## 1 Introduction  
Surface solar radiation (SSR) affects many climatic elements on a planetary scale, such as evapotranspiration [1], the hydrological cycle [2, 3], photosynthesis and plant productivity [4, 5], and the global (land and ocean) energy budget [6, 7], largely affecting temperature and precipitation [8]. Apart from climate, SSR also affects solar energy production [9], which is vital to the economy. The importance of solar radiation has led to the monitoring of SSR levels since the first decades of the 20 th century, the first station being established in 1923 in Sweden. In the decades following, many more stations were established in Europe and around the globe, shaping global networks such as the Global Energy Balance Archive (GEBA) [10], the Baseline Surface Radiation Network (BSRN) [11], the Word Radiation Data Center (WRDC), and regional/national networks. Obviously, these networks cannot offer coverage over every single spot on Earth, a gap filled by satellite measurements during the last four decades (since the early 1980s) [12]. Various satellite missions played a vital role in improving our knowledge on the Earth's energy balance. Down- and up-welling solar radiation at the top of the atmosphere (TOA) is measured directly by satellite sensors, while surface solar radiation fluxes are estimated indirectly, involving the synergistic use of radiative transfer models with satellite- and ground-based measurements of various physical parameters [13, 14, 15, 16, 17, 18, 19]. Among other regions, various aspects of the SSR have been studied so far, by different groups over the Mediterranean Basin, which is a climate change hot spot [20]. In the last decade, the role of aerosols, clouds, and water vapor on SSR levels has been studied, mostly at a coarse spatial resolution, for all- and clear-sky conditions, based on satellite data [21; 22; 23; 24]. Our study focuses on the Eastern Mediterranean, which is a key region at the crossroads of different transport pathways that mix air masses from Europe, Africa, and Asia [25; 26]. SSR in the area depends on a number of parameters, the most important being clouds, aerosols, and water vapor [27]. SSR may also be affected by ozone (O\({}_{3}\)) [28] and gaseous atmospheric pollutants, such as nitrogen dioxide (NO\({}_{2}\)), depending on the season and the local human activities. Specifically, for the effect of tropospheric NO\({}_{2}\), a basic air quality index, to our knowledge, there are only a couple of studies that suggest that this gas may potentially play a significant role in local radiative forcing under certain conditions, and over heavily polluted areas [29; 30]. Here, we investigate, the effect of two basic air quality indexes, aerosols and tropospheric NO\({}_{2}\), on SSR under different sky conditions (all-sky conditions, clear skies, and liquid and ice cloud-covered skies), along with the effect of different cloud types (liquid/ice), using high-resolution state-of-the-art satellite-based observations and climatological data for the 15-year period 2005-2019. We focus on 16 different locations in Greece, in the heart of the Eastern Mediterranean. It has to be noted that only the direct radiative effect of aerosols (scattering and absorption of solar radiation) and tropospheric NO\({}_{2}\) (absorption) is investigated here. Both these parameters affect SSR indirectly as well; aerosols affect SSR by modifying the micro- and macro-physical cloud properties [31], while NO\({}_{2}\) modulates the formation of nitrate aerosols and O\({}_{3}\), and the lifetime of greenhouse gases, such as that of methane ([PERSON] et al., [32] and references therein). The manuscript is organized as follows: in Section 2 we present the datasets used in the radiative transfer calculations, the data aggregation methodology followed, the radiative transfer system developed for the scopes of this research, as well as the methodology followed for the evaluation of the SSR produced by the system, the radiative effect calculation, and the calculation of the trends.