# Air Quality Improvement in Urban Street Canyons:
## 5 Conclusions  
The conducted research presents the degree of difficulty in the implementation of the task aimed at improving air quality in street canyons as far as the two most severe groups of pollutants are concerned, NOx (NO and NO\({}_{2}\)) and particulate matter (PM\({}_{10}\) and PM\({}_{2.5}\)), especially when there is a relatively high background of these pollutants, characteristic of cities such as Krakow. Four street canyons that vary in traffic volume and two groups of traffic reorganization scenarios were analyzed, for which, clear differences were obtained with respect to the emission levels of the pollutants considered and their concentrations in the air. A significant reduction in street canyon air concentrations of NO (about 15-44%) and NO\({}_{2}\) (about 5-14%), as well as PM\({}_{10}\) (about 4-14%) and PM\({}_{2.5}\) (about 3-13%), compared to the adopted base year, was possible only in the scenarios in which there had been a considerable reduction in the traffic volume of cars with combustion engines driving through a given canyon and the elimination of old vehicles from traffic (only vehicles that met the Euro 4-6 emission standards were admitted). The maximum effects of reducing air pollutant concentrations were possible when only vehicles meeting the Euro 6 emission standard were admitted to traffic, with the additional assumption of the limited possibility of replacing banned cars with new combustion vehicles. This type of additional restriction appears to be realistic in light of the observed trend of replacing vehicles with traditional combustion engines with electric vehicles, and ultimately, with hydrogen ones as well. Among the scenarios considered for changes in car traffic organization, variants aimed at calming traffic in a given street canyon seemed to be a promising solution. For example, the reduced capacity of a given road section may encourage the more widespread use of public transport or bicycles and electric scooters (which can move along bicycle paths) or transfer car traffic to other city zones. Nevertheless, simulations carried out with the use of the Krakow Traffic Model in such cases did not exhibit a significant reduction in the stream of motor vehicles. Therefore, the estimated changes in air pollutants emission were usually small and rarely brought about the desired effects. Great caution should also be exercised in the cases of changes in the speed limits, and a detailed assessment should be made of whether this will result in a decrease or increase in the emissions of individual air pollutants. The impact of changes in traffic organization on air quality can be obscured by background pollution resulting from other emission sources to an extent, depending on the background level and the contribution of road transport emissions to the overall air pollution in a given canyon. In low-traffic street canyons (1-4 million vehicles per year), the average background reduction in relative changes in air concentrations can exceed 70% for NO\({}_{2}\) and 90% for PM\({}_{10}\) and PM\({}_{2.5}\), especially with a high background of these pollutants (close to or exceeding the permissible annual average level). The effects of implementing traffic reorganization scenarios (improving air quality) should be much more visible in street canyons with much higher traffic (e.g., 10-25 million vehicles per year), where the initial contribution of NO\({}_{\rm x}\) and fine dust emissions from road transport (before their limitation) in shaping air quality may exceed 60% and 20%, respectively. The results obtained for the model accuracy assessment (22-29%) and the concentration reductions for individual variants are convergent. This means that it is impossible to determine the reliable effects of air quality improvement using emission estimation and air quality modeling methods.