## 3 Results and Discussion  
### Meteorology and Air Quality  
The daily meteorology data are presented in Table S1 (Supplementary Information). During the summer monitoring period, a rainfall event occurred once (28 August 2017, 18:00-21:00) and there was an intensive rainfall event two days before monitoring began (23 August 2017). Temperature ranged from 15.4 to 30.4 \({}^{\circ}\)C, with a mean of 21.2 \(\pm\) 3.3 \({}^{\circ}\)C, and the mean wind speed was 1.8 \(\pm\) 1.3 m\(\cdot\)s\({}^{-1}\) (maximum of 7.8 m\(\cdot\)s\({}^{-1}\)) (data from the KMA AWS, 267 m a.s.l.). The mean wind speed on the ground (Site-1: street canyon, 1.8 m a.g.l.) was 0.6 \(\pm\) 0.3 m\(\cdot\)s\({}^{-1}\) (Table 2). The prevailing winds were north- or south-westerlies in the daytime, and northeies or easterlies at night (representing a weak sea-breze; Supplementary Information Figure S4). The ground wind direction tended to be similar to that of the prevailing winds, likely because the prevailing wind direction coincided with the orientation of the roadway in the street canyon and was also influenced by street canyon effects (Site-1). However, ground wind speeds were minimal when the prevailing winds were easterlies (Figure 1). During the winter monitoring period, a strong cold wave with intense snowfall occurred the day before the winter observation (8 January 2018), and a few snow events also occurred during the monitoring period (13:00-17:00 on 9 January 20:00-24:00 on 12 January and 04:00-08:00 on 22 January 2018). Temperature ranged from \(-\)16.2 to 7.4 \({}^{\circ}\)C (avg. \(-\)2.3 \(\pm\) 5.7 \({}^{\circ}\)C), and the mean wind speed was 2.3 \(\pm\) 1.4 m\(\cdot\)s\({}^{-1}\) (max. wind speed: 10.3 m\(\cdot\)s\({}^{-1}\)) from KMA AWS (prevailing winds). The mean ground-level wind speed observed at Site-2 was 1.1 \(\pm\) 0.8 m\(\cdot\)s\({}^{-1}\), 48% weaker than that of the prevailing winds. The prevailing winds were westerly or easterlies, whereas the ground-level winds were northelies when the prevailing winds were westerly, and a mixture of south- or north- easterly with weak wind speeds (\(<\)1 m\(\cdot\)s\({}^{-1}\)) when the prevailing winds were easterly, likely due to blocking effects by the surrounding tall buildings (Figure 1). In both the summer and winter periods, ground-level air temperature was somewhat higher than at the KMA AWS location (\(\sim\)1.4 and 0.9 km south of the ground AWS locations in the summer and winter, respectively). The higher ground-level temperature in the city could be caused by the differences in land-use (paved roads vs. the foot of Nam Mountain) and/or heat-release from surrounding buildings. However, we cannot exclude the possibility of systematic differences between temperature sensors, because the 1:1 plots had a slope close to one, with an off-set (\(\sim\)3.9 \({}^{\circ}\)C) (Figure 2c,f). When the prevailing winds were westerlies, ground-level wind speeds were \(-\)26% (summer) and \(-\)46% (winter) weaker than that of the prevailing winds due to surface stress (friction) and displacement height of the urban canopy. However, consistent weak ground wind speeds (\(<\)1 m\(\cdot\)s\({}^{-1}\)) occurred when the prevailing winds were easterlies, regardless of their speeds or the locations of ground AWS (Figure 2b,e). We cannot explain the suppressed wind speeds for the easterly prevailing winds, although they could be due to geographic characteristics, but suppressed \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & **Period** & **Temperature** & **Relative** & **Wind Speed** \\  & & & (\({}^{\circ}\)C) & **Humidity (\%)** & **(m\(\cdot\)s\({}^{-1}\))** \\ \hline \multirow{2}{*}{Summer} & Upper & 25–31 August 2017 & 21.2 (3.3) & / & 1.8 (1.3) \\  & Ground & 26–31 August 2017 & 23.4 (3.2) & 49.1 (13.8) & 0.6 (0.3) \\ \hline \multirow{2}{*}{Winter} & Upper & 09–22 January 2018 & \(-\)2.3 (5.7) & / & 2.3 (1.4) \\  & Ground & 10–18 January 2018 & \(-\)1.0 (6.4) & 53.0 (14.2) & 1.1 (0.8) \\ \hline \hline \end{tabular}
\end{table}