## 2 Satellite Images, Ground Measurement, and AERONET Dataset  
The COMS has become a significant player in providing reliable weather data and atmospheric conditions since the Korean government launched its first geostationary satellite in 2011 at 128.2\({}^{\circ}\) E. The satellite system records the Korean peninsula and Asia-Pacific region, including the Australian continent, China, and Southeast Asia.  
The COMS system carries three infrared (10.8 \(\upmu\)m, 12 \(\upmu\)m, and 3.7 \(\upmu\)m), water vapor (6.7 \(\upmu\)m), and visible (0.67 \(\upmu\)m) channels to observe the earth's surface. The images that COMS produced are composed of three levels of quality: level 1A, level 1B, and level 2. The level 1A images are satellite images processed through radiometric, geometric, and georeferencing calibration. In level 1B, images are then screened from distortion and nonuniform pixel intensity. Finally, images in level 2 are extracted into applicable maps, such as for forest fires, fog observation, and solar irradiance maps. Technically, the COMS satellite produces 16-bit visible images at 15-min intervals, and the satellite system provides a spatial resolution of 1 \(\times\) 1 km\({}^{2}\) for the visible sensor and 4 \(\times\) 4 km\({}^{2}\) for the three infrared channels. In this work, we utilized the hourly level 1B of satellite images (Figure 1) obtained from 1 January to 31 December 2018 and selected the pixel from each image that was closest to the ground measurement in order to validate our model.  
This study performed GHI measurements from a ground station located in the northern part of Seoul (latitude, 37.612\({}^{\circ}\) N; longitude, 126.996\({}^{\circ}\) E; and altitude, 141 m). The ground measurement system comprises one pyranometer (MS-802 model) for the GHI and one pyrheliometer (MS-57 model) for direct normal irradiance (DNI). Both of the instruments, manufactured by the Eko company in Japan, have a sensitivity around \(\pm\)7 \(\upmu\)Vm\({}^{2}\)/W for the GHI and DNI, respectively [27; 28]. Identical to the COMS satellite data, hourly intervals for the ground data were also locally archived from 1 January-31 December 2018.  
In this study, quality control was applied based on the procedure from [PERSON] [29] to guarantee reliable data from the ground measurement. In particular, data that did not meet the following criteria were rejected:  
1. \(\theta_{x}>80^{\circ}\)
2. \(\mathrm{GHI}>0\) W/m\({}^{2}\)
3. \(\mathrm{GHI}<1.5G_{\mathrm{ext}}\cos^{1.2}\theta_{x}+100\) W/m\({}^{2}\)
4. \(\mathrm{DNI}\geq 0\) W/m\({}^{2}\)
5. \(\mathrm{DNI}\leq G_{ext}\),  
where \(\theta_{x}\) and \(G_{ext}\) denoted the solar zenith angle and extraterrestrial irradiance on a normal surface, respectively. The data that did not meet the requirements were rejected. Note that the criterion of \(\theta_{x}>80^{\circ}\) was applied to remove GHI values at low sun elevation, and DNI data only were used for the data quality control.  
The aerosol robotic network (AERONET) is a network of ground-based measurements providing continuous data on spectral AOD, precipitable water, and inversion aerosol in various aerosol regimes using a sun radiometer [30]. The sun-sky radiometer system of AERONET quantifies solar spectral measurements every 15 min through eight spectral bands at wavelengths of 340, 380, 440, 500, 675, 870, 940, and 1020 nm [31]. The AERONET dataset has three grades: level 1 for raw data, level 1.5 for data with clouds removed, and level 2 for quality-controlled data. In this study, clear-sky irradiance was calculated through the Linke turbidity factor (\(T_{L}\)) to consider the reduction of solar irradiance owing to atmospheric effects. The level 2 data of AERONET, which were collected from 1 January-31 December 2018, were employed and arranged into hourly intervals to determine the value of \(T_{L}\).