# Air Quality Improvement in Urban Street Canyons:
## 4 Discussion
Background pollution may reduce or increase pollutant concentrations in the air resulting from the interference of local emission sources depending on the prevailing meteorological conditions (wind speed and direction, height of the mixed layer) and whether the instantaneous background level is lower (the effect of dilution and ventilation) or greater (the effect of concentration and accumulation of pollutants) compared to concentration values resulting from emissions only from local sources [102; 103; 104; 105]. In street canyon, additional vortices of air that flow in from outside the canyon can be formed; therefore, in the case of modeling this type of circulation effect, the location of the canyon in relation to the instantaneous or dominant (in long-term analyses) wind direction is important, as well as the building layout and configurations of the roof and walls [35; 37; 79; 106; 107; 108; 109]. As Krakow is dominated by winds from the west, south-west, east and north-east directions, resulting, e.g., from the city's location in the Vistula River valley [110], the KRAS street canyon orientated transversely to these directions is the least ventilated and most susceptible to the formation of air vortices inside the canyon. These turbulences intensify the process of mixing air and exhaust gases as well as the resuspension of road dust. The average annual contribution of traffic-related particles in total PM\({}_{10}\) and PM\({}_{2.5}\) air concentrations in the KRAS canyon in the base variant was 22% and 19%, respectively (Figure 12), that is, at a level similar to the result obtained for the 29-Listopada Av. street canyon in Krakow [26], also located transversely to the dominant wind directions. Due to the reduction in primary and secondary dust emissions implied for variants v1-v3, the contribution of these sources to total PM\({}_{10}\) and PM\({}_{2.5}\) concentrations in the air for the analyzed canyon obtained as a result of modeling decreased proportionally to reduction in their emissions. It should also be noted that the results of the modeling of PM\({}_{10}\) and PM\({}_{2.5}\) air concentrations are also determined in this case by the hourly variability of the background of these pollutants obtained from the air quality monitoring stations and originating from all emission sources that affect the measurement results. Thus, particles formed as a result of chemical transformations of gaseous precursors could also be included with this background pollution. The results of studies carried out using dispersion models taking into account secondary chemical reactions that occur in street canyons or evaluating the origin of fine particulate matter in urban air indicate that gaseous pollutants emitted from road traffic and other sources are important precursors for the formation of secondary inorganic and organic aerosols in the air near the streets [111; 112; 113]. Therefore, the reduction of secondary aerosol precursor emissions should contribute to a further reduction in the background of fine particulate matter in urban air, and thus, to its global background as well, due to the large impact of anthropogenic emission sources located in urban areas on the level of aerosols in the atmosphere [114]. The results of the OSPM model validation carried out for 2017 in the KRAS street canyon (Table 4) were at a level similar to the known results of the air quality model evaluation for street canyons [70; 115; 116], including the results of the evaluation carried out for the same canyon, but earlier [45; 46]. In the literature, there are also cases of even higher accuracy of the predicted results [117; 118; 119], but they were based on the results of 24 h, not 1 h forecasts. The results of the model evaluation indicated that the modeling uncertainty was approximately 22-29% in relation to the observed average. On the other hand, the obtained percentage levels of reduction in air concentration values for some variants were within the range of this uncertainty.