Study On Air Pollution Characteristics And Its Influencing Mechanism In Dazhou City

With the acceleration of China’s economic development and urbanization,the rapid gathering of people in the city brings more and more atmospheric environment pressure to the city,such as cooking fume,motor vehicle exhaust emissions,pollutants discharged from fireworks and firecrackers set off during the Spring Festival and Lantern Festival,road dust and so on.All make the urban environmental air quality face great challenges.Regional pollution transmission,sandstorm weather,severe weather conditions and pollution emissions from industrial enterprises around the city,without effective pollution control,urban environmental air pollution will become more and more serious,which will seriously affect the health of the people.With the growth of motor vehicle ownership and industrial pollutant emissions,the atmospheric environment of Dazhou city has gradually evolved into a primary and secondary,local and regional compound pollution.Particulate matter pollution is the fundamental cause of the decline of air visibility,haze aggravation and deterioration of ambient air quality in Dazhou.In order to further find out the characteristics of air pollution in Dazhou city,for the environmental management department to develop air pollution control measures,this study takes the urban ambient air as the research object.Through the study of the changing characteristics of urban air pollutants,source apportionment of fine particulate matter,cooking fume,and the CMAQ model forecast system,the air pollution characteristics and its influencing mechanism of Dazhou city are preliminarily revealed.A summary of the primary results are as follows.1.According to the ambient air quality automatic monitoring data in the urban area of Dazhou city and its counties（cities and districts）in 2016-2020,the variation characteristics and influencing factors of atmospheric pollutant concentrations were studied.The results showed that,the SO₂ concentration in Dazhou city meeted the first-level annual average ambient air quality standard,indicating that Dazhou city was not polluted by SO₂,coal-burning pollution had been eradicated.In recent years,due to the obvious increase of motor vehicles in Dazhou city,the concentration of NO₂ had gradually increased.PM₁₀ and PM_2.5 fell significantly year on year,but still exceeded the secondary ambient air quality standard.In all counties（cities and districts）of Dazhou city,except SO₂,the concentration of other pollutants had obvious seasonal variation characteristics.The variation characteristics of O₃ concentration were the highest in summer and the lowest in autumn and winter.CO,NO₂,PM_2.5 and PM₁₀ were highest in winter and lowest in summer.The average monthly concentration curve of SO₂ was relatively gentle,and PM₁₀,PM_2.5,NO₂and CO showed high U-shaped curves on both sides and low in the middle.The overall O₃ concentration showed a wavy and convex curve.During the four seasons,SO₂ and O₃ showed unimodal patterns,while peak SO₂concentrations were high in winter and spring and low in summer and autumn.The peak concentration of O₃ was the highest in summer and the lowest in winter.NO₂,PM₁₀,PM_2.5 and CO all were bimodal,and the two peaks showed similar trends in different seasons.Meanwhile,the linear predicting models of PM₁₀ and PM_2.5,NO₂ and CO in four seasons were established,which provided a reference for further understanding the relationship between pollutants and predicting the trend of pollutant concentration.2.Using the air quality monitoring data of state-controlled air automatic station in Dazhou city from 2015 to 2021,the pollution characteristics of PM_2.5 concentration were analyzed,and the source apportionment was carried out on PM_2.5 with single particle aerosol spectrometer（SPAMS）from May to June of 2021.The results showed that,From2015 to 2021,the annual concentration of PM_2.5 in the urban areas of Dazhou city showed a decreasing trend year by year on the whole,but the annual PM_2.5 concentration cannot meet the requirements of Ambient air quality level 2 standard（35g/m³）（GB3095-2012）;The pollution caused by PM_2.5 was heaviest in winter and lightest in summer.In winter,the concentration of PM_2.5 with wind speed above 1 m/s was significantly lower than that of PM_2.5 with wind speed below 1 m/s,and the high concentration of PM_2.5 was concentrated in the wind speed below 0.5 m/s.The daily concentration of PM_2.5 had significant positive correlation with daily air quality index（r>0.90,P<0.01）,the negative correlation between PM_2.5 concentration and precipitation,and the negative correlation between PM_2.5 daily concentration and daily precipitation（|r|>0.27,P<0.01）was greater than that between PM_2.5 hour concentration and hourly precipitation（|r|>0.06,P<0.05）.From 2015 to 2021,the pollution load of PM_2.5 in Dazhou city changed from 27.2%to 33.1%,and the average value was 29.1%.The pollution load of PM_2.5 was the highest in the conventional six parameters of air quality,and the overall pollution load of PM_2.5 showed a downward trend.PM_2.5 had the highest pollution load ratio in January,which accounted for 39.5%,followed by December and February.The source apportionment results showed that the main components of fine particles were element carbon particles（59.9%）and organic carbon particles（10.1%）from May to June 2021.During the rise of the concentration of fine particulate matter,the main components of fine particulate matter were the increase of the proportion of element carbon particles,mixed carbon and heavy metal particles.In addition,during the pollution period,the proportion of mineral particles was also relatively higher than that of the good period.The sources of fine particulate matter pollution were mainly industrial process（22.7%）,dust（19.3%）,biomass combustion（13.9%）and motor vehicle exhaust（13.4%）.It can be seen that the concentration of fine particulate matter during the monitoring period was mainly affected by industrial process sources,dust sources,biomass combustion sources and motor vehicle exhaust sources.3.In order to reveal the air pollution characteristics of cooking oil fume further,we studied the influence of the cooking oil fume at the civic center ambient air automatic monitoring station in dazhou city.the results showed that,The more closely the smoke exhaust from the restaurant,the more obvious the effect of the smoke on the particulates,and vice versa.At the same time,different monitoring points were affected by the kitchen smoke;The minute difference between PM_2.5 and PM₁₀ were most concentrated in the range of[-50,50)when the kitchen smoke was passing,and the distribution of cooking oil smoke had obvious normal characteristics;The effect of kitchen smoke on PM_2.5 was greater than that of PM₁₀;The concentration of particulates in the cooking oil fume was much higher than that of ambient air,and reached tens of times to dozens of times,and the ratio of PM_2.5 to PM₁₀ was about 20%.Each point we studied was affected by different levels of cooking oil smoke,and the increase or decrease of particulates had a certain correlation,the correlation of PM₁₀ was greater than that of PM_2.5.4.The air quality of Dazhou City in 2015-2019 was studied by using Spearman rank correlation coefficient method,pollution calendar,concentration analysis method and CMAQ prediction model.The results showed that,the change trend of O₃ in the main urban area of Dazhou City in 2015-2019 was significantly higher P<0.05),the quarterly change trend of O₃ concentration was obvious,the highest in summer.August was prone to cause mild pollution caused by excessive O₃.The change trend of CO annual concentration in 2015-2019 was significantly reduced（P<0.05）.The average annual concentration of SO₂、PM₁₀ and PM_2.5 showed a downward trend for 5 years,but the change trend was not obvious（P>0.05）.Pollution in Dazhou city in January and December 2019 was the heaviest,PM_2.5 exceeding the standard was the main cause.The situation of PM₁₀ and PM_2.5 annual average concentrations meeting national secondary standard was grim,and more than half of the days of the whole year PM_2.5 exceed the national secondary standard of the annual average concentration,and the PM₁₀ was nearly half.NO₂ annual average concentration showed an upward trend in five years,but had not yet reached a significant level（P>0.05）.The situation of the annual average concentration reaching the national secondary standard in 2019 was grim,and the NO₂daily average concentration exceed the annual average concentration secondary standard for 212 days in a year.CMAQ model predicted different pollution indexes with different accuracy.The accuracy of CMAQ model to predict PM_2.5 concentration,primary pollutant and air quality grade was less than that of manual prediction,while the accuracy of AQI prediction was higher than that of manual prediction.To sum up,this paper analyzed the correlation of air pollutants in dazhou city in different seasons,which provided a reference for further understanding the relationship between pollutants and predicting the development trend of pollutant concentration.The element composition of fine particulate matter in summer in Dazhou city was studied and analyzed,and the source apportionment of fine particulate matter in Dazhou city was analyzed for the first time,which provided a clear direction for the control of fine particulate matter pollution in Dazhou city.This paper studied and analyzed the influence of cooking fume on the environmental air quality,and made clear the necessity and importance of urban cooking fume control and management.The accuracy of PM_2.5concentration,primary pollutants and air quality grade was predicted by CMAQ model,which provided reference for further study of O₃,NO₂,SO₂,CO,rainfall,temperature,humidity and boundary layer height by CMAQ model.In short,the research content of this paper basically covered the main aspects of urban atmospheric environment research,which provided strong support for urban atmospheric environment management.