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Journal of Environmental Sciences 20(2008) 429–435

Morphology of single inhalable particle in the air polluted cityof Shijiazhuang, China

WANG Zanhong1,∗, ZHANG Lingzhi2, ZHANG Yuliang1, ZHAO Zhou1, ZHANG Sumin1

1. College of Resource and Environment Sciences, Hebei Normal University, Shijiazhuang 050016, China.E-mail: wangzanhong@mail.hebtu.edu.cn

2. Environmental Monitoring Center of Shijiazhuang, Shijiazhuang 050016, China

Received 30 May 2007; revised 16 August 2007; accepted 5 December 2007

AbstractIn the typical air polluted city of Shijiazhuang, single inhalable particle samples in non-heating period, heating period, dust

storm days, and snowy days were collected and detected by SEM/EDS (scanning electron microscopy and energy dispersive X-rayspectrometry). The particle morphology was characterized by the 6 shape clusters, which are: irregular square, agglomerate, sphere,floccule, column or stick, and unknown, by quantitative order. The irregular square particles are common in all kinds of samples; sphereparticles are more, and column or stick are less in winter samples; in the wet deposit samples, agglomerate and floccule particles arenot found. The surface of most particles is coarse with fractal edge, which can provide suitable chemical reaction bed in the pollutedatmospheric environment. New formed calcium crystal is found to demonstrate the existence of neutralized reaction, explaining thereason for the high SO2 emission and low acid rain frequency in Shijiazhuang. The three sorts of surface patterns of spheres aresmooth, semi-smooth, and coarse, corresponding to the element of Si-dominant, Si-Al-dominant, and Fe-dominant. The soot particleis present as floccule with average size around 10 µm, considerably larger than the former reported results, but wrapped or capturedwith other fine particles to make its appearance unique and enhance its toxicity potentially. The new formed calcium crystal, the 3sorts of sphere surface patterns, and the unique soot appearance represent the single inhalable particle’s morphology characteristics inShijiazhuang City.

Key words: single particle; morphology; new formed calcium crystal; spheres; soot

Introduction

Air pollution is the major focus of attention in Chinesecities, and suspended particulate matter of aerodynamicdiameter less than 10 µm (PM10) is reckoned as the firstpollutant (Florig et al., 2002). Studies of ambient particu-late pollution have been carried out in some big cities inChina in the recent years (Ning et al., 1996; Davis andGuo, 2000; Kim et al., 2006). Single particulate of PM10in Beijing (Shi et al., 2003), Qingdao (Zhang et al., 2000),Guiyang (Xie et al., 2005), and Shanghai (Li et al., 2003),has been invested with the results showing that mineraldust and soot are dominant, which derive mainly fromanthropogenic influence and discharge. Aerosols producedlocally can influence far-reaching areas by transportationmechanisms (Prospero and Savoie, 1989; Heintzenberg etal., 2000; Teinila et al., 2000). According to Han et al.(2005), the non-local sources to the mineral aerosols inBeijing account for 62% in TSP, 69% in PM10, and 79% inPM2.5. Local air pollution condition cannot be thoroughlyexplained without taking account of the nearby cities’atmospheric environment characteristics. The above case

* Corresponding author. E-mail: wangzanhong@mail.hebtu.edu.cn.

researches involved only a few cities with air pollutionproblems in China, while there still remain other cities,which suffer worse air quality conditions but are rarelydiscussed.

Shijiazhuang City, located in the north plain of China,270 km south of Beijing, ranked in the top ten mostserious air pollution cities in 2003 by the State En-vironmental Protection Administration of China, is theonly provincial capital city in the blacklist. In 2004, theannual average concentration of the primary pollutant–PM10 in Shijiazhuang was 123 µg/m3, exceeding theChinese national atmospheric quality standard criteria by23%, considerably higher than that of the cities mentionedabove. Nevertheless, characteristics of particulate matterin Shijiazhuang City are unexplored. Meanwhile, the SO2release amount and the annual average ambient concentra-tion are both high but the occurrence of acid deposits is lowin Shijiazhuang City, implying some atmospheric antacidmechanism. Actually, by the dense population and seriousatmospheric particulate pollution, Shijiazhuang City canbe the typical one for the air pollution research in northernChina. Therefore, this article aims at the morphologyfeatures of single particulate of Shijiazhuang City bydifferent sampling conditions using SEM/EDS (scanning

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electron microscopy and energy dispersive X-ray spec-trometry) technology, the result of which may be helpful tounderstand atmospheric particulate characteristics in citycluster of northern China.

1 Materials and methods

1.1 Sampling

PM10 samples have been collected at the EnvironmentMonitoring Center of Shijiazhuang City, one of the 6regular air monitoring stations, on the roof of a five storeybuilding surrounded by residential area. A medium volumesampler (TH-150, Wuhan Tianhong, China) was usedunder 30 L/min with a cutoff of 10 µm (aerodynamic diam-eter), using polycarbonate filters (Millipore, UK, 90 mm indiameter with 0.45 µm pores) for the microscopic analysis.Five daily samples in June, August, and October 2005were collected respectively for the period of ordinary dayscondition (viz. non-heating season) and five daily samplesin December 2005 and January 2006 were collected forthe period of heating season. All the above samplings wereconducted under fine, less windy days, and the collectiontime of each sample was controlled in 24 h. On March18–22, 2006, a strong Asian dust storm event occurredin north China and dust float covered Shijiazhuang City.Fortunately, we collected the PM10 sample of this typicaldust storm process. Also, wet deposit of snow in the winterof 2005 was collected and the unsolvable solid particleswere filtered through polycarbonate membranes.

Beside the filter sampling, we hanged vertically a slidecovered by a layer of petroleum jelly, which can revolvefreely with air turbulence, 1.5 m above roof ground, tocollect suspended particles in the air for the particle sizeand shape detection. Three slide samples of the non-heating period and the heating period were collected inOctober 2005 and January 2006, respectively. Table 1

presents the records for the three sampling conditions.

1.2 SEM/EDS detection

From the center of the filters and membranes, a portionof 1 cm2 was cut and mounted on a smooth Cu-alloy stub,and sprayed with a thin film of Au for the SEM (scanningelectron microscope) analysis. The instrument used wasan electronic microscope (Hitachi S-570, Japan), coupledwith an Energy Dispersive X-ray Spectrometer (EDAXpv9900, USA) to obtain the morphology and chemicalcomposition of individual particles. Elements with atomicnumbers less than 11 were not determined, neither were thesuper-tiny particulate less than 1 µm by visual. During theSEM/EDS detection, on each sample portion, around 100particles were scanned and observed, to get totally 3300particles data.

1.3 Particle size and shape detection

The particle size and shape analyzer (CIS-50, Holand)was applied, by its visual channel, slide cell of ACM-110,and lens of CW (0.7-µm pixel and size range of 2–150 µm).The sampling slide was put into the cell slot and moved up-down and left-right to check all parts of the slide. Around5,000 particles were detected on each slide. All slides weremeasured immediately after sampling.

2 Results and discussion

2.1 Particle shape sorts

Based on the images of particle surface shape gainedby the SEM/EDS analysis, 6 clusters of particulate mor-phology of PM10 in Shijiazhuang City have been sortedout (Table 2).

The images of the 6 particle shape sorts and the corre-sponding X-ray spectra in Shijiazhuang City are shown inFig.1.

Table 1 Conditions of the PM10 sampling for the non-heating period, heating period, dust storm days, and wet deposit in Shijiazhuang City

Sample sort Date Filter/slide number Weather description

Non-heating period 10–14 June; 8–12 August; 15/3 Unclouded, average wind speed 1.8–2.1 m/s,15–19 October 2005 with prevailing wind direction of SE, SW, and S

Heating period 20–24 December 2005; 10/3 Unclouded, average wind speed 1.5–1.9 m/s,16–20 January 2006 with prevailing wind direction of N, NW

Dust storm days 18–22 March 2006 2/0 Floating dust, average wind speed 2.9 m/s,with prevailing wind direction of NW

Wet deposit 21–23, 30 December 2004; 6/0 Snowing, snowfall amount ranging5 January 2005 from 3.8 to 21 mm

SE: south east; SW: south west; S:south; N: north; NW: north west.

Table 2 Particle morphology types of PM10 of Shijiazhuang City

Shape description Size Percentage of particle number (%) Main elementsrange (µm) In the non-heating In the In the In the

period heating period duststorm days wet deposit

Irregular square 5–12 +++++* +++ ++++++ ++++ Si, Al, Ca, Mg, Ti, K, FeAgglomerate 4–16 +++ ++++ ++++ ++++ Si, Al, Fe, Ca, K, MgSphere 1–6 ++ ++++ ++ +++ Si, Al, FeFloccule 2–18 ++ + 0 0 -Column or stick 8–12 + + + ++ -Unknown 3–5 + + + + Si, K, Ca, Fe

* The number of signs “+” indicates the particle relative amount.

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Fig. 1 SEM images of single inhalable aerosol particle and corresponding X-ray spectra in Shijiazhuang City. (a) irregular square; (b) agglomerate; (c)sphere; (d) floccule; (e) column or stick; (f) unknown. In the X-ray spectra, element Au is due to the spray dealt for the SEM image observation and thesame in the following figures.

2.1.1 Irregular squareIrregular shaped grains (Fig.1a) are primary quantita-

tively in the samples detected in Shijiazhuang City. Thesurface of these particles is coarse and holed, with averagediameter of 8 µm. EDS (energy dispersive X-ray spectrom-etry) detection shows that the crust elements of Si, Al, Ca,Mg, K, and Fe are the main components. Some irregulardiamonds have rather smooth and flat surface with obviousCa peak in the elemental spectrum. The amount of particleswith holed surface are relatively more in the non-heatingperiod than in the heating period and the most in the duststorm days.

2.1.2 AgglomerateAgglomerate shaped particles (Fig.1b) are less quanti-

tative than the irregular diamond but are little larger insize. Flexuous fractal edge branches or crinkles presentrandomly outside the agglomerate grains’ surface form thelump morphology. Element components of agglomerateparticles are quite similar to that of the irregular squareparticles but with different X-ray counts rate. In someparticle surface of agglomerate, elements S and Cl aremeasured by the EDS. The amount of agglomerate doesnot differ considerably in the three sampling conditions.

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2.1.3 SphereSphere particles (Fig.1c) are generally smaller than

all other particle types with average diameter under 3µm. There are three impressed surface patterns of thesphere particles in Shijiazhuang City (Fig.2): smooth,semi-coarse, and coarse. Each pattern corresponds todifferent elemental components: the smooth sphere to Si-dominant, the semi-coarse to Si-Al-dominant, and thecoarse to Fe-dominant, especially when the spheres areseparated captured. The spheres conglutinated togetherdo not display such patterns. Obviously, sphere particlesappear considerably frequently in the heating-period thanin other days.

2.1.4 FlocculeBy amplified observation, it is found that these grains

are made up of tiny spherical particles normally less than1 µm (Fig.1d). It seems that these floccule particles arestructured loosely and have alterable size, but in thisresearch, most floccule shaped grains possess an apparentsize of about 10 µm. Though almost no elemental peaksemerge in the EDS spectrum (except element of Au forthe spraying operation), C, O, and other elements with lowsequence number cannot be excluded in these samples.The floccule particles are more in the non-heating period,and are not found in the samples of duststorm days andwet deposit, possibly because the loose construction ispropitious for these floccule particles to disaggregate inviolent current environment.

2.1.5 Column or stickShaped long and elliptical, this sort of grain usually

bears some physiological texture externally (Fig.1e), and

can be found in all samples in Shijiazhuang City. TheX-ray detecting counts for these particles are quite low,demonstrating that they are biomass.

2.1.6 UnknownSome deformed or nondescript particles just account for

minority in the samples with elemental spectrum peaks ofSi, K, Ca, and Fe (Fig.1f), which may be explored furtherin the future.

2.2 Sources of the PM10

Based on the morphological features, it can be con-sidered that the irregular diamond particles are assuredlyderived from soil and surface geological deposit as theproduct of mechanical abrasion (Kaegi, 2004); the agglom-erate and sphere particles are from the combustion of coal(Ramesh and Koziski, 1999; Styszko-Grochowiak et al.,2004; Goodarzi, 2006), while the floccule particles arefrom the discharge of vehicles (Colbeck et al., 1997; Shiet al., 2003), and the column or stick shaped particles arefrom bioactivities (Crook and Sherwood-Higham, 1997;Wittmaack et al., 2005), but the numerical ratio of theabove particles (Table 2) cannot directly indicate thesource apportionment. In Shijiazhuang City, a distinct en-vironmental problem is the fugitive dust derived from baresoil, construction activities, vehicle discharge, industryemission, and street deposit. All through the year, there isalways a layer of dust on almost any ground object surface,especially accumulated on the road side. Owing to repeateddepositing, accumulating, and re-suspending processes,the particles from the above sources mix together, thusmaking the dust a serious nuisance. The detection ofPM10 particles by ambient sampling represents intermingle

Fig. 2 Three surface forms and the corresponding components of spherical particles in Shijiazhuang City, China. (a) smooth surface and Si-dominant;(b) semi-coarse surface and Si-Al-dominant; (c) coarse surface and Fe-dominant.

No. 4 Morphology of single inhalable particle in the air polluted city of Shijiazhuang, China 433

of each original discharge source and re-suspension ofstreet dust, and the ratio of particle number percent bycalculation under SEM can only indicate source categories.

2.3 Particle surface characteristics and possible atmo-spheric reaction

Airborne particles play an important catalytic role inthe atmospheric chemical reaction (Boke et al., 1999), andthe particle surface character is influencing (Dall’Osto andHarrison, 2006; Choel et al., 2007). In the atmosphericenvironment, only a few particles possess smooth surface,most of them are fractal. With the enlarged surface areaby the cracked and holed process, these fractal particlescan provide suitable environment and medium for thesecondary atmospheric reaction. In this research, a crystalgrowing process is observed in one of the PM10 samples inthe heating period in Shijiazhuang City (Fig.3), in whichsome spiny like substance can be seen growing from thebed particle and a raised peak of Ca and S, companiedwith other mineral elements such as Al and Si in theEDS spectrum. This demonstrates that an atmosphericreaction has occurred and a new mineral has formed. Thisnewly formed mineral should be the calcium sulfate by themorphological feature according to former studies (Boke

et al., 1999). In the reaction, the Ca may be provided bythe mineral component of suspending particles such asCaCO3, and the S by the SO2 originally. As mentioned,Shijiazhuang City is characterized by both the high SO2emission and serious street dust nuisance, and the precur-sor emission is considerably enough to make the reactioncontinue thoroughly until new mineral is formed. Theformer result obtained by the authors also identifies withthis outcome, which shows that the high ratio of (ρCa2+ +

ρNH+4)/(ρSO2−

4+ ρNO−3 ) with the value 1.93 in the sample of

snow in Shijiazhuang City is responsible for the restraintof acid deposit, revealing strong antacid ability (Wang andLi, 2006). Thus, the reason for the high SO2 emission butlow acid rain frequency in Shijiazhuang City is explainedby this neutralization.

2.4 Unique soot morphology

In Fig.1, the floccule particles are usually sized around10 µm, considerably larger than that in other researchreports (Colbeck et al., 1997; Shi et al., 2003; Xie etal., 2005; Chandler et al., 2007), and some floccule evenlarger by the capturing of other fine particles (Fig.4). Thesesoot particles’ morphology has been seldom described.Considering the sampling condition of continuous calm

Fig. 3 New formed calcium crystal and its X-ray spectrum.

Fig. 4 Unique soot morphology by capturing other fine particles.

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Fig. 5 Particle size distribution and shape factor (SF) of samples in the non-heating (a) and heating period (b) in Shijiazhuang City. a1: mean 4.6571µm, Std 1.7861 µm; a2: mean 0.8375 µm, Std 0.0700 µm; b1: mean 5.3840 µm, Std 3.2785 µm; b2: mean 0.8754 µm, Std 0.8217 µm.

and stable weather days, the unique soot size and shapecharacteristics should indicate a kind of physical agingprocess of aerosol by coagulation or impaction.

2.5 Possible particle water-insolubility

In the snow wet deposit samples, irregular mineralparticles and spherical particles retain the principle part;some biomass like particles are also present; other particlessuch as soot or agglomerates are not detected, whichdemonstrates the water-insolubility of particles derivedfrom fugitive dust and coal combustion.

2.6 Particle size and shape difference in the non-heating and heating period

By the result of CIS-50 measurement (Fig.5), we canfind that the particle size distribution spans wider in theheating period than in the non-heating period, with morecoarse mean size value, indicating alternation of groundemission with more coal combustion. Here, the particleshape factor (S F) is given by:

S F= 4πA/P2 (1)

where, A is the particle area, and P is the particle perimeter.When the particle is round, S F equals to 1, and squareto 0.785, triangle to 0.604, oblong to 0.436, strip to0.160. Also, in Fig.5, particles with S F close to 1 increaseobviously in the heating period, and in the non-heatingperiod, more particles are square like, implying that theirregular and sphere like shape is dominant in the particleshape groups, which is consistent with the estimate bySEM observation.

3 Conclusions

In the typical air polluted city of Shijiazhuang, single in-halable particle’s morphology was characterized by the six

shape clusters, which are: irregular square, agglomerate,sphere, floccule, column or stick, and unknown, by quanti-tative order. The irregular square particles are common inall kinds of samples; sphere particles are more, and columnor stick are less in winter samples; in the wet depositsamples, agglomerate and floccule particles are not foundin this research. The surface of most particles is coarsewith fractal edge, which can provide suitable chemicalreaction bed in the polluted atmospheric environment. Newcalcium crystal is found to demonstrate the existence ofneutralized reaction, explaining the reason for the highSO2 emission and low acid rain frequency in ShijiazhuangCity. Three sorts of surface patterns of spheres are smooth,semi-coarse, and coarse, corresponding to the element ofSi-dominant, Si-Al-dominant, and Fe-dominant. The sootparticle here is present as floccule, considerably larger thanthe former common results, wrapped or captured with oth-er fine particles to make its appearance unique. This mixedprocess can compound particle components to enhanceits toxicity potentially, but to increase its deposit velocityby possible increased weight. The new formed calciumcrystal, the three sorts of sphere surface patterns, andthe unique soot appearance represent the single inhalableparticle’s morphology characteristics in Shijiazhuang City,which are different from those in other cities, and deservefurther research. The particle size distribution and shapefactor alter considerably in different seasons; when in theheating period, the particles suspended are more coarse andmore sphere-like than in the non-heating period.

Acknowledgements

This work was supported by the Natural Science Foun-dation of Hebei Province (No. D2005000176) and theResearch Foundation for the Ph.D of Hebei Normal Uni-versity (No. 130508). We are thankful to the support ofthe Experiment Center of Hebei Normal University and thehelpful discussion with Prof. Li Ji-biao.

No. 4 Morphology of single inhalable particle in the air polluted city of Shijiazhuang, China 435

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