# Air Quality Improvement in Urban Street Canyons:
## 1 Introduction  
Air quality in large cities is determined by the cumulative impact of many sources of emission, among which, road transport is usually of great importance, especially in areas where air emissions from other sources have already been significantly reduced. The direct effect of road traffic emissions is the increase in the concentration of some air pollutants in the vicinity of communication routes, mainly nitrogen oxides (NO\({}_{\rm x}\)) and particulate matter (PM\({}_{10}\) and PM\({}_{2.5}\) fractions), which is confirmed by the results of measurements recorded at communication air quality monitoring stations relative to urban background stations [1; 2; 3; 4; 5]. Street canyons with high traffic volume and poor ventilation are particularly exposed tothis type of impact [6; 7; 8]. Inside these canyons, air emissions occur, including the primary emission of dust and gaseous pollutants due to the movement of motor vehicles (engine emissions and abrasion of brakes, tires, road surfaces, etc.) and the secondary dust emission from the road surface [9; 10; 11; 12]. Furthermore, in street canyons, the mixed zone of pollutants is limited by the compact pattern of buildings, resulting in a greater impact of emissions from road transport on air quality. Its volume depends not only on meteorological conditions, but also on numerous other factors, the most important of which include: the canyon geometry and its location in relation to the dominant wind directions [13; 14; 15], the measures applied to reduce the resuspension of the road silt [16; 17; 10], the presence of additional green infrastructure [18; 19; 20; 21], as well as the type and amount of substances flowing in from neighboring areas, which can trigger chemical transformations of pollutants in the air of [22; 23]. The impact of road transport on air quality can be reduced e.g., with the use of various traffic management activities [24; 25; 26; 27; 28]. Some of these measures may e.g., be aimed at reducing the volume of traffic by shifting transit traffic to ring roads, limiting the capacity of individual road sections or introducing other restrictions or improvements, but sometimes they also have a significant effect on the structure of vehicles travelling in a given area (introducing traffic ban for certain categories of vehicles and creating low-emission zones). The degree of changes in air pollutant concentrations resulting from such actions depends mainly on their type and scale of implementation, as well as on the background level of the analyzed pollutants in relation to the impact of road transport itself. Therefore, the demonstrated effects of improved air quality in the vicinity of communication routes are usually significantly lower than the level achieved in reducing air pollutant emissions from road traffic alone [24; 26]. The selection of a specific traffic management strategy in terms of improving air quality in a given city should be based on the assessment of its potential effects. This could be done by estimating changes in air pollutant emissions resulting from the strategies analyzed and modeling the impact of these changes on air quality. Only after the implementation of a given strategy are the assumed environmental effects usually verified, based on the observed changes in the concentrations of pollutants in the air in the zone covered by these changes [25; 29; 30]. The analysis of environmental effects can also be followed by a health, economic and landscape impact assessment [31; 32; 33]. The purpose of this research study is to assess the effects of the expected reduction in the emissions of specific air pollutants (NO\({}_{\rm x}\), PM\({}_{10}\) and PM\({}_{2.5}\)) from road transport for several hypothetical traffic management scenarios analyzed for selected street canyons of Krakow, one of the most crowded cities in Poland and in the world [34].