## 3 Results and Discussion
### Temporal Variations of Air Pollutants in the Various Micro-Environments
#### 3.3.2 Diurnal Variations  
Sumertime diurnal variations on weekdays showed distinct bimodal distributions for CO and NO\({}_{2}\), with clear differences in pollutant concentrations among the different micro-environments (Figure 6a,b). This is a typical pattern of primary traffic-related pollutants in urban areas due to the effects of morning and evening commuting under relatively stable atmospheric conditions. In this region, daytime traffic rates were not noticeably different from those observed during commuting periods. Therefore, the reduced daytime concentrations of primary pollutants were likely caused by more effective dilution processes due to the increased boundary layer height and more intense turbulence. The O\({}_{3}\) concentration peaked at 13:30 due to photochemical production, with a secondary peak at 17:00 that was likely due to the advection of a large ozone plume from upwind urban regions. The concentration differences in diurnal variations of CO, NO\({}_{2}\), and O\({}_{3}\) among the micro-built environments displayed similar patterns. The concentration differences among the monitoring sub-sites were minimal at night due to there being little traffic but became larger during the morning peak and reached a maximum in the late afternoon due to the different local traffic volumes at each sub-site, followed by rapid dilution of local traffic emissions. The diurnal variations of PM\({}_{2.5}\) were almost identical among the different micro-environments, implying that local (on-site) traffic emissions were not a determining factor of local PM\({}_{2.5}\) levels. However, PM\({}_{2.5}\) also displayed a bimodal distribution, with a primary morning commuting period peak (\(\sim\)6 \(\upmu\)g\(\cdot\)m\({}^{-3}\) elevation) and a smaller nighttime peak (\(\sim\)4 \(\upmu\)g\(\cdot\)m\({}^{-3}\) elevation). Thus, it appears that PM\({}_{2.5}\) was influenced by traffic activities over a larger spatial scale than the monitoring domain. Figure 5: Box plots of the daily variations in air pollutants at each sub-site (dark gray: Site\(-\)1, dark blue: Site\(-\)2, pink: Site\(-\)3, and green: Site\(-\)4). The horizontal dashed lines in the box denote the mean values. Dotted lines with red dots represent the daily mean concentrations from the AQMS. The left panel (**a–d**) shows the daily variations for the summer period, and the right panel (**e–h**) shows those for the winter. The gray shaded area in winter represents the cold wave period, and the red shaded area represents the high PM\({}_{2.5}\) event. Ozone data were used only for the period 11:00–17:00 because of the near-zero concentrations during the night. In winter, there were two distinct weather patterns (a cold wave period with strong winds and a severe PM\({}_{2.5}\) pollution period). Separate diurnal profiles were obtained for the two periods (Figure 6c-h for the cold wave and Figure 6i-l for PM\({}_{2.5}\) pollution). During the cold wave period, CO and NO\({}_{2}\) displayed typical bimodal variations, and the concentration differences between micro-environments were larger during daytime, similar to the summer pattern. Ozone displayed bimodal variation, and the nighttime peak was likely due to low levels of NO (as shown by the minimal NO\({}_{2}\) concentrations), and the sharp decrease in the morning commute period was caused by enhanced NO emissions under the undeveloped boundary layer conditions. The PM\({}_{2.5}\) concentration also had a similar diurnal pattern to that of summer, with a -4 \(\upmu\)g\(\cdot\)m\({}^{-3}\) elevation during the morning commuting period; no concentration differences between micro-environments were observed. During the severe PM\({}_{2.5}\) pollution period, the levels of all pollutants (CO, NO\({}_{2}\), and PM\({}_{2.5}\)) were significantly higher than during other periods. The differences in pollutant  
Figure 6: Weekday diurnal variations of air pollutants at each sub-site (black circles: Site-1, green squares: Site-2, black x: Site-3, blue triangles: Site-4, and orange line: the AQMS).