Particle Size Distribution Characteristics Of Chemical Constituents Of Fine Particulate Matter In Chengdu

With the rapid development of Chengdu's economy and the acceleration of urbanization,air pollution has also become a major environmental and social issue.In order to explore the seasonal characteristics of fine particulate pollution and the mechanism of continuous pollution process in Chengdu,the paper analyzed the mass concentration levels of water-soluble ions and carbonaceous aerosols,acid and alkali,SOR and NOR,particle size distribution in fine particulate matter in the four seasons of Chengdu in 2019 and the heavy pollution in December in 2017.In addition,the hourly monitoring data of concentration of fine particulate matters(PM_2.5)in Chengdu,as well as the meteorological data of MICAPS,ERA-interim reanalysis data of ECMWF,were used in the study to assess the air quality,meteorological factors and atmospheric diffusion capacity in Chengdu during winter from 2013 to2017.The concludes showed that:There was SOC in the PM₁in winter.The aerosol secondary conversion contributes more.secondary water-soluble ions(SIA:SO₄^2-?NO₃^-?NH₄⁺)accounted for 92.0%of total water-soluble ions.The SOR value is higher than the NOR value in all seasons.The proportion of SIA increased significantly in winter.The proportion of SIA in water-soluble ions in winter,spring and autumn increased from 83%,89%and 89%on clean days to 92%,94%and 92%on pollution days respectively.The mass concentration of OC in PM_2.5was 18.7±5.4?g m^-3,10.3±3.9?g m^-3,8.4±2.7?g m^-3and 6.9±2.7?g m^-3in winter,spring,summer,autumn separately,and the mass concentration of EC in PM_2.5was 2.0±0.5?g m^-3,1.3±0.6?g m^-3,1.2±0.4?g m^-3and0.1±0.5?g m^-3in winter,spring,antumn and summer separately.During the continuous pollution process,the proportion of SIA in water-soluble ions increased from 82.2%in the cleaning period and 81.6%in the accumulation phase to 90.2%in the heavy pollution phase.The peak of SO₄^2-,NO₃^-and NH₄⁺in fine particles appeared in 0.65?1.1?m.The peak of OC,EC,Cl^-,K⁺appeared in0.43?0.65?m mainly.There was weak peak of OC in 4.7?5.8?m during heavy pollution.The weak peak of EC appeared in 5.8?9.0?m.The mass concentrations of Ca²⁺,Mg²⁺were mainly concentrated in coarse particles,and Na⁺was mainly concentrated in 0.65?1.1?m and 4.7?9.0?m.The concentration of SIA were higher near the ground and difficult to diffuse while the height of aerosol was lower.The pollutants was transported shortly from the east of Sichuan and then accumulated locally during the accumulated and heavy pollution period.A large number of sand and dust particles was transported from the distance which caused coarse particulate pollution during the dust period.The continuous severe pollution events in winter mainly occurred in the period between the end of December and early January.The number of continuous severe pollution days was less in winter in 2015?2017 than that in 2013?2014.The concentration of PM_2.5presented a decreasing trend at a rate of 9.65%within five years in winter of Chengdu,with the highest level in 2013winter(149.3±72.2?g·m^-3)and lowest in 2015(80.7±44.1?g·m^-3),which indicated that the air quality improved in Chengdu as a whole.There was an obvious scavenging effect on PM_2.5while the daily precipitation was more than 1mm.The concentration of PM_2.5accumulated significantly at the situation of weak winds and low boundary layer height(weighted average).The air quality was poor during winter in 2013 and2016 due to meteorological factors which lead to the accumulation of pollutants.The air quality was better during winter in 2015 due to the meteorological factors which lead to the accumulation of pollutants.The factors such as PM_2.5concentration,boundary layer height,surface wind speed and precipitation were comprehensively calculated to obtain the empirical formula of the threshold of air stagnation in Chengdu.The formula was wind speed<2.2 m·s^-1,boundary layer height<520 m and no effective precipitation(daily precipitation<1 mm).A decreasing trend from 2013to 2017 of the PM_2.5mass concentration was estimated based on the atmospheric diffusion conditions in 2015,using a localized air stagnation criterion.The result indicated that the measures of reducing emission were effective.However,there was a slight decreasing trend of atmospheric diffusion capacity in recent 10 years in Chengdu area,which implied there might be a greater challenge the government has to face in order to improve the air quality in the future.