# Spatiotemporal Variation in Absorption Aerosol Optical Depth over China
## 3 Results and Discussion
### OMI AAOD over Typical Areas of China  
The MODIS reanalysis data are utilized to distinguish sandstorms, fog, and haze, as well as cirrus clouds [56]. The mean AOD values at 550 nm area-averaged over China derived from the MODIS true-color images with a spatial resolution of 1 \(\times\) 1 km in the TB (37.5-41.5\({}^{\circ}\) N, 79.5-86.5\({}^{\circ}\) E), the SB (29.5-31.5\({}^{\circ}\) N, 103.5-106.5\({}^{\circ}\) E), the Pearl River Delta (PRD) (21.5-23.5\({}^{\circ}\) N, 112.5-114.5\({}^{\circ}\) E), Nanjing and Shanghai (NS) (30.5-32.5\({}^{\circ}\) N, 118.5-121.5\({}^{\circ}\) E), the Yellow River Basin (YRB) (32.5-37.5\({}^{\circ}\) N, 114.5-117.5\({}^{\circ}\) E), and BT (38.5-40.5\({}^{\circ}\) N, 115.5-117.5\({}^{\circ}\) E) over 2005-2018 are all higher than 0.5, indicating the impact of aerosols from regional air pollution. The geographical locations of the six regions marked with white boxes are shown in Figure 7. The spatiotemporal properties of the AAOD and AAE are studied in the above six representative regions based on the latest OMI. Figure 6: Annual variation in area-average OMI AAOD\({}_{388}\) and AAOD\({}_{500}\) in China from 2005 to 2018. Figure 7: The mean AOD at 550 nm area-averaged over China derived from MODIS-Aqua data from 2005 to 2018. The white boxes denote the six sampling areas. Figure 8 exhibits temporal variations of the AAOD\({}_{388}\), AAOD\({}_{500}\), and AAE\({}_{388-500}\) in the six typical geographic areas from 2005 to 2018. The effective monitoring days in the TB, the SB, the PRD, NS, the YRB and BT were 4187, 1401, 1425, 1976, 3032, and 2918, respectively. Hence, the temporal changes in the TB region were the most regular. Generally, the annual mean AAOD during 2015-2018 was in the order of TB \(>\) YRB \(>\) BT \(>\) SB \(>\) NS \(>\) PRD. It is not surprising that the maximum AAOD\({}_{388}\) was found over the TB, ranging from 0.073 to 0.140, followed by the YRB (0.053-0.077), BT (0.042-0.066) and the SB (0.039-0.050). However, compared to the aforementioned regions, the overall AAOD\({}_{388}\) of the PRD (0.029-0.051), and NS (0.036-0.062) was lower, implying lighter AAOD pollution, and smaller values are associated with lower population and aerosol loads. The AAOD\({}_{388}\) values display large variabilities between 0-0.8, whereas the AAOD\({}_{500}\) values vary in the range of 0-0.4, nearly half of the AAOD\({}_{388}\). The AAOD values decrease as the wavelength increases, implying a positive AAE will be achieved. The distinctively higher annual mean AAOD (0.088 for AAOD\({}_{388}\) and 0.041 for AAOD\({}_{500}\)) in the TB (Table 2) is intimately linked to dust episodes. The AAE\({}_{388-500}\) depicts regular annual variations, and ranges from 2.207 to 4.332 with the averages of 3.058, 2.821, 2.849, 2.825, 2.806, and 2.770 for the TB, the SB, the PRD, NS, the YRB and BT, respectively. The angstrom wavelength index can be used to quantify the spectral dependence of absorption [57]. The relatively higher AAE value in the TB is likely associated with dust aerosols due to a large dust source from the Taklimakan Desert in the TB region. Figure 8: Temporal variations of the AAOD\({}_{388}\), AAOD\({}_{500}\), and AAE\({}_{388-500}\) between 2005 and 2018 in six sampling areas. To better illustrate the current situation of absorptive aerosol loading, further comparisons in the year scale of the AAOD\({}_{388}\), AAOD\({}_{500}\), and AAE\({}_{388-500}\) are analyzed in the six typical regions (Figure 9). The AAOD over East Asia is mainly caused by dust as well as BC and BrC [29]. Most notably, BC contributes the most to total absorption; BrC and dust contribute significantly at shorter wavelengths or during dust events [54; 55]. The yearly means of AAOD\({}_{388}\) and AAOD\({}_{500}\) display slight variations from 2005 to 2015, while from 2016 to 2018, yearly average AAODs increase obviously for the TB. This may be related to dust with a similar annual trend during 2005 to 2018 (Figure 5b,d) [17; 58; 27].