Size Distributions of Aerosol Particles in the Free Troposphere : Aircraft Measurements in the Spring of 1991-1994 Over Japan

Measurements of number-, mass-, and volume-size distributions of' free tropospheric aerosols were made over Japan in the spring of 1991-1994. Number-size distribution frequently shows a peak in the area of a diam­ eter of D� 1 11.m in the free troposphere during observational periods. A few peaks are identified in the volume-size distribution as estimated on the basis of number-size distribution having single mode in a coarse range. Mass-size distribution frequently indicate enhancement in the coarse size range. This feature of the distribution is more frequent in those measurements made at 4.4 (±0.3) km than those at 2.3 (±0.3) km. On the basis of a backward trajectory anal)rsis of an air mass containing those particles, soil particles originati11g in the Asian continent affect the features found in the size distributions as well as vertical change in those size distributions. (


INTRODUCTION
Airborne rneasure1nents on tl1e size distribution patte1•ns of t'r• ee t1•oposphe1•ic aerosols were made in the .spring of 1991-1994 so as to give cl bette1• understandi11g of the behavior of particulc1te 1nc1tter over Japan.Wester I y prevai 1 i ng w•i nds seem to c .haracterize the nature of the at111ospheric particles over eastern Asia and the weste.rnPacific region since those winds t'requently• carry particulate matter originating on the continent over the ocean and such particles are sometimes expe.cted to be major in those regions (e.g.� Duce <�i.al., 1983� Mei11ert andWinchester, 1977).
Every spring, Asian dust particles are f1•equently transported to Japan by westerly pre \1ailing wi11ds.In particular, the atinospheric e .ffect ot' heavy• dust storms observed near the ground in Japan a1•e familiarly• called 'Kosa' in Japanese which literally means 'yellow sand '. 44 TAO, v'ol. 7, �"lo. 1, l\!Iarch 1996 Many investigations on Kosa particles and their effect on radiative transfer processes and the geochemical budget of soil have been made.Those include observations of particle shape and the chemical composition with an electron microscope (e. g., Okada et al., 1990), chemical analyses of particulate matter (e .g., Gao et al., 1992; Betzer et al., 1988), assessment of the radiative effe ct of particulate matter (e.g., Asano and Shiobara, 1989), estimation of the load of Kosa with a lidar and radiometer (lwasaka et al., 1988;Arao and Ishizaka, 1986) and observations on particle size distributions with a particle counter or sun-photometer ( e.g., Hayasaka et al., 1992;Ta naka et al., 1990).These investigators suggest that the particles play an important role in radiative transfer, global budget and/or the geochemical cycle of various chemical constituents in the region of east Asia and the western Pacific Ocean.
However, investigations of the behavior of Asian dust particles were made mostly on the basis of particle collection near the ground.Accordingly, measurements of particles in the free troposphere are extremely limited due to the difficulty involved in the measurement of free tropospheric Kosa particles.
The main purpose of this paper is to provide particle size distribution patterns measured in the free troposphere over Japan and to discuss the effect of soil particles originating on the Asian continent on the size distributions of the fr ee tropospheric particles.

MEASUREI\1ENT OF NUMBER-SIZE DISTRIBUTION •oF AEROSOLS IN THE FREE ATMOSPHERE
Number-and mass-size distributions of atmospheric aerosols were measured in the free atmosphere in the spring of 199 1, 1992, 1993 and 1994 over the Nagoya and Wakasa-Bay area with a particle counter and an impactor mounted on the aircraft, Cessuna 4028.The distance between Nagoya airport and Wakasa Bay is about 130 km.Additional measurements were made over the Goto Islands area in the southern part of Japan in the spring of 1993.
Sampled air was introduced into a cabin of the aircraft through an isokinetic decelerator and then distributed to the optical particle counter, the QCM cascade impactor and other equipments.The instruments used here are listed in Tab le I.An optical particle counter (DAN Sangyo Co., Model PM-730) with a halogen lamp and a phototube detector of light scattered by particles was mounted on the aircraft to measure the number-size distribution covering fine to coarse particle size.A sizing of the particles was made at 0.34 µm, 0.44 µm, 0.54 µm, 0.69 µm, 0.89 µm, I. 1 µm, 1.3 µm, 1.7 µm, 2.4 µm, 3.4 µm, 4.4 µm, 5.4 µm, 6.9 µm, 8.9 µm and larger than 10.0 µm (diameter).The T�4.0,\1ol. 7,No. l, 1\Iarcb 1996 caliburation of the counter was done according to Japanese Industrial Standard (JIS-Z-8813) instructions as decided by the Japan Ministry of International Trade and Industry.

PEAK IN NUMBER-SIZE DISTRIBUTIONS AT SIZE OF ABOUT 1 µm
In Figures 2a-h, the number-size distributions of aerosols measured during the spring seasons of 1991, 1992, 1993, and 1994 are shown.Most of the curves in Figures 2a-h show small peaks in the range of coarse particle size (mostly 0.89, 1.0, or 1.3 µm diameter) at the altitude of both 2.3 (±0.3) and 4.4 ( ± 0.3) km.
The density peak of coarse size particles in the surface atmosphere is considered to be due to soil and/or sea salt particles which are directly injected into the atmosphere from the local ground sources (e-.g., Gillette and Hanson, 1989; Paterson et al., 1976; Tw omey, 1976).
The-peaks at coarse size range in Figure 2, however, are not only due to local sources but also to other sources since the height of the boundary layer top was at most about 2 km in the spring over Japan (Takeuchi and Kondoh, 1993).Strong westerlies prevail in the middle troposphere over Japan throughout the year except summer (Kawahara, 1992(Kawahara, , 1993;;Takano, 1994).The wind-blown dust injected into the atmosphere over the desert area of the Asian continent is, thus, considered to be a significant source of coarse particles above the planetary boundary layer.
I wasaka et al. ( 19 88) suggested that weak Kosa events were frequently identified in the free troposphe.re, typically in the altitude range of 2 km to 6 km, over Japan in the spring from lidar measurements, even when Kosa events were not detected near the ground.
From the lidar measurements made at Nagoya (the Main characteristics of the lidar are summarized in Table 3), the scattering coefficient of atmospheric aerosols can be obtained.This is defined as:  [ . 8m( z)] 1-/ [p11i] / / which is in the range of 0.02-0.04,when the particle scattering laser beam has a sherical shape (Iwasaka, 1986).Lidar measurement made to collaborate the airborne particle-counter measurements showed an example suggesting the effect of boundary mixing on vertical profiles of aerosol content (Figure 3).
The lidar measurements in Figure 3 indicate the minimum aerosol mixing ratio at I .8km which is expected to correspond to the top of the boundary mixing layer.
Therefore, it is reasonable to suggest that the enhancement of particle number concen tration in the.coarse particle size range above the boundary layer is partially due to the weak Kosa events as described by I wasaka et al. ( 1988).------, '

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. . -1 Mar. 9, 1993 Tolnl NL1nber 4 Sx106 /1113 (4"•00 tn) Diameter (µm} Total Number 1. 1x108 /rn3 (2300 rn)   4).The active transportation of soil particles from the Asian continent to Japan by westerly prevailing winds seems to clearly explain the discrepancy bet\veen the results here and those obtained in the marine free atmosphere over the south west Pacific ocean.
The backward trajectory an a lysis of air mass corresponding to the height of the aerosol layer detected by the lidar was made on the surface of isopotential temperature.The analysis indicates that most of the air mass came from the Asian continent within a few days, as shown in the samples of analyzed trajectories (Figure 5).This trajectory analysi s indicates the possibility that an air 1nass noticeably moves up and down during the long range travel of the air mass.The large upward motion of the ai r mass can compensate for the descending movement of the extremely large size particles contained in the ascending air mass, and consequently super micron particles can also travel a long way.As a good example, Betzer et al. (1988) collected giant mineral particles with D�200 {lm which were transported from the Asian continent to the north-west Pacific ocean by strong westerly winds.The present analysis suggests that the upward motion of the air mass containing extremely large particles is an important process for the long range transport of such particles since it is expected that those particles descend rapidly to the ground without such an upwelling motion of air.
It is thought that the peak in the submicron size range is mainly due to aerosols produced through photochemical reactions including atmospheric sulfur emitted in the urban surface atmosphere through human activities (e.g., Okada et al., 1986), transported from the continent over the ocean (Duce et al., 1983), and produced from the oxidation of dimethylsulfide (DMS) in the marine boundary layer (Patterson et (Ll., 1980;Hoppel et al., 1987; Kreiden we is et al., 1991 ).
The number-size (or volume-size) distributions of 8 March 1993 are the typical exam ple showing the Kosa effect in the free troposphere (Figures 2 and 7).The enhancement in number density was only detected at 4.4 km and not at the 2.3 km level.The mass-size distribution of the 4.4 km level also showed enhancement which corresponds well the feature found in vertical change in aerosol number concentration.The measurement of the volume spectrum of 12 May 1993 indicated the typical pattern under a relatively calm atmospheric condition in which the aerosol concentration decreased according to the increase in the height for all size ranges observe.ct here.However, the volume-size distribution shows a typical sin gle mode shape, as is frequently observed in the boundary atmosphere during a Kosa episode.The measurements of the mass-size distribution during that day show the enhancement of coarse size particles having a diameter of 9.0-17 .6 µm which seems to correspond to the       volume-size pattern.•The c. oncentration of such large particles is so Jo\\' in the • free troposphere that long term collection of particulate matte.r is necessitated so as to obtain accurate.values.
The two-hour particle-collection at the flow rate of about 10 I/min may be insufficient to see the mass-size pattern of a few µm particles in the free troposphere.
Those coarse size particle enhancements in the t' ree troposphere are possibly due to the e .
ffect of the long range transport of soil particles even though local meteorological observatories gave no report on Kosa.This is in view of the fact that many investigations suggest the possible contribution of soil particles to the enhancement of coarse particles in the free troposphere over Japan (l\vasaka et (ll., 1988; Hayasaka et al., 1990; Tanaka et (;,l.,   1990).

VOLUME-SIZE DISTRIBUTION
\l'olume-size distributions frequently show a bimodal pattern with the peak mode in the submicron or micron range in the ambient atmosphere (e. g., Prospero et rLl., 1983).On the basis of measurements of the urban atmosphere with polar nephelometer (Hayasaka et. al., 1992), a bimodal volume spectrum is frequently identified.
The residence time of aerosol particles in the atmosphere strongly depends on the particle size, and the de.nsity of super micron particles is expected to rapidly decrease with an increase in altitudes O\\i'ing to the shorter residence time of coarse particles.Airborne polar nephelometer measurements sho\v that the density of coarse particles decreases drastically above the mixed layer in the summer when the Pacific High pre\'ails over Japan (Hayasaka   et al., 1990).
During Kosa events, the volume spectra with an enhanced single mode (mode diameter of 2-4 11m) is observed in the surface atmosphere over Japan (Tanaka et al., 1989; Arao   and Ishizaka, 1986).In Figure 7a-h (1) two (or three) mode type patterns are sometimes observed at 4.4 and 2.3 kin levels.
(2) single mode function having a mode of coarse range is more frequent])' observed at 4.4   km than at 2.3 km.
Based on the observations with a lidar (Iwasaka et al., 1988) and an airborne polar ne.phelo1neter (Hay as aka et al., 1990), some investigations have suggested the.possibility that soil particles were frequently transported from the Asian continent into the tree tropos_phere over Japan.
The measurements made over the Goto Islands on the southern area of Japan on Mar.
8, 1993 are a good example in that they show a large enhancement of Kosa particles in the free troposphere, and the disturbance ot' Kosa at the lower altitude is apparently small.111 the measurements of May 12, 1993 andApril 26, 1994, t)1pical single mode spectra with a mode diameter of about 1.5 /.tm was observed at the 4.4 km level, while the bimodal d• istribution was identified at the 2.3 km level suggesting a disturbance in the boundary layer.Such stratified layer structure suggests that aerosol concentration above the mixed boundary la)•er is frequently affected by soi I particles transported from the Asian continent.
Therefore, it is speculated that the Kosa episode observed near the surface was caused not onl)' by sedimentation of indi\'idual larger particles during the Kosa appearance in the free :::i.troposphere, but also by the desc.ending motion of an air mass containing lots of Kosa particles.It is necessary to discuss the detailed movement of an air mass to clarify this idea.

ENHANCEMENT OF AEROSOL-LOAD IN THE FREE TROPOSPHERE
As shown in Figures 2 and 7, the number and \'olume concentration of particulate matter observe.cl at about 4.4 km �'as higher than that at 2.3 km in the particle size range larger than 0.54 µ.m.The inversion in the vertical distribution of particulate matter concentration can be hardly expected when the content of the relatively large size aerosols is due to only local ground sources (land or sea surf ace).
The vertical profile of the scattering ratio measured with a lidar on March 8, 1993 shows a broad peak at 2-3 km and 3-4 km (Figure 3).The large depolarization ratio at 4.5 and 3.5 km suggests the existence of nonspherical particles, such as Kosa particles.Combining the vertical distribution of the scattering ratio of particles and the depolarization ratio, the broad peaks of scattering ratio are thought to be an aerosol layer containing nonspherical particles, such as soil particles.This feature corresponds to the observation of a particle counter which showed that the enhanGement of the aerosol load only at 4.4 km and 11ot at 2.3 km (Figures 2e and 7e  � 1.QX10() I---,\.
flight paths of the aircraft used for the measurements are shown in Figure I, and particle collection was made during the flights at 2.3 (±0.3) km and 4.4 (±0.3) km altitude.Observational dates .aresummarized in Tab le 2. A thin hazy layer was fr equently identified in the free atmosphere by the crew's own eyes during the observational flights.

Fig. 1 .
Fig. 1.Flight path used for measurements.All measurements were made over the Nagoya-Wakasa Bay area, except for one over Goto Islands area on March 8, 1993.

.
Particles collecte.dIn• the free troposphere.Irregular shaped particles (see observed during the flight < ) were frequently • the free troposphere In

Fig. 6 .
Fig. 6.Sample of the mass-size distribution function observed with an airborne QCM cascade impactor.•and o correspond to the measui•ements at2.3    km and 4.4 km, respectively.�) means density is lower than the.detection limit.

Table I .
Instruments used for Aircraft Measurements.

Table 2 .
Date and Altitude of Aircraft Measurements. . .