Study On The Diffusion Of Vehicle Emissions And The Influence Of TiO₂-based Photocatalytic Concrete Road In Urban Street Canyon

In recent years,the scale of cities has become larger and larger,and the population of big cities has become increasingly dense.With the expansion of the urban road network,the number of cars is increasing.Such changes aggravate the pollution in the city to a certain extent,and put forward higher requirements for the environmental protection of the city.The urban street canyon is densely populated,and its structural characteristics lead to the accumulation of pollutants easily,which has a great impact on peoples health.Therefore,it is of great significance to study the diffusion characteristics of pollutants in urban street canyon and the prevention plan.Research established a two-dimensional 1:1 scale model of street canyon structure by using computational fluid dynamics（CFD）method.According to the different characteristics,pollutants can be divided into particle pollutants and reactive pollutants to establish the corresponding pollutant diffusion model.The influence of the structure of street canyon on the diffusion of particulate pollutants,the ability of TiO₂ concrete road to reduce nitrogen oxides and related factors are mainly considered.Wind tunnel test data and other scholars simulation data are used to validate the model established in this paper.The applicability of this model in solving the problem in this field is verified by verification.The model and parameters used in this paper can also be used as a reference for subsequent research in this field.After that,the influence of the viaducts and noise barriers on the diffusion of particulate pollutants is simulated.Based on a real road in Shanghai,a simulation model is established to maintain the width height ratio（H/W）of the original street.By changing the height ratio（H1/H2）of buildings on the leeward side and the windward side,the pollutant diffusion law in the symmetry（H1/H2=1）,step up（H1/H2<1）and step down（H1/H2>1）street canyon and the influence of viaduct and noise barriers in the interior of the street canyon are summarized.Next,the ideal street canyon with the same aspect ratio（H/W=1）is used to explore the influence of TiO₂ Concrete Road on the concentration of nitrogen oxides in the street canyon and the influencing factors.It is found that the photochemical reaction between TiO₂ concrete road and NO_xcan effectively reduce the NO_x concentration in street canyon.Its effect is affected by many factors.The higher the irradiance,the more obvious the effect of TiO₂ concrete road in reducing NO_x in street canyon.The lower the relative humidity,the more obvious the effect of TiO₂ concrete road in reducing NO_x in street canyon.When the wind speed is low,the conversion rate of TiO₂ concrete road to NO_x in the canyon will increase due to the higher initial concentration of pollutants.Therefore,the existence of TiO₂concrete road can effectively inhibit the harm of high pollution caused by low wind speed to human health.Finally,if TiO₂ reaction layer is allowed to be paved on all three walls of street canyon,the air purification effect will be better,but the conversion rate of pollutants cannot reach three times of the effect when only paved on the road.Therefore,if the cost performance is discussed,TiO₂ reaction layer only on the road is the best option.If only the purification effect is considered,TiO₂ reaction layer can also be paved on the surface of buildings on both sides to achieve the maximum conversion rate.