Objective In recent years, air pollution problems caused by particulate matter have attracted wide attention. Urban streets are easy to accumulate particulate matter from automobile exhaust. The "canyon" shape formed by the street and buildings on both sides can prevent the diffusion of pollutants. Vertical greening can well retain fine particles and is suitable for dust retention in street canyons, which is expected to be an effective method to improve air quality. This research evaluates, analyzes and compares the influence of relevant factors on the dust retention capacity of vertical greening in urban street canyons.

Methods This research implements a field investigation and measurement of vertical greening facilities in Wuhan City to find out the influence of environmental factors on pollutant concentration and verify the reliability of numerical simulation results. Based on an innovative combination of a number of simulations, the research explores the overall influence of six different factors: street canyon height, street canyon width, relative wind direction, leaf area density (LAD), horizontal density of vertical greening, and layout form of vertical greening. The research selects the dust retention amount at both the windward and leeward sides of vertical greening as the experimental index, and summarizes the morphological characteristics of street canyons and wind directions. Altogether 18 experimental conditions are determined by the orthogonal experimental design. The research adopts Navier-Stokes equations to solve the wind field of the ENVI-met model adopted, whose resolution ranges from 0.5 m to 10 m. Selecting the data of typical summer meteorological days in Wuhan as research data, the research assumes that the inhalable particulate matters are emitted from continuous linear sources where the emission height of automobile exhaust is 0.3 m, and distributed bidirectionally in the middle of street canyons. Additionally, the research has the PM₁₀ emission factor of small passenger cars assumed as 0.72 and the emission rate thereof as 180 μg/m·s, and figures ou the emission factor of PM_2.5 by multiplying that of PM₁₀ by a coefficient of 0.7. Moreover, the research adopts the simple plant module was to set three vertical greening modes with different leaf area densities, with the height and width of vertical greening being respectively set as 6 m and 0.5 m. Street length is set as 60 m to best prolong the retention time of pollutants.

Results The pollutant concentration results of the filed measurement show a trend of "single peak and single valley", with the highest and lowest concentrations occurring at 10:30 and 16:30 respectively, and the pollutant concentration on the side near the driveway being slightly lower than that near the vertical greening side. The results of simulation by ENVI-met software show the concentration distribution of PM_2.5 and PM₁₀ at a height of 1.5 m at 20:00 in the test site. The concentration of particulate matters is highest near the source of pollution, and they accumulate on the windward side of the street canyon. With the southeast wind blowing to the northwest, the concentration of pollutants at the eastern point is relatively low, which is consistent with the measured situation. The simulated value is consistent with the measured data after the dimensionless operation process, indicating a high reliability of the ENVI-met simulation for the purpose of this research. For both the leeward and the windward sides, the optimal combination of relevant factors for the best dust retention effects of vertical greening is as follows: street canyon width, 12 m; street canyon height, 6 m; relative wind direction, 45.0°; LAD, 3.0 m²/m³; layout density, 60%; layout mode, distributed layout. In the orthogonal experiment, the highest dust retention amount is 194, 966 μg on the leeward side and 1, 627, 110 μg on the windward side. The research adopts the range analysis method to evaluate the merits and demerits of each factor in the results of the orthogonal experiment. The analysis results show that the width of street canyon, layout mode and layout density have the strongest influence on the dust retention effect on the leeward side or the windward side. The research also establishes a dust retention capacity model for vertical greening to calculate the dust retention amount of vertical greening for street canyons in Wuhan City.

Conclusion The research has the implementation strategy of vertical greening for urban street canyons summarized as follows. 1) Vertical greening is suitable for narrow urban branch roads, where the environment of street canyon is more enclosed, and the particle pollutants are more difficult to spread. The layout of vertical greening in these streets make the best of the dust retention capacity of vertical greening to absorb particle pollutants and achieve the best purification effect. 2) Vertical greening is suitable for the layout of building facades with more windward hours. The dust retention amount of vertical greening on the windward side can reach more than 85% of the total retention amount on the both sides. The layout design strategy of vertical greening can be described as follows: To ensure the best layout of vertical greening, such factors as leaf area density, layout density and layout mode should be respectively set as 2.0 m²/m³, 60%, and discontinuous layout on the leeward side of street canyon, and as 1.0 m²/m³, 60% and discontinuous layout on the windward side of street canyon. This research provides a theoretical basis and puts forward a referential design strategy for the application of vertical greening application. Vertical greening has great application prospects in the reconstruction of branch roads, and the improvement of space comfort and air quality in street canyons.