Yasunobu Iwasaka

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.

Chemical reaction during the coagulation of ammonium sulphate and mineral particles in the atmosphere

Abstract Until now the climatic effects of mineral dust in the atmosphere has been limited to a consideration of physical processes. Here we present evidence for a chemical reaction occurring upon the coagulation of an Asian dust (kosa) aerosol and ammonium sulphate particles in the atmosphere in which the calcium carbonate in the kosa particles reacted with the ammonium sulphate, at a rate measured in days, to yield koktaite [(NH4)2Ca(SO4)2·H2O] as an intermediate product and, finally, gypsum (CaSO4·2H2O). Koktaite was present in an aerosol sample collected in Beijing, China during a kosa event, but it is absent from the soils from which the kosa aerosol originates. This reaction may have significant climatic consequences.

Chlorine deposition on dust particles in marine atmosphere

A scanning electron microscope and an energy dispersive X-ray spectrometer were applied to analyze the particles collected during dust-storm events at southwestern Japan in spring 2000. Particle morphology and elemental composition were investigated and the weight ratios of different elements in individual particles were obtained. The results indicated that, besides the coagulation of sea-salt and dust particles, chlorine could deposit onto dust particles through the absorption of chlorine-containing gases when the particles passed through the marine atmosphere between China and Japan. The quantitative estimation revealed that the chlorine deposition on many particles was not negligible compared to sulfur deposition, suggesting that dust particles could remarkably modify the chlorine chemistry in marine atmosphere and possibly vise versa.

Mineral dust layers in snow at Mount Tateyama, Central Japan: formation processes and characteristics

Deposition of water-insoluble dust was measured in winter snow deposited at Murododaira (2450 m) on the western flank of Mount Tateyama in central Japan. An analysis of temporal variation in atmospheric aerosol concentration, incremental snow height and concentration of dust in the snow cover suggests that wet deposition is the major process in forming thicker dust layers in the snow cover at Mount Tateyama. Dust layers in the snow cover contain Ca-rich materials typically found in Asian dust (Kosa) particles. Volume size distributions of dust particles in the snow showed single and bi-modal structures having volume median diameters from 6–21 μm. Dust profiles in snow cover over the last 6 yr reveal frequent sporadic high dust concentrations in spring and large year-to-year variations in the amount deposited. The average amount of dust deposition (7.7 g m−2) from winter to spring at the site was close to the long-term averages of annual flux obtained from sea sediment near Japan, implying that dusty precipitation in spring contributes to annual deposition of aeolian mineral dust.

Soot particles and their impacts on the mass cycle in the Tibetan atmosphere

Atmospheric aerosol particles in urban and mountain areas around Lhasa city (29.65°N, 91.13°E) in the Tibetan Plateau were collected in the summers of 1998 and 1999. The particles were analyzed with electron microscopes and an energy dispersive X-ray spectrometer. Individual particle morphology, elemental composition and mixture of sulfate and nitrate were investigated. In the urban area, soot particles emitted from vegetation burning were dominant. These particles were characterized by chain or aggregate morphologies, and an elemental composition of potassium and sulfur. Such particles were frequently detected in mountain areas out of the city, where they formed droplets acting as condensation nuclei. Quantitative estimation indicated that sulfur was accumulated onto the soot particles during their dispersion from the urban area to mountain areas. Sulfate and nitrate detections indicated that soot particles collected in the urban area did not contain nitrate and BaCl2-reactive sulfate, which revealed that the combination of sulfur and potassium in the particles was not K2SO4. In contrast, the particles dispersed to mountain areas contained BaCl2-reactive sulfate and some contained nitrate, suggesting that soot particles emitted from the urban area could increase the buffering capacity of aerosol particles and enhance the formation of particulate sulfate through heterogeneous conversion in the Tibetan atmosphere.

Vertical distribution of atmospheric particles and water vapor densities in the free troposphere: Lidar measurement in spring and summer in Nagoya, Japan

Density distributions of atmospheric water vapor and aerosol particles in the troposphere were observed from April to August 1994 in Nagoya, Japan, with a Raman-scattering lidar. In spring, highly concentrated particle layers were frequently observed in the free troposphere, and in summer air mass with high relative humidity was detected. These suggest the possibility that density distributions of aerosol particles and water vapor over Japan are strongly affected by air mass from the Asian continent in spring and from the Pacific ocean in summer.

Calibration method for the lidar-observed stratospheric depolarization ratio in the presence of liquid aerosol particles

A fine calibration of the depolarization ratio is required for a detailed interpretation of lidar-observed polar stratospheric clouds. We propose a procedure for analyzing data by using atmospheric depolarization lidar. The method is based on a plot of deltaT versus (1 - RT(-1)), where deltaT is the total depolarization ratio and RT is the total backscattering ratio. Assuming that there are only spherical particles in some altitude ranges of the lidar data, the characteristics of the plot of deltaT versus (1 - RT(-1)) lead to a simple but effective calibration method for deltaT. Additionally, the depolarization of air molecules deltam can be determined in the process of deltaT calibration. We compared determined values with theoretically calculated values for the depolarization of air to test the proposed method. The deltam value was calculated from the lidar data acquired at Ny-Alesund (79 degrees N, 12 degrees E), Svalbard in winter 1994-1995. When only sulfate aerosols were present on 24 December 1994, deltam was 0.46 +/- 0.35%. When the particles consisted of sulfate aerosols and spherical particles of polar stratospheric clouds on 4 January 1995, deltam was 0.45 +/- 0.07%. Both deltam values were in good agreement with the theoretically calculated value, 0.50 +/- 0.03%.

Changes in ionic constituents of free tropospheric aerosol particles obtained at Mt. Norikura (2770 m a.s.l.), central Japan, during the Shurin period in 2000

Abstract Free tropospheric aerosols and O 3 concentrations with meteorological parameters were observed at the Norikura Cosmic-Ray Observatory ( 2770 m a.s.l.) near the top of Mt. Norikura, central Japan from 1 September to 8 October 2000. The study period represents the Shurin period in Japan, which is a transition period of dominant air masses over Japan. Automated air sampling was conducted during nighttime: from 00 midnight to 06 a.m. every day to collect free tropospheric aerosols particles under subsidence air flow condition over the mountain. Results of ionic concentrations suggested that the observation period was divided into 2 periods: one was higher equivalent ratios (>1) of non-sea salt (nss) SO 4 2− /NH 4 + until 17 September and the other was slightly above unity of the ratio after that. O 3 concentrations also shifted simultaneously to higher and stable conditions. Concentrations of oxalate, nssK + , and nssCa 2+ were also higher in the later period. Molar ratios of nssSr/nssCa and nssMg/nssCa in the samples were about 0.0024 and 0.172, respectively. These values were very close to ratios found in the desert area in China. Backward air trajectory analysis for the period of high nssCa 2+ suggested that Asian dust with pollutants were not only transported in spring but also in fall from the Asian continent to the free troposphere over Japan.

Variations of constituents of individual sea-salt particles at Syowa station, Antarctica

Sampling of atmospheric aerosol particles was carried out at Syowa station, Antarctica (39.58°E, 69.00°S) in 1998. For a better understanding of sea-salt chemistry in the coastal Antarctic regions, individual sea-salt particles were analysed using a scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM-EDX). Individual particle analysis indicates that more sea-salt particles were modified in fine particles (0.2–2 µm in diameter) through heterogeneous reactions mainly with gaseous sulfur species in the summer and reactive nitrogen oxides in the winter—spring. In particular, sea-salt particles in the coastal Antarctic atmosphere may be modified by heterogeneous reactions with not only SO2 and H2SO4 but also volatile sulfur species (e.g. CH3SO3H, DMS and DMSO) derived from bioactivity on the ocean surface during the summer. Also, low air temperature and a larger extent of sea ice offshore Syowa probably enhanced release of fractionated sea-salt particles (S-rich, Mg-rich, K-rich and Ca-rich) from the surface of snow and sea ice, particularly in September—October 1998. In addition, we attempt to estimate the scavenging rate of atmospheric sulfur species and reactive nitrogen oxides by dry deposition of sea-salt particles. Our estimation suggests that the upper limit of the scavenging rate of atmospheric sulfur species by sea-salt particles could rise to approximately 0.5 nmol m−2 day−1 at Syowa station during the summer. This value corresponded to about 30% of the concentration of particulate sulfur species such as non-sea-salt (nss)-SO2−4 and CH3SO−3 and ~10% of total atmospheric sulfur species (nss-SO2−4, CH3SO−3 and SO2). In contrast, the estimated NO−3 scavenging rate by sea-salt particles was ~0.2 nmol m−2 day−1, which is similar to the dry deposition rate of HNO3+N2O5 (approximately 0.2–0.3 nmol m−2 day−1). Hence, sea-salt particles probably play an important role as scavengers of acidic species in the coastal Antarctic regions.