Characteristics Of Main Air Pollutants In Tianshui

With the rapid development of China’s economy and the acceleration of urbanization and industrialization,many environmental problems have emerged.The urban atmospheric environment issues closely related to people have attracted much attention.Tianshui,located in the zone of“two edges”（the northeast edge of the Qinghai-Tibet Plateau and the northern edge of the Asian summer monsoon）,is sensitive to climate and ecologically fragile.At present,there is no research on its atmospheric environment and aerosol,so it is necessary to study its atmospheric environment quality.Based on the air quality monitoring data of Tianshui from 2015 to2019,this thesis analyzes the pollution characteristics of six major air pollutants.On this basis,the pollution characteristics of PM_2.5,one of the most important characteristic pollutants,are studied,including the characteristics of water-soluble ions and their carbon components.At the same time,the sources of PM_2.5 are investigated in terms of analysis of local sources,the external transportation and potential source areas.The following conclusions are obtained:1)The primary characteristic pollutants in Tianshui are particulate matter,SO₂,NO₂ and CO all reach the standard,and PM_2.5 and PM₁₀ exceed 1.08 times and 1.09times of the second-level standard,respectively.The seasonal variation of pollutants except O₃ is the highest in winter and the lowest in summer,and the diurnal variation is bimodal in the morning and evening.There is a significant negative correlation between SO₂,NO₂,CO and PM_2.5 and temperature.The dry air temperature and strong wind in spring lead to the increase of particle concentration.High relative humidity can inhibit the diffusion of pollutants,but the environment of low temperature and high humidity is conducive to the increase of moisture absorption of particles.The southwest wind can lead to high concentrations of SO₂,NO₂ and CO,and the high concentrations of PM_2.5and PM₁₀ are related to the west wind and northwest wind.2)The SNA（Secondary Inorganic Aerosol）in PM_2.5 in Tianshui accounts for about70%,indicating that the atmospheric environment is greatly affected by human sources.The concentration of water-soluble ions is higher in winter and spring,followed by autumn and lowest in summer.The main forms of NH₄⁺are（NH₄）₂SO₄ and NH₄NO₃ in winter and NH₄HSO₄ and NH₄NO₃ in other seasons.In spring and summer,relatively high SOR（Sulfur Oxidation Rate）indicates that there is obvious oxidation transformation process in the atmosphere,and relatively high NOR（Nitrogen Oxidation Rate）summer indicates that there is a certain secondary transformation process.On the whole,the mass concentration of OC（Organic Carbon）is higher than that of EC（Element Carbon）,and the four-season ratio of OC/TC and OM（Organic Matter）/TCA（Total Carbon Aerosol）is higher than 80%,indicating that OC and organic matter contribute greatly to the atmospheric PM_2.5 of Tianshui.The main sources of carbonaceous aerosols are the exhaust sources of motor vehicles,the sources of biomass combustion and coal combustion.3)The correlation analysis shows that the correlation between PM_2.5 and PM₁₀ is higher in the year and season,indicating that the sources of them are similar.The emission source list shows that PM_2.5 emissions are the highest in winter and autumn and the lowest in summer,with the contribution of civil and industrial sectors to PM2.5emission being the highest,especially in winter,which is more than 80 percent.The principal component analysis of carbon components shows that biomass combustion,coal combustion emissions and motor vehicle exhaust are the main sources of OC and EC in PM_2.5,and the contribution rate to carbon components in carbon is68.32%-79.96%.Using the PMF（Positive Matrix Factorization）model to analyze the local sources of PM_2.5,three types of factors are obtained to better explain the sources of pollutants in the PM_2.5,and the three types of pollution sources are combustion sources（20.26%）,mixed sources（60.14%）and secondary sources（19.6%）.4)The external transport of pollutants shows that Tianshui is easily affected by the north and northwest air flow,and long-distance transportation makes them the most important transport routes affecting the atmospheric fine particulate matter in spring,autumn and winter.Meanwhile,it is susceptible to the east and southeast airflows in summer.PSCF（Potential Source Contribution Function）and CWT（Concentration Weighted Trajectory）analysis shows that the potential source area of PM_2.5 in Tianshui has obvious differences in four seasons,with the largest distribution in winter,followed by spring and autumn,and the smallest in summer.There is a potential source zone in winter,part of the areas covered by it are relatively serious pollution areas in China,and part of it are the main source of dust storms in China.Tianshui and its neighboring cities and counties,southeast Gansu,northeast Sichuan,south-central Shaanxi,the border between Henan,Shaanxi and Hubei,northwest Qinghai,southern Inner Mongolia are the most important potential source areas of fine particulate matter.