## 2 Materials and Methods
### Methods
#### 2.3.3 Data Analysis, Aerosol Contamination Protection  
This research was conducted in two phases, at two different spatial scales, synoptic scale and mesoscale, to determine the most appropriate spatial scale for aerosol contamination monitoring and modeling using Sentinel 5p data. The synoptic scale has a horizontal length of 1000 km. A mesoscale ranges from about one kilometer up to several hundred kilometers. A microscale usually has a horizontal length of about 1 km or less, but it was excluded because of satellite data resolution (Section 2.2.2).  
The possibility of using the Al\({}_{340/380}\) and Al\({}_{354/388}\) in aerosol contamination monitoring on a synoptic scale is explored in a well-known case study involving the movement of Saharan sand over the Mediterranean to the European continent on 19 June 2024. This case study was chosen because a rough AI validation can be conducted. The AI recorded by the Sentinel 5p satellite and the cloud cover images obtained by the EUMESTAT MSG satellite product Dust RGB were compared. Also, data were compared with surface air-quality measurements in the area of interest (Rijeka-Kvarner-Istria: blue box in Figure 1). The AI data are calculated daily, so they were compared with the daily movement of cloud cover (series of images in a 15 min time resolution) and with the series of measurements of PM\({}_{10}\) particle concentrations at an hourly level.  
After the possibilities of using Sentinel 5p data in aerosol contamination monitoring at the synoptic scale were confirmed, this research continued to explore the usefulness of Sentinel 5p data at the mesoscale. It was conducted for the Rijeka-Kvarner-Istria region in Croatia (Figure 1). The correlation between PM\({}_{10}\) particle concentrations and Sentinel 5p products was conducted for the area of interest in its entirety, for the several stations closest to the location of failures and several buffers around each air-quality station. Buffers' radii are 5 km, 10 km, 15 km, and 20 km. The analysis was conducted separately for the whole period of observations, for a seven-day period before the power network outage appeared and on the day of the outage. Observing the period before the failure, the analysis includes the possibility of aerosol sedimentation in a shorter period.  
From the Sentinel 5p product, Al\({}_{340/380}\) and Al\({}_{354/388}\), aerosol mid-pressure and aerosol mid-layer height data were selected. The distribution of the selected data was checked by plotting histograms, and the method for calculating the correlation coefficientswas defined. The outliers were checked separately by comparing aerosol index, layer height, surface measurements, and the synoptical situation. When the comparison was conducted, it was decided whether it was appropriate to mark the data as erroneous or not. After the correlation coefficient calculation, the statistically non-significant results were removed from further analysis (\(p_{0}<0.5\)).  
The PM\({}_{10}\) particle concentration extremes were explored and compared with those recorded on the neighboring air-quality stations and AI and LH values. In this way, it was determined whether extremes were valid or if there were some errors in the measurements. Then, the rest of the data from the Sentinel 5p dataset were compared with a measured concentration of PM\({}_{10}\) particles. The next step in this research includes information about power supply outages and faults in the 25 kV overhead contact network supplying electric traction. In this way, a pretty-broad initial analysis of Sentinel 5p data applicability of the aerosol contamination monitoring and modeling was conducted.