## 1 Introduction  
The problem of deteriorating air quality in urban areas has become one of the major challenges of recent times. With the rapid growth of metropolises, the problem of airborne pollution increases [1]. Considerable attention is therefore given to solutions that may reduce the concentration of harmful compounds, such as nitrogen oxides (NO\({}_{\text{x}}\)), in the air. Short-term exposure to nitrogen dioxide (NO\({}_{2}\)) leads to irritation of the upper respiratory tract, and long-term exposure to NO\({}_{2}\) leads to chronic diseases. Nitric oxide (NO) is significantly less harmful to human health, however, in contact with air it oxidizes to form NO\({}_{2}\). The use of photocatalytic concrete in urban areas can contribute to a reduction in the concentration of NO\({}_{\text{x}}\) in the air. Concrete is the most common construction material in use, hence its adoption to reduce the concentration of NO\({}_{\text{x}}\) is a very promising solution. The implementation of photocatalytic cement-based materials has been the subject of a number of research projects and applications across Europe and North America [2; 3; 4; 5; 6]. The photocatalytic properties of these materials are provided by the use of TiO\({}_{2}\) nanoparticles in cement, or in surface suspension. The mechanism of the photocatalytic reaction of TiO\({}_{2}\) has been described by [PERSON] and [PERSON] [7]. When TiO\({}_{2}\) (semiconductor) is illuminated with high-energy photons, whose energy is equal to or greater than semiconductor band-gap energy, electrons transfer from the valence band to the conduction band. This starts a series of oxidation-reduction reactions with substances adsorbed on the semiconductor surface, which leads to the creation of hydroxyl radicals (OH) (Figure 1). The following process of NOx reduction can be illustrated with the following equations:  
\[\mathrm{NO}+\mathrm{OH}\xrightarrow{\mathrm{hV,~{}TiO_{2}}}\mathrm{NO}_{2}+ \mathrm{H}^{+} \tag{1}\]  
\[\mathrm{NO}_{2}+\mathrm{OH}\xrightarrow{\mathrm{hV,~{}TiO_{2}}}\mathrm{NO}_{3}^{- }+\mathrm{H}^{+} \tag{2}\]  
On a concrete surface, \(\mathrm{NO}_{3}{}^{-}\) is created, which reacts with the cementitious compounds of concrete. Products of this reaction (nitric soils) are removed by rainwater from the concrete surface. As a result of this reaction, the concentration of nitric oxides in the vicinity of the surface is reduced, and the effect of air purification is observed. The photocatalytic properties of concrete with the addition of TiO\({}_{2}\) have been proved in a number of laboratory studies [9; 10; 11; 12; 13; 14]. The results of a research described by [PERSON] [15] indicated that the major factors affecting the reaction are UV light intensity, surface exposure to UV light, pollutant concentration, ambient temperature, and air flow rate. As the research showed, the best results were obtained at high UV intensity, with a temperature above 25\({}^{\circ}\)C, with low relative humidity and low air flow. Another important aspect is the durability of the air purifying capacity over time, especially in the case of pavement materials exposed to abrasion and soil. A case study of a street in Bergamo [16] indicated that paving blocks may still show a satisfactory air purification performance after two years of service. The same study also indicated that the effectiveness of the blocks was strongly dependent on surface cleanliness. Research showed that the reduction of NOx was significantly higher on days when the paving blocks were cleaned. Unfortunately, the number of studies on air purification performance over time is very limited, particularly in relation to the effectiveness of the blocks after long-term usage in a moderate climate, where pavement materials are exposed not only to abrasion and soil, but also to cyclic freezing and thawing through the presence of deicing salt. This paper presents the results of laboratory tests on pavement blocks collected from a bicycle lane in Zielona Gora (Poland).