Chemical Characterization of Acid Fog and. Rain of Akita in Northern Japan

Transcription

1 Int. J. of The Soc. of Mat. Eng. for Resources Vol.7 No (1999) Original Chemical Characterization of Acid Fog and by Nobuaki OGAWA õa), Ryoei KIKUCHIa), Tomoko OKAMURAa), Masahiro KAJIKAWAa), Tetsuya ADZUHATAa), Yoshihiro IWATAb) and Toru OZEKIc) Abstract 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. Key Words: Chemical Characterization, Acid Fog, Acid Rain, Akita, Fog Size. Introduction Acid precipitation is one of the global environmental problems; it affects on terrestrial and aquatic ecosystem. Our research group has recently studied1)-3) the acid precipitation of Hyogo and Akita Prefectures in Japan, combining the chemical analysis of ions with a meteorological situa tion1), and has analyzed the source of pollutant by a factor analysis which has been developed for an acid rain problem by our research group2)3). In general, it is known4) that fog/cloud water is signifi cantly more acidic than rain water and concentrated with respect to chemical components compared with rain water. In this paper, therefore, the fog/cloud samples were collected at Hachimantai Received, May 13, 1999 õ To whom correspondence should be addressed. a) Faculty of Engineering and Resource Sciences, Akita University, Tegata Gakuen-cho, Akita , Japan. b) Faculty of Education and Human Studies, Akita University, Tegata Gakuen-cho, Akita , Japan. c) Hyogo University of Teacher Education, Yashiro-cho, Kato-gun, Hyogo , Japan. 282

2 Vol.7 No.2 (1999) Chemical Characterization of Acid Fog and 283 range in addition to the rain. The chemical components in the fog /cloud and rain water were analyzed and compared with each other. The relationship among ion concentrations in the fog water and its size was discussed. Experimental Fog water samples were collected at the mountainside (39 56'N, 'E, 1465 m, a. s. l.) of Mt. Mokkodake (1578 m, a. s. l.), a mountain ridge of Hachimantai range (Fig. 1, 2). A passive fog sampler with cylindrical Teflon wire screen (Model FWP-500, Usui Kogyo Kenkyusho Inc.) from August to September Rain water samples were also collected at Akita University (39 43'N, 'E, 10m, a. s. l.) in Akita City and at Onuma (39 59'N, 'E, 960m, a. s. l.) in the Hachi mantai range by the bulk sampler during the same periods1). Akita City is located about 6.5km east from the coast of the Sea of Japan (Fig. 1). The Hachimantai range is about 80km east from the sea coast (Fig. 1). At the site of Onuma and the mountain side of Mt. Mokkodake, temperature, humidity and wind direction were also measured (Fig. 1, 2). The Snow Science Laboratory of Akita Fig. 1. Location of sampling places for fog and rain in Akita Prefecture, Japan.

3 284 Nobuaki OGAWA, Ryoei KIKUCHI, Tomoko OKAMURA, Masahiro KAJIKAWA Tetsuya ADZUHATA, Yoshihiro IWATA and Toru OZEKI Fig. 2 Location of sampling sites in the Hachimantai mountain range. This chart was made from the TOPPAN's original chart (of an internet url; co.jp/cgi/m?no= ). University (SSL-AU) at Hachimantai range was used as a stock room of the instruments and as accommodations. The ph and the electric conductivity of the fog and rain water were measured with a ph meter (TOA model HM-30S) and a conductivity meter (TOA model CM-40S), respectively. The ion concentrations (C1-, SO42-, NO3-, Na+, NH4+, K+, Ca2+ Mg2+) of the sample were analyzed using an ion chromatography system (TOA ICA-5000)1). The size of the fog droplets was estimated by the impaction of drops on oil-coated glass slides4). The counting and measuring of the droplets was performed with the aid of a microscope. The chemical components of the insoluble substances (filtered by a 0.45ƒÊm membrane filter (ADVANTEC DISMIC 13 cp)) included in the fog and rain samples were analyzed using Particle Induced X-ray Emission (PIXE) Analysis at Nishina Memorial Cyclotron Center, Japan Radioisotope Association.

4 Vol.7 No.2 (1999) Chemical Characterization of Acid Fog and 285 Results and Discussion Ion concentrations and ph in rain and fog water Figure 3 shows the total and various ion concentrations in rain water at Akita City (A) and at Onuma (B), and in fog water at the mountainside of Mt. Mokkodake (C). The arithmetic mean of the ph in these samples are also shown in Fig. 3. The total ion concentration of fog water was higher than that of rain water at Akita and Onuma. The arithmetic mean of ph was 4.90 (rain) at Akita, 4.80 (rain) at Onuma and 4.07 (fog) at Mt. Mokkodake. The lowest ph of rain water was 4.45 at Akita and 4.31 at Onuma, while that of fog water was Thus the fog water at Mt. Mokkodake were more acidified than the rain water at Akita and Onuma in the same period. Figure 4 shows the ph and concentration change of NO3-, nss (non sea-salt)-so42-, nss-ca2+, NH4+ and Na+ in every rain or fog water. The NO3- is the acid pollution from local sites, the nss-so42- is the acid pollution from China, the nss-ca2+ is the alkaline pollution caused by soil, the NH4+ is the alkaline pollution by biological source at local sites and the Na+ is from sea-salt1). When these ion concentrations, especially NO3- and nss-so42- in fog water, were high, the ph of the samples was low. Figure 5 shows the plots of ph versus the difference in the concentration between acidic ions (NO3-, nss-so42-) and basic ones (NH4-, nss-ca2+) in rain and fog water. We found that as the concentra tion of the acidic ions was in fog and rain increased, the ph decreased, especially that the acidic ion concentration of fog was very high when the ph was low. Therefore, it is concluded that the fog water is acidified mainly by NO3- and nss-so42-. When the concentration of acidic ions equals to that of basic ions, the ph is about 5.6 which is the value of the saturated water of CO2. In other words, this result shows that these four ions could mainly decide ph values of fog and rain water. The plots of [Na+] vs. [Cl-] in rain and fog water were shown in Fig. 6. The results show that the Na+ and Cl- were mainly a sea-salt origin, because the concentration ratio of these ions was close to the sea-salt one Nevertheless Hachimantai range is about 80km east far from the coast of the Sea of Japan, there were some fog samples which had higher concentration of Na+ or Cl- than rain samples at Akita (6.5km from the sea coast). These are very rare cases because it was known7) that the sea-salt concentration in precipitation decreased exponentially with the dis tance from the sea coast. Chemical component of the insoluble substance in fog and rain samples The values in Table 1 shows the weight ratio of various elements to aluminum in the insoluble substance, which remained on the membrane filter (0.45ƒÊm) after filtrating rain and fog water using PIXE analysis, in comparison with that in Kosa from China11). There were some fog and rain samples whose element ratios were close to those of Kosa except for soluble element in water (Na, K, Mg). The 72 h back trajectories at 850 hpa for the selected events (8/28, 8/29, 9/12, 9/13 and 9/14) indicate that air masses of the events, 8/28, 8/29, 9/12 and 9/14 were transported from the basin of the Hwang Ho (Yellow River) in China. However, the mean values of the ratio for all of the samples was not similar to that of Kosa. This result means that depending on an air mass property, the some fog or rain water samples would contain only Kosa as the insoluble substance, but the other samples would contain the other insoluble materials.

5 286 Nobuaki OGAWA, Ryoei KIKUCHI, Tomoko OKAMURA, Masahiro KAJIKAWA Tetsuya ADZUHATA, Yoshihiro IWATA and Toru OZEKI (A) (B) (C) Fig. 3 Total and various ion concentrations in rain water at Akita City (A) and Onuma (B), and in fog water at the mountainside of Mt. Mokkodake (C) from August to September In the panels B and C, the data of observation period were only shown.

6 Vol.7 No.2 (1999) Chemical Characterization of Acid Fog and 287 (A) (B) (C) Fig. 4 The ph and concentration changes of NO-3 nss-so24-, nss-ca 2+, NH+4 and Na+ in all of rain and fog samples from August to September, 1997.

7 288 Nobuaki OGAWA, Ryoei KIKUCHI, Tomoko OKAMURA, Masahiro KAJIKAWA Tetsuya ADZUHATA, Yoshihiro IWATA and Toru OZEKI Fig. 5 Plots of ph vs. the values of {([NO-3]+ [nss-so-24])-([nss-ca2+]+[nh+4])} in rain and fog water. Fig. 6 Plots of [Na+] vs. [Cl-] in rain and fog water.

8 Vol.7 No.2 (1999) Chemical Characterization of Acid Fog and 289 Table 1. Comparison of the chemicalcomponent between the insoluble substance in fog or rain water A and "Kosa" from China (elementweight ratio to aluminum). (A) The mean value and stan dard deviation of all samples. (B)The element ratio of the fog and rain samples which were similar to Kosa. B Relationship among ion concentrations and droplet size of fog samples Figure 7 shows the distribution charts of fog droplet diameters for each fog event during the period from August 23rd to September 28th, The value of arithmetic mean diameter (D= (Di ~Ni)/ƒ Ni) was estimated and was shown in these charts, where D, was a diameter of fog ƒ droplet and N, was a number of fog droplet on a glass slide. The distribution pattern is different for each fog event and D is in the range of ,ƒÊm. Figure 8 shows the plots of ion concentrations ([ion]) versus droplet diameter (D) for all fog samples. The straight line is the fitted curve obtained by the least squares method (log [ion] = a- b log D). There are the fitted values of log a, the slope (b) and the correlation coefficient (r) in the figure legend. The concentration of each ion and the total ion concentration were observed to decrease with mean droplet size. This has been observed in some other measurements"). However, the correlation coefficient was not so high in Fig. 8. Therefore, we picked up the continuous fog events because the continuous sampling in the same air mass was very important for analyzing the growth process of fog droplet, the incorporation mechanism of ion components in fog droplets and the process acidifying fog water. Figure 9 shows the total and various ion concentrations of rain (A : Akita, B: Onuma) and of fog (C), the prevailing wind direction and the droplet diameter of fog from 26th to 28th of Septem ber, The eight fog samples were collected during this period. The total ion concentration

9 290 Nobuaki OGAWA, Ryoei KIKUCHI, Tomoko OKAMURA, Masahiro KAJIKAWA Tetsuya ADZUHATA, Yoshihiro IWATA and Toru OZEKI Fig. 7 The distribution charts of fog droplet diameters for each fog event during 8/23-9/28/1997.

10 Vol.7 No.2 (1999) Chemical Characterization of Acid Fog and 291 Fig. 8 Plots of various ion concentrations ([ion]) vs. mean droplet diameter (D) for all fog samples.

11 292 Nobuaki OGAWA, Ryoei KIKUCHI, Tomoko OKAMURA, Masahiro KAJIKAWA Tetsuya ADZUHATA, Yoshihiro IWATA and Toru OZEKI (A) (B) (C) Fig. 9 The total and various ion concentrations of rain (A: Akita, B: Onuma) and fog (C: Mt. Mokkodake) water, prevailing wind direction and droplet size at 9/26-28/1997.

12 Vol.7 No.2 (1999) Chemical Characterization of Acid Fog and 293 Fig. 10 Plots of various ion concentrations vs. droplet diameter of continuous fog samples in westerly wind at 9/26-28/1997.

13 294 Nobuaki OGAWA, Ryoei KIKUCHI, Tomoko OKAMURA, Masahiro KAJIKAWA Tetsuya ADZUHATA, Yoshihiro IWATA and Toru OZEKI was highest at the 5th sample and the wind direction changes from easterly to westerly from the 4th sample. The 72 h back trajectories show that the air masses for the westerly wind were transported from the continent (China and Russia) and passed the Sea of Japan and the central part of Akita Prefecture. Therefore, in the westerly wind, nss-so24- from China, sea-salt from the sea and NO-3 from Akita City would increase in the fog water. On the other hand, for the easterly wind, the trajectory analysis means that the air masses were transported from the Pacific Ocean and passed over the Japan Island. So, the polutant in the fog would be sea-salt from the sea and NO-3 from the cities in Japan. The fog size was large and the total ion concentration was low at the easterly wind from Iwate compared with the westerly wind from Akita. This result suggests the possibility for that the growth and acidifying process of fog would depend on the wind direction. The same relationship as Fig. 8 are plotted in Fig. 10 for the continuous five samples only in westerly wind during this period. The cloud base of stratocumulus/nimbostatus was m (a.s.l.) during the period of five fog /cloud water sampling. Of course, as the fog size decreased, the total and each ion concentration in it became high. However, the correlation coefficient of each line was much higher than that of Fig. 8. Figure 11 shows the conceptual model for the variation with drop size of the solute concentra tion in cloud/fog drops by Pruppacher and Klett 10). Note that the drop size scale is approximate, and will vary from cloud/fog to cloud/fog. The fog samples in this work was in size interval for the fog size (D) of 8-17um (arithmetic mean diameter). We also obtained the similar relation ship to Fig. 10 for the plots of ion concentration versus mean volume diameter 5). Although, from Fig. 11 Conceptual model for the variation with droplet size of the concentration in cloud/fog drops. Note that the radius scale is approximate and will vary from cloud/fog to cloud/fog10).

14 Vol.7 No.2 (1999) Chemical Characterization of Acid Fog and 295 the size range, the droplets in our fog water samples were categorized by size interval II (D: about 5 ` 50um, growth by condensation) according to the model of Ogren6), the size dependence of ion concentration was inconsistent with this model and the hypothesis that condensation growth con trols this size dependence9). In their model, size interval I by growth to equilibrium is about 0.5 ` 5um in D. However, our results consistent with size interval I (D : about 2 `20um, unactirated or freshly-activated) of the conceptual model established by Pruppacher and Klett11) (Fig. 11). There fore, we conclude that the droplets in the fog samples in this work behaved as in size interval I of the model by Pruppacher and Klett though the droplet size was 8-17um. This kind of dependence of ion concentration on droplet size might be observed in the case of cloud /fog droplets in heavily polluted environments, but no satisfactory explanation exists for such dependence"). We also found that the slope of plots of the concentration versus the droplet size was different from each other. We will collect much more fog/cloud samples in near future and analyze the mechanism of uptake of ion components into cloud droplets. Acknowledgment The authors wishes to express their greatest gratitude to Mr. Tetsuo Muto and Ms. Shinobu Ishi zuka for sampling rain water at Akita University. The authors are grateful to Nishina Memorial Cyclotron Center, Japan Radioisotope Association for PIXE analysis. References 1) N. Ogawa, T. Adzuhata and M. Kajikawa, Seppyo, 60 (2), (1998). 2) T. Ozeki, K. Koike, N. Ogawa, T. Adzuhata, M. Kajikawa and T. Kimoto, Anal. Sci., 13, (1997). 3) N. Ogawa, R. Kikuchi, H. Goto, M. Kajikawa and T. Ozeki, Bunseki Kagaku, 47 (8), (1998) (in Japanese). 4) T. Okita, Tellus, 13, (1961). 5) N. Ogawa, R. Kikuchi, T. Okamura. T. Adzuhata, M. Kajikawa and T. Ozeki, Atmos. Res., 51 (1), (1999). 6) J. A. Ogren, K. J. Noone, A. Hallberg, J. Heintzenberg, D. Shell, A. Berner, I. Solly, C. Kruisz, B. G. Arends and W. Wobrock, Tellus, 44B, (1992). 7) S. Tsunogai and S. Noriki, (ed. M. Nishimura), "Kaiyokagaku: Kagaku de umi o toku (Chemical Oceanography: Chemistry revealed the oceans) ", Sangyo Tosho, pp (1983) (in Japanese). 8) J. W. Munger, J. Jr. Collet, B. Daube and M. R. Hoffmann, Atmos. Environ., 23, (1989). 9) F. Muller and G. Mauersberger, Atmos. Res., 32, (1994). 10) H. R. Pruppacher and J. D. Klett, "Microphysics of Clouds and Precipitation" Academic Publishers, pp , (1997)., Chap. 17, Kluwer 11) S. Kanamori, Y. Kanamori, M. Nishikawa and T. Mizoguti, (ed. Nagoya University), "Kosa no kagaku (Chemical property of Kosa)", Kokon Shoin, pp , (1991) (in Japanese).