Recently, acid precipitation is receiving global interest because it affects terrestrial and aquatic ecosystems. 1,2 Our research group has studied 3-7 the acid precipitation of Hyogo and Akita prefectures in Japan, combining a chemical analysis of the ionic substances with meteorological situations, and has analyzed pollutants by a factor analysis. [3][4][5][6][7] Furthermore, investigations for the fog on a mountain ridge showed by a factor analysis that a fog droplet had some soluble pollutants (e.g. (NH4)2SO4, seasalt, H2SO4 and HNO3), and that the uptake mechanism by a fog droplet would be different for each pollutant.It is well-known that fog/cloud water is more acidic and has a higher concentration of pollutants than rain water. 5,9-11 Nevertheless, prior attempts 15,16 to elucidate the mechanisms of fog acidification, especially the behavior of the ionic substances in the fog droplets, have not been quantitatively conclusive. There has been no investigation, moreover, concerning where the air pollutants specified by employing the oblique rotational factor analysis 3,4,6,12 are transferred.Our purpose here was threefold. First, by using an oblique rotational factor analysis, we examined the observed drop size dependence of the chemical composition during fog events at Akita Hachimantai mountain range from June to September of 1998 and 1999. Second, we evaluated the transport of some air pollutants in combination with ionic substances which were quantitatively extracted from a factor analysis developed by our research group, 3,4 with a 72 h back trajectory at the 850 hPa level and from the point of view of a synoptic weather system. Third, from the feature of chemical composition of fog or rain, which was non-ion-balanced, we tried to estimate unknown soluble chemical species. ExperimentalFog water was collected on a mountainside (39˚56′N, 140˚51′E, 1465 m, a.s.l.) of Mt. Mokkodake (1578 m, a.s.l.) in the Akita Hachimantai mountain range from June to September of 1998 and 1999 (Fig. 1) Fog and rain water samples were collected at the same time in the Akita Hachimantai mountain range in northern Japan from June to September in 1998 and 1999. The various ion concentrations in these samples were analyzed, and the fog droplet sizes were measured for each fog event. As the fog droplet size increased, the ion concentration decreased. The slope of log-log plots of the concentration versus the droplet size differed with the kind of ion. In order to characterize the air pollutant, moreover, these data were quantitatively analyzed by an oblique rotational factor analysis. We found that three factors were extracted as the air pollutant source: (NH4)2SO4, acids (HNO3 + H2SO4) and sea-salt. Combining the factor analysis with the 72 h back-trajectory at 850 hPa level, we found that the contribution of each factor varied with the transport pattern of air masses.

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A statistical method developed by our group, "the constrained oblique rotational factor analysis" enables feature reduction and grouping of the pollutant sources in precipitations from their chemical compositions. A study applied to the precipitations has indicated the necessity of the numerical evaluation of contributions of the extracted sources. In this paper, a procedure for the numerical evaluation is demonstrated; and, as one application, the precipitations collected in Hyogo prefecture are compared with those in Akita prefecture. The analysis has proposed that an ion-balance check of the extracted pollutant source gives a simple criterion about how many pollutant sources should be extracted. As a result, the causes of lowering of the pH of the precipitation of these two areas are explained by two sources: l) an acidic source with sulfuric acid predominant brought over the Sea of Japan from China by winter monsoons; and 2) another acidic source with sulfuric acid and nitric acid predominant emitted domestically. The seasonal changes of acidity of the precipitation of Japan depend upon the amounts of these two contributions.

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Fog/cloud and rain water was collected at the mountainside of Hachiman tai range and rain water was also collected at Akita City. The various ion concentrations (H+, Cl-, SO42-, NO3, Na+, NH4+, K+, Ca2+, Mg2+) of these sam ples were analyzed, and the droplet size and the wind direction were measured for each fog sample. The fog at Hachimantai range had a very high total ion concentration, and was considerably acidified by nss (non sea-salt)-SO42-and NO3-, in comparison with the rain at Akita and Onuma (Hachimantai range). There were some fog and rain samples whose chemical components in insoluble substance, which were analyzed by PIXE analysis, were similar to that of Kosa from China. As the fog droplet size increased, the ion concentration decreased, and the slope of plots of the concentration versus the droplet size was dif ferent from each other. From the negative slope of the plots, we can conclude that the fog samples in this work were in the size interval I (unactivated or freshly-activated) for the droplet growth.