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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D09302, doi:10.1029/2008JD011285, 2009

Transmission electron microscopy study of aerosol particles from the brown hazes in northern China Weijun Li1,2 and Longyi Shao1 Received 14 October 2008; revised 22 February 2009; accepted 3 March 2009; published 6 May 2009.

[1] Airborne aerosol collections were performed in urban areas of Beijing that were

affected by regional brown haze episodes over northern China from 31 May to 12 June 2007. Morphologies, elemental compositions, and mixing states of 810 individual aerosol particles of different sizes were obtained by transmission electron microscopy coupled with energy-dispersive X-ray spectrometry. The phases of some particles were verified using selected-area electron diffraction. Aerosol particle types less than 10 mm in diameter include mineral, complex secondary (Ca-S, K-, and S-rich), organic, soot, fly ash, and metal (Fe-rich and Zn-bearing). Most soot, fly ash, and organic particles are less than 2 mm in diameter. Approximately 84% of the analyzed mineral particles have diameters between 2 and 10 mm, while 81% of the analyzed complex secondary and metal particles are much smaller, from 0.1 to 2 mm. Trajectory analysis with fire maps show that southerly air masses arriving at Beijing have been transported through many agricultural biomass burning sites and heavy industrial areas. Spherical fly ash and Fe-rich particles were from industrial emissions, and abundant K-rich and organic particles likely originated from field burning of crop residues. Abundant Zn-bearing particles are associated with industrial activities and local waste incinerators. On the basis of the detailed analysis of 443 analyzed aerosol particles, about 70% of these particles are internally mixed with two or more aerosol components from different sources. Most mineral particles are covered with visible coatings that contain N, O, Ca (or Mg), minor S, and Cl. K- and S-rich particles tend to be coagulated with fly ash, soot, metal, and fine-grained mineral particles. Organic materials internally mixed with K- and S-rich particles can be their inclusions and coatings. Citation: Li, W., and L. Shao (2009), Transmission electron microscopy study of aerosol particles from the brown hazes in northern China, J. Geophys. Res., 114, D09302, doi:10.1029/2008JD011285.

1. Introduction [2] Aerosols are an important and complex constituent of the atmospheric system. By scattering and absorbing radiation, aerosol particles with different chemical compositions have different impacts on the climate system [Intergovernmental Panel on Climate Change, 2007]. They can act as ice and cloud condensation nuclei (CCN), that can influence the lifetime, formation, and radiative properties of clouds [DeMott et al., 2003; Dusek et al., 2006; Lohmann, 2008]. Aerosol particles from anthropogenic activities, such as biomass burning, industrial activity, and transportation, impact the climate and can overwhelm the natural aerosols within the climate system [Menon, 2004]. In addition, various anthropogenic aerosol particles cause adverse health effects 1 State Key Laboratory of Coal Resources and Safe Mining and Department of Resources and Earth Science, China University of Mining and Technology, Beijing, China. 2 School of Earth and Space Exploration and Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, USA.

Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JD011285

[Roberts et al., 2004; Shao et al., 2006; Reiss et al., 2007] and influence the water budget of the planet [Ramanathan et al., 2005]. Anthropogenic pollutants can be transported from urban and industrial areas to remote continents and oceans [Po´sfai et al., 1999; Seinfeld et al., 2004; Yamanouchi et al., 2005; Peltier et al., 2008]. [3] Rapidly industrializing China is a significant source of anthropogenic aerosol particles on the global scale [Jaffe et al., 1999; Z. Q. Li et al., 2007a; Streets et al., 2008]. With expansion of Beijing and its surrounding area in the past 30 years, severe brown haze episodes have become more frequent [Lau et al., 2008]. The Beijing government has carried out many control measures to reduce local emissions and alleviate urban air pollution [Guinot et al., 2007; Streets et al., 2007]. For example, heavy industries have been removed from urban Beijing. However, the regional haze episodes with typically high concentrations of particulate matter still frequently occur in northern China and these aerosol particles can be transported long distance and influence Beijing air quality [Sun et al., 2006]. [4] Numerous studies have applied various ‘‘bulk’’ techniques to understand the chemical compositions of aerosol particles collected in dusty, hazey, and clean days of Beijing

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Table 1. Samples Collected in the Haze Episodes Over Northern Chinaa Date

Start Time, UTC

T, °C

RH, Average, %

P, hPa

WS, m s1

WD

Visibility, km

31 May 2007 2 June 2007 7 June 2007 7 June 2007 7 June 2007 8 June 2007 8 June 2007 12 June 2007

1000 0200 0200 0400 1030 0300 1345 0715

21 26 29 33 30 31 34 30

73 59 36 40 60 53 31 50

998 998 999 997 996 1001 995 996

2 1 2 1 2 2 3 4

SSE SSE SE SSE SSE SE SE SE

2 1.8 3 4 3.5 3 5 3

a

T, temperature; RH, relative humidity; P, barometric pressure, WS, wind speed; WD, wind direction.

[Bergin et al., 2001; Yao et al., 2002; Duan et al., 2004; Sun et al., 2006; Wang et al., 2006, 2008]. These studies showed that high levels of water-soluble ions (e.g., Ca2+, Mg2+, K+, + SO2 4 , NO3 , and NH4 ) and organic materials occurred in haze episodes. These results implied that emissions from local construction activities and transportation were important as well as nonlocal industrial areas, biomass burning, and natural soil. Whether long-range transported pollutants from biomass burning can significantly affect urban air quality is still an open question. Moreover, these ‘‘bulk’’ techniques only focused on the chemical composition of aerosol particle mass, while properties of individual particles could not be provided. [5] Individual particle analysis using transmission electron microscopy (TEM) provides detailed information on individual particles at higher spatial resolution than scanning electron microscopy (SEM) [Buseck and Po´sfai, 1999; Niemi et al., 2006; Li and Shao, 2009b]. Because of its resolution down to fractions of a nanometer, TEM can provide detailed information on the sizes, compositions, morphologies, structures, and mixing states of individual aerosol particles. Studies have used SEM to determine the sizes and morphologies of aerosol particles collected on dusty and clean days of Beijing [Gao and Anderson, 2001; Shi et al., 2003; Liu et al., 2005; Shao et al., 2007]. Systematic detailed analysis of the individual particles in polluted hazes has not been reported. However, such information is important to understand the sources and properties of individual haze aerosol particles. In particular, their morphologies and mixing characteristics are essential for an assessment of the adverse effects on health and climate effects on a regional scale. [6] Aerosol samples were collected in Beijing city when it was affected by regional hazes over northern China. Our goal is to characterize aggregations, coatings, sizes, and relative abundances of the major aerosol types from multiple anthropogenic and natural sources in haze episodes over northern China. Chemical compositions of individual aerosol particles can provide insight into their sources during haze episodes. Their mixing characteristics can indicate reactions and evolution of internally mixed particles.

Aerosol particles were collected onto copper TEM grids coated with carbon film (carbon type B, 300-mesh copper) using a single-stage cascade impactor with a 0.5-mmdiameter jet nozzle. The airflow rate was 1.0 L min1. This sampler has a collection efficiency of 100% at 0.5 mm aerodynamic diameter if the density of the particles is 2 g cm3. The sampling time for each sample was from 30 to 120 s depending upon the visibility. Meteorological parameters, such as wind speed (WS), wind direction (WD), relative humidity (RH), barometric pressure (P), and ambient temperature (T), were automatically recorded by a Kestral 4000 (Table 1). After collection, each sample was placed in a sealed dry plastic tube. This tube serves to preserve samples prior to analysis and minimize the exposure of aerosol particles collected on the TEM grids to ambient air. These tubes were stored in a desiccator at 25°C and a relative humidity at 20 ± 3%. 2.2. TEM Analysis [8] Aerosol samples were analyzed using a 200 kV Philips CM200 TEM. The distribution of aerosol particles on TEM grids was not uniform. Coarser particles were near the centers of the grid and finer particles on the periphery. Therefore, to ensure that the analyzed particles were representative, three to four areas were chosen from the center and periphery of the sampling spot on each grid. An ellipse was used to fit a particle outline, and the arithmetic mean of the short and long axes of the ellipse was used to determine the particle diameter. Particles examined by TEM were dry at the time of observation in the vacuum of the electron microscope. In our study, the effects of water, other semivolatile organic, and NH4NO3 were not considered.

2. Experiments 2.1. Aerosol Sampling [7] Aerosol samples were collected during five haze episodes from 31 May to 12 June 2007 (Table 1). The collection site (39°590N, 116°200E) was located in northwestern urban Beijing. The samplers were mounted on the roof of a building on the campus of China University of Mining and Technology in Beijing, 18 m above ground.

Figure 1. Frequencies of elements in aerosol particles.

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Table 2. Classification Criteria for Different Particle Groups Particle Groups

Particle Types

Mineral particles

Mineral

Complex secondary particles

Ca-S

Organic particles Soot Fly ash Metal particles

Elemental Characteristics Particles containing O, Si, and Al could be Si-dominated, Si-Al, and Fe-Si. Most particles have visible coatings that include N, O, Na, Mg, Ca, and S. Particles containing O, S, Ca with various K are classified into Ca-S.

S-rich

Particles containing N, O, S, and minor K are classified into S-rich.

K-rich

Particles containing N, O, S, and K are classified into K-rich.

Organic Soot Fly ash Metal

C and minor O C and minor O O, Al, and Si Metals containing O, Ti, Mn, Fe, Zn, Pb, As, Hg, and Co. Particles are classified into Zn-bearing (e.g., Zn-Pb and Zn-Hg) and Fe-rich.

[9] Elemental compositions were semiquantitatively determined by an energy-dispersive X-ray spectrometry (EDS) that can detect elements heavier than C. For some particles, EDS data was combined with selected-area electron diffraction (SAED) to verify their identities. Carbon and copper were not considered in inorganic particles because of effects from the carbon-coated film on the TEM grid. Through a labor-intensive operation, a total of 810 aerosol particles from eight samples were analyzed for size and elemental composition using transmission electron microscopy coupled with energy-dispersive X-ray spectrometry (TEM/EDS). Most of the analyzed aerosol particles (i.e., the mixtures of aerosol particles from different sources) were extremely heterogeneous, internal mixtures. To understand the internally mixed aerosol particles and their sources, compositions of different parts (e.g., coating, inclusion, and coagulation) of 443 individual aerosol particles were analyzed in detail. Intensity of electron beam increases with decrease of electron beam scale. Some aerosol particles (e.g., (NH4)2SO4, K2SO4, and KNO3) sublimated when electron beam scales of TEM were smaller than sizes of the measured particles. The high intensity of electron beam was referred to as strong electron beam in our study. Otherwise, we referred to as weak electron beam. To minimize radiation exposure and potential damage, EDS spectra were collected for 30 s. 2.3. Haze Episodes [10] The meteorological observations during sampling periods indicated low wind speed (1 –4 m s1), humidity (31 – 85%), and visibility (1 – 5 km) (Table 1). Daily mass concentrations of PM10 in Beijing haze episodes were more than 150 mg m3, and reached levels 300 mg m3 in Beijing (Beijing Meteorological Bureau). These values are 2 – 4 times higher than the mean values (70–80 mg m3) for the entire year, suggesting high aerosol loading in the severe haze episodes. [11] Haze episodes persisted for 1 – 3 days, and were ended by the abrupt passage of a northwestern cold front.

Physical Characteristics of Particles Irregular mineral particles. Mostly irregular shape, partly plates or columns. Mineral particles are frequently coated by sulfates and nitrates. Rectangular particles, susceptible to beam damage. Some Ca-S particles mix with mineral, and some mix with S-rich and K-rich particles. No distinct shapes, sensitive to strong electron beam Mixed with soot, organic, fly ash, metal, and fine-grained mineral particles. No distinct shapes, sensitive to strong electron beam. Mixed with soot, organic, fly ash, metal, and fine-grained mineral particles. Spherical or irregular shapes Chain-like aggregates Spherical shape Most Fe-rich and Zn-bearing particles are spherical. Minor other metal particles have irregular shapes.

When the cold front disappeared during the sampling campaign, the severe haze formed again in less than half a day. 2.4. Back Trajectory Analysis and Satellite Observation [12] Back trajectory analysis illustrates the transport paths of air masses arriving at the sampling site. NOAA/ARL Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) (R. R. Draxler and G. D. Rolph, 2003, available at http://www.arl.noaa.gov/ready/hysplit4.html) was used to determine 25 back trajectories of air masses arriving at Beijing at 500 m from 25 May to 20 June 2007. Each trajectory represents the past 48 h of the air mass, with its arrival time at 0000 UTC every day. Twenty trajectories passed over the south and southeast region of Beijing (i.e., Shanxi, Hebei, Shandong, Henan, Jiangsu, and Anhui provinces) (Figure S1 in auxiliary material).1 The air quality of the sampling area was undoubtedly affected by these southerly air masses. [13] Fire maps from the Moderate Resolution Imaging Spectroradiometer (MODIS) (http://rapidfire.sci.gsfc.nasa. gov) showed many fires in Hebei, Shandong, Jiansu, and Anhui provinces from 21 May to 12 June 2007. MODIS images showed that a huge brown haze covered northern China during the sampling period (auxiliary material Figure S2).

3. Results 3.1. Nature of the Aerosol Particles [14] Aerosol particles in the brown haze have complicated compositions. EDS shows more than 18 elements that were detected in aerosol particles (Figure 1). O, Si, and S were 1 Auxiliary materials are available in the HTML. doi:10.1029/ 2008JD011285.

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Figure 2. Morphologies and compositions of mineral and Ca-S particles. Elements of the detected parts of individual particles are in parentheses, and minor elements are indicated in square brackets. (a) Calcite particle with fine calcium sulfate particles and surrounded by a Ca-rich coating containing N, O, Ca, and minor Mg. Phase of CaSO4 was confirmed by SAED and EDS. (b) One illite particle. Phase of illite particle was confirmed by SAED and EDS. (c) Dolomite particle surrounded by a Mg-rich coating containing N, O, Mg, and minor Ca. Phase of dolomite was confirmed by SAED and EDS. (d) Mixtures of Ca-S and S-rich particles coagulated with fly ash. Spacings of Ca-S particle with minor K are different from the phase of CaSO4. detected in more than 85% of the 810 analyzed particles. N, Na, Mg, Al, Ca, and Fe occurred in close to 50% of the analyzed particles. F, P, Cl, As, and heavy metal elements (e.g., Ti, Mn, Zn, and Pb) occurred in less than 27% of the analyzed particles. 3.2. Major Aerosol Particle Types [15] On the basis of the different elemental compositions and morphologies of the individual aerosol particles, we classified them into the following eight groups: mineral, Ca-S, S-rich, K-rich, organic, soot, fly ash, and metal (Table 2). [16] Mineral particles in Beijing air are mainly from road dust, soil, and construction dust [Shao et al., 2007]. Mineral particles show irregular shapes, and their compositions usu-

ally include O, Na, Mg, Al, Si, Ca, and Fe (Figures 2a– 2c). These mineral particles are stable in the electron beam. The SAED and EDS data together indicate the main mineral components are clay, feldspar, quartz, dolomite, and calcite. X-ray diffraction (XRD) of bulk samples collected in Beijing also indicates that these dust components are the major mineral types [Shao et al., 2007, 2008]. Most mineral particles collected in the haze episodes are encapsulated by visible coatings. These coatings mainly contain N, O, Ca (or Mg), minor S, and Cl (Figures 2a and 2c). The coatings are amorphous and can sublimate under strong beam. The oxygen intensities from EDS spectra are larger in the coatings than in the corresponding cores. These properties are consistent with SEM and TEM observations of Ca(NO3)2 and

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Figure 3. TEM images of organic, S-rich, and K-rich particles. Elements of the detected parts of individual particles are in parentheses, and minor elements are indicated in square brackets. (a) S-rich particle including minor K in the weak electron beam. (b) S-rich residue with soot and organic coatings in the strong electron beam. (c) S-rich particle with organic materials in a satellitic ring. Ammonium sulfate was confirmed by SAED. (d) Mixtures of K-S and K-N containing particles. (e) Spherical organic particle (tar ball). (f) An organic particle surrounded by K-rich coating. Mg(NO3)2 [Laskin et al., 2005; Li and Shao, 2009a]. Some fine calcium sulfate particles were detected in the coatings (Figure 2a). [17] Some well-defined Ca-S particles also contain O and minor K (Figure 2d). Most of them are aggregates of two or more Ca-S particles with rectangular shapes. The particles are sensitive to strong electron beam and yield different diffraction patterns than CaSO4 (Figure 2d). [18] S-rich particles also contain N, O, and minor K. Some of they appear spherical on TEM grids (Figure 3a). The particle yields a diffraction pattern of ammonium sulfate ((NH4)2SO4) (Figure 3c). The particles are beam sensitive (Figure 3a and 3b), mostly leaving S-rich residues and one or more inclusions such as soot, fly ash, organic, fine-grained mineral, and metal particles. [19] K-rich particles also contain O, S, and/or N. Some K-rich particles are rectangular or spherical, but most are irregularly shaped (Figure 3d). K-rich particles are sensitive to the electron beam (Figure 3f). Soot, fly ash, organic, finegrained mineral, and/or Fe-rich inclusions can occur in the K-rich particles. Abundant K-rich aerosol particles collected in the cities also serve as a tracer of biomass burning [Niemi et al., 2006; Adachi and Buseck, 2008]. [20] Observed elemental compositions of organic aerosols show abundant carbon and minor oxygen. Organic particles are stable with strong electron beam exposure (Figure 3e).

These amorphous particles do not show reflections in SAED. Some spherical particles were referred to as tar ball from biomass burning [Po´sfai et al., 2004; Hand et al., 2005; Chakrabarty et al., 2006]. In our samples, most organic materials are internally mixed with K- and S-rich particles (Figure 3). EDS analysis is difficult to identify that carbon in the internally mixed particles is exact from organic components because of the effects from carbon substrates. However, organic coatings and inclusions in K- and S-rich particles (Figures 3b and 3f) are frequently observed when K- and S-rich components sublimate under the strong electron beam. [21] Soot particles are chain-like aggregates of carbonbearing spheres. One soot aggregate can contain as few as ten to hundreds of carbon spheres with typical diameters from 10 to 100 nm with some up to 150 nm (Figures 4a and 4b). The soot spheres in high-resolution TEM images display a discontinuous onion-like structure of graphitic layers (Figure 4b). Most soot particles observed are internally mixed with S- and K-rich particles (Figures 3a and 6a). [22] Spherical fly ash particles contain O, Si, and Al with minor Ca, Ti, Mn, and Fe (Figure 2d), and are amorphous. They are typical components of the anthropogenic aerosols from coal combustion for heating and industrial activities in northern China [Shi et al., 2003]. Most fly ash particles are internally mixed within the S- and K-rich particles.

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Figure 4. TEM images of a soot particle. (a) A chain-like soot aggregate. (b) High-resolution TEM image of soot shows the onion-like structure of curved and disordered graphitic layers. [23] Metal particles mainly consist of spherical Fe-rich particles and Zn-bearing particles (Figure 5). Fe-rich particles also contain O with minor Ti, Mn, and Zn. Unlike fly ash, as indicated by SAED, they yield crystalline structures. Most Fe-rich particles are found as inclusions in S- and K-rich particles (Figure 6b). Most spherical Zn-bearing particles consist of Zn-rich coatings and Pb-rich inclusions. The Pb-rich inclusions with diameters from 10 to 400 nm also contain O and S and show various well-defined shapes (Figures 5a and 5b). Zn-rich coatings are also composed of N, O, S, and some containing minor Na and Cl. The particles are sensitive to the strong electron beam. A few Zn-bearing particles also contain Hg (or Ti, As)-rich inclusions. Some Zn-bearing particles are surrounded by small satellitic droplets (Figure 5b). Additionally, we also observed that some of

the Zn-bearing particles also included Zn-rich inclusions. More than 50% of Zn-bearing particles are internally mixed with S- and K-rich particles. 3.3. Relative Abundance of Different Aerosol Particle Types [24] Ca-S, K-, and S-rich particles are often referred to as complex secondary particles in the atmosphere [Vester et al., 2007]. In these samples, it was difficult to identify all of the metal particles because they were frequently internally mixed with complex secondary particles (Figure 6b). In this case, the total number of complex secondary particles, metal particles, and their mixtures were shown in Figure 7. The particles smaller than 2 mm in diameter make up 81% by number of the analyzed aerosol particles (Figure 7). The

Figure 5. TEM images of metal particles. Elements of the detected parts of individual particles are in parentheses. (a) Zn-rich coatings with a rectangular Pb-rich inclusion. (b) Zn-rich coatings with quadrangular Pb-rich inclusions. Zn-bearing particles have a satellitic ring. (c) Aggregates of spherical Fe-rich particles. 6 of 10

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soil have been reported in Beijing air [Shi et al., 2003; Zhang et al., 2003; Zheng et al., 2005; Sun et al., 2006; Xie et al., 2008]. Besides the particle types mentioned above, abundant K-rich and metal particles also occur. Trajectories of air masses with fire maps indicate that the air masses arriving at Beijing have passed over many biomass burning sites in Shanxi, Hebei, Henan, Shandong, Jiangsu, and Anhui provinces. Field burning of agricultural residues, an important source of biomass burning, takes place every year in these regions from May to June [Duan et al., 2004; Yan et al., 2006; X. G. Li et al., 2007]. These provinces are also a part of industrialized northern China [He et al., 2002; Chan and Yao, 2008]. Therefore, fly ash and Fe-rich particles should be from industrial emissions, and K-rich and organic particles were likely from field burning of crop residues. [26] Zn-bearing particles are a frequent anthropogenic aerosol, although they have not been previously reported in Beijing. Similar aerosol particles, detected in other study areas with single-particle mass spectrometry [Murphy et al., 2006; Moffet et al., 2008a], are associated with industrial areas and waste incinerators [Hu et al., 2003; Choel et al., 2006]. Zn-bearing particles that also contain N, S, Pb, minor Na, and Cl indicate that these particles were likely emitted by urban waste incinerators [Moffet et al., 2008b]. In fact, several waste incinerators exist in southern Beijing, and they can incinerate more than 2% of the annual solid waste from Beijing [Xiao et al., 2007]. The Zn-bearing particles with fine Zn- and Fe-rich inclusions suggest that these particles are related to industrial emission. 4.2. Mixing Mechanisms of Aerosol Particles [27] On the basis of the detailed analysis of 443 analyzed aerosol particles, about 70% of the particles are internally

Figure 6. TEM images of internally mixed aerosol particles. (a) Mixtures of S-rich and soot particles. Black arrows show S-rich particles. White arrows show soot inclusions in sulfate particles. (b) Metal particles (Zn-, Pb-, and Fe-rich) with S-rich particles. percentages of complex secondary and metal particles decrease with increasing particle size. In contrast, the percentages of mineral particles increase with particle size, with 84% of them in the range of 2 to 10 mm. Soot, fly ash, and organic particles are mostly smaller than 2 mm.

4. Discussion 4.1. Sources of Aerosol Particles [25] Ammonium salts and Ca-S particles formed through atmospheric reactions, soot from vehicle emissions, fly ash from coal combustion, and mineral particles from crustal

Figure 7. Proportions of aerosol particles in the haze episodes. A total of 810 aerosol particles were identified on the basis of their different morphologies and compositions in TEM. Fly ash, soot, and organic internally mixed with K- and S-rich particles were not included. The number of the analyzed aerosol particles in different size ranges is shown above each column.

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mixed with two or more aerosol components from different sources. Compositions and morphologies of internally mixed particles can suggest that these particles formed through processes, such as condensation, dissolution, aqueous chemistry, and coagulation [Okada et al., 2001; Laskin et al., 2005; Zhang et al., 2005; Jacobson, 2006; Niemi et al., 2006]. The morphologies of mineral particles enclosed by the visible coatings are different than mineral particles without coatings (Figures 2a and 2b). Sullivan et al. [2007] indicated that aging processes can frequently occur on Asian mineral dust particles through heterogeneous chemical modifications by nitric and sulfuric acids and coagulation with ammonium sulfate. Our observation suggests that these coatings likely formed through chemical reactions on the surface of the particles rather than coagulation with preexisting particles. Mineral particles with visible coatings containing N and S (Figures 2a and 2c) probably underwent some heterogeneous chemical modifications with nitric and sulfuric acids in the haze air. Li and Shao [2009a] indicated that these chemical reactions tend to happen on the surfaces of mineral dust particles containing calcite and dolomite components. [28] Soot and organic materials from biomass burning were internally mixed with K- and S-rich particles [Niemi et al., 2005; Adachi and Buseck, 2008]. Johnson et al. [2005] and Shi et al. [2008] also indicate that the rapid aging of fresh soot can occur in polluted urban air, particularly internally mixed with ammonium sulfate. Our results further show that the K- and S-rich particles tend to scavenge any fine insoluble aerosol particles (i.e., fly ash, soot, organic, mineral, and Fe-rich particles). [29] Almost all the Zn-bearing particles collected in the haze episodes consist of Zn-rich coatings and Pb-rich inclusions. The morphologies of Zn-bearing particles from our samples are different to those of the solid Zn-Pb particles as described by Moffet et al. [2008b]. On the basis of TEM observation, the Zn-rich coatings indicate that Zn-bearing particles from industrial activities and waste incineration likely underwent heterogeneous reactions with acidic gases (e.g., SO2, HNO3, and NOX) in the haze air. Heterogeneous reactions on some Zn-bearing particles were also detected in polluted Mexico City [Moffet et al., 2008b]. Additionally, the mixtures of Zn-bearing and K- or S-rich particles suggest that the number of reacted Zn-bearing particles coalesced to the K- and S-rich particles during their transport.

5. Conclusions [30] TEM analysis of 810 aerosol particles collected in regional hazes over northern China has revealed eight particle types: mineral, Ca-S, K-rich, S-rich, organic, fly ash, soot, and metal. Most of soot, fly ash, and organic particles are smaller than 2 mm in diameter. The sizes of most mineral particles (84%) range from 2 to 10 mm while most complex secondary and metal particles (81%) are between 0.1 and 2 mm. Trajectory analysis with fire maps of MODIS indicated that the K-rich and metal particles originated from agricultural biomass burning and industrial activities in Shanxi, Hebei, Shandong, Henan, Jiangsu, and Anhui provinces. Zn-bearing particles likely originated from waste incineration and industrial activities.

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[31] On the basis of the detailed analysis of 443 analyzed aerosol particles, about 70% of the particles are internally mixed by two or more aerosol components from the different sources. Most mineral particles are encapsulated by the visible coatings that include N, O, Ca (or Mg), minor S, and Cl. Soot, fly ash, organic, metal, and fine-grained mineral particles can occur as inclusions in the S- and K-rich particles. Most Zn-bearing particles consist of Zn-rich coatings and Pb-rich inclusions. [32] TEM observations show that most hydrophobic particles (i.e., mineral, certain organic, fly ash, soot, Fe-rich particles) in the brown haze episodes can be coated or coagulated with hygroscopic materials (i.e., nitrates, potassium salts, and ammonium sulfate). Once coated by hygroscopic materials, these internally mixed particles should easily grow larger through the absorption of more water and acidic gases along with increase of relative humidity. These grown particles may explain why the brown haze layer over northern China with high humidity has been associated with strong cooling in the region [Bergin et al., 2001; Z. Q. Li et al., 2007b]. Furthermore, the great amount of fine particles (e.g., metal, nitrates, ammonium sulfate, and potassium salts) transported from the anthropogenic sources and formed in the brown haze air could enhance adverse health impacts of aerosol particles in Beijing air. [33] Acknowledgments. We thank Wei Wang for assistance with sample collection. We are grateful to Peter Buseck, Kouji Adachi, and Evelyn Freney for their discussions of an early version of the manuscript. We thank Jun Wu and Pe´ter Ne´meth for valuable discussions regarding the diffraction identify, and we thank three anonymous reviewers for their constructive comments on this manuscript. We appreciate Peter R. Buseck for his sponsorship of the visit of Weijun Li to Arizona State University. We acknowledge the use of TEM in the LeRoy Eyring Center for Solid State Science at Arizona State University. Financial support was provided by National Basic Research Program of China (2006CB403701), Cultivation Fund of the Key Scientific and Technical Innovation Project of the Ministry of Education of China (705022), and National Science Foundation of China (40575065).

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W. Li and L. Shao, State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, China. ([email protected]; [email protected])

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