Kimio Arao

Ground‐based network observation of Asian dust events of April 1998 in east Asia

We coordinated a ground-based network that has been in use since 1997 to observe Asian dust during springtime. Huge Asian dust events that occurred in the middle of April 1998 were captured by this network. In this paper we present the organization of the network; a description of the instruments, including the lidar, sky radiometer, and optical particle counter; and the results of the observation, and offer discussions regarding the transport mechanism of Asian dust in east Asia using an on-line tracer model. We discussed the time series of the surface concentration and the height distribution of the dust. A cutoff cyclone generated during the dust episode was responsible for trapping and sedimentation during the transportation of the Asian dust, particularly in the southern parts of China and Japan. Horizontal dust images derived from NOAA/AVHRR clearly revealed the structure of the vortex. The lidar network observation confirmed the general pattern of dust height distribution in this event; the height of the major dust layer was about 3 km over Japan but was higher (4 to 5 km) in Seoul and Hefei. A thin dust layer in the upper troposphere was also commonly observed in Hefei and Japan. Evidence of the coexistence of dust and cirrus was shown by the polarization lidar. The lidar network observation of Asian dust and satellite remote sensing provide key information for the study of the transport mechanism of Asian dust. Further extension of the lidar network toward the interior of the continent and the Pacific Rim would reveal the greater global mechanism of the transportation.

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.

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.

Model study on particle size segregation and deposition during Asian dust events in March 2002

[1] A size-segregated aerosol model that includes most of the major physical processes (generation, transport, and dry and wet deposition) is developed. This model is coupled with a Regional Air Quality Model (RAQM) and is applied to simulate Asian dust storms during the 10-day period of 15-24 March 2002. A nonhydrostatic mesoscale model (MM5) is used to provide meteorological fields. Model results are verified by available observational data including surface weather observations and size-segregated particle concentrations. The validation demonstrates a good capability of this model system in capturing most of the key features of dust evolution and reproducing the particle mass size distribution along the transport pathway of soil dust. An apparent feature has been both observed and reproduced by the model, showing a shift of size range with peak mass concentration from coarse mode to finer mode on the pathway from source regions to distant downwind areas. The maximum dust concentration averaged over 10 days is simulated to be 3000 μg m over the southern China-Mongolia border. Total dry deposition of soil dust for 10 days is up to 30 g m -2 in the Gobi desert along the southern China-Mongolia border. Distribution and magnitude of particle deposition are strongly dependent on both concentration and size-segregated dry deposition velocity and scavenging rate. While dry deposition dominates the removal of dust particles in or in the vicinity of source regions, the influence of wet deposition increases along the transport pathway of soil dust, with high removal efficiency for coarser particles (>2 μm) and very low efficiency for particles in the accumulation mode. Of the total dust emission (43.2 megatons), about 71% is redeposited onto the underlying surface by the dry deposition process, 6% is removed by the wet deposition process, and the remaining 23% is suspended in the atmosphere or subject to long-range transport.

Lidar network observation of Asian dust (Kosa) in Japan

We have organized a network campaign for the observation of Asian dust in the spring of 1997 and 1998 in Japan. Through the communication with electric mail, we have successfully observed Kosa events by lidar, sunphotometer, and particle counter, etc. These data must be useful to analyze the transport mechanism of Asian dust and validate the satellite observations.

High-Resolution Measurement of Size Distributions of Asian Dust Using a Coulter Multisizer

Abstract A Coulter Multisizer, which is based on the electrical sensing zone (ESZ) or the Coulter principle, was used to measure the size distribution of Asian dust. Coulter Multisizer analysis provides high-resolution size measurements 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 size distributions fit fairly well with a lognormal distribution. The results show that the WIP size distributions of the same Asian dust air mass varied at each sampling site and the volume mode diameter at the sites reduced from west to east. The derived volume mode diameter ranged from 1.4 to 2.2 μm and was comparatively smaller than those in previous studies on Asian dust. This can be explained by the possible internal mixing of Asian dust with other components and by the breaking of particles and dispersion of aggregations by ultrasonification during extraction. The analysis meth...

Optical Properties of the Turbid Atmosphere in the Yellow Sand Event Over Japan

The optical properties of yellow sand particles were examined by comprehensive measurements carried out during the period of April 21–May 12, 1982 in Nagasaki, Japan. In situ measurements of the phase functions of aerosols were performed for both parallel and perpendicularly polarized components with a portable polar nephelometer, as well as spectral extinction measurements with a sunphotometer, and measurements of the aureole intensity with an aureolemeter. Large depolarization ratio values of about 0.3 and a power-law size distribution with an exponent around 3.0 were characteristically found in the yellow sand event, indicating an abundance of large, irregular sand particles. On the other hand, a powerlaw size distribution with an exponent of around 4.0, i. e., a typical Junge distribution, was well developed in the atmosphere for normal conditions over Japan. The size spectrum and complex index of refraction of aerosols retrieved from the light scattering measurements were found to be useful in parameterizing the phase functions of yellow sand particles, although an unrealistically large value was retrieved for the imaginary index of refraction.

Continuous lidar observations of Asian dust in Beijing, Nagasaki, and Tsukuba

Automated Mie-scattering lidars have been operated since March 2001 at Beijing, Nagasaki and Tsukuba to reveal the time and height distribution of Asian dust and its optical properties. The lidars detect backscattering light from clouds and aerosols at 532nm in both parallel and perpendicular polarization channels. They continuously measure profiles every 15 minutes regardless of weather conditions. At first we eliminated clouds using vertical profiles of intensity, then Asian dust was identified by the depolarization ratio. In Beijing, close to the source region of the dust, Asian dust events occurred 15 times in March April, and May 2001. Each event continued for several days. The aerosol depolarization ratio (ADR) frequently reached up to 40 %. In Nagasaki, located western part of the Japan, Asian dust was confirmed near the surface with a delay of a few days from events in Beijing. However, in Tsukuba, there were few surface dust events and passing dust in the free troposphere was confirmed. The ADR in Tsukuba were lower than those in other two observatories. Internal mixing of mineral dust and anthropogenic aeorols, and changing size distribution may contribute the differences of ADR among observatories. In Beijing, ground sampling of mineral dust was simultaneously carried out. Mass concentration by the sampler at the surface and extinction coefficient near the surface derived from lidar observation were compared to estimate the conversion factor from extinction coefficient to mass concentration. Utilizing this factor we estimated the vertical distribution of the mass of Asian dust in Beijing.