Study On Characteristics And Sources Of PM_2.5 Pollution In Tianjin Urban Area Based On Stable Isotope Technique

In recent years,haze events have ravaged around China,seriously affected people's production and life,as well as the physical and mental health.It is generally an important subject in the research of atmospheric environmental science to qualitatively or quantitatively explore the source,migration and transformation of haze particles and related components.According to previous study,the application of stable isotope technology can effectively deal with the above research topics.As a typical metropolitan area of Beijing-Tianjin-Hebei region,the fine particulate matter pollution in Tianjin during winter is very serious.In the present study,the day and night PM2.5samples were continuously collected for each two weeks during summer?July 2016?and winter?January 2017?in Nankai District,urban Tianjin City,respectively.Based on the analyses of chemical and isotopic compositions,the existing forms of inorganic sulfur and nitrogen components,and the main sources and transformation mechanisms of sulfate and nitrate in PM2.5 were explored through the interpretation of seasonal and diurnal variations of water-soluble inorganic ions,as well as the stable sulfur and nitrogen isotope compositions.The main understanding and conclusions can be drawn as follows:?1?In research area,the average concentration of PM2.5 during winter(160.2?g m^-3)was about 4 times of that in summer(40.18?g m^-3),which was mainly affected by the difference of meteorological conditions?e.g.temperature,humidity and precipitation,etc?in different seasons.The average mass concentration of PM2.5 during daytime was equal to nighttime in winter,while in summer,the values were 37.31?g m^-3 and 43.04?g m^-3 during daytime and nighttime,respectively,and there was no significant difference between them.?2?SO₄^2–,NO₃^–and NH₄⁺were the main water-soluble ions of PM_2.5 in research region during the sampling periods.According to the Pearson correlation analysis,in winter,the water-soluble ions in PM2.5 of Tianjin were mainly in the forms of NH4NO3,?NH4?2SO4?or NH4HSO4?,NH4Cl,KCl and NaCl.While in summer,it may exist mainly in the form of?NH4?2SO4?or NH4HSO4?,Ca?Cl?2 during daytime,and KCl,NH₄NO₃ or?NH₄?₂SO₄ during nighttime.The[NO₃^–]/[SO₄^2–]mass ratios during winter?0.91?was higher than summer?0.50?,which the values were less than 1.0 indicated that the fine particulate matter was still dominated by stationary source,and the contribution of mobile source also cannot be neglected.?3?The?³⁴S-SO4^2–in research region during winter was+5.5‰,and the value was close to the?³⁴S?+6.6‰?of coal in northern China,combined with the actual situation of Tianjin city,we supposed that,the SO4^2–of PM2.5 in winter was mainly affected by coal combustion;However,the?³⁴S value of summer samples was+3.0‰,indicating that biogenic sulfur was also an important source of SO4^2–in particles.?4?The?¹⁵N-NO3^–of PM2.5 were+15.3‰and+1.1‰during winter and summer sampling periods in research region,respectively,and we suspected that the NO3^–in PM2.5 of Tianjin was mainly from the coal combustion source in winter,while in summer,it could be affected by multiple factors,such as vehicle exhaust and soil biogenic source.Otherwise,the?¹⁸O-NO3^–of PM2.5 were+85.1‰and+67.1‰during winter and summer periods,respectively,and this indicated that the NO3^–in summer of Tianjin was mainly formed through the oxidation of NOx by OH·radical,while in winter,the main transformation pathway of NO3^–was mainly through the oxidation of O3 and consecutive hydrolysis reaction.?5?To calculate the contributions of various typical pollution sources of atmospheric NOx,an improved Bayesian mixture model was added,and the results were that,coal combustion source was the main source of NOx,followed by the vehicle exhaust emission and biomass burning,the contribution of microbial activity was the lowest during winter season.However,in summer,microbial activities contributed the most of NOx to form the nitrate in particles,followed by biomass burning and vehicle exhaust emission,and the contribution of coal combustion was the lowest.