# Air Quality Improvement in Urban Street Canyons:
## 3 Results
### Results of Modeling Pollutant Concentrations in the Air
#### 3.3.2 Predicted Changes in Air Pollutant Concentrations in the Analyzed Street Canyons
For some variants that only involve the limitation of the capacity of a given road section, that is, the narrowed street section (SOLI-v1) or the reduction of the current permissible speed (BROD-v1, SOLI-v2), the expected effect of reducing the mean annual concentrations in the air of the pollutants analyzed is very low (below 1 ug\(\cdot\)m\({}^{-3}\)) or even deterioration of air quality may occur (SOLI-v1, SOLI-v2, KRAS-v1), especially regarding NO\({}_{x}\) (Table 5). The highest absolute increase in the mean annual concentration of NO\({}_{x}\) in the air compared to the baseline variant (by approximately 9 ug\(\cdot\)m\({}^{-3}\) converted to NO\({}_{2}\)) was obtained in the case of the KRAS street canyon for variant v1, associated with a reduction in the average vehicle speed by several km\(\cdot\)h\({}^{-1}\) (Figure 9, Table S2) and an increase in NO\({}_{x}\) engine emissions (Table 3). Figure 12 presents the average contribution of pollutant emissions from road transport in the street canyons analyzed in shaping air quality in a given canyon and variant, obtained from modeling using the OSPM model. This contribution is definitely higher in the case of NO\({}_{x}\) and lower in the case of PM\({}_{10}\) and PM\({}_{2.5}\), and also depends on the amount of traffic in a given canyon. The pollution background that obscures the possible relative improvement of air quality in the canyon resulting from the implementation of the analyzed variants of traffic reorganization is approximately constant for a given canyon and substances (Table 5, Figure 13). However, the lower the background level is of a given substance compared to its concentration in the air, which is only the result of emissions from road transport occurring in the analyzed street canyon, the smaller its effect (Figure 12). The average values of the background diminution rate (BDR) of the relative changes in air concentrations of NO, NO\({}_{2}\), NO\({}_{x}\), PM\({}_{10}\) and PM\({}_{2.5}\) in the street canyons with lower traffic (BROD, SOLI) were estimated at the levels of approximately 60%, 80%, 68%, 91% and 93%, respectively (Figure 13). Analogous mean BDR values for the relative changes in the concentrations of these pollutants in the air in the street canyons with heavier traffic (LIMA, KRAS) were obtained at the following levels: 28%, 66%, 39%, 79% and 82%, respectively. Figure 11: The matrix of changes in the mean annual concentrations of the air pollutants analyzed in individual street canyons captured for variants v1, v2 and v3 compared to variant v0 (with 95% confidence intervals listed in parentheses). This means that the pollution background has a large impact on obscuring the effects of improving air quality obtained as a result of reducing traffic-related emissions. This impact is greater when the background level of a given substance is higher and its emission from a given road section is lower. The differences obtained in this respect for the analyzed substances and street canyons indicate that local actions aimed at reducing emissions from road transport should be more visible in terms of lowering air concentrations of pollutants such as NO and NO\({}_{2}\) (NO\({}_{\rm x}\)) than PM\({}_{10}\) or PM\({}_{2.5}\).