## 3 Results
### Radiative Effect of Aerosols, tropospheric NO\({}_{2}\) and Clouds
#### 3.2.2 Seasonal  
The magnitude of the radiative effect of aerosols, tropospheric NO\({}_{2}\), and liquid and ice clouds exhibits a seasonal variability that is similar to that of AOD\({}_{550}\), tropospheric NO\({}_{2}\) VCD, liquid cloud fraction and ice cloud fraction, as shown in Figures S5-S8, for each one of the 16 locations. This is clearer in the case of aerosols and clouds, and less distinct for tropospheric NO\({}_{2}\) where RE\({}_{\rm{NO2}}\) is nearly negligible. Specifically for Athens and Thessaloniki, the two urban centers of Greece, the seasonal variabilities of all the aforementioned parameters are shown in a single comparative figure (Figure 7). The seasonal variability of RE\({}_{\rm{aer}}\) and AOD\({}_{550}\) for each one of the stations is shown in Figure S5. It is well known that AOD\({}_{550}\) in the area peaks in the spring and summer, as a result of the transport of dust from the Sahara Desert and the Middle East, pollution aerosols from Central Europe, and biomass burning aerosols from Southwestern Russia and Eastern Europe [26]. Other factors, such as the low precipitation amounts and the photochemical production of secondary organic aerosols in the summer, also play a substantial role. RE\({}_{\rm{aer}}\) peaks in the spring and summer, with maximum values up to \(-30\) W/m\({}^{2}\) (e.g., \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Location** & **RE\({}_{\rm{aer}}\)** & **RE\({}_{\rm{NO2}}\)** & **RE\({}_{\rm{lc}}\)** & **RE\({}_{\rm{lc}}\)** \\ \hline Antikythera & \(-12\pm 7\) & \(-0.043\pm 0.018\) & \(-31\pm 17\) & \(-16\pm 12\) \\ Argos & \(-12\pm 7\) & \(-0.064\pm 0.023\) & \(-28\pm 14\) & \(-16\pm 11\) \\ Athens & \(-15\pm 12\) & \(-0.166\pm 0.059\) & \(-29\pm 15\) & \(-15\pm 11\) \\ HCMR & \(-14\pm 7\) & \(-0.052\pm 0.027\) & \(-31\pm 19\) & \(-13\pm 11\) \\ Ioannina & \(-10\pm 7\) & \(-0.042\pm 0.016\) & \(-51\pm 24\) & \(-26\pm 16\) \\ Kozani & \(-10\pm 9\) & \(-0.144\pm 0.063\) & \(-43\pm 18\) & \(-21\pm 13\) \\ Mytihene & \(-16\pm 9\) & \(-0.076\pm 0.023\) & \(-22\pm 13\) & \(-16\pm 13\) \\ Olympus & \(-8\pm 6\) & \(-0.046\pm 0.014\) & \(-79\pm 31\) & \(-25\pm 16\) \\ Orestiada & \(-13\pm 10\) & \(-0.073\pm 0.025\) & \(-42\pm 16\) & \(-20\pm 14\) \\ Patras & \(-18\pm 10\) & \(-0.068\pm 0.023\) & \(-23\pm 15\) & \(-18\pm 13\) \\ Preveza & \(-15\pm 9\) & \(-0.053\pm 0.021\) & \(-24\pm 13\) & \(-20\pm 13\) \\ Pylos & \(-12\pm 6\) & \(-0.047\pm 0.020\) & \(-31\pm 14\) & \(-17\pm 13\) \\ Rhodes & \(-15\pm 9\) & \(-0.068\pm 0.029\) & \(-21\pm 13\) & \(-15\pm 12\) \\ Thessaloniki & \(-13\pm 10\) & \(-0.118\pm 0.030\) & \(-35\pm 15\) & \(-19\pm 13\) \\ Volos & \(-13\pm 9\) & \(-0.067\pm 0.024\) & \(-39\pm 18\) & \(-19\pm 14\) \\ Xanthi & \(-10\pm 7\) & \(-0.051\pm 0.022\) & \(-49\pm 22\) & \(-23\pm 16\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Annual average radiative effect of aerosols (RE\({}_{\rm{aer}}\)), tropospheric NO\({}_{2}\) (RE\({}_{\rm{NO2}}\)), liquid clouds (RE\({}_{\rm{lc}}\)) and ice clouds (RE\({}_{\rm{lc}}\)) (in W/m\({}^{2}\)) for the period 2005–2019 with the corresponding \(\pm 1\sigma\). Rhodes, Preveza, Thessaloniki) appearing in August over all the 16 locations. During the winter, especially in December and January, RE\({}_{\rm aer}\) generally drops below \(-5\) W/m\({}^{2}\) and in some cases reaches values around \(-1\) W/m\({}^{2}\) (e.g., Olympus, Kozani, Ioannina). In Figure 7a,b, we see that over Athens, AOD\({}_{550}\) exhibits higher values in the spring compared to Thessaloniki and other locations in the northern part of Greece, where AOD\({}_{550}\) peaks in the summer. This is related to the fact that Northern Greece is closer to the biomass burning hot spots of the area (biomass burning season in July\(-\)August), while Southern Greece is closer to the Sahara Desert (dust transport peaks in the spring). In Figure 7c,d, distinct winter/summer tropospheric NO\({}_{2}\) maxima/minima are observed over Athens and Thessaloniki, with VCDs ranging 2-3 to 5-6 \(\times\) 10\({}^{15}\) molecules/cm\({}^{2}\).