# Trends from 30-Year Observations of Downward Solar Irradiance in Thessaloniki, Greece
###### Abstract  
The shortwave downward solar irradiance (SDR) is an important factor that drives climate processes and energy production and can affect all living organisms. Observations of SDR at different locations around the world with different environmental characteristics have been used to investigate its long-term variability and trends at different time scales. Periods of positive trends are referred to as brightening periods and of negative trends as dimming periods. In this study we have used 30 years of pyranometer data in Thessaloniki, Greece, to investigate the variability of SDR under three types of sky conditions (clear-, cloudy- and all-sky). The clear-sky data were identified by applying a cloud screening algorithm. We have found a positive trend of 0.38%/year for all-sky, \(\sim\)0.1%/year for clear-sky, and 0.41%/year for cloudy conditions. The consistency of these trends, their seasonal variability, and the effect of the solar zenith angle have also been investigated. Under all three sky categories, the SDR trend is stronger in winter, with 0.7, 0.4, and 0.76%/year, respectively, for all-, clear-, and cloudy-sky conditions. The next larger seasonal trends are in autumn\(-\)0.42 and 0.19%/year, for all and cloudy skies, respectively. The rest of the seasonal trends are significant smaller, close to zero, with a negative values in summer, for clear and cloudy skies. The SDR trend is increasing with increasing solar zenith angle, except under cloudy skies, where the trend is highly variable and close to zero. Finally, we discuss shorter-term variations in SDR anomalies by examining the patterns of the cumulative sums of monthly anomalies from the climatological mean, both before and after removing the long-term trend.  
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showing the existence of feedback mechanisms between the two. Similar findings have been shown from the analysis of observations at other locations [8; 9; 10; 11] [and references therein]. In the Mediterranean region, aerosols have been recognized as an important factor affecting the penetration of solar radiation at the surface [12; 13; 14; 15]. These studies investigated the long-term trend in aerosol optical depth, which has been found to decrease in the last three decades, the transport and composition of aerosols, and their radiative effects.  
Due to the significant spatial and temporal variability of the trends and the contributing factors, there is a constant need to monitor and investigate SDR at different sites in order to estimate the degree of variability and its relation to the local conditions. In this study, we examine the trends of SDR using ground-based measurements at Thessaloniki, Greece, for the period from 1993 to 2023. We re-evaluated and extended the dataset used by [PERSON] et al. [16], we applied a different algorithm for the identification of clear-/cloud-sky instances [17; 18], and we derived the SDR trends for the period of study under different sky conditions (all-sky, clear-sky, and cloudy-sky). Finally, we investigated the dependence of the trends on solar zenith angle and season.