Yutaka Ishizaka

Contribution of particulate sulfate and organic carbon to cloud condensation nuclei in the marine atmosphere

To identify the source of cloud condensation nuclei(CCN) in the marine atmosphere, simultaneous measurements of aerosol chemical components and CCN concentration were made over more than a year at Hahajima, the Ogasawara Islands, located on the Northwest Pacific Ocean in the lee of Eastern Asian large pollution sources. Large loading of CCN in continental air mass and long-distance transportation to the remote ocean were confirmed. Non-seasalt sulfate and water soluble organic carbon were dominant in fine aerosols and their concentration revealed significant relationships with CCN concentration, suggesting that not only sulfate but also organic aerosols play a key role in CCN formation, especially in the background atmospheric condition.

Evaluation of chemical composition in fog water near the summit of a high mountain in Japan

Inorganic ionic species contained in fog water and in atmospheric aerosols were analyzed to study the acidification processes of fog droplets in summer near the summit of Mt. Norikura in Japan. Liquid water content (LWC) was also measured during the fog event. Fog water was found to be highly acidic. Nucleation scavenging was less contributive to the acidification of fog water, because NH4+ was contained in aerosols in a sufficient concentration to neutralize the particulate S42− which was the predominant anion in the aerosols. Comparison of the ion concentrations in fog water with those in aerosols showed that the acidification progressed rapidly at the initial stage of fog formation, primarily due to S42− enhancement and secondarily due to NO3− formation. It was supposed that about 60% of S42− in fog water was formed by SO2 gas dissolution followed by liquid phase reaction with H2O2 in the droplets. The results of the observation also suggested that NO3− in fog water was increased by HNO3 gas scavenging. During the mature stage of fog event, LWC could not always explain the temporal variation in the concentrations. This fact also suggests that gas scavenging contributes to S42− and NO3− concentrations in fog water.

Geometrical Thickness, Liquid Water Content, and Radiative Properties of Stratocumulus Clouds over the Western North Pacific

Abstract An algorithm was developed for retrieving cloud geometrical thickness from a measured liquid water path and equivalent width of 0.94-µm water vapor absorption band. The algorithm was applied to aircraft observations obtained by a microwave radiometer and a spectrometer in the winter of 1991 over the western North Pacific Ocean. Retrieved values of the cloud geometrical thickness are apt to be smaller than those observed by eye, especially for horizontally inhomogeneous clouds. Measured cloud albedos in the visible and near-infrared spectral region were also compared with calculated values. For homogeneous clouds there exists a single droplet size distribution that satisfies both spectral regions. However, for inhomogeneous clouds no single size distribution exists that satisfies the albedo observed in both spectral regions.

Sources of sulfate in winter aerosols over the Sea of Japan, as inferred from selenium composition

Abstract Aerosol samples were collected in winter at the Mikuni and Mihonoseki sites facing the Sea of Japan and analyzed for non-sea-salt sulfate (nss-SO42−), Se(IV) and Se(VI). Episodes of a high nss-SO42− concentration were observed 5 times during the observation period of 14 days. Based on the weather conditions as well as the Se/Snss-SO42− and Se(VI)/Se(IV) ratios, these episodes could be classified into three types, i.e., continental-coal, local-coal and local-petroleum types. The continental-coal type was characterized by the high Se/Snss-SO42− (5.0–7.6×10−4) and high Se(VI)/Se(IV) ratios (6.9–8.9). It was produced by a long-range transport of aerosols originating from coal-combustion sources in the Asian Continent. The local-coal type was due to aerosols from local coal-combustion sources around the observation sites together with high Se/Snss-SO42− (3.9×10−4) and low Se(VI)/Se(IV) ratios (0.2). The local-petroleum type was accompanied by low Se/Snss-SO42− (2.4×10−4) and low Se(VI)/Se(IV) ratios (3.3) and it was caused by aerosols from local petroleum-combustion sources. The boundary of the Se(VI)/Se(IV) ratio between continental and local types would be around 2.

Peroxide Concentrations in Fog Water at Mountainous Sites in Japan

Measurements of peroxide concentrations in fog water were conducted near the summit of Mt. Norikura (altitude, 2770m) in central Japan, and at the midslope of Mt. Oyama (altitude, 680m), southwest of the Kanto Plain. The concentrations of peroxide at Mt. Norikura, far from industrial regions, ranged from 3 to 120 µ M during the summer and early autumn in 1993. The potential capacity for SO2 oxidation appears to be very high near the summit of Mt. Norikura. Analysis of the chemical composition of three-stage size-fractionated fog water samples collected at Mt. Norikura showed that the concentrations of peroxide were apparently independent of droplet size, whereas the concentrations of chemical constituents mainly derived from secondary aerosols and the acidity were higher in smaller droplets. Peroxide concentrations in fog water were low (< 5 µ M) at Mt. Oyama, located near heavy industrial areas, and lower than those in rain water sampled simultaneously (0.2–33 µ M). Especially, peroxide was scarcely detected in strongly acidic fogs (< 0.2 µ M). Peroxide might have been decomposed by SO2 (S(IV)) oxidation in the aqueous-phase.

Electron microscope studies of methane sulfonic acid in individual aerosol particles

The electron microscope has been used in studies of the nature of methane sulfonic acid (MSA) particles collected from the air of Sakushima Island in Japan, near the Pacific Ocean. Particles were impacted on electron microscope screens for morphological analysis and thin-film chemical tests, MSA particles could be identified on the basis of their distinctive morphology on the collection surface. In contrast to MSA, sulfuric acid has a similar morphology on a carbon film, but a different morphology on a nitron-barium chloride film. It was found that MSA-containing particles were dominant in the coarse-particle fraction under very humid conditions. The examination also suggested that the MSA particles were present as mixed particles

Aerosol properties around marine tropical cumulus clouds

Airborne measurements of aerosols around convective clouds over the tropical Pacific Ocean show enhanced concentrations of small (Aitken) particles in the vicinity of some of the clouds. In the same regions there were increases in the concentrations of internally mixed particles with diameters around 0.3 pm that contained acidic sulfate. The acidic sulfate is attributed to heterogeneous oxidation of SO 2 in cloud droplets. There was also some evidence of nitrate in the internally mixed particles. The enhancement in the concentrations of ∼0.3 pm diameter particles produced smoother (i.e., more Junge-type) particle size spectra.

Mixtures of nitrate and sulfate in individual particles in acidic fogs

Abstract Chemical tests of individual aerosol particles and residues of fog droplets were performed near the top of Mt. Norikura in Japan in order to examine transformation of aerosol particles during the formation of acidic fog droplets. In this experiment, these particles were collected on electron microscope screens with a two stage impactor, and the presence of sulfate and nitrate ions in individual particles was examined with an electron microscope using thin-film chemical methods. It was found that sulfate ion and nitrate ion can coexist in a particle as internally mixed particle in acid fog. Mixed nitrate and sulfate particles were detected mainly in fog. On the other hand, nitrate-containing particles were found mainly in clear air. Sulfate-containing particles were predominant both in clear and fog conditions. Based on our observations, direct evidence for nitrate scavenging was obtained. Evidence for sulfate production in fog droplets was also presented. Possible mechanisms for the internally mixed particle formation are discussed in light of the field measurements.

Modeling Marine Stratocumulus with a Detailed Microphysical Scheme

A one-dimensional 3rd-order turbulence closure model with size-resolved microphysics and radiative transfer has been developed for investigating aerosol and cloud interactions of the stratocumulus-topped marine boundary layer. A new method is presented for coupling between the dynamical model and the microphysical model. This scheme allows the liquid water related correlations to be directly calculated rather than parameterized. On 21 April 2001, a marine stratocumulus was observed by the Caesar aircraft over the west Pacific Rim south of Japan during the 2001 APEX/ACE-Asia field measurements. This cloud is simulated by the model we present here. The model results show that the general features of the stratocumulus-topped marine boundary layer predicted by the model are in agreement with the measurements. A new onboard cloud condensation nuclei (CCN) counter provides not only total CCN number concentration (as the traditional CCN counters do at a certain supersaturation) but also the CCN size distribution information. Using these CCN data, model responses to different CCN initial concentrations are examined. The model results are consistent with both observations and expectations. The numerical results show that the cloud microphysical properties are changed fundamentally by different initial CCN concentrations but the cloud liquid water content does not differ significantly. Different initial CCN loadings have large impacts on the evolution of cloud microstructure and radiation transfer while they have a modest effect on thermodynamics. Increased CCN concentration leads to significant decrease of cloud effective radius.