high-resolution measurement of size distributions of asian

High-Resolution Measurement of Size Distributions of Asian Dust Using aCoulter Multisizer

HIROSHI KOBAYASHI

Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Yamanashi, Japan

KIMIO ARAO

Faculty of Environmental Studies, Nagasaki University, Nagasaki, Japan

TOSHIYUKI MURAYAMA

Faculty of Marine Technology, Tokyo University of Marine Science and Technology, Tokyo, Japan

KENGO IOKIBE AND RYUJI KOGA

Faculty of Engineering, Okayama University, Okayama, Japan

MASATAKA SHIOBARA

National Institute of Polar Research, Tokyo, Japan

(Manuscript received 18 October 2005, in final form 6 June 2006)

ABSTRACT

A Coulter Multisizer, which is based on the electrical sensing zone (ESZ) or the Coulter principle, wasused to measure the size distribution of Asian dust. Coulter Multisizer analysis provides high-resolution sizemeasurements of water-insoluble aerosol particles (WIPs) and the number concentration at each size bin.Aerosol filter sampling was conducted at four sites in Japan during spring 2003. The measured volume sizedistributions fit fairly well with a lognormal distribution. The results show that the WIP size distributionsof the same Asian dust air mass varied at each sampling site and the volume mode diameter at the sitesreduced from west to east. The derived volume mode diameter ranged from 1.4 to 2.2 �m and wascomparatively smaller than those in previous studies on Asian dust. This can be explained by the possibleinternal mixing of Asian dust with other components and by the breaking of particles and dispersion ofaggregations by ultrasonification during extraction. The analysis method was improved by washing theaerosol particles collected on a filter using a magnetic stirrer, instead of an ultrasonic cleaner. As a result,the WIP size distribution can be measured in the range of 1–10 �m. The measured mode diameters were2.6–3.1 and 4.3–5.6 �m in 2 Asian dust phenomena at Kofu, Japan, in 2004. It is demonstrated that theCoulter Multisizer method can furnish detailed information regarding the spatial and temporal variations inthe mineral dust size distribution.

1. Introduction

The radiative forcing of mineral dust can either bepositive or negative (Houghton et al. 2001). Previousstudies have pointed out that the evaluation of the ra-

diative effect of mineral dust on regional and globalclimates involves large uncertainties (Sokolik and Toon1996; Sokolik et al. 2001; Tegen et al. 1996). The accu-rate estimation of the radiative forcing of aerosols in-cluding mineral dust requires the knowledge of theiroptical properties like complex refractive index in ad-dition to other properties such as size distribution, mor-phology, and the internal mixture ratio of dust andother components. In particular, the size distributioncould vary during long-range transportation due to the

Corresponding author address: Hiroshi Kobayashi, Interdisci-plinary Graduate School of Medicine and Engineering, Universityof Yamanashi, 4-3-11, Takeda, Kofu, Yamanashi 400-8500, Japan.E-mail: [email protected]

194 J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y VOLUME 24

DOI: 10.1175/JTECH1965.1

© 2007 American Meteorological Society

JTECH1965

Unauthenticated | Downloaded 11/16/21 07:29 PM UTC

differences in the terminal settling velocity, which isparticle-size dependent. Liao and Seinfeld (1998) re-ported that shortwave radiative forcing is extremelysensitive to the mass median diameter in the 1.0–5.0-�m region. Claquin et al. (1998) showed that the knowl-edge of the mean radius is required in the determina-tion of the amplitude of the shortwave and longwaveforcings when large particles are involved. Therefore,the size distribution should be measured accurately andwith a sufficiently high resolution.

Several methods exist for measuring the size distri-bution of mineral dust (J. S. Reid et al. 2003). However,each method has its limitations (Sokolik et al. 2001).The optical particle counter (OPC) can be used to mea-sure the amount of light scattered by individual par-ticles (Chun et al. 2001; Iwasaka et al. 2003; Kim et al.2004b; Porter and Clarke 1997). It can measure aerosolnumber concentrations at intervals of several minutes;however, it cannot separate the mineral dust fromwhole aerosols. The OPC contains several channels andis strongly influenced by the complex refractive indexand nonsphericity of the particles, which is fairly sig-nificant for mineral dust. A cascade impactor can beused to segregate particles based on their mass-to-dragcharacteristics (Arimoto et al. 1997; Park et al. 2004;E. A. Reid et al. 2003; Tanaka et al. 1989). By usingchemical analyses, the size distribution for each chemi-cal composition can be obtained; however, the sizeseparation is uncertain, and it exhibits a coarse size binresolution. An electron microscope (EM) can furnishdetailed information on the properties of the individualparticles, such as its morphology; it can also determinethe internal mixing state by elemental analysis if theEM is attached with an energy dispersive X-ray ana-lyzer (EDX; Gao and Anderson 2001; Zhang et al.2003). At present, however, the reports with regard tosystematic measurements are limited due to the needfor extensive and elaborate work on the large numberof different aerosol particles. By using sky radiance dis-tribution measurements obtained with a skyradiometer(Nakajima et al. 1996) and Aerosol Robotic Network(AERONET) sun photometer (Holben et al. 1998), it ispossible to retrieve a column-integrated aerosol sizedistribution using the optical inversion method (Alex-ander et al. 2002; Aoki and Fujiyoshi 2003; Dubovik etal. 2002; Kim et al. 2004a; Murayama et al. 2001;Tanaka et al. 1989; Tanré et al. 2001). However, theanalysis requires various assumptions; further, clear-skyconditions are essential for performing the measure-ments. In addition, with the exception of the EM, it isdifficult to separate the mineral dust from whole aero-sols by using the abovementioned methods, although

the individual size distribution of mineral dust is re-quired for the evaluation of its optical properties.

Coulter Multisizer analysis can be used for measuringthe soil or sediment size. The Coulter Multisizer isbased on the electrical sensing zone (ESZ) or theCoulter principle (Coulter 1956). It is used to deter-mine the number and volume of particles suspended ina conductive liquid. When a particle passes through asmall aperture with two electrodes on either side, itinduces a change in the resistance between the elec-trodes. This process results in an electronic pulse pro-portional to the particle volume. A pulse height analy-sis yields the size distribution of an ensemble of par-ticles. The collected aerosol particles are suspended inan electrolyte solution. In this study, the used electro-lyte solution is for measuring solid materials and is nota near-saturated solution for measuring water-solublematerials. Accordingly, the measurement is possibleonly for the water-insoluble particles (WIPs). Thechemical elements of Asian dust, excluding oxygen,mainly comprise Si, Na, K, Ca, Mg, Al, and Ti, whichare all water-insoluble with the exception of Ca (Kana-mori et al. 1991). For the Asian dust phenomenon atYakushima Island during spring 1989, the total massconcentration of the water-insoluble chemical elementswas about 4 times that of water-soluble Ca (Kanamoriet al. 1991). Therefore, we consider the mineral dust tobe composed of water-insoluble aerosol particles andthat mineral dust particles can be separated from bulkaerosol particles by the dissolution of the collected par-ticles. In addition, this measurement is not affected bythe complex refractive index, shape, and orientation ofthe particle when it passes through the aperture,whereas these parameters strongly affect the size mea-surement in other methods.

The Multisizer-3 Coulter Counter (BeckmanCoulter, Inc.)—the latest instrument based on theCoulter principle—can provide a high-resolution sizemeasurement with a maximum of 300 channels for anarbitrary size range; the maximum size range is 2%–60% of the aperture diameter. Furthermore, since thenumber concentration is determined at each bin in thesolution, the quantitative size distribution with a highresolution and the concentration unit can be obtainedby using volumes of electrolyte solution and sampledair. On the other hand, when the smallest aperture di-ameter of 20 �m is used, the Coulter Multisizer cananalyze the particle size within a diameter range of 0.4–12 �m; therefore, the measurable particle size shouldbe greater than 0.4 �m in diameter. McTainsh et al.(1997) introduced particle size analyses for sedimentsand soil samples using a Coulter Multisizer. In thisstudy, we apply the Coulter Multisizer method to the

FEBRUARY 2007 K O B A Y A S H I E T A L . 195


measurements of the size distribution of Asian dust anddemonstrate the temporal and spatial variation in theWIP size distribution in Japan during the Asian dustphenomena in order to estimate the optical propertiesfor the evaluation of the radiative forcing and for thesatellite remote sensing. Furthermore, we improve theanalytical method and show the results. We then sum-marize the characteristics of the Coulter Multisizermethod.

2. Methods

a. Sampling and size distribution analyses

Filter sampling was conducted at Nagasaki (32°45�N,129°52�E), Okayama (34°39�N, 133°54�E), Kofu(35°39�N, 138°34�E), and Tokyo (35°41�N, 139°45�E),Japan, in 2003 during the spring—the time of the yearwhen the Asian dust phenomena typically occur (Fig.1). Aerosols were collected on Nuclepore filters with apore size of 0.4 �m in diameter (Whatman) through atube with an inner diameter of 10 mm in open air overa sampling period of 1 day. The tube was used due tothe constraint of setting our own sampling instrumentsat each sampling site, although using the tube resultedin sampling losses. At Kofu, when the Asian dust phe-nomenon occurred, the sampling periods were short-ened in order to obtain a higher time resolution. Thedetails of the sampling conditions at each site are listedin Table 1. Blank samples were periodically obtainedfor a qualitative check of the measurement. Simulta-neously, the number concentrations of the aerosol par-ticles were measured at regular intervals of 10 min us-ing optical particle counters at all the sites, excludingOkayama, in order to monitor the abundance of Asiandust and the variation in the total aerosol abundance.At Okayama, the measurements of the suspended par-ticulate matter (SPM) concentrations from the Minami-gata Motor Vehicle Exhaust Gas Monitoring Station,

which is located 2 km south of the Okayama University,were used instead of the OPC measurements.

The WIP size distribution was determined using theMultisizer-3 Coulter Counter equipped with a 20-�m-diameter pore size aperture. The aerosol particles col-lected on the Nuclepore filters were soaked in 90 mL ofthe electrolyte solution (Isoton II, Beckman Coulter,Inc.) and washed using an ultrasonic cleaner with stir-ring for 10 min in order to extract the collected aerosolparticles into the electrolyte. The volume of the elec-trolyte used in each measurement was 0.05 cm3. Themeasurement was repeated around 6–10 times for eachsample. Particles were counted in 256 size bins rangingfrom 0.4 to 12 �m in diameter. The Coulter Multisizerdirectly measures the particle volume; therefore, “thediameter” hereafter refers to the sphere equivalent di-ameter. The counts obtained from four size bins wereaggregated into a single size bin in order to smooth themeasured size distribution. The averaged measure-ments of the blank samples were subtracted from all themeasurements as a background quantity. The detectionlimits were fixed at twice the standard deviation of themeasurement of the blank samples for each size bin.The calibration of the size determination was con-ducted using a latex bead (CC Size Standard L2, Beck-man Coulter, Inc.) whose size is known. The numberconcentration in air for each size bin Nair(Dp) was cal-culated as follows:

Nair�Dp� � Nsuspension�Dp�vsuspension

vair, �1�

where Nsuspension(Dp) is the measured number con-centration in suspension, Dp is the particle diameter,vsuspension is the electrolyte solution volume, and vair isthe air sampling volume. The volume concentrationwas calculated as the product of the number concentra-tion and the volume with the mean diameter of eachsize bin.

Elemental carbon (EC) is a component of water-insoluble aerosol particles and it sometimes aggregateswith Asian dust (e.g., Quinn et al. 2004). However, inthe Coulter Multisizer method, it is essentially impos-sible to separate EC aerosols from dust aerosols.Therefore, the result includes not only Asian dust butalso EC aerosol size distribution, although the EC aero-sol size is generally considerably smaller than that ofdust.

b. Calibration of transportation losses in thesampling tubes

Transportation losses in the sampling tubes were es-timated and used for the calibration of the measured

FIG. 1. Sampling sites in Japan.



size distributions. At all the sampling sites, the samplingtubes were around 1 m in length with total bends of 90°or 270° between the air inlets and filter holders (Table1). The following three loss mechanisms were consid-ered: diffusion to the tube wall, settling, and inertialdeposition in the tube bend (Hinds 1999). With regardto the laminar flow in the tubes, the particle loss bydiffusion is given by

Pdiffusion � �5.50�2�3 � 3.77�, �2�

where � is the deposition parameter:

� �DL

Q

and D is the diffusion coefficient of the particles, L isthe length of the tube, and Q is the volume flow ratethrough the tube. The loss due to settling is given by

Psettling �2�

�2k1k2 � k11�3k2 � arcsin�k1

1�3�, �3�

where VTS is the terminal settling velocity, DS is thetube diameter, U is the air velocity in the tube, and isthe inclination of the tube from the horizontal. Theinertial deposition of particles in the tube bend is givenby

Pbend � �Stk�� 4�

where Stk is the Stokes number:

Stk ��U

DS

and � is the relaxation time. In some cases at Kofu, theReynolds number in the tube was greater than 2000—the maximum limit of the laminar flow—because of anincrease in the sampling flow rate. In these cases, thefollowing equations for turbulent flow were used in-stead of Eqs. (1) and (3):

Pdiffusion � 1 � exp��4VdepL

DSU � �5�

and

Pbend � 1 � exp��2.88�Stk��, �6�

where

Vdep �0.04U

Re1�4 ��gD

� �2�3

and Re is the Reynolds number in the tube, �g is the airdensity, and is the air viscosity. The measured num-ber concentration of each size bin Ncalib was calibratedas follows:

Ncalib �Nmeasured

1 � Pdiffusion � Psettling � Pbend, �7�

where Nmeasured is the measured number concentration.These calculations were performed for each size bin byusing the average flow rate of each sampling period andthe geometry of the sampling tube at each site.

c. Estimation of particles broken byultrasonification

The collected aerosol particles were extracted intothe electrolyte by ultrasonification. Although ultrasonicoscillations facilitate effective extraction, the fracturingof mineral dust particles and aggregation due to thepowerful ultrasonic oscillations leads to an underesti-mation of the WIP size distribution. The extent of thisunderestimation was estimated.

The ultrasonic effect on sizing was evaluated by com-paring the size distribution processed by ultrasonifica-tion with the original one. The test sample must becollected in a manner such that the fracturing of theparticles is avoided. The aerosol particles collected on afilter were required to undergo treatment during theextraction process. The particles collected on a hard

TABLE 1. Filter sampling conditions at each site.

City Nagasaki Okayama Kofu TokyoPlace Nagasaki University Okayama University University of Yamanashi Tokyo University

Mercantile Marine*Inlet MSL (AGL) (m) 20 (5) 27 (17) 312 (12) 27 (24)Inlet tube length (m) 1.5 1.5 1.0 1.2Total bend angle of the

inlet tube (°)270 270 90 90

Flow rate (L min�1) 6.1–6.3 4.7–6.9 4.4–24.1** 2.1–2.2OPC Rion KC-01D SPM data obtained from public

station instead of OPCRion KR-12A Rion KC-01D

* Presently the Tokyo University of Marine Science and Technology.** During the Asian dust event, the flow rate was increased.



plate can be suspended in the electrolyte solution sim-ply by flushing with a wash bottle and do not requireprocessing. Therefore, a cascade impactor (TokyoDylec, LP-20RPA) was used for aerosol particle sam-pling. In this case, no grease was coated on the impac-tion plates because the cascade impactor is not in-tended for aerosol sampling by classification accordingto aerodynamic size. Therefore, the adhesion of thecollected particles with grease on the impaction plate isavoided. Sampling was conducted at Kofu on 3 April2004, during the Asian dust phenomenon. The aerosolparticles from each impaction plate were suspended inthe electrolyte solution without ultrasonification andthen the original size distribution was measured. Afterthe suspended solution was treated by ultrasonificationfor 3 min, the resulting size distribution was measured.The solution was then subjected to further ultrasonifi-cation for 3 min.

d. Improvements in the sampling and analysismethods

The sampling and analysis methods were improved inorder to avoid the possibility of bias in the transporta-tion losses in the sampling tube and particle breakingand aggregation dispersion by ultrasonification. Aero-sol particles were guided to the filter folder directlywithout a tube. The electrolyte solution and extractionprocedure were modified on the basis of the methodused for a clay sample provided by Beckman Coulter,Inc. The aerosol particles collected on Nuclepore filterswere soaked in 90-mL Na3PO4 solution for 2 days andwashed using a magnetic stirrer, instead of an ultrasoniccleaner, for 30 min. A 70-�m-diameter pore size aper-ture, whose measurable size range is 1.4–42 �m in di-ameter, was used to measure particles around 10 �m indiameter with a good signal-to-noise ratio. Aerosolsampling was conducted at Kofu during spring 2004using the improved method. The WIP size distributionwas analyzed using the improved procedure.

e. Estimation of extraction efficiency from a filter

It is possible that the extraction efficiency by using amagnetic stirrer in the improved method is low becausethe agitation is not as powerful as that due to ultrasonicoscillations, although particle breaking and aggregationdispersion might be reduced. Therefore, the extractionefficiency was evaluated.

The extraction efficiency was determined by compar-ing the known particle number on the Nuclepore filterwith the particle number measured using the improvedanalysis method for the filter. The filtered sample wasprepared by the filtration of a standard suspension of

Asian dust and ion-exchanged water for washing theelectrolyte. The particle number on the filtered samplecan be calculated from the filtrated volume of the stan-dard suspension and its concentration. The standardsuspension of Asian dust was prepared from a meltingsnow sample that includes deposited Asian dust; thesampling was performed in Sapporo, Japan, on 12March 2004, by settling coarse particles greater than 20�m and diluting the measured concentration. The par-ticle number concentration with each size bin was mea-sured using the Coulter Multisizer.

3. Results

a. Size distributions of Asian dust particles

The results of the WIP size distributions and OPCmeasurements or SPM data at the four sites around13–14 April 2003, when the Asian dust phenomenonwas recorded in west Japan, are shown (see Figs. 2–5).Each result was previously calibrated with regard to thetransportation loss in the sampling tube. The size dis-tributions were fitted with a lognormal distributionfunction; this function is expressed as follows:

dV

d lnDp�

V

�2� ln�exp��

�lnDp � lnDm�2

2 ln2��,

�7�

where V is the volume concentration, Dp is the particlediameter, Dm is the volume mode diameter, and � is thestandard deviation. The OPC number concentrationwas used to interpret the Asian dust phenomenon be-cause the OPC number concentration in the size rangewith diameter greater than 1–2 �m correlates to thecomponent from the Asian dust (Chun et al. 2001; Ue-matsu et al. 2002).

Figure 2 shows the results for Nagasaki during 11–14April 2003. The number concentration of aerosol par-ticles in the ranges �1 and �2 �m increased from 0600Japan standard time (JST: UTC � 0900) on 12 April;subsequently, the concentration in the range �5 �mincreased after 0800 JST (period 1). The WIP size dis-tribution shows a slight peak in this period. All theconcentrations in each size range stabilized from 1600JST 12 April to 0600 JST 13 April; consequently, all thecounts increased (period 2). The maximum of the num-ber concentrations in the range �1 �m were recordedat 1400 JST (period 3). Furthermore, the peak height ofthe WIP size distribution recorded the maximum valueat this time. The volume mode diameter and standarddeviation were 1.99 �m and 2.26, respectively. In thefollowing period, the concentrations in the range �1



�m decreased gradually (period 4), and the peak valueof the WIP size distribution varied by two-thirds that ofthe maximum with Dm � 2.04 �m and � � 2.04 inperiod 3.

The results for Okayama during 11–14 April 2003,are shown in Fig. 3. The SPM concentration increasedsharply at 1600 JST 12 April. The concentration de-creased gradually after increasing to approximately0.11 mg m�3. The spike of the WIP size distributionappeared to a slight degree at around Dm � 3.6 �m(period 1). After 1000 JST 13 April, the SPM concen-tration began to increase again (period 2). In this pe-riod, the WIP size distribution exhibited a broad log-normal distribution with spikes. The size of the largerspike in period 2 of around 3–4 �m was consistent withthat of the spike in period 1. Hereafter, the SPM con-centration constantly ranged around 0.06–0.08 mg m�3,and the peak value of the WIP size distribution exhib-ited a maximum (period 3). The mode diameter was1.69 �m and the standard deviation was 1.98. In thefollowing period (period 4), the SPM concentrationpartly decreased to around 0.06 mg m�3, and the modeand width of the WIP size distribution increased slightlyto Dm � 1.90 �m and � � 2.30, respectively; this oc-curred despite a decrease of approximately 20% in thepeak height.

Figure 4 shows the results obtained at Kofu during11–17 April 2003. The number concentrations in theranges �2 and �5 �m began to increase from 0000 JST13 April (period 2). In this period, two sharp WIP sizedistributions appeared at around Dm � 2.2 and 3.8 �m.In the following period (period 3), the concentrations inthe ranges �0.3 and �0.5 �m exhibited an inverse cor-

relation with the other ranges, and a WIP size distribu-tion with Dm � 2.10 �m was observed. However, after1700 JST, all the range counts increased (period 4). Themode diameter was smaller by 1.71 �m than that ob-served in period 3. Thereafter, the peak height of theWIP size distribution increased to the maximum withDm � 1.60 �m and � � 2.08 (period 5). In the followingperiod, the concentrations in the range �1 �m began todecrease (period 6). The volume mode diameter de-creased further below 1.40 �m. Although this trendcontinued in period 6, only the count in the range �5�m began to increase after 1530 JST 15 April (period7). Two sharp WIP size distributions appeared in amanner similar to that observed in period 2.

The results for Tokyo are shown in Fig. 5. The num-ber concentrations in the ranges �0.3 and �0.5 �mincreased after 1600 JST 12 April, although the concen-trations in the larger ranges decreased (period 1); thisresulted in the observation of a slight WIP size distri-bution. Thereafter, the concentrations in the range �1�m increased sharply at 2030 JST 13 April, followingwhich the larger range counts recorded their maximum(period 2). Furthermore, the peak of the WIP size dis-tribution showed the highest value with Dm � 1.95 �mand � � 2.14. Around 1330 JST 14 April, all the con-centrations decreased; however, an increase in thecount was observed except for those in the range �5�m (period 3). The peak value of the WIP size distri-bution decreased by �23%. The mode diameter in-creased slightly from Dm � 1.95 to 2.24 �m. In thefollowing period, the increase in the concentrationscontinued, except for those in the �5 �m range (period4). However, the peak of the WIP size distribution de-

FIG. 2. Measured size distribution of (top) WIPs and (bottom) time series of the aerosol numberconcentration using OPC in Nagasaki during April 2003. The sampling periods for the WIP size distri-bution measurement are shown in the bottom panel.



creased. The distribution exhibited a slight decrease:Dm � 1.57 �m.

b. Error estimation for sizing by ultrasonification

Figure 6 shows the results of the fourth and fifthstages of the cascade impactor of the ultrasonificationtest for only the first 3 min because the results for theentire 6-min test period were the same as those for the

initial 3 min. The results greater than one-fourth ofeach maximum were fitted with lognormal distributionsdue to the irregularities of the smaller values. In thefourth stage, the mode diameter changed from 2.97 to2.53 �m by ultrasonification. In addition, the volume,which was smaller than 1.2 �m, increased to more thantwice its original value. This indicates that due to thebroken particles and dispersed aggregates resultingfrom ultrasonification, the number of smaller particles

FIG. 3. Measured size distribution of (top) WIPs and (bottom) time series of the SPM concentrationin Okayama during April 2003.

FIG. 4. Measured size distribution of (top) WIPs and (bottom) time series of aerosol numberconcentration using OPC in Kofu during April 2003.



increased. In the fifth stage, the mode diameter alsochanged from 1.94 to 1.56 �m; furthermore, the ratio ofdecrease in the geometrical mode diameter was 15%–20%.

c. Size distributions measured using the improvedmethod

Figure 7 shows the results of two Asian dust eventsmeasured using the improved method at Kofu duringspring 2004. The size distribution was detected in therange up to 10 �m with smaller scattering than theresults measured using the previous method. In an ear-lier event during 30 March–2 April, the mode diameterof the WIP size distribution appeared at Dm � 2.66 �m(period 2). Thereafter, the mode diameter increased toDm � 3.11 �m as the peak height increased (period 3).Then, the mode diameter decreased with a decrease in

the peak height (period 4). The standard deviationranged from 1.6 to 1.7. A sharp WIP size distributionappeared with a broad distribution in period 1 in amanner similar to period 2 in Fig. 4. In a subsequentevent around 17 April, the WIP size distribution ap-peared at a mode diameter as large as around 5 �m(periods 5 and 6). Thereafter, the peak height in-creased, but the mode diameter decreased to around4.3 �m. In this event, the standard deviations exceeded2 and the size distributions were broader than that inthe earlier event.

d. Extraction efficiency by agitation with amagnetic stirrer

Figure 8 shows the result of the extraction efficiency.The error bar indicates the standard deviation of theresults of the three samples. The value exceededaround 70% in the measured size range and was thelowest around 3 �m. In the size range �8 �m, the ef-ficiency was almost 100%; however, a few values ex-ceeded 100% and the error bars were relatively high.Because the concentration of the standard suspensionof Asian dust in the range �8 �m was one or two ordersof magnitude lower than the smaller range concentra-tion, the measuring error was significant in this sizerange.

4. Discussion

The measured volume-size distributions of water-insoluble particles considered as Asian dust particlesobeyed a lognormal distribution; furthermore, the OPCand SPM results indicated that the air mass was indeedAsian dust.

FIG. 5. Measured size distribution of (top) WIPs and (bottom) time series of aerosol numberconcentration using OPC in Tokyo during April 2003.

FIG. 6. Test sample size distribution collected using a cascadeimpactor before and after ultrasonic treatment. The curves arelognormal distributions fitted with results greater than one-fourthof each maximum.



The results obtained for Nagasaki, Okayama, andKofu showed that the WIP size distribution graduallyincreased in a manner similar to a lognormal distribu-tion. In contrast, a WIP size distribution appearedabruptly at Tokyo. The phenomena were the same asthe results of the OPC number concentrations in therange �1 �m. At Nagasaki, there was a slight variationin the mode diameter and standard deviations. AtKofu, both these parameters exhibited a smooth de-crease, while they increased at Okayama. At Tokyo,however, they first increased and then decreased. De-spite identical or almost similar sampling periods, theresults obtained at each sampling site differed fromeach other. This indicates a high variability in the WIPsize distributions for the same Asian dust air mass. As-suming that the maximum WIP size distribution wastypical for the air mass at each site, the volume modediameter decreases from west to east, except for Tokyo.

This indicates that large Asian dust particles descendbefore the small ones in the airmass transport. At Kofu,where the sampling periods were shorter than those atthe other sites, two sharp peaks appeared both beforeand after the main Asian dust event. The significance ofthese results is not understood at present; however,they are extremely interesting. High-resolution sizemeasurements obtained by using the Coulter Multisizercan assist in understanding these results.

The volume mode diameter of the WIP measuredusing the Coulter Multisizer ranged from 1.4 to 2.2 �m.Observations carried out with the skyradiometershowed that the coarse volume mode diameter is usu-ally around 4 �m at several sites in Japan (Murayama etal. 2001). Aoki and Fujiyoshi (2003) also demonstratedthat the volume size distributions exhibit peaks corre-sponding to diameters in the range of 4–6 �m at Sap-poro. Kim et al. (2004a) indicated that the size distri-

FIG. 7. Size distribution of WIPs measured using the improved method and time series of aerosolnumber concentration using OPC in Kofu during the spring of 2004.



bution shows coarse mode diameters in the ranges of4–6 or 8–10 �m at Anmyon, South Korea. Tanaka et al.(1989) reported that the volume size distribution ob-tained by using the cascade impactor (Andersen sam-pler) shows a sharp peak at around a diameter of 4 �m.If the WIP measured by our method were Asian dustparticles, the measured size distributions would besmaller than those obtained from previous studies. Thereasons can be classified into theoretically and experi-mentally induced factors.

One of the theoretical factors is the possibility of theinternal mixing of Asian dust with other components.Zhang et al. (2003) showed that approximately 60%–85% of the dust particles were internally mixed with seasalt. Although the OPC, cascade impactor, and opticalinversion method using a radiometer measure the in-ternally mixed aerosol particles, the Coulter Multisizermethod only measures the water-insoluble componentsof the particle. Therefore, our results, as compared withthose that employed other methods, showed smallerparticle size distributions. In addition, the combinationof the Coulter Multisizer and other methods listedabove can be used to derive the quantitative informa-tion of the internal mixture.

The experimentally induced factors are considered tobe the dispersion of the aggregations and breaking ofthe particles by ultrasonification during the extractionof aerosol particles from a filter. In particular, the dis-persion of the aggregations occurred only during theimmersion of the aggregated aerosol particles into the

electrolyte solution due to its transience (Leys et al.2005). Nevertheless, it was demonstrated that ultrasoni-fication results in a 15%–20% size reduction.

The improved method can eliminate the ultrasonifi-cation effect as well as calibrate the transportationlosses in the tube. By comparing the results measuredusing the improved method with that of the previousmethod, the concentration in the range �3 �m in-creased and the scattering of its value decreased. Themode diameter measured using the previous method as1.4–2.2 �m increased to as much as 2.6–3.1 or 4.3–5.6�m in the two Asian dust events after the improve-ments, although the phenomenona measured using theimproved and previous methods were distinct. On theother hand, the number size distributions obtained byusing the OPC are compared with those obtained byusing the Coulter Multisizer for both the previous andimproved methods, although the OPC measures wholeaerosol particles, including the water-soluble compo-nents (Fig. 9). It is clear that the consistency of the sizedistributions after the improvements was considerablybetter than that before the improvements in the sizerange from 2 to 5 �m, whose concentrations correlateto the components from Asian dust; however, there isthe possibility of an internal mixture of Asian dust withother components and the uncertainty of OPC countsin this range because of the nonsphericity of the par-ticles and the difference in the refractive index betweenAsian dust and latex particles used in the calibration.These results show that the improved method using theCoulter Multisizer can measure the size distribution ofAsian dust in the range from 1 to 10 �m with highresolution.

If the size reduction by ultrasonification was �20%,the mode diameter measured using the previous

FIG. 8. Extraction efficiency of collected aerosol particles on aNuclepore filter into electrolyte solution stirred with a magneticstirrer.

FIG. 9. Comparison between the number size distributions mea-sured using OPC and the Coulter Multisizer method on both (left)the previous and (right) improved methods at Kofu. The maxi-mum measurable size for the OPC is assumed to be a diameter of10 �m.



method was estimated within the range of 1.8–2.8 �m.This size range is consistent with the smaller mode di-ameter measured using the improved method in 2004.

The extraction efficiency of the collected aerosol par-ticles on a Nuclepore filter into an electrolyte solutionwas measured by using a filter in order to simulate asample of the gathered aerosol particles by using a stan-dard suspension of known concentration. It is recog-nized that the efficiency ranged from approximately70% to 100%. However, it is possible that the physicalproperties of the atmospheric aerosol particles, particu-larly charging, differ from that of the particles oncesuspended in the electrolyte solution. Consequently,the estimated extraction efficiency includes an uncer-tainty.

The dispersion of the aggregated particles is unavoid-able as long as they are suspended in a solution. How-ever, Leys et al. (2005) show the estimation method ofaggregation using a statistical curve-fitting procedure.This method is applicable to the size distribution mea-sured using the Coulter Multisizer because of its highresolution.

5. Conclusions

In this study, we applied the Coulter Multisizermethod to measure the size distribution of Asian dust.This instrument can perform high-resolution measure-ments of size distributions. Aerosol filter samplingswere conducted at Nagasaki, Okayama, Kofu, and To-kyo, Japan, during the Asian dust season in 2003. Thefit between the measured volume size distributions dur-ing the Asian dust phenomena and lognormal distribu-tions is excellent. The measured size distributions var-ied for different sampling sites. This indicates a highvariability in the WIP size distributions for the sameAsian dust air mass. Assuming that the maximum WIPsize distribution was typical for the air mass at each site,the volume mode diameter decreases from west to east,except for Tokyo.

The volume mode diameter of the WIP measuredusing the Coulter Multisizer ranged from 1.4 to 2.2 �m.The size distributions were smaller than those obtainedin previous studies on Asian dust. The reasons for thiscan be classified into theoretical and experimentally in-duced factors. The former, which involves the possibil-ity of the internal mixing of Asian dust with other wa-ter-soluble components, yields smaller size distribu-tions because the Coulter Multisizer method canmeasure only the water-insoluble components of theparticle. The experimentally induced factors includethe breaking of particles and dispersion of aggregationsby ultrasonification. Ultrasonification results in a 15%–

20% size reduction. The sampling and analysis methodswere improved due to the bias possibilities as a result oftransportation losses in the sampling tube and particlebreaking and aggregation dispersion by ultrasonifica-tion. The aerosol particles were guided to the filterfolder directly without a tube. The aerosol particles col-lected on the Nuclepore filter were soaked for 2 daysand washed using a magnetic stirrer, instead of an ul-trasonic cleaner. Aerosol sampling was conducted atKofu in spring 2004 using the improved method. Thesize distribution was detected as approximately 10 �mwith smaller scattering. The mode diameter measuredusing the previous method (1.4–2.2 �m) increased to asmuch as 2.6–3.1 or 4.3–5.6 �m in the two Asian dustevents after the improvements. In addition, the numbersize distribution obtained using the OPC was consistentwith that obtained using the Coulter Multisizer in theimproved method in the size range from 2 to 5 �m.These results show that the improved method using theCoulter Multisizer can measure the size distribution ofAsian dust in the range from 1 to 10 �m. The advan-tages and disadvantages of the Coulter Multisizermethod are as follows:

• higher resolution than other methods;• measurement of only the water-insoluble particles in

the Asian dust, and not the whole aerosol particles;• representation as a concentration unit;• possibility of dispersion of aggregations due to soak-

ing in solution; and• measurable lower size limit of 0.4 �m in diameter.

The Coulter Multisizer method can furnish detailedinformation regarding the spatial and temporal varia-tions in the mineral dust size distribution. In addition, acombination of the Coulter Multisizer and other meth-ods can be used to obtain quantitative information re-garding the internal mixture. Therefore, the opticalproperties of Asian dust can be estimated with a con-siderably higher accuracy.

Acknowledgments. The authors thank K. Yamaguchi,H. Yoshimura, and S. Ozawa for assistance in the analy-sis. We also thank Dr. T. Aoki for providing the snowsample and Okayama prefecture for providing the SPMdata. We are also grateful to two anonymous reviewersfor helpful comments on the earlier drafts of this manu-script. This study was supported by a Grant-in-Aid forScientific Research on Priority Areas under Grant14048228 from the Ministry of Education, Culture,Sports, Science and Technology, Japan.

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high-resolution measurement of size distributions of asian

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