Spatiotemporal Variation Of Urban Air Quality And Its Associated Factors

Background:Air pollution has been an important global issue with the rapid economic development and trend of increasing urbanization especially in China.The level of air pollution is much higher than international standards published by World Health Organization(WTO).Exposure to air pollution has become the largest environmental risk which is associated to many illnesses such as stroke,lung cancer,chronic and acute respiratory diseases and so on.According to the report of WHO that about 7 million people died as a result of exposure to air pollution in 2012and around 6 million to 9 million people died are predicted caused by ambient air pollution by 2060.Air pollution has attracted the attention of the Chinese government in recent years.The Ministry of Environmental Protection(MEP)of People’s Republic of China released "Technical Regulation of Ambient Air Quality Index(HJ633-2012)" and"Ambient air quality standards(GB3095-2012)" to replace air pollution index(API)with the air quality index(AQI)which have more pollutants indicators(e.g.PM2.5)stricter standards and more frequent release.Previous studies just described the spatiotemporal variations but did not explore the behind reason further.As far as we know there was little studies on AQI except that Xu L.J.et al investigated the temporal and spatial characteristics of AQI in 31 Chinese provincial capital cities.But AQI of provincial capital city represent the air pollution situation of the whole province may cause bias because that the capital cities have more population,faster economic growth the other cities in the same province.So explore the spatial-temporal features of AQI in prefecture-level cities are indispensable.This study aimed to investigate the spatial clustering patterns and seasonal variations of AQI across different provinces in 2015.Furthermore,this study intended to analyze the potential factors associated with the AQI spatiotemporal variations including socioeconomic,demographic and meteorological factors in 233 cities in mainland China.Method:Considering the availability of data,the present study includes a total of 335 cities cities in 29 provinces(excluding Taiwan,Hong Kong,Macao,Tibet and Sinkiang)in mainland China.The longitude spans from 98.50°(Jiuquan)to 131.17°(Shuangyashan)and latitude spans from 18.14°(Sanya)to 49.12°(Hailaer).The data of air quality of 335 cities in 2015 were obtained from the website of the MEP of the People’s Republic.Daily meteorological data of 233 cities in 2015 were obtained from the China Meteorological Data Sharing Service System.Meteorological include daily mean temperature,sunshine duration,atmosphere pressure,wind speed,relative humidity and precipitation.Demographical and socioeconomic data including population,Gross Demographic Product(GDP)per capita,proportions of the primary,secondary and tertiary industries,urban greening rate,total gas supply(coal gas,nature gas),liquefied petroleum gas supply,volume of SO2 and volume of industrial soot emission were acquired from the China Statistical Yearbook 2016.We used GIS map to present the spatial pattern of annual AQI in 335 cities in 2015 and added the bar plots of percentage of days with each particular pollutant as the primary pollutant in each city.The Global Moran’s I test was used to determine whether there is any spartial autocorrelation of AQI.And we used Anselin Local Moran’s I statistic to identify statistically significant hot spots,cold spots and spatial outliers.To examine the seasonal variation of AQI,we defined spring(Mar.-May.),summer(June-Aug.),autumn(Sept.-Nov.)and winter(Dec.-Feb).GIS map of monthly AQI was used to show spatial patterns in different cities and heat map was used to show monthly variations of AQI in different seasons.Besides,generalized linear mixed model and Bayesian spatio-temporal model were used to explore the associated factors of AQI.Results:The AQI gradually increasing from south to north.AQI in southeastern coastal areas in China,Yunnan and Guiyang are lowest.Compared with south and southwest cities,cities in the middle-east area of North China Plain faced more serious air pollution.The higher,value of AQI clustered in Beingjing,Heibei,Shandong and Henan province.Similarly,the analyses of Local Moran’s I show that there are two clusters of AQI and no outlier in China.The low values of AQI clustered in south and southwest in China and the low values of AQI clustered in jing-jin-ji area.Heat map shows monthly AQI in each city.It represents consistently seasonal variations and patterns of AQI.Generally,the high AQI dotted in the winter in all cities and the air quality are much cleaner relatively in summer.PM2.5 and PM10 were the most frequent primary pollutants in China which indicated that the air quality was associated with dust ans coal.The national analyses indicate that the ambient air quality in China exhibits significant geographical and seasonal variations which were mainly driven by socioeconomic and meteorological factors.Poulation density and sproportions of the secondary industries had significantly positive effects on the AQI.GDP per capital,sproportions of the primay industries and green rate had significantly negative effects on the AQI.Cities with higher atmospheric pressure and higher annual mean wind speed had a higher level of air pollution.Cities with higher annual relative humidity,higher rainfall and high sunshine had a lower level of air pollution.Besides,temporal variations of air pollution were associated with daily changes in the weather.Conclusion:The ambient air pollution is still serious in China.AQI had obviousily spatial and seasonal variations in China.The air quality was worst in winter and spring,but better in summer and autumn.Temporal variations of air pollution were associated with the daily changes of weather.Daily air quality was better when there are higher tempature,higher wind speed and higher relative humidity and precipitation.And green rate,GDP per capital and other economic factors were significantly associated with AQI.These findings would provide relatively accuracy information to guide government to set up policy.