## 3 Results and Discussion
### Spatial Variations of Air Pollutants
#### 3.4.2 Vertical Distribution of Air Pollutants  
The vertical distributions of air pollutants in the summer and winter periods are shown in Figure 7 (cold wave and polluted periods are displayed separately). Vertical differences in pollutant concentrations were more distinct in summer than winter. However, the vertical differences were smaller than the horizontal ground-level differences among the sub-sites. In summer, there was a more obvious elevation in the ground level NO\({}_{2}\) concentration (69.9% higher relative to 132 m) than the ground level CO concentration (11.4% higher relative to 132 m), likely due to the higher urban background concentrations of CO relative to traffic emission impacts. Consistent with the NO\({}_{2}\) distribution, the ozone concentration was lower at ground level, indicating that the ozone concentration in the urban center was locally controlled by NO\({}_{\rm X}\) emissions from traffic. There were slight vertical differences in mean PM\({}_{2.5}\) concentrations, but the upper 75% and 90% concentrations clearly increased as the altitude increased, implying that photochemical formation in the atmosphere dominated over the direct traffic emissions of PM\({}_{2.5}\) in summer.  
In the cold wave period, the vertical profiles of pollutants were the opposite of those in summer, however, the difference in concentration with altitude was less significant (Figure 7e-h). Ground-level CO was slightly higher than at greater heights, but the concentration at 132 m was higher than at 15 m. The NO\({}_{2}\) concentration was slightly higher at 132 m. We could not fully explain the elevated upper-level NO\({}_{2}\) concentrations with this experimental setup, but the ground level concentrations at this sub-site (Site-1) were lower than at other sub-sites, unlike the summer results, probably due to the more efficient ventilation (higher ground-level wind speeds) caused by persistent westerly prevailing winds during the cold wave period (as discussed in Section 3.1). The relatively high concentrations at 132 m for the primary pollutants (CO and NO\({}_{2}\)), and lower concentrations for O\({}_{3}\) and PM\({}_{2.5}\) might be attributed to the transport of urban plumes around the monitoring domain under the restricted photochemistry and vertical mixing conditions in wintertime.  
In the winter polluted period, the shape of the vertical profiles was similar to those in the cold wave period (Figure 7i-l), but with higher concentrations and slightly smaller differences (median differences of 5%, \(-\)1%, and 6% for CO, NO\({}_{2}\), and PM\({}_{2.5}\), respectively). Ozone was not detected in the whole column due to the very high NO\({}_{\rm X}\) concentration. The high concentrations of primary pollutants at all monitoring heights indicated stagnant me teorological conditions and the regional accumulation of pollutants over the greater Seoul metropolitan area. However, ground-level PM\({}_{2.5}\) was slightly higher than the upper-level concentrations, implying that local traffic effects were added to the regionally produced and/or long-range transported PM\({}_{2.5}\) in highly polluted periods [28].