Mizuka Kido

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.

Diurnal variation of ionic aerosol species and water-soluble gas concentrations at a high-elevation site in the Japanese Alps

Size-separated atmospheric aerosol particles, acidic gases, and ammonia were collected during the daytime and nighttime at Murododaira (36.6°N, 137.6°E, 2450 m above sea level), Mount Tateyama, Japan in early winter. Non-sea-salt (nss) SO42− in fine fractions (<2.1 μm in diameter) was the most dominant ionic constituent and occupied 44% on average of the ionic mass concentration other than H+. Concentration levels of aerosol ions and water-soluble gases were higher during the day and lower at night. Concentrations of the nighttime aerosol particles and gases (except for SO2) were similar to free-tropospheric background levels at various sites of northern midlatitudes in the literature, suggesting that nighttime data at Murododaira were representative of free-tropospheric conditions. High concentration levels of SO2 at Murododaira were close to those over the northwest Pacific Rim region during the winter season. Molar ratios of nssSO42−/SO2 can be explained by the transport time from the Asian continent, suggesting that a significant source of fine nssSO42− aerosols at Murododaira in early winter could be due to the oxidation of anthropogenic SO2 derived from the Asian continent. The equivalent ratios of nssSO42− to NH4+ in fine particles were close to 0.5 under westerly wind conditions, suggesting the existence of half-neutralized sulfate on average.

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.

Nucleation mode particles in upslope valley winds at Mount Norikura, Japan: Implications for the vertical extent of new particle formation events in the lower troposphere

[1] Number-size distributions of atmospheric aerosol particles of 9-300 nm diameter were observed along with SO 2 and NH 3 concentrations at Mount Norikura, Japan (36.1°N, 137.5°E, 2770 m asl), during September 2001 and August-September 2002. Particle size distributions between the free troposphere and the mixed layer were measured at the site under local wind system conditions, comprising downslope mountain and upslope valley winds. The local wind system was well developed under clear-sky conditions and was determined according to temporal variation of the water vapor mixing ratio at the site. Nucleation mode particles were observed on 4 of 23 clear-sky days during our observation periods. Nucleation mode particles were observed in the mixed layer air, but never in the free tropospheric air, suggesting that new particle formation occurred in the air transported from the mixed layer. Nucleation-mode-particle growth rates during new particle formation events were 2.6-3.1 nm h -1 . Preexisting particle concentrations on event days were about half those of nonevent sunny days on average, but no positive correlation was found between the events and either SO 2 or NH 3 concentrations. The events always occurred in a cold air mass advected from the north behind cold fronts. Horizontal advection of the northern cold air mass might bring a clean air mass with low preexisting particle concentration in the mixed layer and trigger a new particle formation event at Mount Norikura.

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.

Temporal variation of water-soluble ions of free tropospheric aerosol particles over central Japan

Free tropospheric aerosol particles were collected at Mt. Norikura (36.1°N, 137.5°E, 2770 m a.s.l.) in Japan during May.October in 2001 and 2002. An automated sequential daily nighttime (00-06 a.m.) sampler collected free tropospheric aerosols. Average, median, SD, minimum and maximum concentrations of total ionic weight of the 114 samples were, respectively, 3.9, 2.8, 3.7, 0.2 and 23.2 μg m-3 Transport conditions were analysed using backward air trajectory with precipitation amounts along the trajectory. Results suggest that low aerosol mass concentration causes are (1) descending trajectories and (2) precipitation scavenging during transport without contacting boundary layer atmosphere until arrival. It is suggested that, without precipitation scavenging after entrainment into the free troposphere, aerosol transport from active emissions at the surface enhances mass concentration at Mt. Norikura. Average concentrations of NO3-, non-sea-salt (nss)K+ and C2O4 2- are high in March.June and low in winter. The highest average nssSO42-. concentration occurs in summer; it is high from spring through fall. Seasonal variation of NH4+ concentrations resembles that of nss SO4 2-, but the concentrations molar ratio (NH4 +/nssSO42-) is high (ca. 2) in spring and decreases to 1 in winter. Seasonal variation of NH4+ + NH3 concentration agrees with that of NH3 emissions in China.

Raman lidar and in-situ aircraft measurements of free tropospheric aerosol particles over Japan

A comparative measurement of the free tropospheric aerosol particles using a ground-based Raman lidar and in-situ sampling instruments on board an aircraft was made over central part of Japan (35-36°N, 136-137°E) in the Asian dust period on April 23, 1996. We compared the aerosol properties measured with the lidar (aerosol backscattering coefficients and the depolarization ratio at the wavelength of 532 nm) and those obtained with the aircraft-based instruments (aerosol shape and surface feature, number size distribution, and chemical composition). The aerosol backscattering coefficients calculated from the aerosol size distribution obtained with the airborne optical particle counters (OPCs) showed good agreements (mean relative difference of 22%) with those measured with the lidar above the altitude of 3.1 km, but showed large difference (mean relative difference of 50%) below 3.1 km. The aerosol depolarization ratios correlated well with the fractions of the backscattering coefficient of the coarse mode particles (radius>0.5 μm) in the total aerosol backscattering coefficient calculated from the OPCs data above 3.4 km. This suggests that the coarse mode particles that consisted mainly of nonspherical mineral dust particles predominantly contributed the depolarized signals in this altitude region. The aerosol depolarization ratio of the coarse mode particles was estimated to be 164% at 5.3 km.