Research 2-Year Study of Chemical Composition of Bulk Deposition in a South China Coastal City: Comparison with East Asian Cities K. M. WAI, P. A. TANNER,* AND C. W. F. TAM Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong S.A.R., P.R. China
Using the emission strengths of the precursor gases, the nature of soil in China, the ventilation power and half value rainout region length, the nss-SO42-, NO3-, Ca2+, and NH4+ concentrations, and pH of rainwater at Hong Kong and other cities of China and Japan are compared and rationalized. The chemical composition of Hong Kong bulk deposition from 1998 to 2000 is taken from our collection and analysis of 156 daily samples. The volume-weighted average (VWA) pH is 4.2 over the whole study period. Nonsea salt- (nss-) sulfate is the most abundant species in the samples, and the pH mostly depended upon the concentrations of the major species nss-SO42-, NO3-, Ca2+, and NH4+. All species concentrations show higher levels in the cold season (especially NO3- and Ca2+), which indicates the dominant dilution effects in the warm season due to heavy rainfall and the influence of the continental outflow of pollutants during the cold season. For Hong Kong bulk deposition, the VWA pH is slightly lower in the cold season, and there is a slight decrease in VWA pH over the period from 1994 to 2000. The impact of acid rain in Hong Kong is briefly discussed.
the acid rain problem and emissions reduction. Phase I was completed in late 1995 (ref 3 and the related studies in the same issue). Approximately 90% of the monitoring stations which found a mean pH of less than 5.6 for bulk deposition are located south of the Yangtze River (located in mid-east of China) (4). The pH based upon the annual volumeweighted average H+ concentration (annual VWA pH) of bulk deposition was 4.1 and 4.2 at Chongqing and Guiyang, respectively. During the northeast monsoon season in Taipei (Taiwan), a high ratio of nonsea salt (nss)-sulfate to nitrate was observed, and this was attributed to long-range transport (5). By contrast, when the Pacific high dominated the region, this ratio was substantially lower and due to the contributions from local sources. In Japan, the VWA pH was 4.8, obtained from a nationwide survey of 29 stations from 1989 to 1993 (6). Analyses of daily rain samples at nine stations in South Korea from 1996 to 1998 indicated that the nss-sulfate and nitrate in the samples were of similar concentrations (but calcium and magnesium were several times higher) just as in Northeastern America and Central Europe (7). The mean pH in South Korea was 4.7. No distinct spatial variations in ionic composition were observed, but seasonal variations were evident. During a year-long (1994-1995) daily monitoring program in Hong Kong, the acidity of rainwater was found to be dependent upon the type of weather system (8). Further daily monitoring programs included the assessment of metal ion bulk deposition fluxes in Hong Kong (9) and the contribution of organic acids to the acidity (10). In this paper, the rain chemistry of a 2-year data set with daily bulk deposition collection and a comprehensive quality assurance program is presented and analyzed. The ammonium ion was not determined previously (2, 8) but is included herein since it plays an important role in the neutralization of acidic species. The comparison is made with available data sets from other cities in Asia during the same sampling period, and the factors governing the differences of the species concentrations among these cities are discussed. Finally, the impact of acid rain in Hong Kong is briefly assessed.
Methodology Introduction Acid deposition is a global environmental issue (1) because of its trans-boundary impacts of the geobiochemical cycles. The increasing SO2 and NOx emission trends during the past two or three decades, associated with the increasing energy use and economy/population growth intensify the acid deposition problem in Asia. Therefore, knowledge of current status (including the acidity) of atmospheric deposition in Asia through monitoring is essential. Bulk deposition is defined as rain samples which include dry deposition deposited before, after, and during rainfall. No significant differences, as probed by t-tests, could be detected in major soluble species concentrations in bulk or wet deposition when sampled on a daily basis in Hong Kong (2). The composition of wet deposition has been studied extensively in Europe and North America but not in Asia where relatively few studies (especially from China) can be found. In particular, quality assurance protocols have not been strictly enforced in early studies. RAINS-Asia, funded by the World Bank, was an Asian scale project which studied * Corresponding author phone: (852) 27887840; fax: (852) 27887406; e-mail:
[email protected]. 6542
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The sampling procedures have been detailed in refs 2 and 8 and are only described briefly here. The sampling site is located at the rooftop (∼50 m elevation) of City University of Hong Kong, situated in a residential area of Kowloon (22°21′ N, 114°10′ E) with no major point sources nearby. Daily bulk deposition samples were collected from October 1998 to October 2000, using two or three polythene bottles (as replicates or triplicates) with 19.5 cm (internal diameter) polythene funnels. All sampling equipment was washed by detergent, soaked in a 10% HNO3 acid bath for 24 h, and finally washed with water and then with double-distilled deionized water before drying and usage. Sampling equipment was replaced after a 24 h period. AR grade chemicals were used throughout. Bulk deposition samples were filtered by suction through a prewashed 0.45 µm pore diameter nylon-66 membrane. The filtrates were divided into portions for the analysis of cations (after acidification by 0.2% HNO3) and anions. Hourly rainfall amounts were measured by Casella CEL Tipping Bucket rain gauge. The procedures for analytical measurements have been described previously (11), and the quality assurance procedures included the routine running of blanks and control 10.1021/es048897d CCC: $30.25
2005 American Chemical Society Published on Web 07/27/2005
TABLE 1. Species Concentrations Measured in Hong Kong Bulk Deposition Collected on a Daily Basis species NH4
K+
Na+
Ca2+
Mg2+
VWA mean (minmax)
22.0 22.5 (1.867.0)
4.2 5.0 (0.717.2)
36.9 29.6 (6.597.7)
16.2 20.0 (2.760.5)
7.0 6.0 (1.219.4)
VWA mean (minmax)
24.9 25.9 (2.767.0)
7.5 8.0 (1.617.2)
33.4 32.5 (11.497.7)
31.6 29.8 (7.360.5)
7.1 6.8 (2.419.4)
VWA mean (minmax)
21.3 19.5 (1.860.9)
3.8 3.1 (0.712.9)
32.5 27.4 (6.588.0)
15.2 14.9 (2.748.9)
6.2 5.4 (1.216.4)
2.2
31.8
15.3
7.8
+
H+
Cl-
NO3-
nss-SO42-
Fo-
Ac-
Na
42.4 34.7 (6.6100.7)
27.6 34.2 (4.8125.0
70.0 66.3 (13.2206.4)
3.1 3.5 (0.422.4)
1.3 1.8 (0.011.5)
156 156 156
39.9 40.5 (14.494.4)
52.1 58.5 (8.3125.0)
72.7 79.8 (22.4163.0)
6.1 5.2 (1.022.4)
2.9 2.5 (0.011.0)
56 56 56
36.7 30.6 (6.6100.7)
22.7 20.3 (4.896.2)
61.9 57.6 (13.2206.4)
2.8 2.7 (0.414.0)
1.0 1.0 (0.011.1)
88 88 88
18.9
48.6d
32
13.2
43.1d
60
Oct 1998-Oct 2000 63.2 61.4 (3.5290.7)
Cold Season 79.1 88.6 (7.5173.1)
Warm Season 52.0 45.8 (3.5290.7)
VWA (1996-1997)b 26.3
VWA 1.9 a
24.8 b
8.9 c
5.7
37.6
(1994-1995)c
23.9
27.8
d
N number of samples. Reference 9. Reference 2. Total sulfate.
samples as well as replicate samples. Briefly, the measurement of pH utilized a Ross combination pH semimicro electrode, with ionic strength adjustor added to triplicate portions of the samples. Soluble anions and cations were determined by capillary electrophoresis. Low and high level Simulated Rain Certified Reference Materials (High Purity Standards) were measured routinely. Ionic balance, defined as the ratio of the equivalent concentrations of total anions to total cations was in the range from 0.77 to 1.21 for all of the samples. However, an even higher (1.00 ( 0.15) standard was adopted herein, so that 156 samples (i.e. 95% of the raw data set) were analyzed.
Results and Discussion Precipitation Chemistry. Table 1 shows the VWA, mean, maximum, and minimum chemical species concentrations of the bulk deposition for the entire sampling period and for the cold (October to March) and warm (May to September) seasons. The mean annual rainfall amount in Hong Kong is 2214 mm, with 64% (13%) falling in the warm (cold) seasons as defined herein (12). The VWA pH value over the whole period is 4.2. The ions SO42- and H+ [VWA (1998-2000) concentrations: 70 and 63.2 µequiv/L, respectively] are the most abundant anion and cation. We have previously given a detailed discussion of the organic acids in bulk deposition, from a parallel yearlong sampling program between 1999 and 2000 (10) (when methanoic and ethanoic acids were present at VWA concentrations of 6.1 and 4.5 µequiv/L, respectively) so these are not discussed further, although the concentrations from the present program are included in Table 1. Na+ and Cl- (and part of Mg2+) mainly come from direct injection of the sea-salt aerosol from the ocean surface into the atmosphere. Figure 1(a) shows the plot of Cl- concentration against Na+ concentration with slope 1.17 (R ) 0.98, p < 0.01, N ) 156), which is virtually identical with the ionic equivalent ratio of sea salt (e.g., refs 13 and 14). Chloride depletion from the reaction of sea salt with mineral acids in particulate matter releases HCl, but this is readily taken up by alkaline species, or subsequently by rain droplets, resulting in minimal loss of Cl in bulk deposition samples. Figure 1(b) shows the relationship of H+ with the concentrations of nss-SO42-, NO3-, Ca2+, and NH4+. The high correlation coefficient (R ) 0.96, p < 0.01, N ) 156) and slope
(∼1) indicate that these four species are the most important species which affect the pH of the bulk deposition. The relationship also indicates that the H+ concentration (or pH) depends not only solely on the absolute concentration of acidic species (or the emission strength of the precursor gases, SO2 and NOx in our case) but also upon other species if significant Ca2+ and NH3 sources exist. Although a detailed discussion about the relationship between the weather system and the species concentrations in bulk deposition is out of the scope of this paper, it should be pointed out that the origins of air masses are very different from cold and warm seasons, which are in general are governed by the East Asian monsoon system. The cold and dry winter monsoon outflows a significant amount of natural and anthropogenic particulate and gaseous species from the continent to Hong Kong, but the summer monsoon brings a relatively clean marine air mass with an increasing amount and frequency of rainfall. In view of the marked diverse seasons, the cold and warm season species concentrations are calculated and tabulated (Table 1). With the exception of sea-salt species at this coastal location, the remaining concentrations in the cold season are higher than those in the warm season, indicating the dominant factors of the dilution effect in warm summer monsoon season and the continental outflow in the cold season. In particular, the high concentrations in bulk deposition of the crustal species Ca2+ and the anthropogenic species NO3- during the cold season are indicative examples of the continental outflow. This finding agrees with those of the aircraft aerosol study (15), which concluded that the boundary layer and lower troposphere of “near Asia” (including Hong Kong) were most strongly influenced by continental outflow and the largest enhancements were seen in calcium and nitrate species in aerosols. In our data set, the concentration of K+ is low and can come from both dust (especially in the cold season) and biomass/biofuel combustion. The VWA pH is slightly lower in the cold (4.1) than in the warm (4.3) season. Comparison with Other East Asian Cities. Here, only the most important species SO42-, NO3-, NH4+, and Ca2+ which affect the acidity of the bulk deposition, as mentioned above, are considered. We focus on the comparison of the concentrations of these species measured at other cities in China as well as in Japan. Monthly VWA data of these species are obtained from the annual data reports in 1999-2000 of the VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Scatter plots of (a) Cl- against Na+ and (b) H+ against SO42- + NO3- - NH4+ - Ca2+, for daily bulk deposition samples collected at City University.
FIGURE 2. Locations of EANET monitoring sites and provinces/cities in China, Korea and Japan mentioned in Table 2 (1. Shanxi; 2. Shandong; 3. Hebei-Henan-Anhui; 4. Jiangsu; 5. Zhejiang; 6. Fujian; 7. Guangdong-Hainan; 8. Sichuan). monitoring activities by Acid Deposition Monitoring Network in East Asia (EANET) (http://www.adorc.gr.jp/index.html). All monitoring stations are located in an ‘urban area’, as defined by EANET activities. These monitoring sites are as follows: Jiancezhan (in Zhuhai, Guangdong Province); Hongwen (in Xiamen, Fujing Province); Shishan (in Xi’an, Shaanxi Province); Guanyinqiao (in Chongqing); and Banryu (in Japan) (Figure 2). For easy reference, the names of cities/ countries instead of the exact site names are used in the following discussion. The EANET program also utilizes a daily sampling protocol except for the weekly sampling program in Japan. 6544
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To understand the variation of the species concentrations a knowledge of the emission strengths of the precursors SO2, NOx, and NH3 over China is useful since China is the major pollutant emitter in Asia and contributes 59.5% SO2, 42.5% NOx, and 49.5% NH3 of total emissions in Asia (16). Table 2 shows the emission inventory of SO2, NOx, and NH3 for all major pollutant emitting provinces/cities in China and in Hong Kong for 1995 and 2000. The direct comparison of these figures can result in bias since each province/city has a different size (Table 2) and a different pollutant source distribution. Xi’an city is assumed to contribute the most emissions in Shaanxi Province because of the dominance of
TABLE 2. Emissions Strengths (kt/yr) at Selected Province/ Cities in Chinaa Province/Cityb Shanxi Shandong Hebei-Henan-Anhui Jiangsu Shanghai Zhejiang Fujian Chongqing Xi’anc Guangdong-Hainan Hong Kong
SO2 NOx area of province NH3 (km2) 1995 2000 1995 2000 2000 150 000 150 000 489 000 100 000 100 000 120 000
214 000
688 1168 3450 2241 544 617 337 1088 971 821 119
1481 471 558 208 1967 545 812 1093 3002 1835 1567 2604 1191 928 693 1007 518 352 383 77 542 306 440 418 192 167 167 334 600 130 64 100 883 439 258 304 880 413 728 790 66 157 102 5
a
The emission inventory in 1995 is obtained from ref 17 except for Hong Kong, from http://www.epd.gov.hk/air. All data in 2000 are obtained from (http://www.cgrer.uiowa.edu/EMISSION_DATA/ index_16.htm#). b Refer to Figure 2. c Xi’an City is assumed to contribute the most emissions in the Shaanxi Province because of the dominance of heavy industrial areas therein.
heavy industrial areas therein. Most of the described cities in China (except Hong Kong) are located in the northeastern part (30°-40° N, 110°-120° E) of China as well as in some areas of Sichuan Province (including Chongqing City) and of Guangdong Province. Their emissions (except those in Southwestern China) have the potential to impact upon the bulk deposition of Hong Kong, especially during the winter months when the long-range transport of pollutants occurs due to the continental outflow. This argument is supported by our 5-day back-trajectory calculations using the HYSPLIT4 model. The areas of the provinces have been shown in Table 2 in order to calculate the emission density (kt/yr-km2), which is an essential parameter for estimation of long-range transport. Xuan and Sokolik (18) stated that northern China is one of the world’s largest sources of atmospheric dust. The sand lands in northeastern China are a potential source for Asian dust (19), with the deserts in northern China including the Gobi, Taklimakan, Gurbantunggut, Badain Juran, Tengger, and Mu Us Deserts, in addition to the loess areas. Dust storm activities peak in spring, associated with the Siberian High and the prevailing westerly flow, but they are still detectable during summer in sandy deserts and the northern Loess Plateau (21 and references herein). Zhang (19) found that Ca2+ is one of the major chemical species in airborne particulate samples collected over the China Dust Storm Research observational network stations. Atmospheric transport/dispersion of chemical species is also important in determining the composition of bulk deposition at different geographical locations. Xu and Zhu (21) rewrote the continuity equation for a steady-state box model (22) as
∆xQa + uH(Cb - C) - ∆x(ud + uw + H/Tc)C ) 0 (1) where ∆x is the along-wind width (m); Qa is the emission strength (mg/m2s); u is the average horizontal wind speed (m/s); H is the mixing height (m); Cb is the background concentration (mg/m3); C is the long-term average concentration within the box (mg/m3); ud is the dry deposition velocity (m/s); uw is the wet deposition velocity (m/s); and Tc is the time constant for chemical decay of a pollutant (s). Based on rawinsonde and routine meteorological data, Xu and Zhu calculated the annual ventilation capacity (uH) and the Half Value Rainout Region Length [HRRL (in km) ) 53 uH/R is a measure of the downwind length that the pollutant can be transported without depletion by wet removal, where R is the rainfall rate (m/s)] over China. Table 3 summarizes the values of ventilation capacity and HRRL
TABLE 3. Values of Ventilation Capacity (m2/s) and HRRL (km) at Different Cities in China Hong Kong Zhuhai Xiamen Xi’an Chongqing ventilation capacity HRRL
2000 100
2000 100
2000 100
1000 200
500 50
TABLE 4. Normalized Species Concentrations at Different Asian Cities, Taking the Species Concentrations at Hong Kong during the Warm Season as Unity season
Hong Kong
Zhuhai
cold warm
2.3 1.0
2.5 1.5
cold warm
1.2 1.0
2.8 1.8
cold warm
1.2 1.0
3.5 1.5
cold warm
2.1 1.0
14.3 7.0
Xiamen
Xi’an
Chongqing
Japan
5.6 3.2
5.3 1.4
0.5 0.4
13.6 8.0
12.5 4.0
0.7 0.3
28.5 18.6
16.8 6.0
0.7 0.5
42.7 29.2
25.5 9.2
0.3 0.4
NO31.1 0.9
nss-SO421.2 1.1
NH4+ 3.7 2.9
Ca2+ 2.9 1.7
at different monitoring locations. Chongqing is located in the most poorly ventilated area (Sichuan Basin) because of the surrounding high mountains (>3000 m altitude to the west) and has a short HRRL, with a large amount of rainfall. The ventilation capacity is very high (about 4 times higher than that in Chongqing) in the coastal regions where Xiamen, Zhuhai, and Hong Kong are located. Xi’an has a moderate ventilation capacity and a relatively large HRRL (2 times higher than that in the coastal region of Southern China). Figure 3(a)-(e) show the cold and warm season nss-SO42-, NO3-, NH4+, and Ca2+ concentrations, and pH, at Hong Kong and the other cities in China and Japan from 1998 to 2000, with the normalized species concentrations shown in Table 4. In general, the highest concentrations for all species were always recorded at Xi’an and Chongqing, while the lowest were in Japan. The emission strengths, ventilation capacity, and HRRL mentioned above can more or less explain the highest concentrations at Xi’an and Chongqing. The cold season species concentrations are normally higher than those of the warm season. During the cold season, prior to the rainfall events, the contribution from the long-range transport of pollutants is important for the cities in Southern China (including Hong Kong) and Japan. Under the influence of continental outflow, driven by the dry East Asian winter monsoon, pollutants from northeast China can be transported for long distances with little wet removal at their sources. The levels of NO3- in Hong Kong are comparable to those in Zhuhai and Xiamen during the cold and warm season, respectively, although they are double those of Xiamen during the cold season. This is attributed to the comparable NOx emissions in Hong Kong and in Fujian (Table 2). It is noted that the concentration of NO3- in Chongqing is very high, especially during the cold season, but the emission strength of NOx is the lowest in Table 2. The poor ventilation capacity and short HRRL mentioned above is the major reason for the high levels in bulk deposition. The levels of nss-SO42- in Hong Kong bulk deposition are comparable to those in Xiamen but are 10 times less than those in Xi’an and Chongqing during the cold season. Such a large difference can be explained by the SO2 emissions (Table 2). The second lowest concentrations of NH4+ were measured in Hong Kong (approaching the lowest concenVOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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trations measured in Japan) for both seasons, since agriculture/livestock activities are negligible there. The concentrations measured in Zhuhai and Xiamen were in the middle range, similar to the cases of nss-SO42- and NO3-. The higher nss-SO42- and NH4+ concentrations in the cold season in Zhuhai than those in Hong Kong may be explained by the higher local emissions of SO2 and NH3 in Zhuhai. Concerning Ca2+, the concentrations measured in Hong Kong and Xiamen are comparable and are among those in the lower range. Large amounts of Ca2+ are present in wet deposition at Zhuhai, due to the intensive local development and construction activities. The mean Ca2+ concentration measured in Xi’an is twice (and 20 times) higher than that in Chongqing (and Hong Kong, respectively) during the cold season. Xi’an is located in close proximity to some large deserts/losses areas of Northern China, for which dust storms occur frequently, as mentioned above. In addition, Xi’an contains about twice (and 10 times) higher Ca2+ content in soil than Chongqing (and Hong Kong, respectively) (23). The warm season Ca2+ concentration is 1.5 times higher than that in the cold season at the Japan site (Figure 3e) due to a bias from an exceptionally high value in May 2000 from an Asian dust event. As mentioned above, NH4+ and Ca2+ play important roles in the neutralization of acidic deposition so that the acidity of rainwater in Xi’an (pH ∼ 5.7) and Chongqing (in the cold season, pH ) 5.3) is not low even though high levels of nssSO42- and NO3- are present. On the contrary, the pH values in Table 2 are about the lowest for Hong Kong, and also for Japan, even though the concentrations of the four species NH4+, Ca2+, nss-SO42-, and NO3- are among the lowest there. The pH value in Zhuhai precipitation during the cold season is about that of natural water. Acid Rain Impacts. The notorious acid rain scenario involves source emissions of NOx and/or SO2 which are distant from sensitive receptor areas but which are oxidized and transported to the ground in raindrops: as illustrated for example by the 1950s transport of the pollutant gases from coal burning in Britain to Scandinavia. The receptor in this case has no pollutant gas sources. A second scenario occurs when the pollutant gases and particulate matter undergo below-cloud washout, so that the source and receptor are at the same location. For example, below-cloud scavenging of SO2 from the combustion of high sulfur-content fuels at low elevation stacks leads to high acidification in urban areas of China (24), although long-range transport is also important in southern China (25, 26). As early as 1988, the damage to soils, forests, crops, and construction materials was assessed as injurious for the agricultural regions near Chongqing and Guiyang (27). More recently, severe forest damage has been reported (28). These serious impacts focus upon the agricultural economy of China. In the north of China the high concentrations of alkaline particulate matter together with ammonia in air reduce the precipitation acidity. In 1987, Galloway et al. (26) commented that the pH of wet deposition was 6.2 in Beijing, whereas that in Guiyang City in the south of China was 4.0. The mean production (in tonnes) of vegetables, fresh fruits, nuts, and other field crops in Hong Kong from 1998 to 2000 was 54 000, decreasing by about 7000 tonnes each year (29). The only other important crops were flowers, with a cash value of about US$37 M per year (29). Thus agriculture is a very minor part of the Hong Kong economy. The construction materials of the modern buildings in Hong Kong are more resistant to damage than limestone, for example. Therefore the impacts of acid rain upon the Hong Kong economy are rather minor. The acidity of Hong Kong rain has been shown to be most marked under several synoptic weather systems: approaching cyclone (AC), cold front (CF), and north/northeasterly 6546
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FIGURE 3. VWA NO3-, nss-SO42-, NH4+, and Ca2+ concentrations and pH of wet deposition measured in East Asian cities in cold (white) and warm (black) seasons in 1998-2000. (The values for Hong Kong are from bulk deposition, this study). monsoon (N/NE) (8). The first of these systems represents a stagnant air condition with a subsidence inversion, whereas the last two types typify long-range transport. In all three cases, the levels of air pollutants are elevated prior to the occurrence of rainfall (30). The presence of air pollutants
TABLE 5. pH of Hong Kong Deposition Measured in Daily Sampling Programs at City University sampling period type of deposition 1994-1995a 1994-1995a 1996-1997 1998-2000 a
wet bulk bulk bulk
N 27 60 29 156
VWA pH mean pH 4.2 4.6 4.6 4.2
4.2 4.5 4.3 4.2
range 3.7-5.6 3.7-6.5 3.6-6.4 3.5-5.5
Simultaneously collected. The bulk data set is more extensive.
such as PM10, SO2, and NOx has been shown in many studies to produce respiratory problems and lead to premature deaths (31-34), with a shortening of life expectancy by 18 months in Hong Kong purely from fine particle concentrations (35, 36). Rain scavenging of these pollutants cleanses the air in Hong Kong and reduces the respiratory problems to humans without adverse economic impacts. Table 5 shows that the pH of Hong Kong bulk deposition has slightly decreased from 1994 to 2000. This is also shown by the daily episodes that we have recorded: the pH was
