# Spatiotemporal Variation in Absorption Aerosol Optical Depth over China
## 3 Results and Discussion
### OMI AAOD over Typical Areas of China
Since 2005, the total consumption of coal and diesel display an overall uptrend, increasing by 91.51 and 7.42 million tons each year, respectively [29]. The uncompleted combustion of fossil fuels is a significant source of BC and BrC [63], and the high concentrations partially illustrate the upward trend of the AAOD in most parts of China during cold winters. The increasing energy consumption (e.g., coal and other fuels) for heating in winter, accompanied by the frequent occurrence of atmospheric inversion and lower boundary layer height, leads to the unfavorable dilution diffusion of BC, thus exacerbating pollution. The winter AAOD values in eastern (NS), northern (the YRB, BT), and central (the SB) China have significantly increased. The northwest (the TB) is sparsely populated and emits less BC throughout the year. On the other hand, spring is prone to sandstorms, and its AAOD values are higher than those of winter. Figure 9: Annual variations of the AAOD\({}_{388}\) (**a**), AAOD\({}_{500}\) (**b**), and AAE\({}_{388-500}\) (**c**) between 2005 and 2018 in six sampling areas. Nevertheless, the AAE\({}_{388-500}\) for six typical regions shows maximum mean values in summer, followed by spring and autumn, whereas winter shows weaker AAEs, being different from the seasonal variation trend of the AAOD. The discrepancy in the AAE is related to two factors: the dominant absorptive aerosol types and emission sources. The AAE over the TB with its desert source obtained in our study (3.088, 3.232, 3.029, and 2.825 for spring, summer, autumn, and winter) is significantly larger than those seen in the other five areas, whereas the values of the AAE\({}_{388-500}\) are found to be near 2.750. The monthly average variations of the AAOD\({}_{3888 nm}\), AAOD\({}_{500 nm}\), and AAE\({}_{388-500}\) over the six sampling areas from 2005 to 2018 are illustrated in Figure 11. Further analysis found that the monthly averaged AAOD\({}_{500}\) in each region is consistently about half of the AAOD\({}_{388}\) (Figure 11a,b). The monthly patterns of the AAOD and AAE\({}_{388-500}\) over the six regions represent almost identical trends and fluctuations, in line with their seasonal patterns. The monthly statistics of the AAOD indicate that the monthly averages of the PRD are lower than those of the other five regions except in March and April. The monthly average AAOD\({}_{388}\) in the TB, SB, YRB, BT, PRD, and NS regions shows the minimal values of 0.072, 0.024, 0.026, and 0.027 in July, 0.024 in June, and 0.025 in September, respectively. Absorptive aerosols can be effectively removed by summer wet precipitation. Moreover, more aerosols from ambient air can be captured by leaves of flourishing trees. The peak events of AAOD\({}_{388}\) occur in January for NS, the YRB and BT with the average values of 0.055, 0.077 and 0.067, respectively, whereas the peaks are in April for the TB, February for the SB, and March for the PRD with 0.123, 0.073, and 0.075, respectively. The monthly mean AAOD values vary two- or three-fold between the lowest and maximum months, and high alert levels of AAOD are preserved from November to the following March. The meteorological conditions in winter and early spring associated with more energy burning facilitate the formation of high BC aerosol contamination. A previous study also declares a significant upward trend in BC and OC emissions after September [64]. For the TB, sandstorm weather is more frequent from March to May, and can explain why the TB can have a high AAOD in spring, rather than in winter. The monthly mean AAE\({}_{388-500}\) is in a range of 2.759-3.254 for the TB, with the largest value in June and the smallest in February. Compared to the monthly variations for the TB, the fluctuation range between the  
Figure 10: Seasonal variations of the AAOD\({}_{388}\) (**a**), AAOD\({}_{500}\) (**b**), and AAE\({}_{388-500}\) (**c**) from 2005 to 2018 in six sampling areas.