# Analysis of Dust Detection Algorithms Based on FY-4A Satellite Data
## 4 Results and Validation
### Analysis of Typical Dust Events  
Figure 3a,g,m,s shows the AGRI true color images and dust detection at a 2 h interval between 02:00 and 08:00 UTC on 15 May 2019. In Figure 3, true color images show a dust storm occurring in northwest China including eastern Xinjiang, the Hexi Corridor, and in central Inner Mongolia in North China. The PM10 concentrations increased significantly in northwest, north, and northeast China (Figure 3b,h,n,t).  
Figure 3: _Cont_.  
Figure 3: FY-4A AGRI true color images (**a,g,m,s**), PM10 (**b,h,n,t**), BTD (**c,i,o,u**), NDDI (**d,j,p,v**), IDDI (**e,k,q,w**), and DST (**f,l,r,x**) corresponding to the times of 15 May 2019 from 02:00 to 08:00 UTC, respectively.  
IDDI and DST from 04:00 to 08:00 UTC showed very similar dust spatial distribution in the FY-4A AGRI true color images and were very similar to the BTD detections. However, there were limitations in detecting dust in parts of the northwest at 02:00 UTC because IDDI and DST are not available when the area is in darkness [46]. The figures show that dust is much more severe in northern China than in northwest China (Figure 3e,k,q,w). Compared to BTD, the DST detection results (over 16) coincided more with the spatial distribution of IDDI.  
Figure 4 shows the AGRI true color images and dust detection results at a 2 h interval from 02:00 to 08:00 UTC on 16 March 2021. The dust affected a wider area originating in the Gobi Desert and affecting northern China and even eastern and central China (Figure 4a,g,m,s). The PM10 concentration even exceeded 1320 \(\upmu\)g\(\cdot\)m\({}^{-3}\) over the eastern part of northwest China during this dusty weather process, and PM10 concentrations over 450 \(\upmu\)g\(\cdot\)m\({}^{-3}\) were observed in central and eastern China. The PM10 concentration dispersion aligned well with the dusty weather process (Figure 4b,h,n,t).  
Figure 4: \(Cont.\)  
Figure 4: _Cont._  
The threshold for BTD was \(-\)0.4 to \(-\)2.8 at 02:00 UTC (Figure 4c) and less than \(-\)0.4 from 04:00 to 08:00 UTC (Figure 4i,o,u). The extent of dust detected by BTD was mostly similar to the spatial distribution of dust in the AGRI true color images. However, the AGRI true color images could not show that central and eastern China was experiencing dust. The detection of dust here with BTD coincided with the distribution of the PM10 concentrations. Thresholds were from 0.05 to 0.30 for NDDI at 02:00 UTC (Figure 4d) and 0.10 to 0.35 for 04:00 to 08:00 UTC (Figure 4j,p,v). At 02:00 and 04:00 UTC, NDDI misclassified large areas of bare ground on the Tibetan Plateau as dust. Some red vegetation in the southwest was also misclassified as dust at 02:00 to 08:00 UTC. There was also widespread misclassification near the dust area boundary between northern and northeastern China.  
IDDI and DST at 04:00 and 06:00 UTC provide a good indication of the spatial distribution of dust (Figure 4k,l,q,r). During this dust event, IDDI showed the strongest dust intensity over the north of western China and the western corridor of the river in Northwest China and relatively weak dust intensity over East China at 06:00 UTC.