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Letter
Evaluation of ionic pollutants in cloud droplets at a
mountain ridge in northern Japan using constrained
oblique rotational factor analysis
Nobuaki Ogawa
a,), Ryoei Kikuchi
a, Tomoko Okamura
a,
Junko Inotsume
a, Tetsuya Adzuhata
a, Toru Ozeki
b,
Masahiro Kajikawa
aa
Faculty of Engineering and Resource Science, Akita UniÕersity, Tegata Gakuen-cho, Akita, 010-8502, Japan b
Hyogo UniÕersity of Teacher Education, Yashiro-cho, Kato-gun, Hyogo 673-1494, Japan
Received 12 January 2000; received in revised form 6 April 2000; accepted 6 April 2000
The scavenging of aerosol particles by precipitation and fogrcloud plays an impor-tant role in the distribution and concentration of polluimpor-tants in the atmosphere. Our research group has recently studied the acid precipitation in Hyogo and Akita Prefec-tures in Japan, combining chemical analysis of ions and analysis of meteorological
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conditions, and has analyzed the ionic pollutants salts by factor analysis Ogawa et al.,
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1998a,b, 1999a,b; Ozeki et al., 1995, 1997 . In general, it is known that fogrcloud water is significantly more acidic and has higher concentrations of chemical components than
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rain water e.g., Waldman et al., 1982, Hosono et al., 1994, Ogawa et al., 1999a,b . But the mechanism of uptake of ion components into cloud droplets, especially for the
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difference between ions, is not completely understood. In our previous works Ogawa et
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al., 1999a,b , cloud water and rain water samples were collected at the Hachimantai mountain range in Akita Prefecture in northern Japan to obtain information on the mechanism of uptake of ion components into cloud droplets. We obtained the informa-tion about relainforma-tionship among ion concentrainforma-tions in the cloud water and droplet size
ŽOgawa et al., 1999a,b ..
In this work, we tried to analyze the seasonal change of ionic pollutants in cloud water due to a variation of mesoscale precipitation system and their dependence upon the cloud droplet size using constrained oblique rotational factor analysis, which has
)Corresponding author. Fax:q81-18-889-2601.
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E-mail address: ogawa@quartet.ipc.akita-u.ac.jp N. Ogawa .
0169-8095r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
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been developed to analyze quantitatively ionic pollutants in precipitation by our research
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group Ogawa et al., 1998b; Ozeki et al., 1995, 1997 .
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Cloud water samples were collected along the mountainside 39856 N, 140851 E,
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1465 m a.s.l. of Mt. Mokkodake 1578 m, a.s.l. , a mountain ridge in the Hachimantai range, while recording wind direction and temperature, using the passive fog sampler
ŽModel FWP-500, Usui Kogyo Kenkyusho during the period from June to September.
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1998 Ogawa et al., 1999a,b . Rain water samples were also collected at Akita City
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Akita University 39843 N, 140808 E, 10 m a.s.l. , at Onuma 39859 N, 140848 E, 960
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m a.s.l. in the Hachimantai range and at the mountainside of Mt. Mokkodake with a
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bulk sampler Ogawa et al., 1998a during the same periods. The concentrations of various ions in the cloud and rain water were analyzed by the method mentioned
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elsewhere Ogawa et al., 1998a, 1999a,b . The size of the cloud droplets was estimated
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by the impaction of drops on oil-coated glass slides Okita, 1961 . The cloud base of stratocumulusrnimbostratus was 1100–1350 m a.s.l. during the period of cloud water sampling.
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The arithmetic average of the pH of the cloud water samples was 4.43 ns64 , and that of the rain water samples collected in Akita City, Onuma and the mountainside of
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Mt. Mokkodake was 4.97 ns46 , 5.22 ns24 and 5.65 ns24 , respectively. The
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standard deviations s of the H concentrations of cloud ss59.3 meqrl and rain
Žss8.6, 8.0 and 2.7 meqrl water were significant different at the 99% probability.
level by F-test. The smallest pH value for cloud water was 3.61 and that for rain water was 4.39 in Akita, 4.44 in Onuma and 5.01 in the mountainside of Mt. Mokkodake. The
w yx w 2yx
arithmetic average of NO3 and nss-SO4 of the rain water samples, at Mt. Mokkodake, was 6.2 and 11.2 meqrl, while that of the cloud water samples was 44.3 and 99.3 meqrl, respectively. That is to say, the cloud water was significantly more acidic than the rain water in Akita City, Onuma and the mountainside of Mt.
Mokko-Ž .
dake as well as for the sample in 1997 Ogawa et al., 1999a,b .
Instead of the ion concentration in each sample of cloud event, the absolute
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equivalent of ion in cloud droplets the product of the concentration and the droplet
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volume calculated from the mean volume diameter, D was used in our factor analysis, because the absolute loading amount of pollutants in the droplet will contribute to the scavenging mechanism. The ion concentration in the cloud water which was collected using the passive fog sampler, can be regarded as a mean concentration of droplets. One can also recognize the reason of using the absolute equivalent by the data which in our
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previous work Ogawa et al. 1999a , the slope of the log–log-plots of the total ion concentration versus the droplet size was near minus three. This result shows that the cloud droplets will grow only by diffusion process without uptaking the ion component after the nucleation scavenging. Since the droplet size of eight samples in all the samples
Žns64 could not be measured during the cloud events, these samples were omitted.
from the factor analysis. In the factor analysis, the data only in westerly wind direction
Žfrom the Sea of Japan. Žns49. Žbecause the sample in easterly wind is only seven and. Ž
the chemical data whose ion-balances the ratio of the sum of equivalents of anions to
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that of cations are from 1.20 to 0.833 1r1.2 are used finally, ns37, Ds8.8–35.5
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and in the case of negative value, the value means OHy. The ‘‘oblique rotational’’ means that each factor is not orthogonal as a mathematical vector. We used the method
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which referred to the ion balance 1.2y 1r1.2 in each factor in order to decide the
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number of factors Ozeki et al., 1995, 1997; Ogawa et al., 1998b . We analyzed with good ion balances in the case that the number of factors was from one to four. However,
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Fig. 1. Chemical compositions of three pollutants a and their contributions vs. date b and vs. the drop size
the fourth factor in the case of four factors consisted of NH OH together with NaCl.4
The combination of ionic substances is unreasonable and unrealistic in chemical and meteorological senses. Therefore, we chose three as the number of factors.
Fig. 1 shows the chemical compositions of their pollutants and their contribution. As a total, the three factors contribute 69.1% to the ionic pollutants contained in all the cloud water samples. Fig. 1a shows the chemical compositions of three pollutants. The
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factor A mainly consists of NH4 2SO and H SO . The factor B consists of sea-salts4 2 4 with H SO and HNO . The factor C is of NH NO . These salts were well-known as2 4 3 4 3
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the composition of the cloud condensation nuclei CCN e.g., Pruppacher and Klett,
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1997 . There is no pollutant the factor which contains mainly Ca ion with OH , comparing the previous results of the factor analysis for the precipitation in Akita City,
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Japan Ozeki et al, 1997; Ogawa et al, 1998b . That is, the cloud water at the mountainside did not contain so much Ca2q unlike in the precipitation in the city site.
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The contribution of the factors A, B and C is 30.8%, 23.6% and 14.7% total: 69.1% , respectively. This result shows that the CCN in the season of June to September would
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be mainly NH4 2SO and H SO . Figure 1b shows plots of the contribution of each4 2 4
pollutant to each cloud event versus date. The pollutants A and C have high contribution
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in July. Plentiful rain especially, much rain and cloudrfog at the mountainside is
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produced by a stationary front Baiu front in the Japanese rainy season from June to
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July especially, in Akita in July . In July, therefore, the CCN would be mainly
ŽNH4 2. SO , H SO and4 2 4 ror NH NO . However, pollutant B contributes strongly in4 3
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September. We also have plentiful rain produced by a stationary front Akisame front in another Japanese rainy season in September. The CCN would be mainly sea-salt because sea-salt was transported to Akita and the mountainside with the westerly wind from the Sea of Japan, which began to become strong from September in Akita Prefecture
ŽOgawa et al., 1998a,b . Fig. 1c shows plots of the contribution of each pollutant to each. Ž .
cloud event versus the droplet size. The factor A of NH4 2SO and H SO has high4 2 4
contribution in the range of small droplet size, while the factor B of sea-salt is high in
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the larger size range Ds20–25mm . The factor C of NH NO is high in the middle4 3
size range between the factor A and B. These results are in agreement with the well-known description of the large CCN, such as the aerosol of NaCl, which results in
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the large droplet, and the small CCN, such as NH4 2SO , which results in the small4
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droplet e.g., Pruppacher and Klett, 1997 . We found in this work that our factor analysis gave the quantitative explanation in the seasonal change and in the droplet size change for cloudrfog water chemistry.
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