Simulation Study Of PM_2.5and O₃ Pollution Formation Mechanisms And Control Strategies In Wuhan Under Typical Scenarios

Regional air compound pollution prevention and control will be the focus of attention for improving regional ambient air quality in China for a considerable period of time in the future.This paper carried out a study on the current air quality characteristics of Wuhan,taking 2019 as the base year and taking into account the COVID-19 epidemic closure period in 2020,to identify the main factors affecting the air quality in Wuhan;combined with multi-source air pollution source emission data,a study of air pollution source emission reduction targets based on environmental capacity to guarantee excellent air quality in Wuhan was carried out;based on the above study,four typical scenarios of air quality changes in Wuhan were designed,and the formation mechanism and control strategy of PM_2.5 and O₃ pollution in Wuhan under typical scenarios and special scenarios of epidemic closure period were investigated using the WRF-CMAQ coupled air quality model,with a view to providing theoretical support for the control of air pollution in Wuhan and the development of a synergistic PM_2.5 and O₃ emission reduction control scheme.The main findings of the study are as follows:1.By evaluating the current air quality situation in Wuhan,it was found that PM_2.5 and O₃ are the main factors affecting the excellent air quality rate in Wuhan in winter-spring and summer-autumn,respectively,while the main urban area is the most seriously affected administrative area within Wuhan by air pollution.The atmospheric environmental capacity of Wuhan in 2019 was obtained using model estimation,and it was found that the emissions of NOx（with NO₂ as the target）,PM_2.5 and PM₁₀ exceeded 8.54%,65.33%and 5.65%of the atmospheric environmental capacity,respectively.And got the long-term emission reduction goals to ensure excellent air quality in Wuhan:the need to focus on the control of power,industrial boilers,metallurgy,building materials and other key point sources,as well as mobile sources,dust sources and biomass combustion sources and other surface sources of NOx and particulate matter.2.By simulating the pollution processes of typical PM_2.5 and O₃ in Wuhan,we found the similarities and differences in the formation mechanisms of different types of pollution.From the perspective of pollution types,the source pathway of pollutant formation in Wuhan is influenced by upwind urban transport during exogenous pollution,which has a significant advection effect;unfavorable meteorological conditions in Wuhan during endogenous pollution make the sink pathway contribution concentration of pollutant formation much lower than the source pathway contribution concentration;and superimposed pollution has both of these characteristics.In terms of pollutants,the contribution of physical processes in the source-sink pathway of PM_2.5 pollution formation reaches over 97%,and the contribution of local emissions in the source pathway is the highest;while the contribution of both physical and chemical processes in the source-sink pathway of O₃ pollution formation is significant,and the contribution of chemical processes in the sink pathway is over 71%.3.The sensitivity characteristics of PM_2.5 and O₃ to precursor emissions in Wuhan during a typical pollution process were obtained using CMAQ-DDM technique simulations.During winter PM_2.5 pollution in Wuhan,more than 50%of the PM_2.5 concentration contribution comes from regional background concentration values,requiring a focus on local emission contribution;the percentage of secondary inorganic aerosols in PM_2.5components exceeds 60%,with nitrate accounting for more than 30%,and secondary inorganic aerosol concentrations show strong sensitivity to NO_X and VOC_S,and to SO₂ and NH₃ sensitivity is weaker.During the summer O₃ pollution in Wuhan,O₃ showed a strong non-linear relationship with NO_X emissions,with O₃ concentrations showing positive sensitivity to NO_X emissions around 13:00 in the midday,but negative sensitivity throughout the day,while O₃ concentrations showed positive sensitivity to VOC_S emissions throughout the day.4.Both the active control（the Military World Games）and passive control（the COVID-19 epidemic closure period）measures of air pollution sources in Wuhan can reduce the atmospheric concentrations of NO₂,PM₁₀,and PM_2.5 by more than 15%,but all will lead to an increase in atmospheric O₃ concentrations.Among them,the active control leads to a significant reduction in the contribution of local emissions to PM_2.5 concentrations in Wuhan,reducing the contribution of background PM_2.5 concentrations in the region to 45.29%,while making O₃ less sensitive to precursors.Passive control makes VOCs emission reduction no longer an effective measure to reduce O₃ concentrations.This indicates that the regional joint prevention and control of air pollution sources is the key to improve regional air quality.5.Based on the sensitivity characteristics of PM_2.5 and O₃ in Wuhan under typical scenarios,the corresponding pollution control strategies were obtained:short-term PM_2.5and O₃ concentrations can be reduced in Wuhan in different seasons by the strategy of controlling VOCs emissions,provided that the conventional control measures of air pollutants remain unchanged.Among them:the emission control strategy of prioritizing the reduction of VOCs in the winter pollution process and NOx and NH₃ in the autumn can reduce the concentration of atmospheric PM_2.5;the emission control strategy of reducing VOCs in the summer pollution process and reducing NOx and VOCs in large quantities in the autumn can effectively achieve the reduction of atmospheric O₃ concentration.