The history and present status of studies on the climatic effect of atmospheric aerosols are briefly reviewed. Pioneering works in the 1970's resulted in the understanding of some of the basic mechanisms related to direct and indirect effects of aerosols on climate. Intensive and extensive discussions on the hypothetical “biosphere-cloud feedback mechanism” in the late 1980's had led the research community to make quantitative estimates of the anthropogenic sulfate distribution on a global scale. Since then, significant improvements have been made in the estimation of direct effect. On the other hand, progresses in both space-borne radiometers and data analytical techniques have allowed us to virtually “see” this indirect effect. However, it is necessary for studies to be done on the indirect effect while keeping in mind the difficulty in modeling the climate system, i.e., our knowledge on the global hydrological cycle is quite fragmentary and incomplete.
Detection of aerosols' indirect effects on global weather by a remote sensing technique is described, focusing on low-level water clouds using passive optical satellite sensors. Remote sensing techniques for cloud properties such as the optical depth and effective particle radius were developed in early 1990s, and modified cloud properties have been reported. With the advent of the successful aerosol satellite retrievals, explicit comparison between aerosol and cloud properties has performed on a global scale. As the aerosol particle increases in number, the cloud particle size decreases and the cloud optical depth and cloud droplet number increase in general. These tendencies are consistent with Twomey effect. Comprehensive research combining other observational methods and numerical models will be needed for further understanding of both the first and the second aerosols' indirect effects.
Recent development in modeling study of aerosol's indirect effect with GCM (General Circulation Model) is reviewed. We introduce several types of parameterization currently used for incorporating the indirect effect of aerosols into GCM in order to evaluate the climatic impact of aerosols on cloud processes. Those parameterizations allow us to determine the cloud droplet number concentration from the aerosol particle number concentration, as well as estimate the precipitation formation from clouds as a function of the cloud droplet number concentration. Previous GCM calculated the effective radius of cloud droplet in order to compare it with satellite-observed values on a global scale. Few works were devoted to the investigation of model-derived correlation statistics between the cloud and the aerosol properties for comparison with the corresponding satellite results. We also introduce previous attempts to evaluate the indirect radiative forcing induced by anthropogenic aerosols.
Water soluble inorganic substances such as (NH4) 2SO4 and NaCl in aerosol particles have been considered to serve as the only cloud condensation nuclei (CCN) in the atmosphere. Recent observational studies, however, indicate that some of the organic compounds in aerosols also play an important role in the activation of CCN comparable to the inorganic particles' contribution. This article reviews recent observational and laboratory experiments conducted to measure various physico-chemical properties of organic substances that determine the high activation capability of pure organic particles and that of organic and inorganic mixtures as CCN.
Artificial Cloud Experiment Systems (ACES) in a vertical shaft of a mine were established at a coal mine in Kami-sunagawa, Hokkaido then at an iron mine in Kamaishi, Iwate. Updraft of air by electric fans induced the adiabatic expansion of air that resulted in generation of artificial clouds. We conducted basic measurements of temperature profiles, updraft air velocity, size distribution of droplets, etc. and investigated the dissolution of SO2 into cloud droplets, the oxidation of sulfite to sulfate, and the flux of droplet deposition onto trees using the stable cloud in ACES. Recent improvements of experimental conditions allowed for precise experiments in studying the influence of aerosol properties on the cloud properties which is the key process of indirect radiative forcing by aerosol particles.
A light-scattering instrument which measures the scattering patterns from an axisymmetric particle irradiated by monochromatic light was developed. The size and orientation of particles were determined through the comparison of measured patterns and those calculated by the computation program developed by Barber et al.1) . Barium sulfate of axisymmetric particle was generated by a spray method and introduced into the scattering instrument. An argon laser was used as the light source. The detector of instrument was further improved so as to measure the scattered light of parallel and perpendicular polarizations in the observation plane simultaneously. As a result, it was found that the particles were spheroids with major axis of 0.74 ∼ 0.75 μm and minor axis of 0.57 ∼ 0.58 μm, with orientation (Θp,Φp) = (45°, 40°) . Although the conventional scattering theory for spheroid was not sufficient, the instrument developed in the present work was confirmed to be useful for the simultaneous measurement of size and orientation of axisymmetric particles.
Cigarette filter consists of crimped fibers with Y-shaped cross-section. In the present work, the influences of fiber crimp angle and fiber cross-sectional shape on the diffusional collection efficiency and the pressure drop of cigarette filter were studied both experimentally and numerically. As a result, the present numerical model was capable of successfully predicting the influences of crimp angle and irregular cross-section on the collection efficiency and the pressure drop. It was further shown that the maximum value of filter performance index lies at the crimp angle of π/6 for fibers with circular and Y-shaped cross-sections. Cigarette filters consisting of fibers with Y-shaped cross-section in the present work had almost the same filter performance index as predicted by the numerical analysis because they had the crimp angles between π/12 and π/4.