Numerical Simulation Of Dust Pollution In Lanzhou Based On Urban Canopy Model

Urban air pollution is closely related to people's lives.As urbanization progresses,urban population explosion and the constant demand for healthy living,the urban air pollution problem is getting more and more attention.Research on large cities and urban agglomerations has led to some understanding of urban air pollution processes,but research on urban air pollution under the influence of complex topography remains one of the important current issues.Lanzhou is an important industrial city in northwestern China,with the main city being a river valley crossed by the Yellow River,which is a typical mountain city.In the special mountainous terrain and the continuous urbanization process,the air pollution in Lanzhou has new changes.Meteorological factors affect the environmental capacity of cities and the dispersion capacity of air,especially in mountainous areas where meteorological factors are changing drastically under the influence of topography and which determine the transport and distribution of air pollutants under certain emission source conditions.In this paper,before conducting the numerical simulation of air pollution in Lanzhou,firstly,the ability of the model to simulate the meteorological elements was tested,and the two weather processes in Lanzhou were simulated using the WRF model coupled with the different urban canopy models,and the simulation results of the different urban canopy models were analyzed by comparing the observation stations.Secondly,the single-layer urban canopy model,which is well simulated,was coupled to the WRF-Chem model and used for numerical simulation of air pollution processes in the Lanzhou region to analyse the impact of the urban canopy model on air pollution and its meteorological elements.Finally,using the WRF-Chem model coupled with the single-layer urban canopy model,a sensitivity test was conducted to analyze the impact of urbanization on air pollution in Lanzhou based on the development trend of Lanzhou Xinqu construction and building height changes in urban areas.The main conclusions were as follows.(1)Different urban canopy models have certain simulation capabilities for the two weather processes of winter cooling and summer high temperature in Lanzhou region,but the simulation capabilities are different in urban and rural areas,and the simulation capabilities for each element are the same in rural sites,but there are differences in simulation results for urban sites.The four models of WRF,coupled single-layer urban canopy,coupled multi-layer urban canopy without considering indoor and outdoor energy exchange,and coupled multi-layer urban canopy with considering indoor and outdoor energy exchange process have inadequate daytime high-temperature and nighttime low-temperature simulation capabilities for the simulation of 2m urban temperature.After coupled urban canopy model,the simulation results of 2m temperatures had a significant impact.After the coupled urban canopy model,the simulation results for 2m air temperature had a significant effect,and the simulation results of coupled single-layer urban canopy model were the best for both weather processes,with better mean deviation,root mean square error and correlation coefficients for the simulation results.And the two coupled multilayer urban canopy models are still somewhat inadequate for meteorological element simulation.All four modes have good simulation capabilities for the 10 m wind direction simulation,while the simulation results of the two models coupled with multi-layer urban canopy are closer to the observations and the model coupled with single-layer urban canopy is the next best.All of the four models show some ability to simulate daily changes of urban heat islands.During the winter cooling process,the model coupled with a single-layer urban canopy has better simulation capability for the urban heat island effect;during the summer high temperature process,the model WRF,the model coupled with a single-layer urban canopy and the model coupled with a multi-layer urban canopy considering indoor and outdoor energy exchange have better simulation capability for the urban heat island effect in Lanzhou City.In addition,during winter cool-down,the night-time boundary layer heights simulated by the two models coupled multilayer urban canopy were high.The average height of the urban boundary layer simulated by the model coupled urban canopy is higher than that of the WRF model,and the high value center is concentrated towards the southern part of the city,and the deviation of the model coupled single-layer canopy is small;During summer high temperatures,the urban boundary layer height simulated by the model coupled with the single-layer urban canopy is low during the day and high at night,but the deviation is not significant.The average height of the urban area boundary layer simulated by the model coupled multi-layer urban canopy considering indoor-outdoor exchange is higher than that in the WRF model and the rest models are lower.In summary.The simulation results of the single-layer urban canopy model have minimal bias,and the WRF model coupled single-layer urban canopy has a better simulation capability.(2)The simulation of air pollution in Lanzhou based on the urban canopy model,using the single-layer urban canopy model with better integrated simulation capability of meteorological elements.In this paper,,in order to analyse the impact of coupled urban canopy patterns.the WRF-Chem model and the coupled monolayer urban canopy model are used to simulate the dust pollution process in the Lanzhou region.The results show that before the dust pollution,the near-surface wind speed increased,the convective motion was obvious,the humidity decreased,and the height of the boundary layer increased;with the pollutant concentration increased,the near-surface wind speed decreased,the convection decreased,the layer stability was strengthened,and the height of the boundary layer decreased.The WRF-Chem model coupled single-layer urban canopy model simulated high near-surface contaminant concentrations,long contamination durations,and contaminant concentrations in the contamination persistence phase are closer to the observed results.The WRF-Chem model coupled with the single-layer urban canopy model simulates a stronger and longer duration of structural strength of the inverse thermosphere over the urban area of Lanzhou,as well as an overall lower wind speed below the inverse thermosphere.At the beginning of the air pollution process,the air pollutants simulated by the model coupled single-layer urban canopy are more widely distributed and the air pollutants accumulate in the upwind direction of the city,intensifying localized air pollution.In the middle of the air pollution process,air pollutants are elevated in urban areas,and after coupling the urban canopy model,local circulation is evident in urban areas,but the pollutant concentration distribution near the ground is less affected by local circulation.Later in the air pollution process,after coupling the urban canopy model,the air pollutants in the urban center are less diffuse,causing air pollution for longer periods.(3)In order to analyze the impact of urbanization development on air pollution,sensitivity experiments on the urbanization of Lanzhou New District and the height perturbation of urban buildings were conducted in this paper.After widespread urbanization,Lanzhou New District 2m temperature has significantly increased,the daily warming time in advance,the cooling time delay;The wind speed in the new district has been significantly reduced and there is a "bypass" phenomenon around the new district.After the urbanization of the Lanzhou New District,when the Lanzhou region has a large northerly wind,the wind direction of the Lanzhou urban area from northerly wind to easterly wind,that is,the construction of the Lanzhou New District affects the Lanzhou regional wind farm to a certain extent.In addition,the construction of the Lanzhou New District will cause the local boundary layer to increase in height.After the construction of the Lanzhou New District,the local pollutant concentration in the New District decreases at the beginning of the air pollution process,but the air pollutant concentration in the surrounding areas increases,covering the Lanzhou urban area,the pollution level in the urban area increases,in the middle of the air pollution,the impact of the construction of the New District decreases,and in the later stages of the air pollution,the construction of the New District will cause pollutants to stay in the New District and prolong the local air pollution time.Because of increased building height perturbations in urban Lanzhou,temperature and wind speeds in the urban canopy have increased,and the height of the daytime boundary layer has increased.In addition.The rate of transport of air pollutants upstream to urban areas slows down during the early stages of the air pollution process,and the pollutants accumulate on the upwind side and spread to a wider area.By the middle of the air pollution process,air pollutants are transported to urban areas with northerly winds and accumulate more at urban fringe locations,resulting in increased air pollution in urban Lanzhou.During the air pollutant dissipation phase,air pollutants are trapped in urban areas due to the increased height of urban buildings,prolonging the duration of air pollution.