Single-particle Characterization Of Ambient Fine Particles During A Heavy Pollution Event In Wintertime In Suburb Of Beijing

Recently,severe air pollution events occurred frequently in China,and particulate pollutions have become environmental major issues in multiple areas.Beijing-Tianjin-Hebei(Jing-Jin-Ji)area is one of the most polluted regions.These pollution events affect visibility,climate,environment and human health.A Single Particle Aerosol Mass Spectrometer(SPAMS)was deployed on a rural site of Beijing during the winter of 2016,and chemical composition,temporal variation,size distribution,mixing state,and possible source of ambient particles were analyzed.The study focused on the chemical composition,sources,and mixing state of regional aerosols in the Beijing suburb and the North China Plain,contributing to the scientific understanding of air pollution in this area.(1)The average concentration of PM2.5 was 208.4±176.7 μg/m3 during observation,26.0±28.6 μg/m3 during the clean period,and 365.9=102.5 μg/m3 during the haze period.The hourly maximum of PM2.5 was up to 588.4 μg/m3.During the clean period,air masses mainly transported from the north and northwest;while during the haze period,air masses were mostly from the west north-west.The dry and cold air masses from the north and northwest dispersed the haze.Potential source contribution function(PSCF)analysis showed that the PM2.5 were mainly from the area with a range approx.100 km around the sampling site.(2)Using an ART-2a clustering algorithm,the SPAMS dataset could be clustered to nine major particles types,including fresh organic carbon(OC-fresh),aged organic carbon(OC-aged),high molecular organic carbon(HMOC),elemental carbon(EC),internally mixed elemental-organic carbon(ECOC),biomass burning(BB),K-rich,Na-rich and Fe-rich.℃(OC-fresh,OC-aged and HMOC)accounted for 51.5%of the total particles,which was the most abundant particle type.The carbonaceous species(OC,EC and ECOC)accounted for 72.1%of the total particles.Over 70%of particles internally mixed with sulfate as well as nitrate,except OC-fresh.Less than 40%of OC-fresh particles were found internally mixed with nitrate.(3)OC-fresh,HMOC and BB were the most abundant(75%)particle types during the clean period.OC-fresh and HMOC particles were mainly emitted from coal combustion for the heat supply.BB particles were mainly from biomass burning which was used in local catering and cooking.The incomplete burning of coal or biomass was major sources of EC,and Fe-rich particles was originated from fly ashes of coal or biomass combustion.The secondary carbonaceous species(OC-aged and ECOC)and secondary inorganic salt(K-rich and Na-rich)were dominant in haze period,accounting for 63.6%of total particles.They were mainly transported from the nearby area around the sampling site.(4)Particles with vacuum aerodynamic diameters(dva)<0.5 μm were mainly from primary sources(OC-fresh,HMOC and BB),and particles with dva>0.5 μm were mainly from secondary sources(OC-aged、EC、ECOC、K-rich、Na-rich and Fe-rich).(5)Particles of all types encountered aging processes,which were mainly affected by photochemistry and relative humidity.The aging processes of EC,K-rich,Na-rich and Fe-rich particles were mainly affected by the photochemical reactions,while aging of OC-aged particles was more possibly affected by relative humidity.And aging of ECOC particles was affected by both.(6)The proportion of ammonium-containing particles in the total particulate matters was about 31%in average,and varied during the pollution event.During the clean period,the proportion was only 2.6%,but increased to 36.8%during the haze period.According to the chemical composition,ammonium-containing particles clustered to be rich in sulfate(S-rich),rich in nitrate(N-rich),internally mixed sulfate-nitrate(NS-rich)and other(Other).NS-rich was the most abundant particle type,which accounted for 64.9%of ammonium-containing particles.(7)Ammonium particles were mainly existed in the form of NH4Cl,(NH4)2SO4 or NH4HSO4 during the clean period.With the increase of pollution level,a great amount of NH4NO3 yielded in the conditions of ammonia-rich and high relative humidity,and NH4C1 was uptaken by condense H2SO4 and HNO3.At last,NH4NO3,(NH4)2SO4 or NH4HSO4 became the main form of ammonium.(8)The size distribution peak of ammonium-containing particle was larger than all particles(MASS).The peak of particle types followed an order of S-rich<NS-rich<N-rich.(9)The diurnal variation of ammonium-containing particles had typical double-peaks pattern,which were affected by relative humidity and photochemistry.