{500}\), respectively. The AAOD value of the PRD is generally the lowest in the six regions, and reaches the smallest annual average in 2008. Apart from the PRD, the TB, the SB and the YRB values also descend to a lowest point in 2008, and then demonstrate an uptick again in recent years. The phenomenon may be mainly related to anthropogenic activities and natural changes. Along with rapid industrial development and urbanization, China has witnessed a fast-growing economy as well as energy consumption. Subsequently, as a coping mechanism against air-pollution hazards and to protect public health, the government and departments have taken various environmental measures to prevent and control air pollution [59]. The BC concentration fluctuates in a downward trend but remains at a high level during 2016-2018 (Figure 5a). On the other hand, the enhanced AAOD from 2016-2018 may indicate increased dust aerosol pollution over China, especially in the TB (Figure 5b,d), possibly induced by the enhanced frequency of sandstorms. In Figure 9c, the range of the AAE\({}_{388-500}\) in the TB, the SB, the PRD, NS, the YRB and BT is 2.936 to 3.116, 2.781 to 2.861, 2.798 to 2.890, 2.786 to 2.863, 2.750 to 2.852 and 2.722 to 2.802, respectively. The TB is significantly larger than other regions and reaches its peak value in 2015, indicating an increase in dust aerosols, possibly induced by the enhanced frequency of sandstorms. BT owns the lowest value among the six regions, and drops down to the lowest point in 2018. Meanwhile, the PRD, NS and the YRB all reach their minimum in 2017. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **Season** & **TB** & **SB** & **PRD** & **NS** & **YRB** & **BT** \\ \hline \multirow{6}{*}{AAOD\({}_{388}\)} & Annual & 0.088 \(\pm\) 0.073 & 0.047 \(\pm\) 0.046 & 0.036 \(\pm\) 0.032 & 0.042 \(\pm\) 0.045 & 0.058 \(\pm\) 0.064 & 0.052 \(\pm\) 0.059 \\  & Spring & 0.105 \  
Despite large variations in the time scale, the AAOD\({}_{388}\) and AAOD\({}_{500}\) and AAE\({}_{388-500}\) over the six regions during 2005-2018 exhibit palpable seasonality, as demonstrated in Figure 10. Since the AAOD\({}_{3888\text{nm}}\) and AAOD\({}_{500}\) have a similar linear trend, herein, we only discuss AAOD\({}_{388}\) as a representative. In the TB, the highest AAOD\({}_{388}\) occurs in spring (0.105), and the magnitude has rapidly decreased in summer and autumn (Table 2). Drylands are characterized by high AAOD in spring, which can be attributed to the frequent dust activity [60; 61]. The seasonally averaged AAOD\({}_{388}\) in the YRB peaks in winter (0.074), followed by autumn, spring, and summer. NS has a similar seasonal trend with 0.051, 0.043, 0.042, and 0.031 for winter, autumn, spring, and summer, respectively. In the SB and BT, the average AAOD\({}_{388}\) is alleviated to some extent in summer compared to the average values in the other seasons. For the PRD region, 0.06, 0.031, 0.029, and 0.026 are observed in spring, winter, autumn, and summer, respectively. The mean AAOD\({}_{388}\) in the PRD in the four seasons is analogous, and the variance is smaller as compared with the other five regions, indicating less absorptive aerosol loading. The largest seasonal averages of AAOD\({}_{388}\) for all six regions occur in winter or spring, whilst the smallest seasonal averages are observed in summer without exception. Strong seasonal means are observed during 2016-2018, and weak values during 2005-2015 are seen. Absorbable aerosol contributes more in cold seasons, and the seasonal variation trends of the AAOD are in line with the results obtained by other studies [38; 62].