# Trends from 30-Year Observations of Downward Solar Irradiance in Thessaloniki, Greece
## 2 Data and Methodology  
The SDR data were measured with a Kipp & Zonen CM-21 pyranometer operating continuously at the Laboratory of Atmospheric Physics of the Aristotle University of Thessaloniki (40\({}^{\circ}\) 38\({}^{\prime}\) N, 22\({}^{\circ}\) 57\({}^{\prime}\) E, 80 m a.s.l.). Here, we used data for the period from 13 April 1993 to 13 April 2023. The monitoring site was located near the city center, thus we expect that measurements were affected by the urban environment, mainly by aerosols. During the study period, the pyranometer was independently calibrated three times at the Meteorologisches Observatorium Lindenberg, DWD, verifying that the stability of the instrument's sensitivity was better than 0.7% relative to the initial calibration by the manufacturer. Along with SDR, the direct beam radiation (DNI) was also measured with a collocated Kipp & Zonen CHP-1 pyrheliometer since 1 April 2016. The DNI data were used as auxiliary data to support the selection of appropriate thresholds in the clear-sky identification algorithm (CSid), which is discussed in Section 2.1. It is noted that the limited dataset of DNI was not used for the identification of clear-sky cases in the entire SDR series to avoid any selection bias due to the unequal length of the two datasets. There are four distinct steps in the creation of the dataset analyzed here: (a) the acquisition of radiation measurements from the sensors, (b) the data quality check, (c) the identification of \"clear sky\" conditions from the SDR data, and (d) the aggregation of data and trend analysis. For the acquisition of radiometric data, the signal of the pyranometer was sampled at a rate of 1 Hz. The mean and the standard deviation of these samples were calculated and recorded every minute. The measurements were corrected for the zero offset (\"dark signal\" in volts), which was calculated by averaging all measurements recorded for a period of 3 h, before (morning) or after (evening) the Sun reaches an elevation angle of \(-10^{\circ}\). The signal was converted to irradiance using a ramped value of the instrument's sensitivity between subsequent calibrations. A manual screening was performed to remove inconsistent and erroneous recordings that can occur stochastically or systematically during the continuous operation of the instruments. The manual screening was aided by a radiation data quality assurance procedure, adjusted for the site, which was based on the methods of [PERSON] and [PERSON] [19; 20]. Thus, problematic recordings have been excluded from further processing. Although it is impossible to detect all false data, the large number of available data, and the aggregation scheme we used, ensures the quality of the radiation measurements used in this study. Due to the significant measurement uncertainty when the Sun is near the horizon, we have excluded all measurements with solar zenith angle (SZA) greater than 85\({}^{\circ}\). Moreover, due to obstructions around the site (hills and buildings) that block the direct irradiance, we excluded data with azimuth angle in the range 58\({}^{\circ}\)-120\({}^{\circ}\) and with SZA greater than 78\({}^{\circ}\). To make the measurements comparable throughout the dataset, we adjusted all one-minute data to the mean Sun-Earth distance. Subsequently, we adjusted all measurements to the total solar irradiance (TSI) at 1 au in order to compensate for the Sun's intensity variability using a time series of satellite TSI observations. The TSI data we used are part of the 'NOAA Climate Data Record of Total Solar Irradiance' dataset [21]. The initial daily values of this dataset were interpolated to match the time step of our measurements. In order to estimate the effect of the sky conditions on the long-term variability of SDR, we created three datasets by characterizing each one-minute measurement with a corresponding sky-condition flag (i.e., all-sky, clear-sky, and cloudy-sky).