# Modeling and Optimization of NO2 Stations in the Smart City of Barcelona
## 5 Computational Results
### Behavior of Measured NO\({}_{2}\) Concentrations  
The obtained results highlight the importance of temporal and spatial variability in air quality analysis, demonstrating how local conditions and weekly traffic patterns can have a significant impact on NO\({}_{2}\) concentrations. Figure 3 shows the average value per year for each time slot at each station. Although most stations exhibit a similar pattern, with peak concentrations observed around 9:00 a.m. and another local peak in the late hours, the concentration ranges differ across stations. Similarly, Figure 4 shows the average value per year for each time slot, considering  
the day of the week. This is because it is known that traffic patterns depend on the day of the week, which directly affects NO\({}_{2}\) concentrations [63]. In this case, the values from Monday to Friday are similar, while weekends show a different behavior, with significantly lower concentrations. Considering the adaptation periods proposed by the WHO to reduce NO\({}_{2}\) concentration levels (40 \(\upmu\)g/m\({}^{3}\) for adaptation level 1, 30 \(\upmu\)g/m\({}^{3}\) for adaptation level 2, and 20 \(\upmu\)g/m\({}^{3}\) for level 3), it can be concluded that the city of Barcelona currently falls within adaptation level 2 regarding the NO\({}_{2}\) concentration limits established by the WHO. The WHO recommends a daily average NO\({}_{2}\) concentration of 25 \(\upmu\)g/m\({}^{3}\) (level 3 in the air quality guidelines), which should not be exceeded on more than 3 to 4 days per year. Figure 6 illustrates the daily average values during the study period alongside the recommended daily average limit (dashed red line). As observed, the number of days surpassing the threshold per year exceeds the 3-4 days limit. Taking a more flexible approach and considering the adaptation period to reduce NO\({}_{2}\) concentrations, the maximum daily concentration could be considered to be 120 \(\upmu\)g/m\({}^{3}\) (adaptation level 1) and 50 \(\upmu\)g/m\({}^{3}\) (adaptation level 2). If we consider the adaptation periods rather than the strict guidelines, it can be concluded that the city of Barcelona is at adaptation level 2 but still far from achieving compliance with the guidelines. Figure 5: Annual mean of each station with the WHO limit value as the 24 h average (yellow) and limit annual average 2023 (red). Figure 6: Daily average of NO\({}_{2}\) concentrations in the period under study. Figure 7 presents a wind rose diagram depicting the average wind direction trends for each station throughout the study period. The differences in altitude among the stations, combined with their proximity to the sea, result in varying wind rose diagrams, despite the stations being relatively close to one another. Station X4, located nearest to the sea, experiences winds from multiple directions. In conjunction with the other variables studied, a similar behavioral pattern is observed across the stations, with stations X4 and X8 exhibiting more comparable behavior than station D5, which is situated further inland. The dendrogram illustrating the average concentrations at the air quality stations reveals interesting clustering patterns among the stations (Figure 8a). Station 58 emerges as a distinct cluster, with a different behavior from any other station due to its location in the city's peripheral areas. On the other hand, stations 44, 50, and 43 exhibit similar behavior, indicating their central location in the city's high-traffic zone with increased concentrations. Likewise, stations 4 and 42 demonstrate identical patterns, while stations 57 and 54 share similar behavior. These two groups of stations are clustered together, forming a distinct cluster defined by their proximity to the city's peripheral highways. For the analysis of the meteorological stations, the representation of wind speed along the x-axis and y-axis is utilized (Figure 8b). This method demonstrates the existence of two distinct groups with different behaviors.