## 3 Results
### Radiative Effect of Aerosols, tropospheric NO\({}_{2}\) and Clouds
#### 3.2.2 Seasonal
This is most probably related to the increased consumption of fossil fuel and biomass burning for heating purposes in the winter, and the decreased traffic in the summer. On the contrary, over island locations where tourism peaks in the summer, we observe either an opposite seasonal variability with a clear summer maximum (e.g., Rhodes) or a secondary summer peak (e.g., Mytilene) (Figure S6). The radiative effect of tropospheric NO  
Figure 5: Maps with the annual average MODIS/Aqua (**a**) liquid cloud fraction, (**b**) ice cloud fraction, (**c**) liquid cloud optical thickness and (**d**) ice cloud optical thickness over the area of interest for the period 2005–2019. is overwhelmed by the effect of other parameters, especially aerosols, water vapor and clouds, peaking in July over all the locations examined here (Figure 7c,d and Figure S6). The highest RE\({}_{\text{NO2}}\) value appears over Athens in July (\(-0.215\) W/m\({}^{2}\)), which is still very low to play a substantial role in the local radiative budget. Figure 6: Annual average radiative effect of aerosols and tropospheric NO\({}_{2}\) for **(a,b)** clear, **(c,d)** liquid and **(e,f)** ice cloud-covered skies over 16 locations in Greece for the period 2005–2019. Mind that a different scale is used for the two parameters and that the radiative effect of tropospheric NO\({}_{2}\) is smaller by two orders of magnitude. Figure 7: Seasonal variability of AOD\({}_{580}\), tropospheric NO\({}_{2}\) VCD, liquid cloud fraction and ice cloud fraction (black color) with the corresponding aerosol, tropospheric NO\({}_{2}\), liquid cloud and ice cloud radiative effect (grey color) for Athens (**a**,**c**,**e**,**g**) and Thessaloniki (**b**,**d**,**f**,**h**) during the period 2005–2019. Aerosol- and cloud-related parameters are from MODIS/Aqua and tropospheric NO\({}_{2}\) from OMI/Aura. The liquid cloud fraction in the area exhibits a winter peak (values up to 0.5) with a summer minimum, especially in July-August (Figure S7), being almost zero over specific locations, mostly in Southern Greece (e.g., Rhodes). RE\({}_{\rm ic}\) exhibits a quite similar seasonal variability with liquid cloud fraction, with clear summer minima over all the locations. The lowest RE\({}_{\rm ic}\) levels appear in July over the island locations of Rhodes (\(-1\) W/m\({}^{2}\)) and Antikythera (\(-3\) W/m\({}^{2}\)). The highest RE\({}_{\rm ic}\) appears in May over the mountainous location of Olympus (\(-123\) W/m\({}^{2}\)). Over Olympus, RE\({}_{\rm ic}\) takes values higher than \(-40\) W/m\({}^{2}\) throughout the year. As shown in Figure 7e,f, over Athens and Thessaloniki, RE\({}_{\rm ic}\) peaks in the winter-spring (values up to \(-40\) to \(-50\) W/m\({}^{2}\)), with a clear summer minimum. The seasonal variability of the ice cloud fraction is similar to the liquid cloud fraction, with a winter peak (values up to 0.3) and a summer minimum when the presence of ice clouds is very limited, over all the 16 locations (Figure S8). In general, RE\({}_{\rm ic}\) exhibits quite a similar seasonal variability with ice cloud fraction, with clear summer minima over all the locations and peaks in the spring. In July, RE\({}_{\rm ic}\) is zero over the island/coastal locations of Rhodes/HCMR, while for others it is lower than \(-2\) W/m\({}^{2}\) in July and/or August (Antikythera, Argos, Athens, Mytilene, Patras, Pylos, Volos). The highest RE\({}_{\rm ic}\) appears in May over Ioannina (\(-44\) W/m\({}^{2}\)) and Olympus (\(-43\) W/m\({}^{2}\)), while in spring RE\({}_{\rm ic}\) takes values between \(-20\) and \(-40\) W/m\({}^{2}\) over all the 16 stations (Figure 7g,h and Figure S8).