Kikuo Okada

X-ray spectrometry of individual Asian dust-storm particles over the Japanese islands and the North Pacific Ocean

Abstract Individual aerosol particles were collected during spring 1986 near the surface over the Japanese islands (Nagasaki and Nagoya) and the North Pacific Ocean near Hawaii. Asian dust-storm particles found in these samples were examined by use of an electron microscope equipped with an energy-dispersive X-ray analyzer (EDX). These dust-storm particles usually consisted of Mg, Al, Si, Ca, Ti and Fe, together with S and Cl. For the individual particles collected over Japan, changes in morphological features and in the amounts of elements before and after the dialysis (extraction) of water-soluble material were studied. The examination indicated that the dust particles were present as mixed particles (internal mixture of water-soluble and -insoluble material), wheras the the water-soluble material mainly contained Ca and S. Over the North Pacific Ocean, the dust-storm particles were present internally in sea-salt particles. It is suggested that the internal mixture of minerals and sea-salt is probably due to interaction within clouds. Formation of CaSO4 on the dust particles was also suggested on the basis of quantitative results obtained by the use of the EDX.

Shape of atmospheric mineral particles collected in three Chinese arid‐regions

The shape of atmospheric mineral particles of 0.1–6 µm radius was studied by electron microscopy applied to the samples collected in three arid regions in China (Qira in the Taklamakan Desert, Zhangye near the southern border of the Badain-Jaran Desert and Hohhot in northern China). In all three regions, the mineral particles showed irregular shapes with a median aspect ratio b/a (ratio of the longest dimension b to the orthogonal width a) of 1.4. Although the aspect ratio exhibited no clear size dependence, the circularity factor (4πS/l²; S is surface area and l is periphery length) tended to decrease with increasing radius, suggesting the presence of aggregated mineral particles at larger sizes. The ratio of particle height-to-width h/a was also evaluated by measuring the shadow length. The median ratio h/a was 0.49 in Hohhot, 0.29 in Zhangye and 0.23 in Qira. Analytical functions were fitted to the grand total of the frequency distributions of aspect ratios, height-to-width ratios and circularity factors allowing parametric calculations of radiative effects and calculations of optical and sedimentation behavior of mineral particles.

Mineral particles collected in China and Japan during the same Asian dust-storm event

Aerosol particles were collected at China and Japan in the same spring Asian dust-storm event of 1991 in order to study the change in composition of mineral aerosol particles during long range transport. It was found on the basis of a microchemical analysis that dust particles containing Mg, Al, Si, K, Ca and Fe were a major fraction of the aerosol particles collected at Hohhot and Beijing, China, and Nagasaki, Japan. A large fraction of the mineral aerosol particles was internally mixed with sea salt in Nagasaki. Present study suggests that the internal mixed particles were mainly produced by cloud processes through droplet coalescence.

The mixture state of individual aerosol particles in the 1997 Indonesian haze episode

Abstract The mixture state of individual aerosol particles collected at altitudes of 1– 5 km on 23 and 25 October 1997, from an aircraft flying over southern Kalimantan during the 1997 Indonesian forest fires, has been examined using the dialyses of water-soluble material with water, and organic material with benzene in conjunction with electron microscopy. Individual aerosol particles in the radius range of 0.1– 2 μm were mainly present as an internal mixture of water-soluble organic material and inorganic salt (mainly ammonium sulfate). Although material comprised of chain aggregations of electron-opaque spherules (elemental carbon) was also found, the proportion of these was small.

Nature of coarse nitrate particles in the atmosphere : a single particle approach

Abstract Aerosol particles were collected in Nagoya, a coastal city of Japan. Individual nitrate-containing particles were identified with a vapor-deposited thin film method by electron microscopy. Elemental composition of individual coarse nitrate-containing particles was determined with X-ray spectrometry. Nitrate was detected in the coarse particles in all the samples. It was observed that chemical modification of sea-salt particles by nitric acid occurred frequently in the coastal urban area. It is found that nitrate also existed on the surface of dust particles during the period of “Kosa” phenomenon, a yellow sand storm originating from East Asian deserts or loess areas observed in Japan. Nitrate formation through heterogeneous reactions on the dust particles may provide a removal mechanism for the nitrogen oxides in the atmosphere in ‘Kosa’ phenomenon.

Some optical properties of smoke aerosol in Indonesia and tropical Australia

Aerosol light-scattering coefficient at 530 nm and its hygroscopic growth were determined in biomass-burning smoke in the lower atmosphere over Kalimantan and northern Australia during the 1997 dry-season fires. Both in and away from plumes, light-scattering was considerably greater in the Indonesian region and hygroscopic growth in scattering was also consistently greater. The relative increase in scattering, from 20% to 80% relative humidity, was typically 1.37 in northern Australian and 1.65 in Kalimantan. Limited aerosol light absorption data indicate relatively small absorption in the Indonesian smoke. In part these differences can be explained by different combustion phases, mixed flaming and smoldering in the Australian savanna fires compared with predominantly smoldering in Indonesia, although these and other concurrent measurements suggest that underground peat combustion may have made a significant contribution to the Indonesian smoke.

On the composition of non-volatile material in upper tropospheric aerosols and cirrus crystals

Abstract As part of an airborne cirrus experiment over the Alps, samples of non-volatile material in upper tropospheric aerosols and cirrus crystals were taken at 9.4 km altitude in January 1992. At that time high backscattering coefficients were measured by Lidars over Europe in this altitude region. Aircraft samples showed that minerals were dominant in micrometer sized interstitial and out-of-cloud particles and in cirrus crystal residues. The results were compared to similar chemical signatures from source samples prepared from Pinatubo ash and Saharan dust. The chemical signatures of interstitial and out-of-cloud particles were similar to those of the source samples, with a depletion of heavier elements. The crystal residues were depleted in lighter elements (NaS) and enriched in the heavier elements TiCd.

Shattering and modification of sea‐salt particles in the marine atmosphere

Sampling of sea-salt particles was carried out over the western Pacific Ocean. The collected particles were studied individually by X-ray spectrometry. Sea-salt particles with radii r ≥ 0.5 μm show a considerable range in their weight ratios of S/Na and Ca/Na, while Cl/Na ratios remain constant at the seawater value. Moreover, there is a tight correlation between S/Na and Ca/Na. This property is attributable to fractionary recrystallization within evaporating drops. Particles with r < 0.5 μm exhibit a similar trend. However, these particles tend to show smaller Cl/Na ratios and larger S/Na ratios than those with r ≥ 0.5 μm. Chemical reaction between sea salt and acidic material, where Cl− is replaced by (SO4)2− and (NO3)−, is the most plausible explanation.

On the state of mixing of particles due to Brownian coagulation

A numerical model is used to study the mixing of atmospheric aerosol particles due to Brownian coagulation only. We related the model results to atmospheric measurements by assuming the model aerosol to consist of water-soluble and water-insoluble materials. The state of mixing of particles resulting from the computer simulations of the coagulation process is compared to measurements of volume fractions of water-soluble material. Solubility effects of the aerosol material are not included in the model. It is found that coagulation is a very important process in forming the resulting distribution of mixed particles as a function of particle size and amount of soluble material in a particle.

Number-size distribution and formation process of submicrometer sulfate-containing particles in the urban atmosphere of Nagoya

Abstract Individual aerosol particles were collected in the summertime urban atmosphere of Nagoya, Japan, with an electrostatic aerosol sampler and a low-pressure impactor in order to obtain the number-size distribution and some properties of subμm particles by electron microscopy. Two characteristic types of number-size distribution of submicrometer particles were obtained in the daytime and they were discussed in relation to the formation process of sulfate-containing particles. In a type A distribution particles with radii more than 0.1 μm were present in a low number concentration and the concentration tended to increase strongly with decreasing radius in the Aitken size range. Type B distribution was characterized by a large number concentration of particles in ‘accumulation mode’. In both cases sulfate-containing particles often strongly controlled the number-size distribution. Although hygroscopic particles comprised 70–80 % of particles of 0.03–0.35 μm radius for each type of distribution, those with water-insoluble inclusions (mixed particles) were present in high number fractions of particles (> 40%) collected from type B distributions. Type A distribution was considered to be closely related to the formation process of sulfate-containing particles by gas-to-particle conversion through homogeneous nucleation in the presence of a small proportion of pre-existing particles. Conversely, heterogeneous processes on pre-existing particles were important in forming the type B distribution.