Mixing Structures Of Individual Aerosol Particles And Their Influences On Particle Hygroscopicity

The mixing state of individual particles can affect their optical and hygroscopic properties through numerous physical and chemical processes.Therefore,the knowledge of mixing state is crucial for providing information about particle aging,reaction histories and sources which can bridge field and laboratory studies in atmospheric chemistry.This study investigated the particle size-and age-dependent mixing structure of individual particles in clean and polluted air.To clarify this issue,we classified the particle types and summarized the mixing states of individual particles collected at urban,suburban,rural,clean background,and mountain-top sites and from two emission sources:biomass burning and coal combustion.Meanwhile,we focused on a specific problem-mixing state of dust particles before leaving Asian continent and compared it with mixing structure of haze particles.Based on microscopic analysis method such as transmission electron microscopy coupled with energy-dispersive X-ray spectrometry(TEM/EDS),aerosols were classified into eight components:sea salt,mineral dust,fly ash,metal,soot,sulfates,nitrates,and organic matter(OM).A particle that consists of two or more types of aerosol components can be defined as an internally mixed particle.Otherwise,it is considered to be an externally mixed particle.Within the internally-mixed particle class,we identified four heterogeneous mixing structures:core-shell,dumbbell,OM-coating,and dispersed-OM,as well as one homogeneous-like mixing structure.Homogeneous-like mixing mainly occurred in fine particles(<1 ?m),while the frequency of heterogeneously mixed particles increased with particle size.Our study demonstrated that particle mixing structures depend on particle size and location,and evolve with time.OM-coating and core-shell structures are important indicators for particle aging in air as long as they are distant from specific emission sources.Long-range transported particles tended to have core-shell and OM-coating structures.We found that secondary aerosol components(e.g.,sulfates,nitrates,and organics)determined particle mixing structures because their phases change following particle hydration and dehydration at different relative humidities.Once externally mixed particles are transformed into internally mixed particles,they cannot revert to their former state,except when semi-volatile aerosol components are involved.Categorizing mixing structures of individual particles is essential for studying their optical and hygroscopic properties and for tracing the development of their physical or chemical properties over time.Here we observed an Asian dust storm event and compared the mixing states of dust particles with haze particles which indicated how particles reacted and existed in different polluted events.In order to determine physical and chemical properties of aerosol particles before leaving the Asian continent,we carried out a field campaign from 14 April to 2 May,2011 at a background site in the path of Asian dust and haze outflows.Pollutant concentrations(BC,4 ?g m-3;CO,808 ppb;SO2,24 ppb;NO2,37 ppb)were highest during haze periods,except for the PM2.5 mass,which was highest(131 ?g m-3)during a dust storm.OM-coating mixing structure was dominant during haze events.Some haze particles were internal mixtures of sulfates and refractory fine particles(e.g.,soot,metal/fly ash,and mineral particles)which formed core-shell structure.During a dust storm,fresh mineral particles existed as external mixing structure.Some mineral particles stuck with soot,mental and fly ash,forming dumbbell structure.In an aged air mass,dust particles were found to be coated with Ca(NO3)2 which was identified as core-shell mixing structure.These results indicated that both natural and anthropogenic aerosol particles in Asian outflow can undergo significant physical and chemical processes before leaving the Asian continent.Particle mixing state can influence its hygroscopicity and vice versa.Hygroscopicity has a key impact on the physical properties of particles,including size,morphology,and wavelength dependent refractive indices which influences their atmospheric lifetimes,reactivity,effects on air quality,and,ultimately,their effects on human health.Here an individual particle hygroscopicity(IPH)system was built to investigate the hygroscopic properties of individual particles at different RH?Combined with TEM-EDX,we analyzed hygroscopic property of laboratory-generated Ca(NO3)2 samples,dust particies and haze particles which can bridge laboratory and field experiment together.We can also obtain different hygroscopicity of particles in different polluted events which can induce how particle mixing state affects their phase changes with varying RH.We found that 79%of haze particles started to deliquesced at 68-70%relative humidity(RH).The existence of Ca(NO3)2 coatings on dust particles changed them from hydrophobic into hydrophilic.Therefore,we can conclude that the nitrate coatings on dust mineral surfaces likely remained in the liquid phase throughout the ambient humidity range(above 30%).The differences in particle mixing state lead to changing of particle hygroscopicity and the humidity-dependent phase transitions of particles have important implications on the formation of secondary aerosols during haze events.