Physico-chemical Properties And Aging Process Of Individual Narticles With Different PM_2.5 Mass Levels In Urban Beijing

Atmospheric particulate matter,especially the fine particles PM2.5,is an important component of ambient air.Airborne particles have serious impacts on human health due to their association with toxic matters.They can also exert important effects on environment and climate through hygroscopic growth,cloud condensation nuclei activity and light scattering and absorption.The long-range transport of particulate matter can affect the regional and global geochemical and biochemical cycles.In recent years,frequent severe air pollution episodes occurred in China due to large emissions of anthropogenic pollutants with the rapid development of industrialization and urbanization.Beijing,one of the severe air pollution cities in China,was selected to study the physico-chemical characteristics of individual aerosol particles by using atmospheric chemistry and individual particle analysis methods.We collected and analyzed different particle samples in Beijing,including the winter particle collection campaign,summer particle collection campaign,two heavy dust storm periods in spring,and two stringent emission control periods(e.g.,APEC and Military Parade emission control periods),and we also compared the particles at ground level and above the mixed layer height(MLH).According to the morphology,elemental composition,mixing state,and the response after beam irritation,individual particles were classified into seven types,including soot particles,organic particles,S-rich particles,mineral particles,metal particles,organic and S-rich mixed particles(OP-S),and other mixed particles(others).Wintertime particle analysis showed that at ground level mineral particles accounted for a larger relative number percentage in non-hazy days compared with hazy days,while the mixed particles accounted for a larger number percentage in hazy days than in non-hazy days,which might because the higher relative humidity(RH)in hazy days favor the heterogeneous reactions of aerosol particles.Most of the organic particles in winter showed spherical or near-spherical shapes,and they were more than 1/5 of the total analyzed particles in winter,higher than the samples collected in other seasons.Compared with ground level,there were fewer relative number percentages of mineral particles and more relative number percentages of organic particles above the MLH.The core-shell particles showed a higher relative number percentage above the MLH than at ground level in hazy days.The core/shell ratio(the equivalent diameter of core divided the equivalent diameter of the whole particle including the shell in core-shell structured particles)above the MLH was less than those at ground level,suggesting that the particles above the MLH were more coated by secondary aerosols and thus underwent more aging process.Summertime particle analysis showed that RH substantially affected the physico-chemical characteristics of particles even in non-hazy days with lower PM2.5 mass concentration.There were more relative number percentage of S-rich particles but less relative number percentage of mineral particles at higher RH.The equivalent diameter of S-rich particles was larger and the content of K+of S-rich particles was less at higher RH.Mineral particle analysis showed the occurrence of S and the weight ratio of S on each mineral particle was higher at higher RH,suggesting that higher RH favors the heterogeneous reactions of mineral particles.Summertime particle analysis also showed that the MLH affected the vertical distribution of individual particles;when the particles collected at a higher altitude represented the particles within the MLH,the vertical difference was less,while when the particles collected at a higher altitude represented particles above the MLH,the vertical difference was obvious.There were fewer mineral particles but more S-rich particles and mixed particles above the MLH,suggesting that the particles above the MLH underwent more aging process.When the dust storm reached Beijing,the mineral particles were predominant,with a relative number percentage of 85.3%and 95.4%in the first and second dust storms,respectively.With the evolution of time after the severe dust storms,the mineral particles decreased and other particles including S-rich particles and organic particles increased,suggesting that the relative number percentage of anthropogenic emission increased.Clay minerals were predominant during the two dust storm periods,with a relative number percentage of more than 50%.However,the relative number percentage of feldspar and carbonate particles were different,which might because they were from different dust sources.When the severe dust reached Beijing,the occurrence and weight ratio of S of individual mineral particles were extremely low,suggesting that less heterogeneous reactions occurred on high concentration mineral particle surface.When these mineral particles were suspended for some time in Beijing,the occurrence of S of mineral particles increased substantially,and the weight ratio of S of mineral particles increased but less than the summer Mineral particles during non-hazy days,suggesting that most of the mineral particles surface underwent heterogeneous reaction but the reaction rate was not much high.During the APEC periods,particles were mainly composed of primary particles,such as mineral particles and organic particles,while during the Military Parade periods,particles were predominated by S-rich particles,with mineral particles only accounted for a few relative number percentage.The results suggested that the particles could show different relative number percentages in different emission control periods,which might because of the different emission sources and meteorological conditions.More core-shell structured particles were observed during APEC hazy days but only a few core-shell particles were found during APEC non-hazy days.However,more core-shell particles were found during the Military non-hazy days,which is different from other kinds of non-hazy days.The cores of core-shell particles during APEC periods were mainly primary particles while those were mainly secondary S-rich particles during Military Parade periods.The core/shell ratio of particles showed larger values during the Military Parade periods than during the APEC periods,suggesting that the particles during the APEC periods were more coated,and thus more aged.