## 3 Results
### Radiative Effect of Aerosols, tropospheric NO\({}_{2}\) and Clouds  
#### 3.2.1 Annual  
The annual all-sky radiative effect of aerosols, tropospheric NO\({}_{2}\), and liquid and ice clouds over the 16 locations examined here is shown in Figure 4. It is obvious that liquid clouds dominate, with an average radiative effect (RE\({}_{\rm ic}\)) of \(-36\) W/m\({}^{2}\). Ice clouds exhibit a smaller radiative effect (RE\({}_{\rm ic}\)) of \(-19\) W/m\({}^{2}\), with the radiative effect of aerosols (RE\({}_{\rm aer}\)) following (\(-13\) W/m\({}^{2}\)). It has to be highlighted that the radiative effect of tropospheric NO\({}_{2}\) (RE\({}_{\rm NO2}\)) is smaller by two orders of magnitude (\(-0.074\) W/m\({}^{2}\)) compared to that of aerosols and clouds and hence is negligible. Aerosols exhibit a stronger radiative effect, by \(\sim\)2 W/m\({}^{2}\), over the urban locations (14 W/m\({}^{2}\)) compared to the rural locations (12 W/m\({}^{2}\)). The annual RE\({}_{\rm aer}\) ranges from \(-18\) W/m\({}^{2}\) (Patras) to \(-8\) W/m\({}^{2}\) (Olympus). It has to be highlighted here that even over the Olympus location, a high-altitude background area entailing the top of Mount Olympus (2917 m), RE\({}_{\rm aer}\) is still \(-8\) W/m\({}^{2}\), which is very close to other rural (Xanthi and Ioannina; \(-10\) W/m\({}^{2}\)) and even urban locations (Kozani; \(-10\) W/m\({}^{2}\)). The annual RE\({}_{\rm aer}\) values for all the 16 locations of interest can be found in Figure 4a and in Table 2. The values presented in this work are in accordance to previous studies in the area. [PERSON] et al. [21] reported an average RE\({}_{\rm aer}\) value of \(-17.4\) W/m\({}^{2}\) over the Eastern Mediterranean, in the electronic supplement of their paper. For the same area, and based on climatological calculations, [PERSON] et al. [27] suggested that aerosols reduce incoming shortwave solar radiation by \(\sim\)18 W/m\({}^{2}\). However, both these studies refer to later periods. Hence, taking into account the decreasing AOD trends in the area (see Section 3.3.2), the smaller RE\({}_{\rm aer}\) values we obtain here make sense. The radiative effect of tropospheric NO\({}_{2}\) over urban locations (\(-0.113\) W/m\({}^{2}\)) is double that over rural locations. The highest (still negligible) RE\({}_{\rm NO2}\) values appear over the following three pollution hot-spot locations in the area (see Figure 2d): Athens (\(-0.166\) W/m\({}^{2}\)), Kozani (\(-0.144\) W/m\({}^{2}\)) and Thessaloniki (\(-0.118\) W/m\({}^{2}\)) (Figure 4b). Athens is the capital city, and industrial and commercial center of Greece, with a population of approximately four million people in its metropolitan area. Thessaloniki is the second largest city, with a population of approximately one million people in its metropolitan area, while the greater area of Kozani is a power-producing center entailing lignite-burning power plants. However, the tropospheric NO\({}_{2}\) levels in those areas are still quite low (\(-4\times 10^{15}\) molecules/cm\({}^{2}\)) to exert a significant radiative effect on an annual basis compared to the heavily polluted areas of Eastern China where tropospheric NO\({}_{2}\) levels can be up to \(-10\) times higher [32]. It can be seen in Figure 5a,c that higher annual liquid cloud fractions, up to 0.4, and liquid cloud optical thicknesses up to 30 are seen over high-elevation areas over land (Figure 2a). It comes as no surprise that the location with the highest RE\({}_{\rm ic}\) is, by far, Olympus (\(-79\) W/m\({}^{2}\)), with Ioannina (\(-51\) W/m\({}^{2}\)) and Xanthi (\(-49\) W/m\({}^{2}\)) following. The lowest RE\({}_{\rm ic}\) values, from \(-21\) to \(-23\), appear over island (Rhodes and Mytilene) and coastal locations (Patras and Preveza) (Table 2). Ice clouds exhibit a south-to-north increasing cloud fraction gradient over both the land and sea areas, reaching values up to 0.2-0.25 (Figure 5b). Ice cloud optical thickness is higher over high-elevation areas, reaching values up to 20-25 (Figure 5d).