Seasonal dependence of the longrange ... - Wiley Online Library

4 downloads 0 Views 805KB Size Report
Aug 13, 2003 - range transport of Asian dust particles inferred from isentropic trajectory analysis ... on the aeolian transport of mineral dust to the vast regions.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D23, 8663, doi:10.1029/2002JD003266, 2003

Seasonal dependence of the long-range transport and vertical distribution of free tropospheric aerosols over east Asia: On the basis of aircraft and lidar measurements and isentropic trajectory analysis Atsushi Matsuki,1 Yasunobu Iwasaka,1 Kazuo Osada,1 Katsuji Matsunaga,1 Mizuka Kido,1 Yayoi Inomata,1 Dmitri Trochkine,1 Chiharu Nishita,1 Takayoshi Nezuka,1 Tetsu Sakai,2 Daizhou Zhang,3 and Soung-An Kwon4 Received 1 December 2002; revised 20 February 2003; accepted 18 April 2003; published 13 August 2003.

[1] Seasonal changes in the vertical structure of free tropospheric aerosols over east Asia,

on the basis of aircraft-borne and lidar measurements, and on the pathway of the longrange transport of Asian dust particles inferred from isentropic trajectory analysis are discussed. Aircraft-borne measurements held in situ in the free troposphere over central Japan in 2000–2001 revealed a small in scale yet steady transport of dust in the lowermiddle free troposphere (2–6 km altitude) during spring including days with no evident dust outbreak. Such dust, found as background, was observed even in summer in the regions higher than 4 km under the influence of remaining westerly winds but not in the lower regions. From a series of lidar observations over Nagoya (35N, 137E), Japan, noticeable changes in aerosol characteristics were obtained in the free troposphere from spring to summer. Taklimakan desert is suggested as possible important source of the INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and background dust. particles (0345, 4801); 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; KEYWORDS: aerosol, aircraft, Asia, dust, long-range transport, troposphere Citation: Matsuki, A., et al., Seasonal dependence of the long-range transport and vertical distribution of free tropospheric aerosols over east Asia: On the basis of aircraft and lidar measurements and isentropic trajectory analysis, J. Geophys. Res., 108(D23), 8663, doi:10.1029/2002JD003266, 2003.

1. Introduction [2] Over the past decades, many reports have been made on the aeolian transport of mineral dust to the vast regions of the Northern Pacific, and it is now well recognized that it originates in the continental arid regions upwind [Duce et al., 1980; Gao et al., 1992; Leinen et al., 1986; Merrill et al., 1989; Rex and Goldberg, 1958; Uematsu et al., 1983]. There is a growing concern over the impact which the dust may have on the global climate system and biogeochemical cycle because of the vast scale involved in the transport. While they travel in the atmosphere, not only do dust particles interact directly with incoming radiation to the atmosphere [Sokolik and Toon, 1996; Tegen et al., 1996], 1

Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan. 2 Japan Science and Technology Corporation, Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan. 3 Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto, Japan. 4 Environmental Technology Center, National Institute of Environmental Research, Seoul, Republic of Korea. Copyright 2003 by the American Geophysical Union. 0148-0227/03/2002JD003266

ACE

they also provide an effective reaction surface (hence a sink) for acidic gaseous species such as SOx and NOy [Denterner et al., 1996], further increasing the dust’s potential to act as a giant Cloud Condensation Nuclei: CCN [Levin et al., 1996]. These features may cause changes in the cloud cover and precipitation patterns [Albrecht, 1989; Pincus and Baker, 1994; Twomey, 1974], thereby affecting hydrological and geochemical cycles. In addition, it has been suggested by a number of investigators that the aeolian input of soil dust in remote ocean sites distant from any estuary is the key source of trace metals, especially Fe, which critically limits the primary production of phytoplankton [Falkowski, 1997; Gao et al., 2001; Martin et al., 1989]. This implies a significant role played by the dust in controlling not only the marine ecosystem, but also the level of atmospheric CO2. [3] Dedicated investigations in the past on the long-range transport of dust from the Asian continent revealed that the maximum transport takes place in spring when dust outbreaks most frequently occur. However, most measurements have been made from the ground and have tended to focus only on severe individual events or on the spring period alone. Iwasaka and his colleagues frequently detected lidar signals suggesting the presence of weak dust layers (i.e.,

31 - 1

ACE

31 - 2

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

background KOSA) in the free troposphere over Japan during periods with no evident dust outbreak or even in seasons other than spring [Iwasaka et al., 1983, 1988; Iwasaka and Kwon, 1997; Kwon et al., 1997; Sakai et al., 2000]. As can be inferred from this description of previous measurements, there is a lack of in situ free tropospheric measurements, as well as cross-seasonal measurements to fully characterize the spatial and temporal variabilities of free tropospheric aerosols over the region. [4] The distinct seasons of eastern Asia create strong monsoons that cause variation not only in the passing of the westerly jet, but also in the source strength of continental aerosols. Hence the long-range transport of aerosols by the westerly wind in this particular region should exhibit large spatial and temporal variations. Knowledge on the aerosol distribution in the free troposphere is essential since this is where the long-range transport actively takes place. In the following sections of this report, we will discuss the seasonal changes in the vertical structure of the free tropospheric aerosols over east Asia, on the basis of aircraft-borne and lidar measurements, and the transportation pathway and origin of the Asian dust particles inferred from the isentropic trajectory analysis.

2. Measurements and Analysis 2.1. Aircraft-Borne Measurements [5] Six series of aircraft-borne measurements were conducted over Japan during 2000– 2001. Measurements took place on 10 March (16:24 –19:30 Japan Standard Time, JST), 11 March (8:56 – 12:19 JST), 29 April (10:57 – 13:51 JST), and 19 July (9:09– 12:26 JST) of 2000, and 21 March (9:11 – 13:23 JST) and 20 July (17:25 – 20:29 JST) of 2001. For the sake of safety, days of unstable weather conditions, such as strong frontal activities, were avoided. The flight tracks are shown in Figure 1. The four series of measurements made in the spring were conducted mostly over Wakasa bay (36000N, 135300E) on the Japan Sea side of Japan, and the two series of measurements made in the summer were taken over the sea off the eastern coast of Kii peninsula (33300N, 136300E) on the Pacific side. In every flight, a Cessna 404 aircraft took off from Nagoya (35N, 137E) and followed several straight tracks at multiple levels over the destination covering the range from the top of the boundary mixing layer to the mid free troposphere (up to 5– 6 km) to obtain a vertical structure of the aerosols (Figure 1b). [6] Ambient air was introduced into the aircraft cabin through an inlet as shown in Figure 2. A manifold placed directly below the inlet distributed sampled air to various instruments. The pressure inside the aircraft cabin was maintained to be the same as the ambient air. The inlet was aligned parallel to the aircraft axis, and airflow entered the thin-walled tip at a rate of 640 L/min; subsequently, the conic diffusing section decelerated the axial velocity from 60 m/s to 17 m/s. Stainless steel was selected as the inlet material to minimize the formation of electrical charges on the smoothed wall surface, and no anti-bounce coating was applied to the inlet walls. Particle loss is inevitable in any measurement involving inlet sampling. Generally, larger particles are more prone to loss. For example, the 90 bent tube of the inlet may have caused large particles to impact upon the inner walls. To examine the extent of loss caused

by the bend, we modeled the transmission characteristics using calculations from Pui et al. [1987]. The curvature ratio, which is the ratio of the bend radius to the radius of the tube cross-section, in our case was 12. This is well within the range (5 < Rbend/Rtube < 30) where the curvature effect has been proven to be insignificant. Calculations were made by assuming a flow velocity of 17 m/s and particle densities of 1.0, 2.0, and 3.0 g/cm3. The density of dust is assumed to be around 2.6 g/cm3 [Ishizaka, 1972]. Table 1 shows the diameter at which particles of various densities are transmitted with 90%, 50%, and 10% efficiency. Theoretically, 50% of particles with a 5.3 mm diameter and 3 g/cm3 density will be deposited in the bend. Given the high Reynolds numbers (Re) of 40,000 inside the diffuser and 32,000 inside the tube, turbulence is also likely to have occurred inside our inlet, thus causing particle deposition. Calculations from Muyshondt et al. [1996] were used to estimate the turbulent loss caused by the inlet length. The transmission efficiencies of the diffuser cone (5.2 cm) and straight tube sections (50 cm, including the connection between inlet and manifold) were calculated separately and were then multiplied to deduce the overall transmission efficiency of the entire straight sections. For simplification, the diffuser section was assumed to be cylindrical with the diameter being the mean of the tip and the tube diameters. Transmission efficiencies calculated for particles of various sizes and densities are shown in Table 2. According to these results, 5 mm particles with 3 g/cm3 density would pass through the straight sections of the inlet with 65% efficiency. The above estimations indicate that particle deposition would have been slightly more pronounced in the bend than in the straight tube section. Particle residence time for the entire inlet length was estimated to be about 0.05 s. Hence even a 10 mm, 3 g/cm3 density particle with sedimentation velocity of 0.9 cm/s would fall only 0.04 cm inside the inlet, which makes gravitational sedimentation insignificant. Difficulties in maintaining isoaxial sampling at the inlet tip, or obtaining a representative aerosol size distribution from external probes, remain as potential sources of uncertainty in our observational results. However, depositions in bends and high Re flow are often considered the most important loss mechanisms [Blomquist et al., 2001; Okazaki et al., 1987]. Overall, the fairly slow aircraft (60 m/s) used in our study allowed the inlet to be short in length, thereby helping to avoid any significant loss of large particles inside the inlet. [7] Aerosol particles were sampled directly with an onboard two-stage low-volume impactor (LVI). Particles were directly collected onto sampling substrates placed inside the impactor. Carbon-coated nitrocellulose (collodion) films supported by Ni or Cu grids were used as the substrates. The jet diameters of the first and second stages of the impactor were 1.3 mm and 0.4 mm, respectively. Approximately 1000 cm3 of sampled air was introduced into the impactor every minute. By assuming a particle density of 1.0 g/cm3 under standard atmospheric conditions (1013 hPa, 20C), the diameters at which particles would be collected with 50% efficiency were expected to be 1.6 mm and 0.2 mm at the respective stages. The morphology and chemical constituents of individual particles were analyzed with a scanning electron microscope (SEM) (HITACHI, S-3000N) equipped with an energy dispersive X-ray (EDX) analyzer (HORIBA, EMAX-500). Particle size was estimated from

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

ACE

31 - 3

Figure 1. Flight tracks of Cessna 404 aircraft in the six airborne measurements held over central Japan in 2000 – 2001 (a) horizontal sections. The aircraft takes off from Nagoya (35N, 137E) and four measurements in spring were conducted over Wakasa bay (36000N, 135300E) on the Japan Sea side of Japan, and the remaining two in summer were made over the sea off the eastern coast of Kii peninsula (33300N, 136300E). (b) A typical vertical structure from 19 July 2000. Multilevel flights covered the range from the top of the boundary mixing layer to the mid free troposphere (up to 5 – 6 km) over the destination.

the direct appearance of individual particles in the electron micrograph. The maximum length of a particle was considered the diameter for an irregularly shaped particle [Trochkine et al., 2002]. In parallel with the particle collection, the number-size distribution was measured by two optical particle counters (OPC). The KC18 counter (RION) divided 0.1- to 0.5-mm particles into five channels and the TD-200S counter (SIGMATEC) divided 0.3- to 10-mm particles into 15 channels. 2.2. Lidar Measurements [8] The lidar observation site in the eastern part of Nagoya city, Japan (35N, 137E), allowed continuous monitoring of vertical profiles of the free tropospheric aerosols through the period of March to August 1994. The lidar used here consisted of a pulsed Nd:YAG laser with wavelengths of 1064, 532, and 355 nm, a receiving telescope with a diameter of 1000 mm, and a multichannel photon-counter (dual five channels). (The main characteristics of the lidar system are summarized in Table 3.) [9] The scattering ratio of atmospheric particulate matter, R (Z), is defined as   RðZÞ ¼ Bm ðZÞ þ Bp ðZÞ =Bm ðZÞ;

ð1Þ

where Bm(Z) and Bp(Z) are the atmospheric molecular and particulate backscattering coefficients, respectively, at altitude Z. Equation (1) is rewritten as RðZÞ ¼ 1 þ Bp ðZÞ=Bm ðZÞ ð10 Þ RðZÞ  1 ¼ Bp ðZÞ=Bm ðZÞ:

From (10), we can regard the value of R(Z)-1 as a parameter indicating the mixing ratio of particulate matter at an altitude of Z measured with an optical technique. We assumed the height of the matching point (the aerosol-free atmosphere) to be above 25 km when deducing the backscattering coefficient of particles. [10] The total depolarization ratio Dt is defined by Dt ðzÞ ¼ Bjj ðzÞ=B? ðzÞ h i   ¼ Bp jj ðzÞ þ Bmjj ðzÞ = Bp ? ðzÞ þ Bm? ðzÞ ;

ð2Þ

where B is the backscattering coefficient, subscripts jj and ? refer to measurement made with the plane of the polarization of backscattered light parallel and orthogonal to that of the transmitted laser pulse, respectively, and subscripts p and m refer to components due to atmospheric aerosol particles and air molecules, respectively.

31 - 4

ACE

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

Figure 2. Schematic diagram of the inlet used in the aircraft-borne measurements. [11] For single scattering by spherical particles, the polarization of the incident wave is retained in the backscattering. The particle depolarization component in the lidar return would be 0 if only that scattering mechanism was present. When the depolarization ratio Dp is high, nonspherical particles are expected to be present in the atmosphere. It has been widely accepted that dust particles have strong nonsphericity and show a depolarization ratio 0 [Iwasaka et al., 1988; Kwon et al., 1997; Sakai et al., 2000]. Here we assumed that the particle backscattering effect is a major component in backscattering light from the lower atmosphere (the boundary mixing layer and the lower troposphere), and ignored the contribution of atmospheric molecule backscattering in our estimation of the depolarization ratio of particles, as follows;

3.1. Particle Number-Size Distribution Measured by Aircraft-Borne OPCs [12] Figures 3a and 3b show the particle number-size distributions observed over central Japan by the onboard OPCs, and distributions from spring and summer are com-

pared. In the middle of the free troposphere at altitudes greater than 4 km, the distributions from both seasons were quite similar and all distributions showed a bimodal structure with concentration peaks at 1 to 2 mm in diameter. However, in the lower part of the free troposphere (below 4 km), while the spring distributions showed no significant change with altitude, the summer distributions showed a marked drop throughout the size range and the peak found in the coarse range was somewhat obscured. Vertical variation of the coarse (D > 1 mm) particle numbers (Figure 3c) provided a better picture of this seasonal trend where there were similar numbers of particles at higher altitudes, and a summer decrease in lower altitudes. On average, the summer drop accounted for 86% of the number of coarse particles measured in spring. Whereas in the case of the mid free troposphere, the drop was only 9%. Emphasis is on the fact that the series of aircraft measurements were conducted under relatively stable atmospheric conditions, and hence these distributions may represent the ‘‘background’’ state of the atmosphere. Values of dN/dlogD values in the event of a dust outbreak, on the basis of measurements by Xu and Kai [2002] are compared in Figures 3a and 3b. The highest points of the vertical bars indicate values measured in Nagoya during the prominent 8 April 2000 episode. The

Table 1. Estimated Transmission Efficiencies for the Bent Section of the Inlet (at 20C, 1013 hPa)

Table 2. Estimated Transmission Efficiencies for the Straight Sections of the Inlet (at 20C, 1013 hPa)

Dt ðzÞ ¼ Bjj ðzÞ=B? ðzÞ ’ Bpjj ðzÞ=Bp? ðzÞ ¼ Dp ðzÞ:

ð20 Þ

3. Results and Discussion

Particle Diameter Transmitted With Various Efficiencies, mm

Transmission Efficiency With Various Densities, %

Particle Density g/cm3

90%

50%

10%

Particle Diameter, mm

1.0 g/cm3

2.0 g/cm3

3.0 g/cm3

1.0 2.0 3.0

3.6 2.5 2.1

9.2 6.5 5.3

16.7 11.8 9.6

3.0 5.0 7.0

91 84 74

87 73 60

83 65 54

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE Table 3. Main Specifications of the Raman Lidar System at Nagoya Type/Value Transmitter Laser type Laser output Laser output Laser output Repetition rate Laser beam divergence Receiver Optics Diameter Field of view Photon counting method: Multichannel counter Detection system PMT-1 (404 nm) PMT-2 (375 nm) PMT-3 (532 nm) PMT-4 (532 nm) PMT-5 (1064 nm) Observational target Raman scattering (355 nm) Mie/Rayleigh scattering (532 nm) Mie/Rayleigh scattering (1064 nm)

Nd:YAG >350 mJ/pulse (1064 nm) >100 mJ/pulse (532 nm) >150 mJ/pulse (355 nm) 10 Hz 1 mm are also presented as examples from each season.

ACE

31 - 6

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

Figure 4. (a) Representative electron micrograph and X-ray spectrum of a dust particle collected over Japan, 10 March 2000. The Cu* peak is due to the Cu grid used inside the collection surface. (b) Representative electron micrograph and X-ray spectrum of a sea-salt particle collected over Japan, 21 March 2001. Peaks with asterisks are due to elements found inside the collection surface. as mentioned above, and the absence of dust was the most likely cause of this decrease. [16] Figure 5 implies that in much of the free troposphere during spring, mineral dust is always major in the coarse particle range even when there are no visual signs of major dust outbreaks. Even though the concentration will be too small to be detected as a dust outbreak, the persistence and high speed of the westerly jet will create a significant flux of dust from the continent to the vast regions farther east. Matsuki et al. [2002] pointed out the possibility that the steady state in spring alone could generate a horizontal mass flux of dust comparable to that transported in the event of a dust storm. [17] The most striking information contained in Figure 5 is that the particle concentration in the higher part of the free troposphere remained comparable to that in spring and the dust remained dominant even in summer. This indicates that dust transport is not a phenomenon confined to the spring, but continues on a modest scale that we cannot visually detect through, for example, satellite images or ground observations. 3.4. Relative Humidity Distribution [ 18 ] Figure 6 shows the relative humidity profiles obtained during the aircraft measurements. In spring, a dry air mass with relative humidity of less than 20% was dominant in the free troposphere, usually at an altitude greater than 2 km. The one exception occurred on 11 March, 2000 when humidity increased at an altitude of over 4 km because of overlaying cloud. Summer profiles were quite different in structure. A layer with high humidity of 50 to 70% extended from the boundary mixing layer to as high as 4.5 km, and humidity dropped markedly above that to a level comparable to that in the spring free troposphere. A

dry air mass observed in the spring free troposphere and a similar one observed in the mid-higher free troposphere in summer coincided with the dust-dominant layer mentioned above; we observed similar coincidence in the presence of humid air masses and a relatively dust-free air mass. The most likely cause of this spatial variation was the difference in the origin of the observed air parcels. 3.5. Backward Trajectory Analysis [19] We used isentropic backward trajectory analysis to estimate the origin of the air parcels. An air parcel was traced backward from the point when and where particle collection was actually done. The meteorological field used in the analysis was global objective analysis data provided by the Japan Meteorological Agency (GAPLX). The 6 hourly data included geopotential height, horizontal wind, temperature, and humidity (available below 300 hPa) at 18 pressure levels with horizontal resolution of 1.25. An air parcel was traced backward on the isentropic surface by using the fourth-order Runge-Kutta scheme [Sakai, 2001]. Trajectories were calculated for a duration of 5 days with a time step of 1 hr. The isentropic assumption cannot stand in saturated air or convective conditions as are often encountered inside the planetary boundary layer. For this reason, we stopped the calculation in cases where the air parcel became saturated with respect to water or ice, or intersected a vertically unstable layer or the Earth’s surface. Figure 7 shows the estimated trajectories of air parcels arriving at the sampling sites. As is evident in Figure 7, for the sampling points higher than 4 km, air parcels were estimated to always originate within the Asian continent regardless of season. For the sampling points lower than 4 km, though, there was a clear seasonal difference in air parcel origin, as we expected from the differences in the observed aerosol

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

ACE

31 - 7

Figure 5. Observed seasonal change in the vertical structure of coarse (D > 1 mm) aerosols over Japan. Relative change in the total number of coarse particles with season is shown as 100% being the mean value for spring. Fractions for the different particle types making up the particle population are superimposed. and humidity characteristics. In spring, air parcels also approached from the continent carrying dust as was the case for the higher free troposphere. However, in summer, air parcels became more maritime in origin and approached from the sea south-west of Japan, thus bringing humid and clean air.

[20] Given the limited number of our measurements (four series in spring and two in summer), the question arises as to whether we have observed representative states of the two seasons, or rather, some sporadic cases. Daily trajectories of the air parcels approaching Nagoya in April and July during 2000 and 2001 are shown in Figure 8. Two test

Figure 6. Relative humidity profiles obtained from the six aircraft-borne measurements held over central Japan. Distributions from spring (open symbols) and summer (solid symbols) of 2000 – 2001 are compared.

ACE

31 - 8

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

Figure 7. Trajectories of air parcels traced backward from the points of aircraft-borne measurements. The isentropic assumption was used in the analysis. Trajectories approaching (a) above and (b) below the altitude of 4 km are compared. Markers on the trajectory denote position every 24 hours. Tracks indicated with open symbols are for air parcels arriving in spring, and tracks with solid symbols are for those arriving in summer. areas, dust (25 – 50N, 80– 105E) and marine (10 – 35N, 120 –145E) source areas, were set to better distinguish the arid continental and marine origin of the air parcels. The fraction of trajectories passing through each area is summarized in Table 4. Because Nagoya is close to the marine source area, trajectories passing over both test areas were regarded as continental. At every point of air parcel arrival, the relative humidity over Nagoya was obtained from the meteorological field used in the calculation; these are presented in the table as monthly mean values. The mean transit time of air parcels from the continent is given in the table as an average value taken from those that arrive after crossing the 90E meridian. [21] Air parcels approaching Nagoya at higher altitudes, 7 km in this case, both in April (spring) and July (summer)

exhibited similar features. Pathways were mostly from over the Asian continent, centering over the arid regions of China and Mongolia. Table 4 shows that the majority (77 – 80%) of air parcels came after passing over the dust source region in spring. In summer, about half (39 – 61%) were still from the dust source regions. Air parcels were relatively dry in both seasons with a similar average humidity of about 40%. However, the mean transit time from a point at 90E to Nagoya was only 2.7– 3 days in spring, while it was 4.6– 5 days in summer because of the weaker westerly wind. [22] The pathways of air parcels approaching at 3 km over Nagoya showed a distinct seasonal change, as was also the cases shown in Figure 7. In spring, air parcels took a path that was probably affected by the high pressure system stationed over Siberia; the parcels then descended and headed south at

31 - 9

ACE

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

Figure 8. Daily backward trajectory of air parcels arriving 7 km and 3 km over Nagoya (35N, 137E) in April and July of 2000 and 2001. Hypothetical dust (25 – 50N, 80– 105E) and marine (10 – 35N, 120– 145E) source areas are indicated as squares in the top panels. about 110E. Despite the strong continental tendency, only a small fraction of the air parcels passed through the dust source area (10 – 30%), and most pathways followed a northerly detour. In summer, on the other hand, the paths were mostly along the edge of the extending subtropical high stationed over the Pacific, and the majority (65 – 84%) of the trajectories originated in the marine source area. Also, the average relative humidity was higher at about 60%. [23] Since the trajectories of observed air parcels (Figure 7) well reflected the main path for each season (Figure 8), it seems likely that our results represent the typical aerosol characteristics of each season. 3.6. Seasonal Changes in Aerosol Vertical Profiles Measured by Lidar [24] Figure 9 shows the seasonal change in the vertical profiles of the scattering ratio, R(z) at a laser wavelength of 1064 nm and the aerosol depolarization ratio, Dp(z) at 532 nm, derived from a series of lidar measurements made

Table 4. Fraction of Trajectories Passing Through Hypothetical Dust (25 – 50N, 80 – 105E) and Marine (10 – 35N, 120 – 145E) Source Areas Before Arriving 3 and 7 km Over Nagoya (35N, 137E)a April 2000

July 2001

2000

2001

Dust, % Marine, % RH, % Transit time, days

Altitude: 7 km 80 77 3 10 43.4 37.6 2.7 3.0

61 29 45.2 4.6

39 58 39.3 5.0

Dust, % Marine, % RH, % Transit time, days

Altitude: 3 km 30 10 27 47 46.6 42.7 4.0 4.6

0 84 60.9 -

10 65 54.8 -

a Monthly mean relative humidity at the point of arrival. Mean transit time required for an air mass to reach Nagoya from the 90E meridian.

ACE

31 - 10

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

Figure 9. Seasonal change in the vertical profiles of the scattering ratio at a laser wavelength of 1064 nm (top panel) and the aerosol depolarization ratio at 532 nm (bottom panel) derived from a series of lidar measurements in Nagoya (35N, 137N) during the period of March to August 1994. Tropopause heights are indicated by white horizontal lines in the top panel. A vertical line divides the spring and summer months. See color version of this figure at back of this issue. at Nagoya (35N, 137E) from March to August 1994. The white horizontal lines in the top panel of Figure 9 indicate tropopause heights obtained from upper air sounding at Hamamatsu station (47681; 34450N, 137420E). Tropopause heights ranged from 10– 13 km during spring months, but increased with time and eventually exceeded the heights covered in Figure 9. Such layers having a high scattering ratio (>5) and depolarization ratio (>3) were frequently observed throughout the free troposphere during the spring months (March – May). As the season changed, these layers gradually shifted to higher altitudes. Layers with a high scattering ratio inside the planetary boundary layer (1000 m ASL) of the basin floor would efficiently warm the air directly above, and is therefore likely to cause a very strong thermal plume, possibly carrying dust to high elevations along the northern slopes of the Kunlun mountains. [30] In the case of the Gobi desert, on the other hand, even dust outbreaks may not be able to lift dust as high as the mid-troposphere. Sun et al [2001] compiled 40 years of dust storm reports in China and concluded that 90% of the dust raised from the Gobi would remain in the lower part of the troposphere and would be the main source of aeolian deposits in the proximal regions. Their conclusion was further supported by the depositional record which showed the highest limit of loess deposition on the slopes of the downwind Chinese Loess Plateau to be less than 2850 m. This is in good agreement with the observational findings by Iwasaka et al. [1983]. In this sense, although the contribution of dust from this source is still not entirely deniable, the Gobi is less likely to be a significant source of the background dust observed in the mid free troposphere. 3.7.3. Overall Likelihood of Causing Long-Range Transport [31] A significant fraction of the dust observed over Japan in the steady state of spring and in summer probably has its origin in the Tarim basin. This is further supported by Bory et al. [2002], who reported that the mineral composition and the isotopic ratios of Sr and Nd from the dust entrained in the snow deposits of Greenland indicated the dust’s origin was mainly the Taklimakan desert. It is reasonable to think the dust can be transported at higher altitudes where the westerly flow is more intense when explaining such long-range transport extending literally halfway around the globe. However, the local circulation system and accompanying

ACE

31 - 11

vertical flux of dust in the mountainous regions are not well understood. Since it is also possible that the background dust originated directly from high mountain ranges where it could have been generated by weathering processes [Sun, 2002], investigation of the behavior of the boundary mixing layer between the Tarim basin and Kunlun mountain ranges, as well as chemical identification of the dust source, remain as future tasks necessary to fully prove the Taklimakan desert’s role in generating the background dust.

4. Conclusions [32] An ensemble of aircraft-borne measurements, lidar measurements, and isentropic trajectory analysis, has revealed the characteristic seasonal variations in the vertical structure of aerosols over east Asia. [33] Through in situ measurements from an aircraft, we found coarse particles in much of the springtime free troposphere over Japan to always consist of mineral dust even at times when there were no visual signs of major dust outbreaks. This led us to conclude that there is a steady transport of dust in the lower-middle free troposphere (2 – 6 km altitude) during spring, and its persistence and the high speed of the westerly jet would together create a significant flux of dust from the continent to vast regions farther east. [34] In the mid free troposphere (>4 km), we discovered that the particle concentration in summer remained comparable to that in the spring, and the dust remained dominant, contrary to the general understanding that the summertime free troposphere over the region should be dust free because of the prevailing subtropical high. This indicates that dust transport is not a phenomenon confined to the spring season, but continues on a modest scale in the layer under the persisting westerlies. [35] Layers with high scattering and depolarization ratios in the dry free troposphere were frequently observed by lidar, and we confirmed that these layers were mainly composed of dust coming from the continent. The upward shift of such layers toward summer illustrated the monsoonal transition characteristic of this region. We suggest that the replacement of the continental air mass of winter by the subtropical air mass of summer does not occur uniformly with regard to altitudes, but proceeds from the lower troposphere depending on the elevation of the intruding subtropical high. [36] Tarim Basin is a stable dust source at the southern rim of the Taklimakan desert, and taking into account a possible dust uplifting mechanism created by the unique topography and the strong persisting westerlies hanging over the high mountains, we speculate that it is a major source of the dust observed over Japan under steady conditions of spring and summer. However, investigation of the local circulation system inside the basin and chemical identification remain as future tasks necessary to determine if this is indeed a major source location. [37] Acknowledgments. This investigation was supported by the Japan Ministry of Education, Culture, Sports, Science and Technology (Grant-in-Aid for Specially Promoted Research, 10144104).

References Abe, O., L. Wang, W. Wei, and X. Zhang, Local circulation over upstream regions of the Qira River, Kunlun Mountains, China, J. Arid Land Stud., 11, 223 – 227, 2002.

ACE

31 - 12

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

Albrecht, B. A., Aerosols, cloud microphysics, and fractional cloudiness, Science, 245, 1227 – 1229, 1989. Blomquist, B. W., B. J. Huebert, S. G. Howell, M. R. Litchy, C. H. Twohy, A. Schanot, D. Baumgardner, B. Lafleur, R. Seebaugh, and M. L. Laucks, An evaluation of the community aerosol inlet for the NCAR C-130 research aircraft, J. Atmos. Oceanic Technol., 18, 1387 – 1397, 2001. Bory, A. J.-M., P. E. Biscaye, A. Svensson, and F. E. Grousset, Seasonal variability in the origin of recent atmospheric mineral dust at NorthGRIP, Greenland, Earth Planet. Sci. Lett., 196, 123 – 134, 2002. Denterner, F. J., G. R. Carmichael, Y. Zhang, J. Lelieveld, and P. J. Crutzen, Role of mineral aerosols as a reactive surface in the global troposphere, J. Geophys. Res., 101, 22,869 – 22,889, 1996. Duce, R. A., C. K. Unni, B. J. Ray, J. M. Prospero, and J. T. Merrill, Long-range atmospheric transport of soil dust from Asia to the tropical North Pacific: Temporal variability, Science, 209, 1522 – 1524, 1980. Falkowski, P. G., Evolution of the nitrogen cycle and its influence on the biological sequestration of CO2 in the ocean, Nature, 387, 272 – 275, 1997. Gao, Y., R. Arimoto, M. Y. Zhou, J. T. Merrill, and R. A. Duce, Relationships between the dust concentrations over eastern Asia and the remote North Pacific, J. Geophys. Res., 97, 9867 – 9872, 1992. Gao, Y., Y. J. Kaufman, D. Tanre´, D. Kolber, and P. G. Falkowski, Seasonal distributions of aeolian flux to the global ocean, Geophys. Res. Lett., 28, 29 – 32, 2001. Ishizaka, Y., On materials of solid particles contained in snow and rain water: Part 1, J. Meteorol. Soc. Jpn., 50, 362 – 375, 1972. Iwasaka, Y., and S.-A. Kwon, Lidar measurements of free tropospheric aerosols in Nagoya, Japan, 1994 – 1995, Asian duststorms and their effects on radiation and climate, part IV, Tech. Rep. 3134, pp. 51 – 58, Sci. and Technol. Corp., Hampton, Va., 1997. Iwasaka, Y., H. Minoura, and K. Nagaya, The transport and spatial scale of Asian dust-storm clouds: A case study of the dust-storm event of April 1979, Tellus, Ser. B, 35, 189 – 196, 1983. Iwasaka, Y., M. Yamato, R. Imasu, and A. Ono, Transport of Asian dust (KOSA) particles; importance of weak KOSA events on the geochemical cycle of soil particles, Tellus, Ser. B, 40, 494 – 503, 1988. Kurosaki, Y., and M. Mikami, Seasonal and regional characteristics of dust event in the Taklimakan desert, J. Arid Land Stud., 11, 245 – 252, 2002. Kwon, S.-A., Y. Iwasaka, T. Shibata, and T. Sakai, Vertical distribution of atmospheric particles and water vapor densities in the free troposphere: Lidar measurement in spring and summer in Nagoya, Japan, Atmos. Environ., 31, 1459 – 1465, 1997. Leinen, M., D. Cwienk, G. R. Heath, P. E. Biscaye, V. Kolla, J. Thiede, and J. P. Dauphin, Distribution of biogenic silica and quartz in recent deepsea sediments, Geology, 14, 199 – 203, 1986. Levin, Z., E. Ganor, and V. Gladstein, The effects of desert particles coated with sulfate on rain formation in the eastern Mediterranean, J. Appl. Meteorol., 35, 1511 – 1523, 1996. Martin, J. H., R. M. Gordon, S. Fitzwater, and W. W. Broenkow, VERTEX: Phytoplankton/iron studies in the Gulf of Alaska, Deep Sea Res., 36, 649 – 680, 1989. Matsuki, A., Y. Iwasaka, D. Trochkine, D. Zhang, K. Osada, and T. Sakai, Horizontal mass flux of mineral dust over east Asia in the spring: Aircraft-borne measurements over Japan, J. Arid Land Stud., 11, 337 – 345, 2002. Merrill, J. T., M. Uematsu, and R. Bleck, Meteorological analysis of longrange transport of mineral aerosols over the North Pacific, J. Geophys. Res., 94, 8584 – 8598, 1989.

Mikami, M., T. Fujitani, and X. Zhang, Basic characteristics of meteorological elements and observed local wind circulation in Taklimakan desert, China, J. Meteorol. Soc. Jpn., 73, 899 – 908, 1995. Muyshondt, A., N. K. Anand, and A. R. McFarland, Turbulent deposition of aerosol particles in large transport tubes, Aerosol Sci. Technol., 24, 107 – 116, 1996. Okazaki, K., R. W. Wiener, and K. Willeke, The combined effect of aspiration and transmission on aerosol sampling accuracy for horizontal isoaxial sampling, Atmos. Environ., 21, 1181 – 1185, 1987. Pincus, R., and M. Baker, Precipitation, solar absorption, and albedo susceptibility in marine boundary layer clouds, Nature, 372, 250 – 252, 1994. Pui, D. Y. H., F. Romay-Novas, and B. Y. H. Liu, Experimental study of particles deposition in bends of circular cross section, Aerosol Sci. Technol., 7, 301 – 315, 1987. Rex, R. W., and E. D. Goldberg, Quartz contents of pelagic sediments of the Pacific Ocean, Tellus, 10, 153 – 159, 1958. Sakai, T., Optical properties of free tropospheric aerosol particles related to the relative humidity as derived from Raman lidar observations at Nagoya: Contributions of aerosols from the Asian continent and the Pacific Ocean, doctoral thesis, Nagoya Univ., Nagoya, Japan, 2001. Sakai, T., T. Shibata, S.-A. Kwon, Y.-S. Kim, K. Tamura, and Y. Iwasaka, Free tropospheric backscatter, depolarization ratio, and relative humidity measured with the Raman lidar at Nagoya in 1994 – 1997: Contributions of aerosols from the Asian continent and the Pacific Ocean, Atmos. Environ., 34, 431 – 442, 2000. Sokolik, I. N., and O. B. Toon, Direct radiative forcing by anthropogenic airborne mineral aerosols, Nature, 381, 681 – 683, 1996. Sun, J., Provenance of loess material and formation of loess deposits on the Chinese loess plateau, Earth Planet. Sci. Lett., 203, 845 – 859, 2002. Sun, J., M. Zhang, and T. Liu, Spatial and temporal characteristics of dust storms in China and its surrounding regions, 1960 – 1999: Relations to source area and climate, J. Geophys. Res., 106, 10,325 – 10,333, 2001. Tegen, I., A. A. Lacis, and I. Fung, The influence on climate forcing of mineral aerosols from disturbed soils, Nature, 380, 419 – 422, 1996. Trochkine, D., Y. Iwasaka, A. Matsuki, D. Zhang, and K. Osada, Aircraft borne measurements of morphology, chemical elements, and number-size distributions of particles in the free troposphere in spring over Japan: Estimation of particle mass concentrations, J. Arid Land Stud., 11, 327 – 335, 2002. Twomey, S., Pollution and the planetary albedo, Atmos. Environ., 8, 1251 – 1256, 1974. Uematsu, M., R. A. Duce, J. M. Prospero, L. Chen, J. T. Merrill, and R. L. McDonald, Transport of mineral aerosol from Asia over the North Pacific Ocean, J. Geophys. Res., 88, 5343 – 5352, 1983. Xu, B., and K. Kai, The observation of Nagoya dust event occurred in April 2000, paper presented at the first Aeolian Dust Experiment on Climate Impact Workshop, Minist. of Educ., Culture, Sports, Sci. and Technol., Tokyo, 2002. 

Y. Inomata, Y. Iwasaka, M. Kido, A. Matsuki, K. Matsunaga, T. Nezuka, C. Nishita, K. Osada, and D. Trochkine, Graduate School of Environmental Studies, Nagoya University, Nagoya, Aiti 464-8601, Japan. (matsuki@ stelab.nagoya-u.ac.jp) S.-A. Kwon, Environmental Technology Center, National Institute of Environmental Research, Seoul, Republic of Korea. T. Sakai, Meteorological Research Institute, Japan Science and Technology Corporation, Japan Meteorological Agency, Tsukuba, Japan. D. Zhang, Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto, Japan.

MATSUKI ET AL.: SEASONAL SHIFT OF ASIAN DUST PASSAGE

Figure 9. Seasonal change in the vertical profiles of the scattering ratio at a laser wavelength of 1064 nm (top panel) and the aerosol depolarization ratio at 532 nm (bottom panel) derived from a series of lidar measurements in Nagoya (35N, 137N) during the period of March to August 1994. Tropopause heights are indicated by white horizontal lines in the top panel. A vertical line divides the spring and summer months.

ACE

31 - 10