Chemical characteristics of precipitation in Okinawa Island, Japan

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Atmospheric Environment 42 (2008) 2320–2335 www.elsevier.com/locate/atmosenv

Chemical characteristics of precipitation in Okinawa Island, Japan Hideaki Sakihama, Maki Ishiki, Akira Tokuyama Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japan Received 2 February 2007; received in revised form 9 November 2007; accepted 7 December 2007

Abstract This study examined the chemical composition and characteristics of precipitation collected from March 2003 to February 2005 in the central part of Okinawa Island, Japan. Chloride ions contributed most to the total ion equivalent   2+ + concentration, and followed in order by Na+4Mg2+, SO2 4H+, NH+ 4 4Ca 4 , K 4NO3 , HCO3 . Concentrations of + + 2+ 2+  2 2 Na , K , Ca , Mg , Cl , and sea salt SO4 (ss-SO4 ) increased from summer to autumn and decreased from winter to  2 2 + spring. In contrast, concentrations of NH+ 4 , NO3 (except for July 2004), non-sea salt SO4 (nss-SO4 ), and H were lower in summer and higher in winter. During periods with typhoons, concentrations of sea salt components, such as Na+ and 2 Cl, increased while NO concentrations decreased. Wet deposition driven by typhoons accounted for 3 and nss-SO4 about 77% of the total annual wet deposition. The pH values ranged from 3.89 to 7.61. Acid rain (pHo5.6) occurred in 72% of the collected samples, even though Okinawa Island is considered to be an unpolluted area. Principal component analysis indicated three main origins of the chemical components in precipitation: (1) sea salt generated from local 2+ surrounding ocean (Na+, K+, ss-Ca2+, Mg2+, Cl, and ss-SO2 and 4 ), (2) soil generated from local land (nss-Ca  2 D-SiO2), and (3) anthropogenic source of Asian Continent (NH+ , NO , and nss-SO ). 4 3 4 r 2007 Elsevier Ltd. All rights reserved. Keywords: Subtropical area; Chemical composition; Typhoon; Acid rain; Principal component analysis

1. Introduction Precipitation contains substances of natural origin such as sea salt and soil dust, and of anthropogenic origins such as gases emitted from industrial areas and vehicles. Precipitation in coastal areas is strongly affected by sea salt (Ozeki et al., 2006), while precipitation in inland areas contains proportionately more substances originating in soil (Zhang et al., 2003; Mouli et al., 2005). Past studies have reported very high concentrations of anthroCorresponding author. Tel./fax: +81 98 895 8526.

E-mail address: [email protected] (A. Tokuyama). 1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.12.026

2 pogenic substances such as NO 3 and SO4 in urban + or industrial areas compared to other areas (Gulsoy et al., 1999; Tu et al., 2005). Many factors (e.g., geographic condition, weather, climatic conditions, degree of industrialization) influence the concentrations of chemical components in precipitation. The chemical composition of precipitation reflects all local characteristics. Okinawa Islands that are located in the southwestern part of Japan have subtropical climate and unique island ecosystems including rare vegetation, insects, and animals. In summer, typhoons from the Pacific Ocean occasionally approach the islands, accompanied with high salinity deposition.

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In winter, prevailing northwesterly wind brings acid components to Okinawa from the Asian Continent (Tomoyose et al., 2003). In recent years, atmospheric transport of pollutants emitted from Asian Continent has become the noticeable problems in many areas far from the source (Zdanowicz et al., 2006; Hsu et al., 2005; Chung et al., 2001; Perry et al., 1999). Since in summer, Okinawa Islands are affected by air masses from the Pacific Ocean considered unpolluted background area but in winter they are affected by air masses containing pollutants from Asian Continent, Okinawa Islands are specific area from the viewpoint of atmospheric chemistry. Furthermore, because high salinity deposition and acid rain can degrade concrete buildings and culture properties, inhibit the growth of agricultural crops, damage forests, and acidify aquatic and terrestrial ecosystems, precipitation chemistry and related issues have been studied worldwide (Xie et al., 2004; Driscoll et al., 2003; Yamada et al., 2001; Walna and Siepak, 1999; Bini and Bresolin, 1998; Komai et al., 2001). Even though Okinawa Islands have regional characteristics and the need for study on impact of atmospheric deposition on various environment, few studies have considered the chemical composition of rainwater in Okinawa (Nagamine et al., 2002; Tomoyose et al., 2003; Agata et al., 2006), where available data to evaluate effect of precipita-

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tion on the environment have been limited. This study will characterize precipitation chemistry in Okinawa Island and will augment the quality and quantity of data for Okinawa. Precipitation was collected in the central part of Okinawa Island from March 2003 to February 2005, and its chemical components were analyzed. 2. Site description As shown in Fig. 1, Okinawa Island is located between the East China Sea and the Pacific Ocean (Planning and Coordination Division, Okinawa Prefecture, 2006). The island has a subtropical climate, with annual average atmospheric temperature and precipitation at the meteorological observatory in Naha City, Okinawa Island, being 22.7 1C and 2036.9 mm (National Astronomical Observatory 2004), respectively. Heavy rains are associated with Bai-u (rainy season) rainfall events from early May to mid-June and typhoons in summer and autumn. Fig. 2 shows variation in monthly precipitation and annual precipitation in the study period. Annual precipitation totals were 1610 mm (from March 2003 to February 2004) and 1927.5 mm (from March 2004 to February 2005). These totals were smaller than the annual average precipitation. Seven typhoons approached Okinawa Island (Naha

50°

N 0

40°

The Pacific Ocean

Japan

30°

1000km

The East China Sea

Univ. of the Ryukyus

Naha City Okinawa Is.

0 20°

120°

130°

140°

150°

20km 160°

Fig. 1. Map of Okinawa Island showing the sampling site (K) and meteorological observatory (’).

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Fig. 2. Variation of monthly precipitation and amount of annual precipitation in Okinawa Island.

City) in both 2003 and 2004, where approach means that the center of the typhoon passed within 300 km of Okinawa meteorological observatory in Naha. 3. Sampling and analytical methods Precipitation was sampled from March 2003 to February 2005 on the rooftop of the Faculty of Science building at the University of the Ryukyus in the central part of Okinawa Island (Fig. 1). Samples were collected in two 10 L polyethylene buckets fixed on blocks. Bucket spouts were about 75 cm above the floor. Precipitation samples were generally collected in daily cycles. During some typhoons, however, the sampling intervals were shortened to a few hours. Since a sampling interval was 1 day at a maximum, the effect of dry deposition on precipitation chemistry can be ignored and thus the samples can be considered as wet deposition (Wai et al., 2005). After sampling, two samples were immediately carried to the laboratory and combined. Then a sample volume, pH and electrical conductivity (EC) were measured with a measuring cylinder, pH meter (TOA HM-21), and EC meter (TOA DKK CM-20J), respectively. If foreign substances such as insects were in the bucket, the volume was measured separately, and EC, pH, and concentrations of chemical components were measured in an unpolluted sample. Then the sample was filtered through a membrane filter with 0.45 mm pore size.

Sodium ion and K+ were measured using flame photometry (NIPPON Jarrell Ash AA-782), and Ca2+ and Mg2+ were measured using atomic absorption spectrometry (NIPPON Jarrell Ash AA-782). Ammonium ion was measured by indophenol-spectrophotometry (HITACHI U-1500). 2 Major anions, such as Cl, NO were 3 , and SO4 measured by ion chromatography (HITACHI L-7000 series). Alkalinity was measured using a titration method (the end point was pH 5.2) with 0.02 M HCl. Dissolved silica was measured by molybdate blue-spectrophotometry (HITACHI U-1500). 4. Results and discussion 4.1. Measurement accuracy The number of collected samples totaled 189 (98 in 2003 and 91 in 2004). The accuracy of the datasets was checked by the ion balance (R1) and EC (R2) with the method used by Acid Deposition Monitoring Network in East Asia (Network Center for EANET, 2005). The following formulas were used to calculate R1 and R2: CA ; CþA Lcal:  Lmeas: R2 ð%Þ ¼ 100  ; Lcal: þ Lmeas:

R1 ð%Þ ¼ 100 

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where C is the sum of equivalent concentrations of cations, A is the sum of equivalent concentrations of anions, and Lcal. and Lmeas. are calculated and measured electrical conductivity, respectively. Data were assessed with the standard used by EANET. Four inadequate datasets were discarded; therefore, 185 datasets are considered for the following discussion. 4.2. Chemical composition of precipitation The chemical composition of precipitation in Japan is generally affected by sea salt particles generated from sea spray (Hara, 1997). In coastal areas, the dominant component is NaCl, and that concentration exponentially decreases with distance from the coast (Fujita et al., 2000). The Okinawa Islands, surrounded by ocean, are likely strongly affected by sea salt as well. Fig. 3 considers the effect of sea salt, showing the relationship between concentrations of Cl and Na+. Equivalent concentration ratios of Cl/Na+ for precipitation are extremely close to the ratio of seawater. Thus, precipitation in Okinawa Island is affected by sea salt throughout the year. Calcium ion and SO2 concentrations were 4 separated into sea salt (ss-) and non-sea salt (nss-) by assuming that all Na+ was supplied from sea salt. The nss-fraction of the component (nss-[X]) was calculated as nss-½X ¼ ðtotal-½XÞ  ðss-½XÞ;   ½X þ ss-½X ¼ ½Na prec:  ; ½Naþ  sea

Fig. 3. Relationship between Cl and Na+ concentrations in precipitation.

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where total-[X] is the concentration of X in precipitation, ss-[X] is the fraction of X that is sea salt, [Na+]prec. is the concentration of Na+ in precipitation, and ([X]/[Na+])sea is the concentration ratio in seawater; [Na+] and [X] indicate equivalent concentrations. In this study, concentrations of sea salt components accounted for approximately 99% of the total concentration for precipitation most influenced by sea salt. Fig. 4 shows the contribution of each ion to the sum of cations, sum of anions, and total ion equivalent concentration for precipitation most affected by sea salt. The order of equivalent concentrations of cations was Na+ (77%)4Mg2+ (17%)4Ca2+ (4.6%)4K+ (1.7%); that of anions was Cl (89%)4SO2 (10%)4HCO 4 3 (0.3%). The order of equivalent concentrations of total ions was Cl(43%)4Na+(39%)4Mg2+ 2+ (8.8%)4SO2 (2.4%). Concentra4 (4.8%)4Ca tions of sea salt components accounted for only about 10% of the total concentration for that precipitation least influenced by sea salt (i.e., most influenced by non-sea salt components). Fig. 5 shows the contribution of each ion to the sum of cations, anions, and total ion equivalent concentrations for such precipitation. The order of equivalent concentrations of cations was H+ (39%)4NH+ 4 (29%)4Ca2+ (20%)4Na+ (6.5%)4K+ (3.4%)4 Mg2+ (2.4%); that of anions was SO2 (79%)4 4  NO 3 (12%)4Cl (8.9%). The order of equivalent concentrations of total ions was SO2 4 (39%)4 2+ H+(19%)4NH+ (9.8%)4NO 4 (15%)4Ca 3 (6.2%) 4Cl(4.5%)4Na+(3.2%)4K+(1.7%), Mg2+(1.2%). These results show that chemical composition of precipitation in Okinawa widely varies through the year. Table 1 shows the volume weighted mean (VWM) concentrations for the rainwater in this study and in other coastal areas. Fig. 6 shows the contribution of each ion to the sum of cations, sum of anions, and total ion equivalent concentration as calculated from the VWM for entire samples. The order of equivalent concentrations of cations was Na+(72%) 4Mg2+(15%)4Ca2+(5.9%)4H+(2.8%),NH+ 4 (2.2%), K+(2.2%); that of anions was Cl(84%)4SO2 4  (13%)4NO 3 (1.7%), HCO3 (1.5%). The total ion concentration was Cl(42%)4Na+(36%)4Mg2+ 2+ (7.5%), SO2 (3.0%)4H+(1.4%), NH+ 4 (6.4%)4Ca 4 +  (1.1%), K (1.1%)4NO3 (0.83%), HCO 3 (0.75%). Chloride ion and Na+ were dominant species, accounting for about 78% of the total ion concentration. In Okinawa (including the results of Hedo),

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Fig. 4. Contribution of each ion to (a) the sum of cations, (b) the sum of anions, and (c) total ion concentrations in equivalent values for the sample most influenced by sea salt.

Cl and Na+ concentrations in precipitation were  higher, and NH+ 4 and NO3 concentrations were lower, than those in other coastal areas (Table 1). Sea salt particles are the source of the sea salt component in precipitation. The particles are generated as small air bubbles burst in the foam of breaking waves at the ocean surface (Berner and Berner, 1987). Coral reefs surround the island of Okinawa, and wave breaking, which generates sea salt particles, may occur more frequently around Okinawa than in other areas. Typhoons are also accompanied with high salinity deposition. Both phenomena help elevate Na+ and Cl concentrations compared to other areas.

Fig. 5. As in Fig. 4, but for the sample most influenced by nonsea salt components.

4.3. Temporal variation of each component The variations in monthly VWM concentrations of the components in precipitation are presented in Fig. 7. Each year has been divided into four seasons (spring is March–May, summer is June–August, autumn is September–November, and winter is December–February). Concentrations of Na+, K+, Ca2+, Mg2+, Cl and ss-SO2 4 showed similar variations. Concentrations increased from summer to autumn and decreased from winter to spring. The Na+ concentration had the most variability of the species, ranging from

Sites

NH+ 4 (meq L1)

Na+ (meq L1)

K+ (meq L1)

Ca2+ (meq L1)

Precipitation (mm year1)

pH

2003–2005

1769

4.93

2004

1778.2

4.86

12.9

594

16.5

31.2

2004

1408.6

4.80

10.8

298

10.5

23.8

2004

945.4

4.84

17.1

229

6.00

2004

1123.9

4.74

36.0

57.8

Kangwha Yangyang

1991–1997 1991–1997

1177 1364

4.89 5.05

66.7 41.4

China Hong Kong

1994–1995

2735

4.62

Japan This study (Okinawa Island) Hedo (Okinawa Island) Oki (Shimane) Rishiri (Hokkaido) Korea Cheju

9.52

308

9.39

25.2

Mg2+ (meq L1)

63.9 125

Cl (meq L1)

351

NO 3 (meq L1)

7.01

SO2 4 (meq L1)

References

53.9

675

10.1

106

Network Center for EANET (2005) Network Center for EANET (2005) Network Center for EANET (2005)

69.4

325

16.9

58.6

17.2

52.8

256

11.6

57.0

8.10

15.4

14.6

79.1

24.8

47.6

36.9 62.8

7.6 7.4

27.6 23.0

11.6 16.6

41.5 73.4

25.9 19.1

76.6 57.6

Network Center for EANET (2005) Kang et al. (2004) Kang et al. (2004)

24.8

1.9

8.9

5.7

27.8

13.2

43.1

Tanner (1999)

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H. Sakihama et al. / Atmospheric Environment 42 (2008) 2320–2335

Table 1 Volume weighted mean concentration of chemical components of precipitation at some coastal areas and Islands in East Asia

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study, concentrations of chemical components usually decreased with increasing rainfall amount. However, both concentrations of chemical components and rainfall amount were high in typhoon period. It is known that the abundances of chemical species in high solute events are largely influenced by below-cloud scavenging (Rastogi and Sarin, 2007). It is thought that concentration peaks in summer or autumn are likely due to sea salt supplied by typhoons and wind-blown soil particles taken in precipitation by below-cloud scavenging.  Concentrations of NH+ 4 , NO3 (except for July + 2 2004), nss-SO4 , and H tended to decrease in summer and increase in winter. Hara (1997) suggested that concentrations of NO 3 and nssSO2 are higher in winter at some places facing 4 the Sea of Japan because of transport of these components from the Asian Continent by prevailing northwesterly winds. Monthly wind rose data for the past 10 years show that the prevailing wind direction in winter at Okinawa (Naha) is north or north–northeast. Therefore, back-trajectory analysis was performed with HYSPLIT4 (Draxler and Rolph, 2003; Rolph, 2003). For each sampling interval, three back-trajectories of air masses arriving at Okinawa were computed for start time at beginning, middle and end of sampling interval. Trajectories were classified into the following four zones:

  Fig. 6. Contribution of each ion to (a) the sum of cations, (b) the sum of anions and (c) total ion concentrations in equivalent values calculated from the VWM of whole samples.

 

a minimum concentration of 17.3 meq L1 (April 2003) to a maximum concentration of 2112 meq L1 (October 2004). The fluctuation range (maximum monthly VWM concentration divided by the minimum monthly VWM concentration) was 122. Concentrations of nss-Ca2+ also peaked in the same seasons, but the peak did not vary widely like the sea salt components. Some meteorological factors such as wind speed, wind direction and rainfall amount affect concentrations of chemical components in precipitation (Andre´ et al., 2007). Generally, heavy precipitation causes low concentrations of components in precipitation (Zhang et al., 2007; Rastogi and Sarin, 2005). Also in this

Zone 1: Oceanic air mass transported from the Pacific Ocean. Zone 2: Air mass transported from Philippines and the South China Sea. Zone 3: Continental air mass transported from Asian Continent. Zone 4: Air mass transported from the mainland of Japan.

Fig. 8 showed the result of the classification of trajectories. Many of the air masses that came to Okinawa in winter passed through the Asian Continent before arriving to Okinawa. Thus,  NO 3 and nss-SO2 are likely transported from the Asian Continent to Okinawa in winter. It is considered that ammonium ion is also transported from the Asian Continent as NH4NO3 and (NH4)2SO4 because NH+ concentration of 4 precipitation correlated with concentrations of NO 3 or nss-SO2 4 in this study, and China is one of the largest ammonia emitters in Asia (Dianwu and Anpu, 1994).

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Fig. 7. Variations of monthly VWM concentrations of chemical components in precipitation from 2003 to 2005.  Concentrations of NH+ 4 and NO3 were high in July 2003, but only one sample was collected in this month. The high concentrations do not reflect

pollutant transport from the Asian Continent because the air mass came from south. Rather, high concentrations are likely the result of local sources

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No. of trajectories

150

No. of trajectories

150

Zone 3 100

Zone 4 100

50

0 Spring Summer Autumn Winter Season

50

0 Spring Summer Autumn Winter Season

Okinawa Is.

No. of trajectories

150

No. of trajectories

150

Zone 2 100

50

Zone 1 100

50

0 Spring Summer Autumn Winter Season

0 Spring Summer Autumn Winter Season

Fig. 8. Classification of back-trajectories of air masses arriving at Okinawa.

such as ammonia from fertilizer and NOx from vehicular emissions. 4.4. Effect of typhoons on precipitation chemistry and annual wet deposition Typhoons occasionally approach the Okinawa Islands in summer and autumn bringing heavy rain to this area. Precipitation samples during two typhoon periods (June and September 2004) were collected at intervals of a few hours to clarify variations in the concentrations of chemical components during typhoon periods. Fig. 9 shows the variations in concentrations of the components and wind speeds observed in September 2004. The

VWM pH values of precipitation during the typhoons were 5.56 for June and 5.69 for September 2004; these values are higher than those observed in other seasons. The maximum pH value was observed as a typhoon approached Okinawa and wind speeds reached a maximum; pH subsequently decreased as wind speeds relaxed as the typhoon moved away from Okinawa. The variations of sea salt components such as Na+ and K+ were similar to pH variation. In contrast, concentrations of  2 NH+ 4 , NO3 , and nss-SO4 were smallest around the time of maximum wind speed and subsequently increased as wind speeds decreased. Although  2 NH+ 4 , NO3 , and nss-SO4 showed low concentrations around the time of maximum wind speed

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Fig. 9. Temporal variations of pH, concentrations of chemical components and wind speed from 5 to 8 September 2004.

during the June typhoon, no clear relationships were found between concentrations of other components and wind speed. Yu et al. (1998) reported that SO2 and NO 4 3 concentrations in typhoon precipitation were lower than during other seasons and that acidity decreased in the southern coastal area of China. Sakihama and Tokuyama (2005)

have suggested that wind speed has a large effect on the chemical composition of rainwater and that the chemical composition changes drastically at a certain wind speed during a typhoon. Maximum wind speed of 14.5 m s1 during the typhoon in June was relatively small compared with that of 27.1 m s1 in September, so the supply of sea salt

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Fig. 11. Frequency distribution of precipitation pH. Fig. 10. Relative contribution of typhoon deposition to annual wet deposition in molarity.

components did not increase in June. Therefore, no clear relationship between ion concentrations and wind speed was found. Typhoons play an important role in the deposition of chemical components. According to Ishijima et al. (2004), typhoon precipitation accounts for 23% of the annual precipitation. In one case, a single typhoon produced one-third of the annual deposition (Tsunogai, 1975). Therefore, annual wet deposition was calculated for each component at Okinawa Island to determine the contribution of wet deposition by typhoons to annual wet deposition. Molarity was used in the calculation to include the contribution of dissolved silica. Wet deposition caused by only typhoons accounted for 77% of the total annual wet deposition. Deposition from each typhoon varied with the strength and course of the typhoon. Wet deposition was large for strong typhoons that hit Okinawa Island directly. The maximum contribution from one typhoon in this study was about 50% of the annual wet deposition. Fig. 10 shows the relative contribution of typhoon deposition to annual wet deposition. Typhoon precipitation was about 34% of the annual precipitation (H2O). About 70–80% of the annual wet depositions of Na+, K+, Ca2+, Mg2+, and Cl were caused by typhoons. In contrast, only 10% of  the annual wet depositions of H+, NH+ 4 , and NO3 2 were caused by typhoons. Wet depositions of SO4 and D-SiO2 by typhoons were at intermediate levels. Concentrations of sea salt components such as Na+ and Mg2+ increase with increasing wind speed in the typhoon, and those of non-sea salt components  such as NH+ 4 and NO3 decrease with increasing

Fig. 12. Variation of monthly volume weighted mean pH for precipitation from 2003 to 2005.

wind speed. Thus, wet depositions of Na+, K+, Ca2+, Mg2+, and Cl increase and those of H+,  NH+ 4 , and NO3 decrease during the typhoon period. Sulfate in precipitation originates from both sea salt and non-sea salt sources. Because concentrations of sea salt component increase and those of non-sea salt components decrease, typhoon deposition of SO2 was intermediate. Dissolved silica in 4 precipitation is mostly supplied by soil particles caught by strong winds, such as those during typhoon periods. Therefore, much D-SiO2, albeit a non-sea salt component, is deposited during a typhoon. 4.5. pH and behaviors of acid and basic components Acid air pollutants such as SO2 and NOx are transported over long range and can cause acid rain (Pongkiatkul and Oanh, 2007). Such air pollutants

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2 Fig. 13. Variation of NO 3 /nss-SO4 ratio in precipitation from 2003 to 2005. Dotted line represents average value of NO 3 /nssSO2 4 ratio.

can therefore cause air pollution both locally and transboundary. Acidification of precipitation is a global problem that has been the focus of many studies (e.g., Sequeira and Peart, 1995; Reynolds et al., 1999; Okuda et al., 2005). Fig. 11 shows the frequency distribution of pH of precipitation in Okinawa Island. The pH ranged from 3.89 to 7.61. The modal class was 4.5–5.0, which accounted for 28% of the distribution. Most of the collected samples (72%) were of acid rain (pHo5.6). Furthermore, 38% of the collected samples had pH values o4.8 that suggests anthropogenic influences (Hu et al., 2003). Okinawa Island experiences frequent acid rain even though the islands are considered to be unpolluted. Fig. 12 shows the variation of monthly VWM pH. The monthly VWM pH ranged from 4.52 to 6.16. The VWM pH in the study period was 4.93, but pH showed a clear seasonal variation, increasing in summer and decreasing in winter. This result is similar to results of studies on precipitation in western Japan (Seto and Hara, 2006). The pH of rainwater is determined by the content of acid components and basic components (Topc- u et al., 2002). Generally, dominant acids are sulfuric and nitric acids in atmospheric chemistry (Seto et al., 2004). Therefore, the indices of acid compo2 nents in precipitation are of NO 3 and nss-SO4 .  Fig. 13 shows temporal variations in the NO3 /nssSO2 4 ratio in equivalent concentration to reveal the contribution of acid components to precipitation 2 acidification. Although NO was some3 /nss-SO4 times large, the ratio was relatively constant. The 2 average value of NO in this study was 3 /nss-SO4

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Fig. 14. Variation of nss-Ca2+/NH+ 4 ratio in precipitation from 2003 to 2005.

Fig. 15.PRelationship P ( acid base).

between

pH

and

the

D-value

0.406. Thus, sulfuric acid was responsible for about 71% of the acidity in precipitation. This percentage was slightly larger than those (60–66%) at rural sites in Japan (Seto et al., 2001). Major base components in precipitation are derived from gaseous ammonia and particulate basic calcium salts where calcium carbonate would be the most prevailing calcium species of interest (Seto et al., 2004). Therefore, the indices of base compo2+ nents in precipitation are of NH+ . 4 and nss-Ca + 2+ Fig. 14 traces variations of the nss-Ca /NH4 ratio in equivalent concentration to highlight the contri-

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Table 2 Principal component analysis for precipitation samples in Okinawa Island Variables

Factor loading PC1

Zone Zone Zone Zone

1 2 3 4

NH+ 4 Na+ K+ ss-Ca2+ nss-Ca2+ Mg2+ Cl NO 3 ss-SO2 4 nss-SO2 4 D-SiO2

PC2

PC3

0.590

0.632 0.952

0.640

PC4

0.540 0.592

0.802 0.969 0.976 0.969 0.693 0.972 0.968 0.838 0.970 0.862 0.549

Eigenvalue Total variance % Cumulative %

6.68 44.5 44.5

3.28 21.8 66.4

Source

Natural Local

Anthropogenic Continental

1.46 9.76 76.1

1.19 7.92 84.0

Fig. 16. Distribution of factor loadings of PC1 and PC2.

bution of base components in neutralizing precipi2 tation acidity. Unlike the NO ratio, the 3 /nss-SO4 + 2+ nss-Ca /NH4 ratio showed a distinct seasonal

variation. The nss-Ca2+/NH+ 4 ratio was large in summer and autumn. Thus, basic calcium salt (mainly CaCO3) is the most important factor of neutralizing acid in precipitation in summer and autumn.  The sum of equivalent concentrations P Pof NO3 2 and nss-SO4 is expressed as P acid ( acid ¼ 2 NO +nss-SO ). Likewise, base indicates the 3 4 sum ofP equivalent concentrations of NH+ 4 and P nss+ 2+ 2+ Ca ( base ¼ NH +nss-Ca ). D-value ( acid 4 P base) was calculated (Kitamura et al., 1991). Fig. 15 shows the relationship between the pH and D. D-values ranged from 156 to 179 meq L1. When the D-value is 0, pH is about 5.5, which is nearly equal to the pH of water equilibrated with atmosphere (pH ¼ 5.6). This result suggests that the pH of precipitation in Okinawa Island is controlled by major acid (sulfuric and nitric acids) and basic (ammonia and calcium salts) components. 4.6. Principal component analysis Principal component analysis (PCA) is a statistical method that is used to deduce the sources of precipitation components (Seto et al., 2000; Balasubramanian et al., 2001; Ozeki et al., 2004). PCA was applied to the result of back-trajectory analysis of air mass and daily ionic concentrations with correlation matrix. Principal components with eigenvalues 41 were extracted, and factor loadings that exceeded 0.5 were considered significant. Table 2 shows PCA results. Four principal components were extracted, and the contribution of PC1 and PC2 were 44.5% and 21.8%, respectively. These two components together explained 66.4% of the total variation of the data. PC1 with high loadings for Na+, K+, ss-Ca2+, Mg2+, Cl, and ss-SO2 4 and moderate for nss-Ca2+ and D-SiO2 is associated with natural sources. PC2 with high loadings  2 for NH+ is associated with 4 , NO3 , and nss-SO4 anthropogenic sources. PC2 also includes Zone 3. These results suggest that those anthropogenic substances are derived from Asian Continent to Okinawa Island by air mass transportation. Fig. 16 shows the distribution of factor loadings of PC1 and PC2. It demonstrates that natural sources can be classified into two categories. Moreover, it is found that the components included in PC1 (Na+, K+, ssCa2+, Mg2+, Cl, and ss-SO2 4 ) are hardly affected by any air masses. This fact suggests that these components are generated from local sources such

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as surrounding ocean and land. Finally, chemical components in the precipitation can be categorized as having three origins: (1) sea salt generated from local surrounding ocean (Na+, K+, ss-Ca2+, Mg2+, Cl, and ss-SO2 4 ), (2) soil generated from local land (nss-Ca2+ and D-SiO2), and (3) anthro pogenic source of Asian Continent (NH+ 4 , NO3 , 2 and nss-SO4 ). 5. Conclusions The chemical composition of precipitation collected from 2003 to 2005 at the University of the Ryukyus in the central part of Okinawa Island, Japan, was studied to clarify the chemical characteristics of precipitation in that area. Results are summarized as follows: 1. The equivalent concentration ratios of Cl/Na+ of the precipitation closely matched the ratio of seawater. Precipitation in Okinawa Island is clearly affected by sea salt throughout the year. 2. The chemical composition of precipitation in Okinawa varies widely throughout the year. On average, Cl ions contribute most to the total ion equivalent concentration, followed in order 2+ + by Na+4Mg2+,SO2 4H+,NH+ 4 4Ca 4 ,K 4   NO3 ,HCO3 . 3. Monthly VWM concentrations of Na+, K+, Ca2+, Mg2+, Cl and ss-SO2 increased from 4 summer to autumn and decreased from winter to spring. It is thought that concentration peaks in summer or autumn are likely due to sea salt supplied by typhoons and wind-blown soil particles taken in precipitation by below-cloud scavenging. In contrast, concentrations of NH+ 4 , + 2 NO 3 (except for July 2004), nss-SO4 , and H decreased in summer and increased in winter. This is caused by air mass transported from Asian Continent. 4. Wet deposition from typhoons accounted for about 77% of the total annual wet deposition in molarity. Typhoons caused approximately 70–80% of the annual wet depositions of Na+, K+, Ca2+, Mg2+, and Cl, but only 10% of the annual wet depositions of H+,  NH+ 4 , and NO3 . Typhoons brought intermediate levels of wet depositions of SO2 and D-SiO2. 4 Thus, typhoons have a great effect on the chemical composition of precipitation in the study area.

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5. The pH of precipitation ranged from 3.89 to 7.61. The modal class was 4.5–5.0, comprising 28% of the total. Acid rain (pHo5.6) was observed in 72% of the collected samples. The relationship between the pH and D-value showed that the pH of precipitation in Okinawa Island is controlled by major acid (sulfuric and nitric acids) and basic (ammonia and calcium salts) components. 6. PCA suggests three main origins of the chemical components of precipitation: (1) sea salt generated from local surrounding ocean (Na+, K+, ssCa2+, Mg2+, Cl, and ss-SO2 4 ), (2) soil generated from local land (nss-Ca2+ and D-SiO2), and (3) anthropogenic source of Asian Continent  2 (NH+ 4 , NO3 , and nss-SO4 ). Acknowledgments The authors appreciate Dr. T. Arakaki’s review of the manuscript and valuable comments. The authors also thank all laboratory colleagues for their cooperation and useful comments. This study was financially supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology (No. 1620 1009 to M.T.). References Agata, S., Kumada, M., Satake, H., 2006. Characteristics of hydrogen and oxygen isotopic compositions and chemistry of precipitation on Ishigaki Island in Okinawa, Japan. Chikyukagaku (Geochemistry) 40, 111–123 (in Japanese with English abstract). Andre´, F., Jonard, M., Ponette, Q., 2007. Influence of meteorological factors and polluting environment on rain chemistry and wet deposition in a rural area near Chimay, Belgium. Atmospheric Environment 41, 1426–1439. Balasubramanian, R., Victor, T., Chun, N., 2001. Chemical and statistical analysis of precipitation in Singapore. Water, Air, and Soil Pollution 130, 451–456. Berner, E.K., Berner, R.A., 1987. The Global Water Cycle Geochemistry and Environment. Prentice-Hall Inc., USA, p. 56. Bini, C., Bresolin, F., 1998. Soil acidification by acid rain in forest ecosystems: a case study in northern Itary. The Science of the Total Environment 222, 1–15. Chung, Y.-S., Kim, H.-S., Park, K.-H., 2001. Acidic precipitation and large-scale transport of air pollutants observed in Korea. Water, Air, and Soil Pollution 130, 367–372. Dianwu, Z., Anpu, W., 1994. Estimation of anthropogenic ammonia emissions in Asia. Atmospheric Environment 28 (4), 689–694. Draxler, R.R., Rolph, G.D., 2003. HYSPLIT (HYbrid SingleParticle Lagrangian Integrated Trajectory) model access via NOAA ARL READY website /http://www.arl.noaa.gov/

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