CHEMICAL COMPOSITION OF PRECIPITATION IN A FOREST AREA

OF CHONGQING, SOUTHWEST CHINA

F U Z H U Z H A N G 1, J INGYANG Z H A N G 1, HONGRUI ZHArNG 1, NORIO OGURA 2 and AKIKUNI USHIKUBO 3

1 Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100080, China; z Tokyo University of Agriculture and Technology, Faculty of Agriculture, Fuchu, Tokyo 183,

Japan; 3 Tokyo University of Agriculture, Setagaya-Ku, Tokyo 156, Japan

(Received 11 October, 1994; accepted 27 March, 1995)

Abstract. Experiments were carried.out in Chongqing- a city seriously damaged by acid precipitation in southwest China - to explore chemical compositions of open bulk precipitation, throughfall and stemflow in a Masson pine (Pinus massoniana) forest. The results showed that annual mean pH values of. and annual ion depositions in the three types of rain water were 4.47 and 50.6 g m -2, 3.82 and 69.7 g m -2, and 2.92 and 0.215 g m -2 respectively, pH values demonstrated an obvious seasonal variation; they were lower in winter than in the rest of the year. Ca 2+ and NH + together made up more than 80% of thetotal cation, while SO]- alone contributed over 90% to the total anion. This high level of SOl- in rain water in Chongqing, which outran those found in other cities in China, was closely related to the combustion of locally produced coal that contains 3 to 5% sulphur. Thus, acid precipitation in Chongqing is of a typical sulphuric-acid type.

Key words: chemical composition, precipitation, Masson pine forest

1. Introduction

Chongqing is the largest industrial city in southwest China, where combustion of coal accounted for more than 75% of the regional energy supplies in recent years. The low grade local coal contains roughly 3 to 5% sulphur, 25% ash and 30% volatile combustibles. Large-scaled consumption of this type of coal, along with unfavorable meteorological and topographical factors, has caused rapid changes in the atmospheric environment of Chongqing and become a major reason for the increase in acidity of precipitation and acid rain frequency. Damages caused by acid precipitation have been observed in forest areas of Chongqing (Feng et al., 1990; Feng and Shan, 1991). In order to investigate the impacts of acid pollution on the compositions of open bulk precipitation (hereinafter referred to as "precipitation"), throughfall and stemflow in a Masson pine (Pinus massoniana) forest, experiments were carried out on mountains in Chongqing, as a part of the cooperative research between China and Japan on acid rain and its impacts on terrestrial ecosystems.

2. Experimental Sites and Methods

The locations of the three experimental sites are shown in Figure 1. Far from urban Chongqing, Jinyun Mountain and Simian Mountain were believed to be "clean" in

Water, Air, and Soil Pollution 90: 407-415, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.

4 0 8 FUZHU ZHANG ET AL.

Ymyun Mountain

The Central District of Chongqing Zhuwu Zhuwu i/ N

't J

Legend

border of Chongqing Simian

2 " " " ' ~ , Mountain general

• x . \ direction

~ ? / 11.5 krn,

Fig. 1. Locations of the experimental sites.

the sense of air pollution. So precipitation was collected on these two mountains to be compared with that gathered near the industrial areas of Chongqing. The main part of the research was undertaken on Zhenwu Mountain, which is covered by a Masson pine forest. Throughfall and stemflow were collected under canopies and from stems of trees of the forest respectively on both windward and leeward slopes of the mountain, while precipitation was collected outside the forest.

Rain water of every 10-day period was taken as a sample. Ions analyzed and methods used are as follows. SO ]- , NO 3 and C1-: ion chromatogrphy; Na +, K +, Ca 2+ and Mg2+: ion emission spectrometry; and NH+: colorimetry. Rain water pH was measured with a pH-meter (PHL-20).

CHEMICAL COMPOSITION OF PRECIPITATION IN CHONGQING 409

TABLE I

Monthly average pH values of rain water on Zhenwu Mountain, Chongqing, 1991-1992

Months June July Aug. Sept. Oct. Nov. Dec. Jan. Feb. Mar. April May

Precipitation 4.30 4.09 4.80 5.80 5.20 4 .41 4.45 2.61 4.13 3.93 4.70 5.25

Throughfall 4.10 3.53 4.75 4.63 4.05 3.35 3.05 - 3.35 3.56 3.60 4.15

Stemflow 3.20 2.93 3.30 3.07 3.15 2.98 2.51 - 2.30 2.83 2.80 3.00

- = Not determined.

3. Results and Discussions

3.1. VARIATION IN RAIN WATER pH

pH values of rain water collected and analyzed throughout the year are shown in Table I. Precipitation of each month, except September 1991, whose pH was 5.8, had a high acid content. It can be observed that pH values of the throughfall and the stemflow were much lower than those of the precipitation, pH values were found to have an obvious seasonal variation - - in winter (from November 1991 through March 1992) they were significantly lower than in the rest of the year.

This seasonal variation of rain water pH closely resembled that of air pollution, i.e. in winter when there was more SO2 pollution [averaging 220 #g/m 3 (Shen et al., 1993)] the rain water pH decreased, while in other months [SQ ranging from 110 #g/m 3 to 200 #g/m 3 (Shen et al., 1993)], vice versa.

3.2. CHEMICAL COMPOSITIONS OF RAIN WATER

Annual average ion concentrations in rain water are given in Table II, from which it can be seen that SO]- had the highest concentrations of all the measured anions. Its annual average concentrations in precipitation, throughfall and stemflow were 469.1 #eq/L, 1008.9 #eq/L and 4083.8 #eq/L respectively. The second highest anion, NO~-, had concentrations of 45.0 #eq/L, 69.8 #eq/L and 192.6 #eq/L respectively. Concentrations of C1- were 27.6 #eq/L, 64.6 #eq/L and 115.9 #eq/L respectively. Proportions of SOl- , NO~- and C1- was 10:0.7:0.5. SO]- was therefore the dom- inant anion, contributing over 90% to the acidity of rain water. This indicated that acid rain pollution in Chongqing is of a sulphuric-acid type, which has a close rela- tionship with the fact that coal is the major energy resource there. Concentrations of the measured cations in rain water came in the following order, from highest to lowest: Ca 2+, NH +, K +, Mg 2+ and Na +. Ca 2+ and NH4 + were the main cations, making up over 80% of all those measured.

It should be pointed out that, despite the low rain water pH and high acid rain frequency, rain water contained high levels of Ca 2+, NH + and K +, and the total cations were found to be higher than the total anions except in stemflow. Possible

410 FUZHU ZHANG ET AL.

TABLE II Annual average ion concentrations (#eq/L) in rain water on Zhenwu Mountain (1991-1992)

Ions C1- SOl- NO~- H + K + Na +

Precipitation 27.64 469.08 45.00 33.63 82.59 23.92 Throughfall 64.58 1008.94 69.84 150.20 170.81 34.80 Stemlow 115.90 4083.84 192.59 1192.49 272.58 94.83

Ions Ca 2+ Mg 2+ NH + ~3+ ~_ (~2+)/(~_)

Precipitation 417.66 40.31 105.66 703.78 541.72 1.299 Throughfall 629.24 88.84 258.43 1332.32 1143.36 1.165 Stemlow 1505.46 211.41 1072.28 4349.05 4392.33 0.990

explanations to this unexpected phenomenon are that some anions such as F - , Br- , PO 3, HCOO- and CH3OO- were not analyzed and that pollution from nearby small concrete and fertilizer plants might be responsible. Whether these hypotheses are correct, or whether there exist some other causes requires further study.

3.3. SEASONAL VARIATION IN CHEMICAL COMPOSITIONS OF RAIN WATER

Monthly average ion concentrations in rain water on Zhenwu Mountain are shown in Table III. The highest monthly anion concentration was that of SO ] - , followed by those of NO 3 and C1-. The highest cation concentrations were those of Ca 2+

and NH + . Corresponding to acidity, SO ] - , Ca 2+ and NH + were higher in autumn and winter than in spring and summer. Figure 2 shows their seasonal variations.

3.4. COMPARISON OF CHEMICAL COMPOSITIONS OF RAIN WATER ON ZHENWU

MOUNTAIN AND OTHER MOUNTAINS IN CHONGQING

Results of this part of the study are given in Table IV. Simian Mountain and Jinyun Mountain have no significant local pollution source and therefore are supposed to be clean areas as far as air pollution was concerned. However, seen from the analytical results, rain water of these areas had fairly high SO ] - concentrations,

especially in Jinyun Mountain, where SO24 - was found to be as high as 261.6/zeq/L. It is obvious that these areas have also been affected by human activities. The facts that rain water pH values were under 4.8 and that SO 2- was the main anion clearly indicate the effects of sulphuric acid pollution in both urban and relatively clean areas of Chongqing.

CHEMICAL COMPOSITION OF PRECIPITATION IN CHONGQING 41 ]

p.eqlL

1200

1000

800

600

400

200

0

Precipitation ,A~

m

summer auttann winter spring

i~eqlL

2500,

2000

1500

1000

500

0

Throughfall , ~

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10000 W /~ Stemflo

9000

8000

7000

6000

peqlL 300040005000 ~ ~

ooo

lOOO -- ~11

o summer autumn winter spring

S04 "--B'- NH4 ~Jr-- C.,a

Fig. 2. Seasonal average ion concentrations in rmn water on Zhenwu Mountain.

412 FUZHU ZHANG ET AL.

TABLE III

Monthly ion concentrations (mg/L) in precipitation, throughfall and stemflow Zhenwu Mountain (1991-1992)

in the forest on

Months C1- SO]- NO 3 K + Na + Ca 2+ Mg z+ NH +

June'91 Precipitation 0.60 16.05 1.91 2.29 0.23 2.54 0.17 2.01 Throughfall 0.85 22.69 2.63 3.04 0.21 3.47 0.29 3.79

Stemflow 2.04 116.30 5.96 5.23 0.25 10 .87 0.60 17.90

July '91 Precipitation 0.56 10.84 1.25 2.70 0.22 2.79 0.15 -1.40 Throughfall 1.89 39.14 2.30 5.49 0.78 7.87 0.85 3.40

Stemflow 3.52 170.08 4.56 8.19 0.38 25.20 2.13 16.90

Aug. '91 Precipitation 0.48 8.76 1.73 4.15 0.14 4.46 0.23 1.76 Throughfall 1.74 27.74 1.50 9.71 0.52 9.87 1.11 2.52

Stemflow 3.50 269.19 26.15 14.09 0.39 30.53 3.25 16.36

Sept. '91 Precipitation 0.97 30.63 2.65 2.67 0.20 6.15 0.40 2.25 Throughfall 2.47 50.06 3.33 9.67 0.37 12 .05 1.26 6.33

Stemflow 4.36 158.14 8.14 11.75 0.36 24.85 2.27 21.61

Oct. '91 Precipitation 0.63 16.12 2.14 0.97 0.24 6.61 0.33 1.56 Throughfall 1.76 39.70 2.02 2.98 0.44 7.59 0.50 2.65

Stemflow 4.02 106.10 7.85 6.95 0.48 22.54 2.04 18.33

Nov. '91 Precipitation 2.20 44.84 4.76 1.36 2.83 11 .15 0.88 0.96 Throughfall 13 .66 244.88 19.33 20.57 6.62 70.66 5.95 12.87

Stemflow 8.72 287.01 20.24 27.34 6.46 133.58 8.02 30.80

Dec. '91 Precipitation 2.34 57.22 7.05 8.30 3.98 1 9 . 7 9 1.01 1.84 Throughfall 3.99 110.59 8.87 12.74 3 . 8 5 30.00 2.51 9.63

Stemflow 6.61 589.72 25.36 21.75 3.22 130.99 7,20 33.80

Jan. '92 Precipitation 2.61 61.19 7.41 1.71 0.85 16 .40 0,87 3.88

Throughfall . . . . . . . . Stemflow . . . . . . . .

Feb. '92 Precipitation 1.43 41.00 3.54 0.92 0.43 13 .45 0.55 2.82 Throughfall 5.55 109.94 10.02 5.33 1 . 8 7 23.79 1.66 4.68 Stemflow 4.79 393.79 19.82 15.38 1 . 6 9 17 .66 4.68 18.90

Mar. '92 Precipitation 1.12 21.62 4.45 0.69 0.48 17 .61 0.82 2.25 Throughfall 3.33 41.93 9.61 3.92 0.80 13 .21 1.19 5.21 Stemflow 7.50 287.70 27.60 13.00 1 . 6 0 35.30 3.90 21.60

Apr. '92 Precipitation 1.47 29.30 4.01 6.49 0.54 13 .17 0.71 2.69 Throughfall 1.01 41.07 4.93 9.91 0.52 11.42 1.13 3.48 Stemflow 1.75 88.63 7.42 10.50 0.69 12 .69 1.48 7,19

May '92 Precipitation 0.72 12.76 1.73 4.78 0.23 7.88 0.66 1.47 Throughfall 0.97 27.55 2.46 6.36 0.20 14 .43 0.68 2.79 Stemflow 1.52 71.70 5.35 9.76 0.29 14 .01 1 . 5 7 18.20

- = Not determined.

CHEMICAL COMPOSITION OF PRECIPITATION IN CHONGQING 413

TABLE IV

pH values and chemical compositions (#eq/L) of rain water on different mountains of Chongqing (1991-1992)

Mountains pH SO]- NO 3 C1- K + Na + Ca 2+ Mg 2+ NH +

ZhenwuMountain 4.47 469 .1 45.0 27.6 82.6 24.0 417.7 40.3 • 105.7

Jinyun Mountain 4.80 261.6 35.0 6.5 - 6.5 90.0 13.5 26.0

Simian Mountain 4.33 118 .9 16.0 14.2 6.3 7.7 29.6 2.4 73.6

- = Not determined.

TABLE V

Chemical compositions (#eq/L) of rain water in several major cities of China

Cities years C1- SO]- NO 3 K + Na + Ca 2+ Mg 2+ NH +

Guangzhou 1985 66.4 205.3 26.8 24.5 60.8 154.4 25.8 87.5

Beijing 1981 157.4 273 .1 50.2 40.2 140 .9 184.0 - 141.1

Tianjin 1981 183 .1 317.7 29.2 59.2 175 .2 287.0 - 125.6

Guiyang 1981 8.2 411.0 21.0 26.4 10.1 231.2 56.6 78.9

Mianyang 1985 11.0 86.0 10.0 9.9 18.0 62.0 21.0 46.0

- = Not determined.

3.5. COMPARISON OF THE SITUATIONS OF CHONGQING AND SEVERAL OTHER

CITIES

Table V shows the chemical compositions of acid rain and nonacid rain in several typical coastal and inland cities in China (Zhao and Sun, 1986; Quan and Li, 1988; Chen et al., 1989). SO ]- , NO3, Ca 2+ and NH + concentrations in rain water were higher on Zhenwu Mountain than in the other cities. This may have resulted from the climate and topography of Ch6ngqing, which is located on small mountains in the southeast of the Sichuan Basin. The subtropical monsoon climate of Chongqing is symbolized by warm winters and hot summers, abundant but unevenly distributed rainfall (more in summers than in winters), high humidity and high frequency of foggy days. Factories of various sizes are built in the valleys along the Yangtze River and the Jialing River, and on the hillsides facing these valleys are the residential areas of Chongqing. The mountainous terrain retards convection of air, causing pollutants such as SO ]- to accumulate in high concentrations at the bottom of the valleys.

Table V also shows that in all the examined cities, both coastal and inland, the most concentrated ions in rain water were SO]-, Ca 2+ and NH +. In most of these cities, total cation concentrations were higher than the total anion concentrations.

414 FUZHU ZHANG ET AL.

200

180

160

E E 14o

~ 120

• ~ 100

_~ 0o e ~

4O

20

r ' - ~ l

I O

June July Aug. Sept.

Fig. 3.

I " - - '1 precipitation

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@ stemf low

I !

, , I F ' ]

Oct. Nov. Dec. Jan.

,-n t l

I I

I i

: , . : '

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Monthly rainfalls on Zhenwu Mountain.

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TABLE VI

Annual ion depositions in precipitation, throughfall and stemflow (mg.m -2) on Zhenwu Mountain

Ions Precipitation Throughfall Stemflow

C1- 1222.40 1969.13 3.16 SO~- 27942.51 41764.20 152.89 NO~- 3454.49 3729.15 9.33 K + 3990.19 5756.50 8.74 Na + 683.61 713.07 0.69 Ca 2+ 10376.09 10867.01 23.54 Mg 2+ 604.91 924.65 2.00 NH + 2369.86 4015.60 15.04 ~+ 18024.66 22276.83 50.01 ~_ 32619.40 47462.48 165.38 (~+) + (~_) 50644.06 69739.31 215.39 (~,+) % 35.59 31.94 23.22 ( ~ - ) % 64.41 68.06 76.78

This bears a c lose relat ion with the high level o f SO 2 - and dust pollut ion, and is one o f the na t ionwide trends o f precipi tat ion in China.

CHEMICAL COMPOSITION OF PRECIPITATION IN CHONGQING 415

3.6. DEPOSITION OF IONS IN RAIN WATER ON ZHENWU MOUNTAIN

Monthly rainfalls, including precipitation, throughfall and stemflow, are shown in Figure 3, from which it can be seen that rainfall mainly occurred in spring and summer. There was much less rain in fall and winter, especially in January 1992, when the precipitation was only 10.23 mm and the throughfall and the stemflow were too little to collect. Ion concentration was significantly affected by the amount of rainfall, and their product is defined as the ion deposition (See Table VI). Annual ion deposition in precipitation was 50644.06 mg m -2, 35.59% of which was made up by cation deposition and 64.41% by anion deposition; annual ion deposition in throughfall was 69739.31 mg m -2, 31.94% of which was made up by cation deposition and 68.06% by anion deposition; and annual ion deposition in stemflow was 215.39 mg m -2, 23.22% of which was made up by cation deposition and 76.78% by anion deposition. Anions, SO ] - in particular, were obviously the main contributors to ion depositions.

Acknowledgments

The study was financially supported by the Grant-in-Aid for Scientific Research (Creative Fundamental Research) of the Ministry of Education, Science and Culture of Japan. We are very grateful to the officials of the Forestry Bureau of Chongqing Municipality, especially Mr. Liangqing Ma, Mr. Shicai Du and Mr. Shuguang Peng, who rendered us with great help in maintaining the experimental sites and collecting the samples.

References

Chen, P., Wang, Y., Wan, X., Liu, G., Liu, Z., Ye, Q. and Zhang, J.: 1989, in Papers on Acid Rain, • compiled by the AssociatiOn of Environmental Sciences of China, pp. 250-264.

Feng, Z., Chen, C., Zhao, J., Wang, K., Zhang, J. and Deng, S.: 1986, Atmospheric Environment and Acid Rain 2, 3, 38-45.

Feng, Z. and Shan, Y.: 1991, J. Environ. Sci. (China) 3, 2, 61-68. Quan, W. and Li, Y.: 1988, inAcidRain and Agriculture, pp. 55-61. Shen, J., Zhao, Q., Ji, G., Tang, H., Feng, Z., Okita, T. and Ogura, N.: 1993, Proc. China-Japan Joint

Symposium on Impacts and Control Strategies of Acid Deposition on Terrestrial Ecosystems, Nov. 1-4, 1992, BeOing, China (edited by Feng, Z. and Ogura, N.), pp. 34-40.

Zhao, D. and Sun, B.: 1986, Ambio 15, 1, 2-5.