# Evaluation of NOx Reduction Effect and Impact on Asphalt
## 1 Introduction  
Recently, air pollution has negatively affected various social, environmental, and industrial aspects. In particular, in cities with a high population density, roadside air pollution caused by automobile exhaust gas is emerging as a serious environmental problem [1, 2]. Nitrogen oxide (NOx), which is emitted in the highest quantities from automobiles, is known to be harmful not only to the air environment but also to human health. It is also well known as a harmful gas component that causes diseases of the respiratory system as well as photochemical smog and acid rain [3]. To solve this problem, research has been conducted to reduce exhaust gas emissions from automobiles, and solutions such as the use of clean fuel with fewer pollutants, and the installation of automobile exhaust system purification equipment, have been proposed. However, owing to the continuous increase in the number of vehicles operated, the emission of nitrogen oxides continues to steadily increase, and various alternative technologies for reducing NOx emissions are required. Photocatalysts are used to purify air pollution caused by NOx, and titanium dioxide (TiO\({}_{2}\)) is known to be the most effective material among various photocatalysts [4, 5, 6, 7]. TiO\({}_{2}\) has better chemical stability than other photocatalysts and is not corroded by most acids, bases, and organic solvents, so it is actively used in various fields [8, 9, 10, 11, 12]. The application of TiO\({}_{2}\) in the construction field is being studied for application to large surface areas, such as the exterior of buildings and road pavements [13, 14, 15, 16]. In the case of road pavements applied with TiO\({}_{2}\), as shown in Figure 1, electrons (e\({}^{-}\)) and holes (h\({}^{+}\)) are formed due to a photocatalytic reaction when exposed to ultraviolet (UV) light. The generated electronsand holes react with O\({}_{2}\) and H\({}_{2}\)O in the air, respectively, to generate active oxygen of superoxide anion (O\({}_{2}\)\({}^{-}\)) and hydroxyl radical (OH\({}^{-}\)) on the surface of TiO\({}_{2}\), thereby decomposing NOx into nitrate (NO\({}_{3}\)\({}^{-}\)). This nitrate is washed away by rain in the form of an aqueous solution of nitric acid (HNO\({}_{3}\)). This complex reaction mechanism is used to reduce NOx in the atmosphere and remove NO\({}_{3}\) through biological denitrification in the groundwater zone [17; 18]. There are two possible methods for applying TiO\({}_{2}\) to the asphalt pavements that occupy the majority of urban roads. One method is to produce an asphalt mixture by adding TiO\({}_{2}\) powder during the mix design, and the other is to coat the existing asphalt pavement surface with TiO\({}_{2}\) mixed with liquid adhesive [19; 20; 21]. The method of directly mixing TiO\({}_{2}\) with the asphalt mixture is not economical because exposure to UV is limited, and a large amount of photocatalyst is used [22; 23; 24]. To overcome these shortcomings, various methods of coating the surface of an asphalt pavement with TiO\({}_{2}\) solution have been recently attempted. These include: mixing TiO\({}_{2}\) with an aqueous solution, or asphalt binder, and then spraying it on the road surface; using TiO\({}_{2}\) as an asphalt modifier; coating TiO\({}_{2}\) on waste rubber and then spraying the rubber mixture on the pavement surface; and coating the road surface with an asphalt emulsion mixed with TiO\({}_{2}\)[25; 26; 27; 28; 29; 30; 31]. However, these methods are complicated and limited in manufacturing and construction and do not provide sufficient durability and NOx removal efficiency for field applications. In addition, because TiO\({}_{2}\) uses UV as an energy source, it may promote the aging and oxidation of binders used for coatings and existing asphalt pavements [32; 33]. Meanwhile, one of the main challenges in testing construction materials (paint, cement, tile, asphalt, concrete, etc.) with TiO\({}_{2}\) is finding a way to determine the NOx decomposition effect.