# Spatiotemporal Variation in Absorption Aerosol Optical Depth over China
## 3 Results and Discussion  
### Validation of OMI AAOD with AERONET AAOD  
There are approximately 35 unevenly distributed ground-based measurement AERONET sites in China, many of which only have several days' or months' measurements. In addition, ground-based observations are scarce in central and southwestern China, with more along the eastern coastal region. Unlike AERONET, the OMI AAOD can cover the whole of China and provide long-term AAOD values. Four AERONET sites with the most observation data were chosen for comparison of OMI remote sensing. Table 1 displays the detailed geographical location, longitude, latitude, and altitude of Xianghe, Taihu, Hongkong Polytechnic Univ, and Sacol stations, and summary data related to the AAOD and AAE. Figure 1 shows the comparison of monthly average AAOD\({}_{500}\) data between OMI products and AERONET during 2005-2018. The adjacent grid was extracted from the OMI according to the longitude and latitude of each site, and then the relevant data were employed for validation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Site** & **Xianghe** & **Taihu** &
\begin{tabular}{c} **Hongkong** \\ **Polytechnic Univ** \\ \end{tabular} & **Sacol** \\ \hline Location & North China & East China & South China & Northwest China \\ Latitude (Degree) & 39.8\({}^{\circ}\) N & 31.4\({}^{\circ}\) N & 22.3\({}^{\circ}\) N & 35.9\({}^{\circ}\) N \\ Longitude (Degree) & 117.0\({}^{\circ}\) E & 120.2\({}^{\circ}\) E & 114.2\({}^{\circ}\) E & 104.1\({}^{\circ}\) E \\ Elevation (m) & 36 m & 20 m & 30 m & 1965 m \\ AERONET AAOD\({}_{40}\) & 0.052 & 0.050 & 0.039 & 0.038 \\ AERONET AAE\({}_{440-870}\) & 1.288 & 1.310 & 1.272 & 1.553 \\ AERONET AAOD\({}_{500}\) & 0.044 & 0.042 & 0.033 & 0.031 \\ OMI AAOD\({}_{500}\) & 0.023 & 0.021 & 0.018 & 0.021 \\ OMI AAE\({}_{388-500}\) & 2.766 & 2.826 & 2.850 & 2.793 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic information of the four studied AERONET sites during 2005–2018. The correlation coefficient (R) for all sites is significant at the 0.01 level coherently. The R between the OMI and AERONET fluctuated around 0.55. Relative mean bias (RMB) is utilized to express the accuracy of AAOD data, and values greater than 0.5 are considered reliable [50]. Compared with the monthly average data, some more robust results were obtained for daily data as the sample size increased. For example, the R and RMB slightly increased from 0.54 and 0.53 to 0.56 and 0.58 in Xianghe from 2005 to 2018, respectively (Figure 2a). The total slope was 0.21-0.35, deviating from the predetermined 1:1 path (red dashed line), implying that the OMI product underestimated the AAOD\({}_{500}\) in Xianghe (RMB = 0.53), Taihu (RMB = 0.50), Hongkong Polytechnic Univ (RMB = 0.55), and Sacol (RMB = 0.68). The underestimation was caused by the overestimation in the estimated surface reflectance [26]. Correlation analysis on the daily dataset was also performed between the OMI AAOD\({}_{500}\) and AERONET AAOD\({}_{500}\) with Level 1.5 and Level 2.0 quality assurance (Figure 2). The sample size of Level 2.0 is only one-third of that for Level 1.5, and the R value has increased to a certain extent but the RMB has decreased from 0.61 to 0.48. Figure 1: Comparison of the monthly average AERONET AAOD\({}_{500}\) (Level 1.5) and OMI AAOD\({}_{500}\) in (**a**) Xianghe, (**b**) Taihu, (**c**) Hongkong Polytechnic Univ, (**d**) Sacol. Figure 2: Comparison of the daily average OMI AAOD\({}_{500}\) and AERONET AAOD\({}_{500}\) with Level 1.5 (**a**) and Level 2.0 (**b**) quality assurance in Xianghe during 2005-2018. Through comparison with ground-based observations from AERONET, OMI retrievals have achieved a great deal of research and approval. [PERSON] et al. reported a good correlation with the R of 0.81 between the OMI AOD retrieved using the OMAERUV algorithm and AERONET AOD over 2005-2008 [30]. [PERSON] et al.