## 5 Evaluation of FFNN-Based VIIRS Dust Detection off CALIOP Track
### Off-Track Case Studies  
For the selected two days, subsets of granules are selected for both off-track FFNN and PHYS to compare with images from Wisconsin's VIIRS quicklook, including RGB, AOD, and Angstrom Exponent parameter ([[https://sips.ssec.wisc.edu/#/](https://sips.ssec.wisc.edu/#/)]([https://sips.ssec.wisc.edu/#/](https://sips.ssec.wisc.edu/#/))). The Angstrom Exponent is a retrieved parameter to show the size of the retrieved aerosol. It shows how coarse the particle is within the aerosol plume that can best match the the observed radiance. The larger the value is, the smaller the particle size is. Generally, we consider less than 0.4 of the Angstrom Exponent to be dust. Because of the lacking of ground truth (CALIOP) for the off-track data, we use the RGB images, the magnitude of AOD, which shows the amount of aerosol loading, along with the Angstrom Exponent, which indicates dust or not to compare with off-track FFNN and PHYS results. Among a few cases, we select three different cases that show the characteristics of FFNN and PHYS. Figure 9 shows a case over the Arabian peninsular on day 224 where FFNN and PHYS agree very well with each other in terms dust detection. The large value of AOD in  
Figure 8: Dust distribution of day 224 for (**a**) on-track CALIOP product, (**b**) on-track FFNN prediction, (**c**) off-track FFNN, and (**d**) off-track PHYS. Missing observations where there are no CALIOP overpasses are marked as white. Figure 7: Dust distribution of day 075 for (**a**) on-track CALIOP product, (**b**) on-track FFNN prediction, (**c**) off-track FFNN, and (**d**) off-track PHYS. Missing observations where there are no CALIOP overpass are marked as white. Figure 9e suggests this case is a high dust loading case. Figure 9e also shows the CALIOP track and its dust product that are residing in this target region (see solid green and black line in panel e). As it can be seen in the figure, FFNN agrees very well with the CALIOP product on the CALIOP track, and predicts a high dust loading event in this region. PHYS also captures the similar pattern of dust, although the overall magnitude of dust loading is less significant compared to FFNN. Figure 10 shows a case in Southern Australia on day 075, where the FFNN detects significant dust loading (Figure 10e) and PHYS detects almost none (Figure 10f). As seen in the Figure, the dust loading event is more likely  
from Figure 10c, the aerosol optical thickness in this case is quite small, mostly around 0.1. This thin layer of dust is not captured in PHYS, but it is captured both by CALIOP, albeit on its track only, and by FFNN both on-track and off-track. This is also expected from Figures 7 and 8, where CALIOP and FFNN predicted a high frequency of dust over Australia, which is not seen in PHYS. Since CALIOP also captures dust in this regions, as shown in Figure 10e, the difference is probably due to the different definitions of dust event applied to CALIOP and PHYS, where CALIOP is more sensitive to a small amount of dust layers. However, it is debatable if this should be considered as a dust event, given the small amount of dust loading. Finally, Figure 11 shows a case in the Sahara region on c, where the FFNN detects substantially more dust than the PHYS model. As seen from Figure 11b,c, the AOD in this  
Figure 10: A case study over Arabian peninsular on day 075, at 04:54. (**–d**) depict an image from Wisconsin’s VIIIRS quicklook, where (**a**) represents the true color, (**b**) Clear Sky confidence, (**c**) Aerosol Optical Thickness, and (**d**) Angström Exponent; (**e**) FFNN results on the same geolocation. Results are plotted in a \(0.1\times 0.1\) latitude and longitude grid, and the percentage of dust within the grid are shown as colors. Thin magenta lines show the CALIOP swath of this region. Pixels within the magenta lines are the CALIOP product. (**f**) same as (**e**), but for a PHYS product. case is quite high and the Angstrom exponent is small, indicating this to be a high dust loading case.