Study On The Impact Of Meteorological Factors And Weather System On Ozone Pollution In Chengdu

Ozone is a kind of trace gas,which has strong oxidizing properties.When the ozone concentration is high,it will cause a certain degree of damage to human health.Previous studies have shown that meteorological factors have an important effect on ozone pollution in Chengdu.Most of the existing studies have analyzed the relationship with them by using the linear methods,such as multiple regression and correlation coefficient,and few involved the mechanism of O₃generation and the interaction between meteorological factors.However,according to the measured data,it is not the linear relationship between meteorological factors and O₃concentration.Therefore,further studies are needed to fully and accurately reveal the effects of meteorological factors on ozone in Chengdu.In addition,as a unique weather system occurring in the southwest of China,the research on the southwest vortex at home and abroad mostly focuses on the definition,time variation characteristics,movement characteristics and the relationship with precipitation.There is little research on the influence of the southwest vortex on the concentration of atmospheric pollutants.Therefore,the air quality monitoring data,meteorological monitoring data and sounding data in Chengdu from 2015 to 2019 are used in this study.Firstly,on the basis of analyzing the time distribution characteristics of O₃concentration,the effects of temperature,solar radiation intensity,relative humidity,mixed layer height,air pressure,wind direction and wind speed on O₃concentration in Chengdu were qualitatively analyzed by using the statistical methods,combining with the chemical mechanism of O₃generation.Then,based on the monitoring data,the BP neural network ozone prediction model was established.The sensitivity index of the ozone to the meteorological factors in Chengdu was calculated by the conditional entropy analysis and the global sensitivity analysis method based on the BP neural network simulation,so as to determine the primary and secondary order of the meteorological factors that have significant effects on the ozone concentration in Chengdu.In addition,combining with the statistical data of the southwest vortex,the influence of the southwest vortex on the O₃concentration in Chengdu is analyzed based on the statistical method,in order to provide a scientific basis for the prevention and control of O₃pollution in Chengdu and the formulation of subsequent corresponding emission reduction plan.The main research results of this paper are as follows:The concentration of O₃in Chengdu varies greatly in different seasons,with the highest concentration in summer,followed by spring,autumn and winter.The diurnal variation of O₃concentration in Chengdu was single peak and single valley,and the peak appeared at about15 o’clock.Meteorological factors play an important role in the O₃concentration change in Chengdu.The change of O₃concentration in Chengdu is positively correlated with solar radiation and temperature.When the relative humidity is less than 50%,the concentration of O₃is positively correlated with the relative humidity,and while the relative humidity is greater than 50%,the concentration of O₃is negatively correlated with it.The concentration of O₃is negatively correlated with the change of air pressure.Because the enhancement of solar radiation promotes the generation of O₃more than the diffusion of the mixing layer to O₃,the height of the mixing layer is obviously positively correlated with the concentration of O₃,which conceals the real relationship between the two.When the wind speed is less than2m/s,the O₃concentration increases with the wind speed increasing.When the wind speed is greater than 2m/s,the O₃concentration decreases gradually.The ozone pollution in Chengdu mainly appears in the direction of east wind,southeast wind and south wind.The number of nodes in the input layer,the output layer and the hidden layer were set as6,1 and 4,respectively,the activation function was adopted as Re LU function,the learning rate was set as 0.001,and the number of iterations was set as 20,000.Then taking the O₃concentration data and meteorological monitoring data of Southwest Jiaotong University Station from 2016 to 2018 as the learning sample data set,a BP neural network model was established.It has been proved that the prediction results of the model can well reflect the variation trend of O₃concentration,and the model can be used for subsequent sensitivity analysis.The results of conditional entropy analysis and global sensitivity analysis based on BP neural network simulation are consistent.Both show that temperature,relative humidity,solar radiation intensity and mixing layer height are the main meteorological factors affecting ozone concentration in Chengdu,while air pressure and wind speed have no significant effect on O₃concentration in Chengdu.The second-order sensitivity index analysis results show that ozone concentration is sensitive to the interaction between temperature,solar radiation intensity and mixing layer height.The activity of southwest vortex will lead to the decrease of O₃concentration in Chengdu,and the O₃concentration in the summer half year vortex days is significantly lower than that in the non-vortex days.There was no significant difference in O₃concentration between vortex days and non-vortex days in winter half year.The number of the wet vortex is more than the number of the dry vortex in the southwest vortex which affecting Chengdu in the summer half year from 2015 to 2018.When wet vortex occurs,the distribution of O₃concentration in Chengdu is significantly lower than that in dry vortex.The main reason for the decrease of O₃concentration caused by wet vortex is the wet cleaning effect.When the wet vortex is active,the influence of low vortex on O₃concentration is less than that caused by precipitation.Even if there is no precipitation,the activity of vortex will lead to the decrease of O₃concentration in Chengdu.