The Influence Of Urban Viaducts On The Diffusion Of Fine Particles In Street Canyons

With the improvement of living standards,the number of motor vehicles increase quickly,so motor vehicle exhaust has become one of the major sources of urban air pollution.In particular,particulate matter in motor vehicle exhausts could cause great harm to the human's respiratory system.The continuous advancement of urbanization process puts forward a higher demand for the development of urban transportation.Obviously,viaducts can expand urban road space and improve transport efficiency.However,viaducts will affect the microenvironment within the canyon and cause a large accumulation of particulate matter in the canyon,which not only threatens the health of pedestrians,but also has an adverse effect on the indoor air quality.In this paper,the field monitoring of particle mass concentration and CFD numerical simulation were used to study the influence of urban viaducts on the pedestrian exposure and fine particulate matter dispersion in street canyons.The results provide a reference for evaluating the environmental effects of urban road design and planning.Field monitoring of the concentration of particulate matter and meteorological factors was conducted at 1.5m above the ground on the sidewalks in the street canyons with viaduct coverage and no viaduct coverage.The results indicate that the concentration of PM10,PM2.5 and PM1 were higher on the sidewalks in the street canyons with viaduct coverage.In terms of pedestrian exposure doses,the difference in respiratory deposition of PM10 is relatively larger between two cases,so viaduct coverage has a greater influence on the deposition of particulate matter in the nasal cavity.The mass concentration of particulate matter is positively correlated with wind speed and humidity,and negatively correlated with temperature.The smaller the particle size is,the higher the correlation is,and there is a certain degree of synergy between meteorological factors.Due to the limitation of field monitoring situation,this paper used Fluent combined with discrete phase model to numerically simulate the influence of viaduct noise barriers and temperature difference between ground and air on the air flow field in street canyons and PM2.5 concentrations in streetside buildings.Results show that the concentration of PM2.5 at the height of 1.5m above the sidewalk with viaduct coverage is 49% higher than that without viaduct coverage,which is in consistent with the observed trend.PM2.5 concentration in the leeward building is higher than the windward side.In the vertical direction,the living environment on the middle floor is better,the tenth floor and the eleventh floor are slightly inferior,and the first and second floors is the worst.Viaduct will hinder the spread of particles in the street canyons,resulting in the concentration of indoor particulates increased and the concentration of particles in the floors that are at the same high level with viaduct increased too.Solar radiation will affect ground temperature,and the ground heating can improve the air recirculation in the canyon.As the ground-air temperature difference(?T)increases,the thermal buoyancy increases,so the concentration of indoor PM2.5 on both sides of the canyon with no viaduct and only viaduct coverage decreases.However,when the viaduct with barrier,the situation is different.When ?T=10K,the PM2.5 concentration of indoors increases compared with ?T=0K,therefore,when the temperature difference is about 10 K,the barrier cannot improve the bad effect of viaduct particulate matter on the indoor environment.