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An intensive measurement of ambient AD particles was performed at a ground-based site in Fukuoka, Japan during an AD period in April 2005. Because of its closeness to the Asian mainland, this area can be directly exposed to the outflow of air masses from the Asian continent during springtime (see Fig. 1). Therefore, this area is one of the well suited sites to monitor the occurrences AD in Japan. A ground-based sampling station (with a height of 15 m above ground) is located at one of the buildings of Fukuoka Women’s University (33.40°N; 130.26°E) and is marked as an empty square in Fig. 1.

Fig. 1. 
Map showing the sampling location of ambient AD particles.

For the sampling of ambient AD particles, a low pressure Andersen impactor (LPAI) samplers (Tokyo Dylec Co., LP-20) was operated on April 20 (12:00-15:00), 2005. As one of multi-stage particle sampling instruments, this LPAI with multi-orifice was designed to measure the size distribution and mass concentration of particulate matter. Particles are sampled directly on substrate material (80 mm diameter Nucleporefilter^®) arranged behind the jet-nozzles of an impactor. The detailed cut-off particle diameter of each stage for the LPAI was already mentioned in other papers by Ma et al. (2004c) and Ma and Kim (2008). During the sampling period, the ranges of temperature and relative humidity (RH) were 18.8-21.7°C and 41-56%, respectively.

An isentropic backward-trajectory analysis was carried out in order to obtain the transport path of the dust-storm particles. This backward trajectories for air parcel that arrived at our sampling site calculated by the National Oceanic Atmospheric Administration (NOAA) HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) dispersion-trajectory model “backwards” (http://www.arl.noaa.gov) are illustrated in Fig. 3. The prevailing wind on April 20, 2005 was coming from the desert and loess regions in northern China and southern Mongolia. It was extended to the Japanese Island after passing through the domestic areas of China, the Yellow sea, and the Jeju Island, Korea. It could therefore be suggested that the AD particles were long-range transported (i.e., from the dust source areas to the receptor of this study).

Fig. 3. 
Backward trajectories drawn by the NOAA HYSPLIT model. Source point (★) and label interval are 33.40°N; 130.26°E and 6 hours, respectively. Each symbol means meters (triangles: 500 m, squares: 1,000 m, and circles: 3,000 m) above model ground level (AGL).

A weather chart depicting the meteorological conditions over East Asia area at 09:00 on Apr. 20, 2005 is also piled up Fig. 3. A low pressure accompanying a wide cold front was distributed at west Japan and moved to east. This frontal type and location caused very high speed wind at west side of Japan (a maximum wind speed of 33.9 m s^-1 at Totori city). And a dense AD covered over all Kyushu area (Japan Meteorological Agency).

An example of the micro-PIXE spectra taken on single AD particle is displayed in Fig. 4. As shown in Fig. 4, by means of the double detectors system of micro-PIXE it was possible to draw the broaden range of analyzed elements including the low Z elements (≥Si). Through the combination of two kinds micro-PIXE spectra we can thoroughly realize the interpretation of the mixing state of individual AD particles with sea-salt.

Fig. 4. 
An example of two kinds micro-PIXE spectra for a AD particle (5.07 μm aerodynamic diameter). It was driven by a HP-Ge X-ray detector (top) and a Si(Li) detector (bottom).

Fig. 5 shows the micro-PIXE elemental spectra for the seven arrow-marked individual AD particles that were collected on the 2nd stage (5.07μm aerodynamic diameter) of LPAI. Outer blackly filled and inner greyly filled spectra were formed by scanning the whole area retaining seven particles (a blackly filled square at right side of Fig. 5) and each indicated particle, respectively. The scanning of microbeam on each individual particle enabled us to draw more clearly separated peaks of each element including light elements (e.g., Na, Mg, and Al). First of all, the micro-PIXE spectra displayed in Fig. 5 shows a severe particle-to-particle variation of the elemental composition among individual coarse particles collected during our intensive field measurement campaign.

Fig. 5. 
Micro-PIXE elemental spectra for individual AD particles collected on the 2nd stage (5.07 μm aerodynamic diameter) of LPAI.

From the inner greyly filled spectra, individual particles can be classified into two distinct groups (i.e., above four particles (group 1) and below three particles (group 2) groups). In the particles belonged to group 1, Na and Cl coexist with soil-derived components such as Al, Si, K, Ca, and Fe. The particles of group 1 are the typical example of internally mixed particles which contain a uniform mixture of components from each of the sources. The coexistence of sea salt and mineral components also indicates that AD particles were experienced aging processes namely, chemical transformation, in the course of their long-range transport. Whereas, only marine components (e.g., Na, Cl, S, and Mg) were detected in the particles of group 2.

Fig. 6 displays an example of micro-PIXE elemental maps taken on two particles (5.07 μm aerodynamic diameter) collected during AD event. Two combined masks of total elements are schematically shown on the last insert. The particle displayed at top portion of scanning area is containing marine components (e.g., Cl, Na, and S). Whereas, several crustal elements (e.g., Si, Al, and Ca) are principally distributed in and/or on a particle located at bottom of scanning area. On the other hand, Mg and Ca, which are minor (or relatively minor) composition in both sea-salt (Mg: 0.128 wt.% and Ca: 0.0418 wt.%) and dust sand (Mg: 1.57 wt.% and Ca: 5.83 wt.%) (The standards of China loess and Simulated Asian mineral dust, 1998) are weakly coexisting in and/or on two particles. From this elemental map and mask replayed corresponding to individual two particles we can reasonably suggest that two particles are externally mixed particles (i.e., each particle arises from only one source).

Fig. 6. 
An example of micro-PIXE elemental maps taken on two particles collected during AD event. Scanning area of microbeam is 25×25 μm².

Micro-PIXE analysis has the scanning ability of microbeam with a 1-2 μm beam spot size. By scanning this microbeam, we can obtain spatial distribution of trace elements in individual particles. As a result, it is possible to know the portion-to-portion variation of elemental compositions in a particle. Fig. 7 shows an example of the micro-PIXE elemental spectra for two different portions of an AD particle with 5.07 μm aerodynamic diameter. Outer white and inner black spectra were formed by scanning the whole and each square portion on a particle, respectively. According to this analytical result, the top portion of particle contains both sea-salt and silicon. However, only sea-salt components were detected in the bottom portion of particle. It can therefore be said that the chemically transformed AD particle shows heterogeneous elemental distribution. This heterogeneous elemental distribution in an AD particle might be caused by two dissimilar aging processes (i.e., (a): collision (or coagulation) of dust particles and sea-salts and (b): in-cloud scavenging of both particles). The modification of AD particles occurred most probably due to the collisions and coagulations of dust particles and sea-salts (Zhang and Iwasaka, 2004). Zhang and Iwasaka (2004) suggested that the interaction of AD particles and sea-salt is an important process in size and compositional changes of AD particles during their long range transport, consequently affecting mass transformation and optical properties in the atmosphere. Moreover, a cloud formation process contributes significantly to the chemical modification of AD particles (Ma et al., 2004b). From the study on individual cloud droplets by an X-ray fluorescence microprobe analysis, Ma et al. (2004b) reported that a large number of crustal particles were incorporated into cloud droplets during their long-range transport.

Fig. 7. 
Micro-PIXE elemental spectra for two different portions of an AD particle.

Fig. 8 illustrates a presumable explanation for the transformation processes of the AD particles with sea-salt by heterogeneous nucleation. Two types of transformation processes of the AD particles can be identified, i.e., the coalescence of two different droplets formed by heterogeneous nucleation on a separated AD particle and a sea-salt ((a) of Fig. 8) and the collision of an AD particle to the droplet formed by a sea-salt nucleation ((b) of Fig. 8). Although AD particles are insoluble in water, they begin to serve as centers with the condensation of water vapor. AD particles are less soluble than marine of anthropogenic aerosols, their solubility weathered by condensation-evaporation cycles can be increased (Desboeufs et al., 2001). In addition, these homo/heterogeneous nucleation processes can be homomolecular (involving a single species) or heteromolecular (involving two or more species) (Desboeufs et al., 2001).

Fig. 8. 
An illustrative explanation of the transformation processes of the AD particles with sea-salt via nucleation scavenging.

According to the transformation processes of the AD particles with sea-salt (as illustrated in Fig. 8), the possible reasons of portion-to-portion dissimilarity shown in Fig. 7 (i.e., (a) upper scanned portion: Na, Si, and Cl and (b) below scanned portion: Na and Cl) of elemental distribution on and/or in an AD particle can be potentially evaluated by below three aging processes of the AD particles.

As the first aging process ((a) of Fig. 8), both AD and sea-salt particles were transferred into different cloud droplets via nucleation scavenging. This in-cloud scavenging of particles is called rainout. During this rainout processes, sea-salt particles were perfectly melted; however, hydrophobic AD particle were keeping a horizontal water film on its surface (Desboeufs et al., 2001). Both cloud droplets were then consolidated, when they grew and diffuse in cloud layer. Finally, descending air parcel by high pressure and evaporation of water from cloud droplet made a new particle which contained partially sea-salt crystal on its surface.

As the second aging process ((b) of Fig. 8), an AD particle that did not take up any water (i.e., it cannot be dissolved in cloud droplet by itself) was scavenged by a cloud droplet (i.e., through collision) formed by rainout of sea-salt. Then a chemically different particle was born.

As distinct from nucleation scavenging, the collisions and coagulations of an AD particle and a sea-salt can be thought as the third aging process. This homogeneous particle-to-particle reaction is probably due to the modification of AD particle.

Besides the morphological grouping of individual AD particles discussed above, we made another attempt to classify individual AD particles by micro-PIXE net counts. Energy of the detected X-ray is converted in the pulse (count) processing electronics to the voltage pulse delivered at the output of the spectroscopy amplifier. This is an analog signal (pulse amplitude) which has to be converted to a digital number in a device called multichannel analyzer (MCA). The MCA spectrum consists of a certain number of channels which number is a linear function of X-ray energy. Each channel is actually a counter that counts events in the corresponding energy interval, forming the final energy spectrum on the MCA screen (IAEA-TEC-DOC-1190, 2000).

In order to obtain the actual net count originated from an AD particle, the integrated X-ray counts of background (filter blank) was subtracted from the gross count containing that of an AD particle. Then the net count was normalized by the current (count/nC) that was measured directly from the rotating vane which intercepts the beam certain percentage of the total time. From the normalized net count, the quantitative classification of individual AD particles was attempted and the result was summarized in Table 2. From the calculation and comparison of the relative ratios of X-ray net count for three elements (Na, Si, and Cl) in AD particles, individual AD particles (50% cut off size: 3.5 μm) can be classified into three distinct groups (i.e., Aging types 1-2, and non-aged type). The aging processes of each type can be classified as follows;

Aging type 1: AD particle was coated by the gaseous Cl evaporated by the reaction between artificial acids (e.g., sulfuric acid & nitric acid) and sea salt.

Aging type 2: AD particle was mixed with sea salt but no additional reaction with artificial acids was done.

As another possible aging type, it can be thought that AD particle is mixed with sea salt and then Cl is partially evaporated after reaction between this mixed particle and artificial acids. However, this aging type of AD particle was not observed for the target particles of this study.

Fig. 9 shows the relative number fraction of each particle type as a function of aerodynamic particle size. 150 particles were randomly selected from each size of particle and were radiated by a microbeam. The chemically transformed AD particles with marine components accounted for 49.6% to 75.9% (average level of 60.4%) in full size-range (2.1-7.7 μm) of particles. Larger particles (greater than 5.1μm in diameter) tend to be effectively aged. On the other hand, relatively smaller fraction (<3.5 μm) did not go through sufficient transformation processes. On the basis of this particle size dependence of AD particle’s mixing ratio, Zhang et al. (2003) reported the similar results obtained from SEM-EDX analysis of size-resolved particles collected during AD event at Nagasaki located at the west of the island of Kyushu.

Fig. 9. 
Relative number fraction of each particle type in the size-classified particles collected during AD event.

As mentioned earlier, the chemical modification of AD particle with sea-salt occurred most probably by the particle-to-particle collisions, coagulations, and coalescence of cloud droplets. In order to thoroughly estimate the particle-to-particle (or cloud droplet-to-cloud droplet) collision and coalescence of cloud droplets, though one has to consider a large number of parameters such as particle (cloud droplet) radii, particle (cloud droplet) number concentrations, particle (cloud droplet) settling speeds, vigorousness of turbulence in ambient (cloud), vertical velocities generated by turbulence in ambient (cloud), horizontal velocity of the fluid in ambient (cloud), uniformity of fluid mixing in ambient (cloud) (Blanchette, 2003), it can be suggested that larger particles might be expected to be more frequently experienced collision with sea-salt.