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Characterization of water-soluble ion species in urban ambient particles
Environment International 28 (2002) 55 – 61 www.elsevier.com/locate/envint
Characterization of water-soluble ion species in urban ambient particles Jim Juimin Lin* Department of Safety, Health, and Environmental Engineering, National Kaohsiung First University of Science and Technology, 1 University Road, Yanchau, Kaohsiung 824, Taiwan, ROC Received 11 July 2001; accepted 7 January 2002
Abstract Concentrations and distributions of water-soluble ion species contained in ambient particles were measured in a coastal urban area, Kaohsiung City, Taiwan. PM10 and PM2.5 samples were collected using a dichotomous sampler from November 1998 to April 1999 and were analyzed for water-soluble ion species with ion chromatography. On the average, ion species measured in this study accounted for 42.2% of the PM2.5 and 35.7% of the PM10. It was found that SO42 , NO3 , and NH4 + dominated the identifiable components within both fine (PM2.5) and coarse (PM2.5 – 10) fractions, and occupied 90.0% and 80.6% of total dissolved ionic concentrations for PM2.5 and PM10. The secondary aerosol formed through the NOx/SO2 gas-to-particle conversion was estimated based on the oxidation ratio of sulfur and nitrogen (SOR and NOR, respectively), i.e., sulfate sulfur/nitrate nitrogen to total sulfur/total nitrogen. The average SOR/NOR values were 0.25/0.07 and 0.29/0.12 for PM2.5 and PM10, respectively. The high SOR and NOR values obtained in this study suggested that there existed a secondary formation of SO42 from SO2 along with NO3 from NOx in the atmosphere. D 2002 Elsevier Science Ltd. All rights reserved. Keywords: Particulate matter; Water-soluble ion species; SOR; NOR; Secondary aerosol
1. Introduction Particulate matter with aerodynamic diameter less than 10 mm (PM10), especially the fine particle fraction (aerodynamic diameter < 2.5 mm, PM2.5), have been found to be associated with ambient air quality problems in urban areas including visibility reduction (Larson and Cass, 1989; Larson et al., 1989) and health problems, such as mortality and asthma (Dockery and Pope, 1994; Schwartz et al., 1996). In the atmosphere of many urban and heavily industrialized areas, particulate matter contains a major proportion of inorganic constituents. As reported by previous studies, soil accounts for the largest identifiable component of the coarse fraction (PM2.5 – 10), whereas SO42 , NO3 , and NH4 + dominate the identifiable components within the fine fraction (PM2.5)(Maln et al., 1994; Brook et al., 1997; Chan et al., 1997; Turnbull and Harrison, 2000). In general, these components account for about 20 – 45% of the PM2.5.
* Tel.: +886-7-601-1000x2312; fax: +886-7-601-1061. E-mail address: [email protected] (J.J. Lin).
Sulfate, nitrate, and ammonium are also the most common components of secondary particles in the atmosphere. These particles are formed in the atmosphere from direct emissions of sulfur dioxides (SO2), oxides of nitrogen (NOx), and ammonia (NH3) gases, respectively. Chemical transformation and equilibrium processes for inorganic secondary aerosols have been extensively studied. Equilibrium between gas and particle species was clearly described using models (Bassett and Seinfeld, 1983, 1984; Wexler and Seinfeld, 1991; Watson et al., 1994). Larson and Cass (1989) and Larson et al. (1988) also provided a scheme for the speciation of ionic material for ambient aerosol. The degree of oxidation of gas species to particle species could be utilized as an index of occurrence as well as level of secondary aerosol formation (Colbeck and Harrison, 1984; Ohta and Okita, 1990; Kaneyasu et al., 1995). Air quality in the Kao-ping area (Kaohsiung City, Kaohsiung County, and Pingdong County) is the worst in Taiwan and has become a serious concern due to high air pollutant emissions in the area. For example, Kao-ping area emitted about 100.221 Gg PM10, 92.794 Gg SOx, 109.841 Gg NOx, and 153.274 Gg NMHC (nonmethane hydrocarbon) in 1997 alone. The Kaohsiung City is the
0160-4120/02/$ – see front matter D 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 0 - 4 1 2 0 ( 0 2 ) 0 0 0 0 4 - 1
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second largest city in Taiwan and highly urbanized and industrialized. The city is densely populated, with approximately inhabitants totaling to 9606 km 2 (with about 1,475,505 inhabitants distributed over 153.6 km2). Currently, there are more than 1,228,429 automobiles and motorcycles together with a total of 1911 factories including 2 fossil power plants, 2 cement mills, 9 sizable steel plants, and 14 large-scale petrochemical plants in Kaohsiung City. Moreover, the city is surrounded by four heavily polluted industrial parks namely Jen-wu, Ta-shei, Ta-fa, and Lin-yuan located in the adjacent Kaohsiung County. As for the current air quality, the ambient particulate matter and other pollutants loads are high (annual average concentration is 84 mg m 3 for PM10, 9.67 ppb for SO2, 24.58 ppb for NOx, and 25.68 ppb for O3 in 1997). To understand the air pollution levels and their impact, water-soluble ion species in these particles were measured and their characteristics compared. In this paper, we have presented the concentrations and distributions of ion species in ambient particles in Kaohsiung City. Airborne particulate samples (PM2.5 and PM10) were collected at six ambient air quality sites distributed over the city during November 1998 and April 1999. In Taiwan, data on fine particle mass concentrations and chemical compositions are limited, particularly for urban areas. The objective of this study was to investigate the chemical composition distributions of the water-soluble inorganic ion species in fine (PM 2 .5 ) and coarse (PM2.5 – 10) particles in the urban atmosphere. The analyses provide information that is useful to air quality as well as health effect studies and to regulatory assessment in terms of the particle issue.
been conditioned at 25 C and 40% relative humidity for 24 h. After collecting from the field, sampling filters were stored in a refrigerator at 4 C before chemical analysis to prevent the loss of semivolatile species, especially ammonium nitrate. 2.2. Sampling program Particle samples were collected at six ambient air quality monitoring stations (Nan-tze, Tso-ying, San-min, Chienchin, Hsiao-kang, and Chien-chen) in Kaohsiung City, Taiwan ROC. These stations are part of the Taiwan Air Quality Monitoring Network, which was established by the Environmental Protection Administration of Taiwan ROC in 1993. These ambient stations are generally situated in populated areas and are intended to provide information pertaining to populations exposure to particles in the area. Specifically, the Chien-chen station was defined as an industrial district air-monitoring station located within or immediately downwind from the industrial districts with significant emission sources. The locations of the six stations are shown in Fig. 1. The observations were carried out from November 1998 to April 1999 in Kaohsiung City. Samples were collected during periods of no rain. Concentrations of ambient particle samples (PM2.5 and PM10) were calculated as 24-h average (approximately 0700– 0700 h next day). Hourly wind speed was recorded and then the average wind speed during each sample period was calculated. Fifty-one samples were
2. Experimental methods 2.1. Ambient particle size distributions and concentrations Ambient particle concentrations were measured with a dichotomous sampler (Graseby Andersen G241). This dichotomous sampler was equipped with an inlet having a 10-mm cut point. The particles that are below the 10-mm aerodynamic diameter (PM10) were divided into two size fractions using a virtual impactor with a 2.5-mm cut point. These two fractions are classified as a coarse fraction (2.5 mm < diameter < 10 mm, PM2.5 – 10) and a fine fraction (diameter < 2.5 mm, PM2.5). The dichotomous sampler was operated at a total flow rate of 16.7 l min 1. Particles were collected using 37-mm filters (Pallflex 2500 QAT-UP, 37 mm) supported by polyolefin rings. Quartz fiber filters were used to minimize the influence of background material from collection filters for carbonaceous analysis that proceeded simultaneously within this study (Lin and Tai, 2001). Before and after field sampling, filters were weighed on an electrical balance (Mettler Toledo AT261) with a reading precision of 10 mg to determine the mass concentration after its having
Fig. 1. Location map of the sampling sites (.) in the Kaohsiung City, Taiwan, and industrial parks (~) in the adjacent Kaohsiung County.
J.J. Lin / Environment International 28 (2002) 55–61
collected from the six sampling sites. Detailed information is shown in Fig. 1 and Table 1.
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3. Results and discussion 3.1. Particle concentrations
2.3. Water-soluble ion species analysis The collected aerosol filters were ultrasonically extracted for 1.5 h into 30 ml of deionized distilled water and filtered through a Teflon filter of 4.5-mm nominal pore size. Ion chromatography (Shimadzu HIC-10ASP) was used to analyze the concentrations for three anions (Cl , NO3 , and SO42 ) and four cations (Na + , NH4 + , Ca2 + , and Mg2 + ) species. The anions were analyzed with a Shim-pack ICSA1(G) guard column and a Shim-pack IC-SA1 analytical column. The eluent for anion analysis was 14 mM sodium hydrogen carbonate (NaHCO3). The cations were analyzed with a Shim-pack IC-SC1(G) guard column and a Shimpack IC-SC1 analytical column. The eluent was 6.5 mM methane sulfuric acid (CH4O3S). Injection of the samples was done manually using a 50-ml loop. Method detection limit was estimated from the repeated analyses of predefined quality control solutions. The detection limits of those ion species of Cl , NO3 , SO42 , Na +, NH4 +, Mg2 +, and Ca2 + were 39, 26, 22, 14, 31, 5, and 24 ppb, respectively. Performance testing via submission of standards was used to estimate the recover efficiencies for species analyzed in this study. Recoveries for species ranged from a low of 87% for Mg2 + to a high of 108% for NO3 with an overall average of 97%. The relative standard deviations of the fraction recovered were 9% (less than 10%), representing a high degree of reproducibility.
Descriptive statistics for all valid observations of PM10 and PM2.5 concentrations from six urban sites in Kaohsiung City operating from November 1998 to April 1999 are included in Table 1. The 24-h average concentration of PM10 varied from 68 to 200 mg m 3 with an average and standard deviation of 111 ± 38 mg m 3. Among the six sites, Tso-ying and Hsiao-kang had the highest 24-h average PM10 concentrations over the ambient air quality standard of 125 mg m 3. This high concentrations could be attributed to the local man-made sources (industrial emissions and heavy vehicular traffic) adjacent to the sampling sites. The 24-h average concentrations PM2.5 at the six sites varied from 34 to 118 mg m 3 with an average and standard deviation of 68 ± 24 mg m 3. Among the six sites, Hsiaokang had the highest 24-h average with PM2.5 concentration being 86 ± 33 mg m 3. This high concentration could be attributed to the local man-made sources (combustion sources in industrial park nearby) of particulate matter. Linear regression results relating PM2.5 to PM10 are shown in Fig. 2. There is a relatively strong correlation (correlation coefficient =.77) between PM10 and PM2.5 samples obtained at these six sites in Kaohsiung City. The strength of the correlation of PM2.5 to PM10 reveals that the temporal variations in fine particles have a significant influence on the observed variability in PM10 (Brook et al., 1997; Eldred et al., 1997).
Table 1 Average species concentrations (mg m 3) of the PM2.5 and PM10 samples at the six sampling sites in Kaohsiung City Sampling sitea
Nan-tze
Tso-ying
Chien-chin
San-min
Hsiao-kang
Chien-chen
Average
Number of samples
9
9
9
9
7
11
51
NA
1.93
NA
1.76
Wind speed (m s
1
1.01
2.33
PM2.5 Mass Cl NO3 SO42 Na+ NH4+ Mg2 + Ca2 +
66 ± 23 0.78 ± 0.64 10.53 ± 5.05 13.97 ± 6.84 0.91 ± 0.42 6.70 ± 3.09 0.11 ± 0.10 0.42 ± 0.35
71 ± 22 2.43 ± 0.87 12.37 ± 6.30 14.34 ± 5.63 2.08 ± 0.72 7.62 ± 3.40 0.08 ± 0.0 0.03 ± 0.03
47 ± 12 2.27 ± 1.44 8.20 ± 3.18 9.93 ± 1.29 2.28 ± 0.97 5.46 ± 2.60 0.09 ± 0.05 0.02 ± 0.03
72 ± 21 1.99 ± 0.73 12.84 ± 5.62 16.37 ± 4.17 1.29 ± 0.45 9.11 ± 2.70 0.11 ± 0.08 0.48 ± 0.48
86 ± 33 3.07 ± 0.24 12.78 ± 3.84 17.82 ± 8.56 2.79 ± 0.57 9.54 ± 3.71 0.09 ± 0.07 0.03 ± 0.05
64 ± 20 2.19 ± 0.71 10.87 ± 3.05 13.59 ± 2.19 2.01 ± 1.18 8.55 ± 3.30 0.05 ± 0.06 0.02 ± 0.04
68 ± 24 2.08 ± 0.99 11.31 ± 4.42 14.34 ± 5.10 1.84 ± 0.93 7.89 ± 3.09 0.09 ± 0.07 0.17 ± 0.30
PM10 Mass Cl NO3 SO42 Na + NH4+ Mg2 + Ca2 +
113 ± 31 1.54 ± 1.15 15.63 ± 6.47 19.15 ± 7.76 1.80 ± 0.52 11.17 ± 5.39 0.30 ± 0.18 1.72 ± 1.09
126 ± 48 5.74 ± 2.60 17.59 ± 9.93 19.27 ± 8.59 4.66 ± 2.02 11.06 ± 4.52 0.19 ± 0.08 0.09 ± 0.10
83 ± 16 3.77 ± 1.95 11.20 ± 3.58 12.95 ± 2.30 3.75 ± 1.51 8.66 ± 3.55 0.23 ± 0.12 0.06 ± 0.06
102 ± 35 3.07 ± 0.95 16.47 ± 7.29 20.53 ± 6.51 2.20 ± 0.82 13.12 ± 3.97 0.27 ± 0.16 1.34 ± 1.08
137 ± 47 5.00 ± 0.80 15.92 ± 4.77 21.94 ± 8.67 5.32 ± 1.12 13.69 ± 3.73 0.22 ± 0.03 0.10 ± 0.11
102 ± 29 3.56 ± 0.67 14.43 ± 3.23 17.90 ± 3.77 3.78 ± 1.38 12.37 ± 3.73 0.11 ± 0.08 0.06 ± 0.09
111 ± 38 3.72 ± 1.90 15.31 ± 5.94 18.69 ± 6.38 3.51 ± 1.71 11.75 ± 4.03 0.22 ± 0.13 0.58 ± 0.92
a b
)
Locations of the sampling sites are shown in Fig. 1. NA: data are not available at the site.
1.92
b
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Fig. 2. Relationships between PM2.5 and PM10 concentrations for all samples observed at the six sampling sites in Kaohsiung City.
The ratio of PM2.5 to PM10 concentrations are fairly uniform, ranging from 57% to 71%, with an average and standard deviation of 62 ± 10% (as shown in Fig. 3). In other words, PM2.5 concentrations are almost always larger than that of the coarse particle (PM2.5 – 10), with an average PM2.5 being 1.6 times larger than the average coarse particle concentration. Samples from Kaohsiung City indicate a comparable PM2.5/PM10 ratio (0.62) as compared with samples from the other urban cities like Azusa (0.51), Anaheim (0.67), Downtown Los Angeles (0.74), Long Beach (CA, USA) (0.66) (Chow et al., 1994), Montreal (0.52), Toronto (0.60), and Vancouver (Canada) (0.60) (Brook et al., 1997). 3.2. Concentrations of water-soluble ion species Table 1 summarizes the average and standard deviation of the ionic species concentrations for PM10 and PM2.5 by site. Overall, sulfate is the most abundant inorganic species measured in PM2.5. The sulfate concentrations ranged from 9.93 (Chien-chin) to 17.82 (Hsiao-kang) mg m 3 (average 14.34 mg m 3), and accounted for 20.4 –24.6% (average 22.7%) of the PM2.5 mass concentration. Nitrate is another major soluble inorganic species. The nitrate concentrations range from 8.20 (Chien-chin) to 12.78 (Hsiao-kang) mg m 3 (average 11.31 mg m 3), and were 9.6 – 10.8% (average 10.1%) of the PM2.5 mass concentration. The ammonium concentrations ranged from 5.46 (Chien-chin) to 9.54 (Hsiao-kang) mg m 3 (average 7.89 mg m 3), and were 8.2 –10.5% (average 9.5%) of the PM2.5 mass concentration. The total concentrations of both sea salt particle components (sodium and chloride) and soil particle contents (calcium and magnesium) were only 4.1 – 5.6% (average 4.7%) of the PM2.5 mass concentration. On the average, three major soluble inorganic species (sulfate, nitrate, and ammonium) accounted for 42.2% of the PM2.5 mass con-
centration and 90.0% of the total dissolved ionic concentration measured. The composition for soluble inorganic components of PM10 was similar to the composition of PM2.5. The sulfate concentrations at six sites were between 12.95 (Chien-chin) and 21.94 (Hsiao-kang) mg m 3 (average 18.69 mg m 3), and were 15.0– 20.5% (average 17.0%) of the PM10 mass concentration. The concentrations of nitrate ranged from 11.20 (Chien-chin) to 17.59 (Hsiao-kang) mg m 3 (average 15.31 mg m 3), and accounted for 9.5– 12.6% (average 11.4%) of the PM10 mass concentration. The ammonium concentrations were ranged from 8.66 (Chien-chin) and 13.69 (Hsiao-kang) mg m 3 (average 11.75 mg m 3), and were 6.0 – 9.0% (average 7.4%) of the PM10 mass concentration. The total concentrations of both sea salt particle components (sodium and chloride) and soil particle contents (calcium and magnesium) were only 7.5– 9.3% (average 8.6%) of the PM2.5 mass concentration. On the average, three major soluble inorganic species (sulfate, nitrate, and ammonium) accounted for 35.7% of the PM10 mass concentration and 80.6% of the total dissolved ionic concentration measured. 3.3. Mass size distributions of water-soluble ion species The mass size distributions of water-soluble ion species have been widely studied (Wall et al., 1988; John et al., 1990; Pakkanen, 1996; Zhuang et al., 1999; Kerminen et al., 2000). The size distributions of the water-soluble ion species in this study were calculated based on the concentration ratio of PM2.5 to PM10. Fig. 4 demonstrates the averages and deviations of the size distributions of these species measured at the six sampling sites. Sodium and chloride, the two sea salt particle components, were found to be associated with fine particles (PM2.5), and they had (as an average) PM2.5/PM10 concentration ratios of 0.72 ± 0.16 and 0.59 ± 0.13, respectively. Kerminen et al. (2000) reported that most of the sea salt
Fig. 3. Distributions of PM2.5 at the six sampling sites in Kaohsiung City. The box plots indicate the mean (dot line within box) and median (solid line within box) distributions and the 90th, 75th, 25th, and 10th percentiles.
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molar concentration ratio of chloride to sodium in PM2.5 and PM10 were on the average 0.79 ± 0.16 and 0.95 ± 0.17, respectively. The correlation coefficients (R2) between chloride and sodium concentrations for PM2.5 and PM10 were .78 and .85. This suggests that there is some transport of sea salt particles from the coast to the urban area. To observe both variation of chloride loss over different seasons and chloride replacement for different sizes of particles, the chloride residual rate was applied and defined as follows (Ohta and Okita, 1990; Kaneyasu et al., 1999; Kerminen et al., 2000): chloride residual rate ¼ Cl =Cl s chloride loss ¼ Cl s Cl
Fig. 4. Distributions of analyzed water-soluble ionic species at the six sampling sites in Kaohsiung City. The box plots indicate the mean (dot line within box) and median (solid line within box) distributions and the 90th, 75th, 25th, and 10th percentiles.
particle mass is found with size ranging from 1 to 5 mm in particle diameter. However, for high sea salt particle loadings, a significant amount of sea salt particles were found in particles greater than 5 mm in diameter. On the contrary, sodium and chloride were associated in fine particle mode (PM2.5) for low sea salt particle concentration samples. The average concentration levels and distributions of sodium and chloride obtained in this study is situated in between the two extremes mentioned above, it suggesting that sea salt particles contribute to ambient particles at the coastal city. Sulfate and ammonium were found to be in fine particle mode (PM2.5) and had the average PM2.5/PM10 concentration ratios of 0.82 ± 0.07 and 0.79 ± 0.07. Nitrate can be found in both fine and coarse modes, and it had an average PM2.5/PM10 concentration ratio of 0.55 ± 0.10 in this study. Results obtained in this study are in agreement with those found in other researches (Meng and Seinfeld, 1994; Kerminen and Wexler, 1995; Zhuang et al., 1999). For example, Zhuang et al. (1999) reported that sulfate and ammonium particles at a coastal site in Hong Kong existed primarily in the submicron size range of condensation and droplet modes. Concentrations with respect to particles less than 1.8 mm were 88 ± 5% and 96 ± 5% of the concentrations for PM10 concerning sulfate and ammonium, and they were 33 ± 17% for nitrate. Coarse nitrate was often observed in coastal areas such as in China (Zhuang et al., 2000; Cheng et al., 2000), Finland (Pakkanen, 1996), and USA (Savoie and Prospero, 1982). 3.4. Sea salt particle contribution Regarding the origin of chloride in atmospheric aerosols collected near coast areas, the contribution of sea salt particles has to be considered. In this study, we assumed that suspended Na + in the atmosphere was derived only from the sea salt particles (Ohta and Okita, 1990). The
þ Cl s ¼ 1:80 Na
where Cls is the amount of Cl contained originally in the sea salt. Average chloride residual rates were 0.67 ± 0.15 and 0.81 ± 0.14 for PM2.5 and PM10, respectively, during sampling period (winter and spring) in this study. The average chloride residual rate for PM10 in this study was comparable to those measured in spring season by Ohta and Okita in 1990 (averaged 0.76 between January and April for PM8). The average chloride residual rate decreased from 0.81 for PM10 to 0.67 for PM2.5 in this study. This trend was in agreement with the findings found by Kerminen et al. (2000). Kerminen et al. (2000) reported that the percentage of chloride lost from sea salt particles is decreased with increasing particle size. The average chloride loss was slightly above 50% for particles larger than about 3 mm. 3.5. Chemical conversion of species Sulfate and nitrate are major components contained in the atmospheric aerosols. To determine the degrees of atmospheric conversion of SO2 to SO42 and of NOx to NO3 , the sulfur and nitrogen oxidation ratios (SOR and NOR, respectively) were employed and defined as follows (Colbeck and Harrison, 1984; Ohta and Okita, 1990; Kaneyasu et al., 1999): SOR ¼
NOR ¼
Sex:SO2 4 Sex:SO2 þ SSO2 4 NNO3 NNO3 þNNOx
where ex.SO42 is the excess sulfate that was calculated by subtracting the amount of SO42 of marine from that of SO42 in the atmosphere (Kaneyasu et al., 1995; Cheng et al., 2000). The unit of ex.SO42 and S(SO2) are in mg-S m 3; the unit of N(NOx) and N(NO3 ) are mg-N m 3. Concentrations of SO2 and NOx are the average values in each sampling period and were obtained from the six ambient air quality stations. The SOR expresses the degree
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of oxidation of sulfur in terms of the ratio of sulfate sulfur to total sulfur (in sulfate plus sulfur dioxide). Similarly, the NOR expresses the degree of oxidation of nitrogen in terms of the ratio of nitrate nitrogen to total nitrogen (in nitrate plus nitrogen dioxide). Higher values of SOR and NOR suggest that the photochemical oxidation would have occurred and more secondary aerosols can exist in the atmosphere (Colbeck and Harrison, 1984; Ohta and Okita, 1990; Kaneyasu et al., 1995). In this study, average values of SOR and NOR for PM2.5 and PM10 were shown in Fig. 5. The SOR value was higher in the northern part of the urban area (Nan-tze and Tso-ying sites) than those in the southern area (Hsiao-kang and Chien-chen sites). Since the major emissions of SO2 were mostly distributed in the southern part of the Kaohsiung City, we would expect areas with high emissions of SO2 to have low values of SOR. The average SORs at Kaohsiung City were 0.25 ± 0.12 and 0.29 ± 0.13 for PM2.5 and PM10, respectively. The paired t test indicated that the SORs for PM2.5 and PM10 were statistically the same ( P < .05) since the secondary SO42 formed was in association with the PM2.5 mode. Earlier studies (Pierson et al., 1979; Truex et al., 1980) have reported that in the primary pollutant the value of SOR is < 0.10. Shaw and Rodhe (1982) indicated that the median values of SOR in source areas were 0.14 in winter and 0.28 in summer, and were 0.29 in winter and 0.50 in summer in receptor areas. Ohta and Okita (1990) suggested that when the ratio value is greater than 0.10, then there would be the occurrence of the photochemical oxidation of SO2 in the atmosphere. The SOR values obtained in this study were comparable with the resulted measured in Taichung metropolitan basin, Taiwan (Tsai and Cheng, 1999). The average SOR of PM2.5 and PM10 during hazy days were 0.30 and 0.32, respectively, while the values
Fig. 5. Calculated SOR and NOR at the six sampling sites in Kaohsiung City.
during clear days were 0.12 and 0.14, respectively. The high SOR values obtained in this study suggest that secondary formation of SO42 from SO2 occurs in the atmosphere during the sampling period in Kaohsiung City. The distribution of NOR values between sampling sites was similar to those of SOR values. The average NORs in Kaohsiung City were 0.07 ± 0.03 and 0.12 ± 0.05 for PM2.5 and PM10, respectively. Although NOR values for PM2.5 and PM10 were not statistically different, NORs for PM10 were averaged 1.7 times higher than for PM2.5. The difference between PM2.5 and PM10 for NOR might be due to the fact that 45% of the secondary NO3 formed within the coarse particle mode (PM2.5 – 10). Values of the NOR are generally lower than those for SOR (Colbeck and Harrison, 1984). The NOR values obtained in this study were also similar to the resulted measured in Taichung metropolitan basin, Taiwan (Tsai and Cheng, 1999). The average NOR of PM2.5 and PM10 during hazy days were 0.09 and 0.10, respectively, while the values acquired during clear days were 0.03 and 0.04. The high NOR values obtained in this study suggest that secondary formation of NO3 from NOx occurs in the atmosphere during the sampling period in Kaohsiung City.
4. Conclusion Measurements of PM10, PM2.5, and water-soluble ion species collected from six ambient air quality monitoring sites in the urban area, Kaohsiung City, were examined. The important findings of this study are described as following. During the study in the winter, the 24-h concentration of PM10 was ranged from about 68 to 200 mg m 3 with an average of 111 mg m 3, and for PM2.5 it was ranged from about 34 to 118 mg m 3 with an average of 68 mg m 3. When all sampling sites were considered, on average, PM2.5 constituted 62% of the PM10 mass. On the average, three major soluble inorganic species (sulfate, nitrate, and ammonium) accounted for 42.2 and 35.7% of the PM2.5 and PM10 mass concentrations, respectively, and 90.0 and 80.6%, respectively, of the total dissolved ionic concentrations measured. The size distributions of the water-soluble ion species in this study were calculated based on the concentration ratio of PM2.5 to PM10. Sodium and chloride (sea salt particle components) were found to be associated with fine particles (PM2.5), and had average PM2.5/PM10 concentration ratios of 0.72 and 0.59, respectively. In this study, the average chloride residual rate decreased from 0.81 for PM10 to 0.67 for PM2.5, indicating the replacing the particulate chloride occurred. Sulfate and ammonium were found to be primarily in fine particle mode (PM2.5) and had the average PM2.5/ PM10 concentration ratios of 0.82 and 0.79. Nitrate had an average PM2.5/PM10 concentration ratio of 0.55. SOR and NOR were used to investigate the degree of atmospheric conversion of both SO2 to SO42 and NOx to
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NO3 . The average SORs were 0.25 and 0.29 for PM2.5 and PM10, respectively. The distribution of NOR values between sampling sites was similar to those of SOR. The average NORs were 0.07 and 0.12 for PM2.5 and PM10, respectively. The high SOR and NOR values obtained in this study suggest that secondary formation of SO42 from SO2 and NO3 from NOx occurs in the urban atmosphere. The results obtained in this study indicate that further studies regarding the origin and composition of the primary and secondary particles are needed to construct reduction plans for emission sources control and to improve ambient air quality.
Acknowledgments This work was supported by the Environmental Protection Bureau of the Kaohsiung City Government, Taiwan, ROC. The support is gratefully acknowledged.
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sition of atmospheric aerosols and gaseous species in Sapporo, Japan. Atmos Environ 1995;29:1559 – 68. Kaneyasu N, Yoshikado H, Mizuno T, Sakamoto K, Soufuku M. Chemical forms and sources of extremely high nitrate and chloride in winter aerosol pollution in the Kanto Plain of Japan. Atmos Environ 1999; 33:1745 – 56. Kerminen VM, Wexler AS. Grow laws for atmospheric aerosol particles: an examination to the bimodality of the accumulation model. Atmos Environ 1995;29:3263 – 75. Kerminen VM, Teinila K, Hillamo R. Chemistry of sea-salt particles in the summer Antarctic atmosphere. Atmos Environ 2000;34:2817 – 25. Larson SM, Cass GR. Characteristics of summer midday low-visibility events in the Los Angeles area. Environ Sci Technol 1989;23L:281 – 9. Larson SM, Cass GR, Hussey KJ, Luce F. Verification of image processing based visibility models. Environ Sci Techol 1988;22:629 – 37. Larson SM, Cass GR, Gray HA. Atmospheric carbon particles and the Los Angeles visibility problem. Aerosol Sci Technol 1989;10:118 – 30. Lin JJ, Tai H-S. Concentrations and Distributions of carbonaceous species in ambient particles in Kaohsiung City, Taiwan. Atmos Environ 2001; 35:2627 – 36. Maln WC, Sisler JF, Huffman D, Eldred RA, Cahill TA. Spatial and seasonal trends in particle concentration and optical extinction in the United States. J Geophys Res 1994;99:1346 – 70. Meng Z, Seinfeld JH. On the source of the submicrometer droplet mode of urban and regional aerosols. Aerosol Sci Technol 1994;20:253 – 65. Ohta S, Okita T. A chemical characterization of atmospheric aerosol in Sapporo. Atmos Environ 1990;24A:815 – 22. Pakkanen TA. Study of formation of coarse particle nitrate aerosol. Atmos Environ 1996;30:2475 – 82. Pierson WR, Brachaczek WW, Mckee DE. Sulfate emissions from catalystequipped automobiles on the highway. J Air Pollut Control Assoc 1979; 29:255 – 7. Savoie DL, Prospero JM. Particle size distribution of nitrate and sulfate in the marine atmosphere. Geophys Res Lett 1982;9:1207 – 10. Schwartz J, Dockery DW, Neas LM. Is daily mortality associated specially with fine particles? J Air Waste Manage Assoc 1996;46:927 – 39. Shaw RW, Rodhe H. Non-photochemical oxidation of SO2 in regionally polluted air during winter. Atmos Environ 1982;16:2879 – 88. Tsai YI, Cheng MT. Size distribution and chemical composition of urban aerosols during autumn PM10 episodes. J Aerosol Sci 1999;30:S523 – 4. Turnbull AB, Harrison RM. Major component contributions to PM10 composition in the UK atmosphere. Atmos Environ 2000;34:3129 – 37. Truex TJ, Pierson WR, Mckee DE. Sulfate in diesel exhaust. Environ Sci Technol 1980;14:1118 – 21. Wall SM, John W, Ondo JL. Measurement of aerosol size distributions for nitrate and major ionic species. Atmos Environ 1988;22:1649 – 56. Watson JG, Chow JC, Lurmann FW, Musarra SP. Ammonium nitrate, nitric acid, and ammonium equilibrium in wintertime Phoenix, Arizona. J Air Waste Manage Assoc 1994;44:405 – 12. Wexler AS, Seinfeld JH. Second-generation inorganic model. Atmos Environ 1991;25A:2731 – 48. Zhuang H, Chan CK, Fang M, Wexler AS. Size distributions of particulate sulfate, nitrate, and ammonium at a coastal site in Hong Kong. Atmos Environ 1999;33:843 – 53. Zhuang D, Shi GY, Iwasaka Y, Hu M. Mixture of sulfate and nitrate in coastal atmospheric aerosols: individual particle studies in Qingdao (36040N, 120210E), China. Atmos Environ 2000;34:2669 – 79.
Characterization of water-soluble ion species in urban ambient particles Jim Juimin Lin* Department of Safety, Health, and Environmental Engineering, National Kaohsiung First University of Science and Technology, 1 University Road, Yanchau, Kaohsiung 824, Taiwan, ROC Received 11 July 2001; accepted 7 January 2002
Abstract Concentrations and distributions of water-soluble ion species contained in ambient particles were measured in a coastal urban area, Kaohsiung City, Taiwan. PM10 and PM2.5 samples were collected using a dichotomous sampler from November 1998 to April 1999 and were analyzed for water-soluble ion species with ion chromatography. On the average, ion species measured in this study accounted for 42.2% of the PM2.5 and 35.7% of the PM10. It was found that SO42 , NO3 , and NH4 + dominated the identifiable components within both fine (PM2.5) and coarse (PM2.5 – 10) fractions, and occupied 90.0% and 80.6% of total dissolved ionic concentrations for PM2.5 and PM10. The secondary aerosol formed through the NOx/SO2 gas-to-particle conversion was estimated based on the oxidation ratio of sulfur and nitrogen (SOR and NOR, respectively), i.e., sulfate sulfur/nitrate nitrogen to total sulfur/total nitrogen. The average SOR/NOR values were 0.25/0.07 and 0.29/0.12 for PM2.5 and PM10, respectively. The high SOR and NOR values obtained in this study suggested that there existed a secondary formation of SO42 from SO2 along with NO3 from NOx in the atmosphere. D 2002 Elsevier Science Ltd. All rights reserved. Keywords: Particulate matter; Water-soluble ion species; SOR; NOR; Secondary aerosol
1. Introduction Particulate matter with aerodynamic diameter less than 10 mm (PM10), especially the fine particle fraction (aerodynamic diameter < 2.5 mm, PM2.5), have been found to be associated with ambient air quality problems in urban areas including visibility reduction (Larson and Cass, 1989; Larson et al., 1989) and health problems, such as mortality and asthma (Dockery and Pope, 1994; Schwartz et al., 1996). In the atmosphere of many urban and heavily industrialized areas, particulate matter contains a major proportion of inorganic constituents. As reported by previous studies, soil accounts for the largest identifiable component of the coarse fraction (PM2.5 – 10), whereas SO42 , NO3 , and NH4 + dominate the identifiable components within the fine fraction (PM2.5)(Maln et al., 1994; Brook et al., 1997; Chan et al., 1997; Turnbull and Harrison, 2000). In general, these components account for about 20 – 45% of the PM2.5.
* Tel.: +886-7-601-1000x2312; fax: +886-7-601-1061. E-mail address: [email protected] (J.J. Lin).
Sulfate, nitrate, and ammonium are also the most common components of secondary particles in the atmosphere. These particles are formed in the atmosphere from direct emissions of sulfur dioxides (SO2), oxides of nitrogen (NOx), and ammonia (NH3) gases, respectively. Chemical transformation and equilibrium processes for inorganic secondary aerosols have been extensively studied. Equilibrium between gas and particle species was clearly described using models (Bassett and Seinfeld, 1983, 1984; Wexler and Seinfeld, 1991; Watson et al., 1994). Larson and Cass (1989) and Larson et al. (1988) also provided a scheme for the speciation of ionic material for ambient aerosol. The degree of oxidation of gas species to particle species could be utilized as an index of occurrence as well as level of secondary aerosol formation (Colbeck and Harrison, 1984; Ohta and Okita, 1990; Kaneyasu et al., 1995). Air quality in the Kao-ping area (Kaohsiung City, Kaohsiung County, and Pingdong County) is the worst in Taiwan and has become a serious concern due to high air pollutant emissions in the area. For example, Kao-ping area emitted about 100.221 Gg PM10, 92.794 Gg SOx, 109.841 Gg NOx, and 153.274 Gg NMHC (nonmethane hydrocarbon) in 1997 alone. The Kaohsiung City is the
0160-4120/02/$ – see front matter D 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 0 - 4 1 2 0 ( 0 2 ) 0 0 0 0 4 - 1
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second largest city in Taiwan and highly urbanized and industrialized. The city is densely populated, with approximately inhabitants totaling to 9606 km 2 (with about 1,475,505 inhabitants distributed over 153.6 km2). Currently, there are more than 1,228,429 automobiles and motorcycles together with a total of 1911 factories including 2 fossil power plants, 2 cement mills, 9 sizable steel plants, and 14 large-scale petrochemical plants in Kaohsiung City. Moreover, the city is surrounded by four heavily polluted industrial parks namely Jen-wu, Ta-shei, Ta-fa, and Lin-yuan located in the adjacent Kaohsiung County. As for the current air quality, the ambient particulate matter and other pollutants loads are high (annual average concentration is 84 mg m 3 for PM10, 9.67 ppb for SO2, 24.58 ppb for NOx, and 25.68 ppb for O3 in 1997). To understand the air pollution levels and their impact, water-soluble ion species in these particles were measured and their characteristics compared. In this paper, we have presented the concentrations and distributions of ion species in ambient particles in Kaohsiung City. Airborne particulate samples (PM2.5 and PM10) were collected at six ambient air quality sites distributed over the city during November 1998 and April 1999. In Taiwan, data on fine particle mass concentrations and chemical compositions are limited, particularly for urban areas. The objective of this study was to investigate the chemical composition distributions of the water-soluble inorganic ion species in fine (PM 2 .5 ) and coarse (PM2.5 – 10) particles in the urban atmosphere. The analyses provide information that is useful to air quality as well as health effect studies and to regulatory assessment in terms of the particle issue.
been conditioned at 25 C and 40% relative humidity for 24 h. After collecting from the field, sampling filters were stored in a refrigerator at 4 C before chemical analysis to prevent the loss of semivolatile species, especially ammonium nitrate. 2.2. Sampling program Particle samples were collected at six ambient air quality monitoring stations (Nan-tze, Tso-ying, San-min, Chienchin, Hsiao-kang, and Chien-chen) in Kaohsiung City, Taiwan ROC. These stations are part of the Taiwan Air Quality Monitoring Network, which was established by the Environmental Protection Administration of Taiwan ROC in 1993. These ambient stations are generally situated in populated areas and are intended to provide information pertaining to populations exposure to particles in the area. Specifically, the Chien-chen station was defined as an industrial district air-monitoring station located within or immediately downwind from the industrial districts with significant emission sources. The locations of the six stations are shown in Fig. 1. The observations were carried out from November 1998 to April 1999 in Kaohsiung City. Samples were collected during periods of no rain. Concentrations of ambient particle samples (PM2.5 and PM10) were calculated as 24-h average (approximately 0700– 0700 h next day). Hourly wind speed was recorded and then the average wind speed during each sample period was calculated. Fifty-one samples were
2. Experimental methods 2.1. Ambient particle size distributions and concentrations Ambient particle concentrations were measured with a dichotomous sampler (Graseby Andersen G241). This dichotomous sampler was equipped with an inlet having a 10-mm cut point. The particles that are below the 10-mm aerodynamic diameter (PM10) were divided into two size fractions using a virtual impactor with a 2.5-mm cut point. These two fractions are classified as a coarse fraction (2.5 mm < diameter < 10 mm, PM2.5 – 10) and a fine fraction (diameter < 2.5 mm, PM2.5). The dichotomous sampler was operated at a total flow rate of 16.7 l min 1. Particles were collected using 37-mm filters (Pallflex 2500 QAT-UP, 37 mm) supported by polyolefin rings. Quartz fiber filters were used to minimize the influence of background material from collection filters for carbonaceous analysis that proceeded simultaneously within this study (Lin and Tai, 2001). Before and after field sampling, filters were weighed on an electrical balance (Mettler Toledo AT261) with a reading precision of 10 mg to determine the mass concentration after its having
Fig. 1. Location map of the sampling sites (.) in the Kaohsiung City, Taiwan, and industrial parks (~) in the adjacent Kaohsiung County.
J.J. Lin / Environment International 28 (2002) 55–61
collected from the six sampling sites. Detailed information is shown in Fig. 1 and Table 1.
57
3. Results and discussion 3.1. Particle concentrations
2.3. Water-soluble ion species analysis The collected aerosol filters were ultrasonically extracted for 1.5 h into 30 ml of deionized distilled water and filtered through a Teflon filter of 4.5-mm nominal pore size. Ion chromatography (Shimadzu HIC-10ASP) was used to analyze the concentrations for three anions (Cl , NO3 , and SO42 ) and four cations (Na + , NH4 + , Ca2 + , and Mg2 + ) species. The anions were analyzed with a Shim-pack ICSA1(G) guard column and a Shim-pack IC-SA1 analytical column. The eluent for anion analysis was 14 mM sodium hydrogen carbonate (NaHCO3). The cations were analyzed with a Shim-pack IC-SC1(G) guard column and a Shimpack IC-SC1 analytical column. The eluent was 6.5 mM methane sulfuric acid (CH4O3S). Injection of the samples was done manually using a 50-ml loop. Method detection limit was estimated from the repeated analyses of predefined quality control solutions. The detection limits of those ion species of Cl , NO3 , SO42 , Na +, NH4 +, Mg2 +, and Ca2 + were 39, 26, 22, 14, 31, 5, and 24 ppb, respectively. Performance testing via submission of standards was used to estimate the recover efficiencies for species analyzed in this study. Recoveries for species ranged from a low of 87% for Mg2 + to a high of 108% for NO3 with an overall average of 97%. The relative standard deviations of the fraction recovered were 9% (less than 10%), representing a high degree of reproducibility.
Descriptive statistics for all valid observations of PM10 and PM2.5 concentrations from six urban sites in Kaohsiung City operating from November 1998 to April 1999 are included in Table 1. The 24-h average concentration of PM10 varied from 68 to 200 mg m 3 with an average and standard deviation of 111 ± 38 mg m 3. Among the six sites, Tso-ying and Hsiao-kang had the highest 24-h average PM10 concentrations over the ambient air quality standard of 125 mg m 3. This high concentrations could be attributed to the local man-made sources (industrial emissions and heavy vehicular traffic) adjacent to the sampling sites. The 24-h average concentrations PM2.5 at the six sites varied from 34 to 118 mg m 3 with an average and standard deviation of 68 ± 24 mg m 3. Among the six sites, Hsiaokang had the highest 24-h average with PM2.5 concentration being 86 ± 33 mg m 3. This high concentration could be attributed to the local man-made sources (combustion sources in industrial park nearby) of particulate matter. Linear regression results relating PM2.5 to PM10 are shown in Fig. 2. There is a relatively strong correlation (correlation coefficient =.77) between PM10 and PM2.5 samples obtained at these six sites in Kaohsiung City. The strength of the correlation of PM2.5 to PM10 reveals that the temporal variations in fine particles have a significant influence on the observed variability in PM10 (Brook et al., 1997; Eldred et al., 1997).
Table 1 Average species concentrations (mg m 3) of the PM2.5 and PM10 samples at the six sampling sites in Kaohsiung City Sampling sitea
Nan-tze
Tso-ying
Chien-chin
San-min
Hsiao-kang
Chien-chen
Average
Number of samples
9
9
9
9
7
11
51
NA
1.93
NA
1.76
Wind speed (m s
1
1.01
2.33
PM2.5 Mass Cl NO3 SO42 Na+ NH4+ Mg2 + Ca2 +
66 ± 23 0.78 ± 0.64 10.53 ± 5.05 13.97 ± 6.84 0.91 ± 0.42 6.70 ± 3.09 0.11 ± 0.10 0.42 ± 0.35
71 ± 22 2.43 ± 0.87 12.37 ± 6.30 14.34 ± 5.63 2.08 ± 0.72 7.62 ± 3.40 0.08 ± 0.0 0.03 ± 0.03
47 ± 12 2.27 ± 1.44 8.20 ± 3.18 9.93 ± 1.29 2.28 ± 0.97 5.46 ± 2.60 0.09 ± 0.05 0.02 ± 0.03
72 ± 21 1.99 ± 0.73 12.84 ± 5.62 16.37 ± 4.17 1.29 ± 0.45 9.11 ± 2.70 0.11 ± 0.08 0.48 ± 0.48
86 ± 33 3.07 ± 0.24 12.78 ± 3.84 17.82 ± 8.56 2.79 ± 0.57 9.54 ± 3.71 0.09 ± 0.07 0.03 ± 0.05
64 ± 20 2.19 ± 0.71 10.87 ± 3.05 13.59 ± 2.19 2.01 ± 1.18 8.55 ± 3.30 0.05 ± 0.06 0.02 ± 0.04
68 ± 24 2.08 ± 0.99 11.31 ± 4.42 14.34 ± 5.10 1.84 ± 0.93 7.89 ± 3.09 0.09 ± 0.07 0.17 ± 0.30
PM10 Mass Cl NO3 SO42 Na + NH4+ Mg2 + Ca2 +
113 ± 31 1.54 ± 1.15 15.63 ± 6.47 19.15 ± 7.76 1.80 ± 0.52 11.17 ± 5.39 0.30 ± 0.18 1.72 ± 1.09
126 ± 48 5.74 ± 2.60 17.59 ± 9.93 19.27 ± 8.59 4.66 ± 2.02 11.06 ± 4.52 0.19 ± 0.08 0.09 ± 0.10
83 ± 16 3.77 ± 1.95 11.20 ± 3.58 12.95 ± 2.30 3.75 ± 1.51 8.66 ± 3.55 0.23 ± 0.12 0.06 ± 0.06
102 ± 35 3.07 ± 0.95 16.47 ± 7.29 20.53 ± 6.51 2.20 ± 0.82 13.12 ± 3.97 0.27 ± 0.16 1.34 ± 1.08
137 ± 47 5.00 ± 0.80 15.92 ± 4.77 21.94 ± 8.67 5.32 ± 1.12 13.69 ± 3.73 0.22 ± 0.03 0.10 ± 0.11
102 ± 29 3.56 ± 0.67 14.43 ± 3.23 17.90 ± 3.77 3.78 ± 1.38 12.37 ± 3.73 0.11 ± 0.08 0.06 ± 0.09
111 ± 38 3.72 ± 1.90 15.31 ± 5.94 18.69 ± 6.38 3.51 ± 1.71 11.75 ± 4.03 0.22 ± 0.13 0.58 ± 0.92
a b
)
Locations of the sampling sites are shown in Fig. 1. NA: data are not available at the site.
1.92
b
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Fig. 2. Relationships between PM2.5 and PM10 concentrations for all samples observed at the six sampling sites in Kaohsiung City.
The ratio of PM2.5 to PM10 concentrations are fairly uniform, ranging from 57% to 71%, with an average and standard deviation of 62 ± 10% (as shown in Fig. 3). In other words, PM2.5 concentrations are almost always larger than that of the coarse particle (PM2.5 – 10), with an average PM2.5 being 1.6 times larger than the average coarse particle concentration. Samples from Kaohsiung City indicate a comparable PM2.5/PM10 ratio (0.62) as compared with samples from the other urban cities like Azusa (0.51), Anaheim (0.67), Downtown Los Angeles (0.74), Long Beach (CA, USA) (0.66) (Chow et al., 1994), Montreal (0.52), Toronto (0.60), and Vancouver (Canada) (0.60) (Brook et al., 1997). 3.2. Concentrations of water-soluble ion species Table 1 summarizes the average and standard deviation of the ionic species concentrations for PM10 and PM2.5 by site. Overall, sulfate is the most abundant inorganic species measured in PM2.5. The sulfate concentrations ranged from 9.93 (Chien-chin) to 17.82 (Hsiao-kang) mg m 3 (average 14.34 mg m 3), and accounted for 20.4 –24.6% (average 22.7%) of the PM2.5 mass concentration. Nitrate is another major soluble inorganic species. The nitrate concentrations range from 8.20 (Chien-chin) to 12.78 (Hsiao-kang) mg m 3 (average 11.31 mg m 3), and were 9.6 – 10.8% (average 10.1%) of the PM2.5 mass concentration. The ammonium concentrations ranged from 5.46 (Chien-chin) to 9.54 (Hsiao-kang) mg m 3 (average 7.89 mg m 3), and were 8.2 –10.5% (average 9.5%) of the PM2.5 mass concentration. The total concentrations of both sea salt particle components (sodium and chloride) and soil particle contents (calcium and magnesium) were only 4.1 – 5.6% (average 4.7%) of the PM2.5 mass concentration. On the average, three major soluble inorganic species (sulfate, nitrate, and ammonium) accounted for 42.2% of the PM2.5 mass con-
centration and 90.0% of the total dissolved ionic concentration measured. The composition for soluble inorganic components of PM10 was similar to the composition of PM2.5. The sulfate concentrations at six sites were between 12.95 (Chien-chin) and 21.94 (Hsiao-kang) mg m 3 (average 18.69 mg m 3), and were 15.0– 20.5% (average 17.0%) of the PM10 mass concentration. The concentrations of nitrate ranged from 11.20 (Chien-chin) to 17.59 (Hsiao-kang) mg m 3 (average 15.31 mg m 3), and accounted for 9.5– 12.6% (average 11.4%) of the PM10 mass concentration. The ammonium concentrations were ranged from 8.66 (Chien-chin) and 13.69 (Hsiao-kang) mg m 3 (average 11.75 mg m 3), and were 6.0 – 9.0% (average 7.4%) of the PM10 mass concentration. The total concentrations of both sea salt particle components (sodium and chloride) and soil particle contents (calcium and magnesium) were only 7.5– 9.3% (average 8.6%) of the PM2.5 mass concentration. On the average, three major soluble inorganic species (sulfate, nitrate, and ammonium) accounted for 35.7% of the PM10 mass concentration and 80.6% of the total dissolved ionic concentration measured. 3.3. Mass size distributions of water-soluble ion species The mass size distributions of water-soluble ion species have been widely studied (Wall et al., 1988; John et al., 1990; Pakkanen, 1996; Zhuang et al., 1999; Kerminen et al., 2000). The size distributions of the water-soluble ion species in this study were calculated based on the concentration ratio of PM2.5 to PM10. Fig. 4 demonstrates the averages and deviations of the size distributions of these species measured at the six sampling sites. Sodium and chloride, the two sea salt particle components, were found to be associated with fine particles (PM2.5), and they had (as an average) PM2.5/PM10 concentration ratios of 0.72 ± 0.16 and 0.59 ± 0.13, respectively. Kerminen et al. (2000) reported that most of the sea salt
Fig. 3. Distributions of PM2.5 at the six sampling sites in Kaohsiung City. The box plots indicate the mean (dot line within box) and median (solid line within box) distributions and the 90th, 75th, 25th, and 10th percentiles.
J.J. Lin / Environment International 28 (2002) 55–61
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molar concentration ratio of chloride to sodium in PM2.5 and PM10 were on the average 0.79 ± 0.16 and 0.95 ± 0.17, respectively. The correlation coefficients (R2) between chloride and sodium concentrations for PM2.5 and PM10 were .78 and .85. This suggests that there is some transport of sea salt particles from the coast to the urban area. To observe both variation of chloride loss over different seasons and chloride replacement for different sizes of particles, the chloride residual rate was applied and defined as follows (Ohta and Okita, 1990; Kaneyasu et al., 1999; Kerminen et al., 2000): chloride residual rate ¼ Cl =Cl s chloride loss ¼ Cl s Cl
Fig. 4. Distributions of analyzed water-soluble ionic species at the six sampling sites in Kaohsiung City. The box plots indicate the mean (dot line within box) and median (solid line within box) distributions and the 90th, 75th, 25th, and 10th percentiles.
particle mass is found with size ranging from 1 to 5 mm in particle diameter. However, for high sea salt particle loadings, a significant amount of sea salt particles were found in particles greater than 5 mm in diameter. On the contrary, sodium and chloride were associated in fine particle mode (PM2.5) for low sea salt particle concentration samples. The average concentration levels and distributions of sodium and chloride obtained in this study is situated in between the two extremes mentioned above, it suggesting that sea salt particles contribute to ambient particles at the coastal city. Sulfate and ammonium were found to be in fine particle mode (PM2.5) and had the average PM2.5/PM10 concentration ratios of 0.82 ± 0.07 and 0.79 ± 0.07. Nitrate can be found in both fine and coarse modes, and it had an average PM2.5/PM10 concentration ratio of 0.55 ± 0.10 in this study. Results obtained in this study are in agreement with those found in other researches (Meng and Seinfeld, 1994; Kerminen and Wexler, 1995; Zhuang et al., 1999). For example, Zhuang et al. (1999) reported that sulfate and ammonium particles at a coastal site in Hong Kong existed primarily in the submicron size range of condensation and droplet modes. Concentrations with respect to particles less than 1.8 mm were 88 ± 5% and 96 ± 5% of the concentrations for PM10 concerning sulfate and ammonium, and they were 33 ± 17% for nitrate. Coarse nitrate was often observed in coastal areas such as in China (Zhuang et al., 2000; Cheng et al., 2000), Finland (Pakkanen, 1996), and USA (Savoie and Prospero, 1982). 3.4. Sea salt particle contribution Regarding the origin of chloride in atmospheric aerosols collected near coast areas, the contribution of sea salt particles has to be considered. In this study, we assumed that suspended Na + in the atmosphere was derived only from the sea salt particles (Ohta and Okita, 1990). The
þ Cl s ¼ 1:80 Na
where Cls is the amount of Cl contained originally in the sea salt. Average chloride residual rates were 0.67 ± 0.15 and 0.81 ± 0.14 for PM2.5 and PM10, respectively, during sampling period (winter and spring) in this study. The average chloride residual rate for PM10 in this study was comparable to those measured in spring season by Ohta and Okita in 1990 (averaged 0.76 between January and April for PM8). The average chloride residual rate decreased from 0.81 for PM10 to 0.67 for PM2.5 in this study. This trend was in agreement with the findings found by Kerminen et al. (2000). Kerminen et al. (2000) reported that the percentage of chloride lost from sea salt particles is decreased with increasing particle size. The average chloride loss was slightly above 50% for particles larger than about 3 mm. 3.5. Chemical conversion of species Sulfate and nitrate are major components contained in the atmospheric aerosols. To determine the degrees of atmospheric conversion of SO2 to SO42 and of NOx to NO3 , the sulfur and nitrogen oxidation ratios (SOR and NOR, respectively) were employed and defined as follows (Colbeck and Harrison, 1984; Ohta and Okita, 1990; Kaneyasu et al., 1999): SOR ¼
NOR ¼
Sex:SO2 4 Sex:SO2 þ SSO2 4 NNO3 NNO3 þNNOx
where ex.SO42 is the excess sulfate that was calculated by subtracting the amount of SO42 of marine from that of SO42 in the atmosphere (Kaneyasu et al., 1995; Cheng et al., 2000). The unit of ex.SO42 and S(SO2) are in mg-S m 3; the unit of N(NOx) and N(NO3 ) are mg-N m 3. Concentrations of SO2 and NOx are the average values in each sampling period and were obtained from the six ambient air quality stations. The SOR expresses the degree
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of oxidation of sulfur in terms of the ratio of sulfate sulfur to total sulfur (in sulfate plus sulfur dioxide). Similarly, the NOR expresses the degree of oxidation of nitrogen in terms of the ratio of nitrate nitrogen to total nitrogen (in nitrate plus nitrogen dioxide). Higher values of SOR and NOR suggest that the photochemical oxidation would have occurred and more secondary aerosols can exist in the atmosphere (Colbeck and Harrison, 1984; Ohta and Okita, 1990; Kaneyasu et al., 1995). In this study, average values of SOR and NOR for PM2.5 and PM10 were shown in Fig. 5. The SOR value was higher in the northern part of the urban area (Nan-tze and Tso-ying sites) than those in the southern area (Hsiao-kang and Chien-chen sites). Since the major emissions of SO2 were mostly distributed in the southern part of the Kaohsiung City, we would expect areas with high emissions of SO2 to have low values of SOR. The average SORs at Kaohsiung City were 0.25 ± 0.12 and 0.29 ± 0.13 for PM2.5 and PM10, respectively. The paired t test indicated that the SORs for PM2.5 and PM10 were statistically the same ( P < .05) since the secondary SO42 formed was in association with the PM2.5 mode. Earlier studies (Pierson et al., 1979; Truex et al., 1980) have reported that in the primary pollutant the value of SOR is < 0.10. Shaw and Rodhe (1982) indicated that the median values of SOR in source areas were 0.14 in winter and 0.28 in summer, and were 0.29 in winter and 0.50 in summer in receptor areas. Ohta and Okita (1990) suggested that when the ratio value is greater than 0.10, then there would be the occurrence of the photochemical oxidation of SO2 in the atmosphere. The SOR values obtained in this study were comparable with the resulted measured in Taichung metropolitan basin, Taiwan (Tsai and Cheng, 1999). The average SOR of PM2.5 and PM10 during hazy days were 0.30 and 0.32, respectively, while the values
Fig. 5. Calculated SOR and NOR at the six sampling sites in Kaohsiung City.
during clear days were 0.12 and 0.14, respectively. The high SOR values obtained in this study suggest that secondary formation of SO42 from SO2 occurs in the atmosphere during the sampling period in Kaohsiung City. The distribution of NOR values between sampling sites was similar to those of SOR values. The average NORs in Kaohsiung City were 0.07 ± 0.03 and 0.12 ± 0.05 for PM2.5 and PM10, respectively. Although NOR values for PM2.5 and PM10 were not statistically different, NORs for PM10 were averaged 1.7 times higher than for PM2.5. The difference between PM2.5 and PM10 for NOR might be due to the fact that 45% of the secondary NO3 formed within the coarse particle mode (PM2.5 – 10). Values of the NOR are generally lower than those for SOR (Colbeck and Harrison, 1984). The NOR values obtained in this study were also similar to the resulted measured in Taichung metropolitan basin, Taiwan (Tsai and Cheng, 1999). The average NOR of PM2.5 and PM10 during hazy days were 0.09 and 0.10, respectively, while the values acquired during clear days were 0.03 and 0.04. The high NOR values obtained in this study suggest that secondary formation of NO3 from NOx occurs in the atmosphere during the sampling period in Kaohsiung City.
4. Conclusion Measurements of PM10, PM2.5, and water-soluble ion species collected from six ambient air quality monitoring sites in the urban area, Kaohsiung City, were examined. The important findings of this study are described as following. During the study in the winter, the 24-h concentration of PM10 was ranged from about 68 to 200 mg m 3 with an average of 111 mg m 3, and for PM2.5 it was ranged from about 34 to 118 mg m 3 with an average of 68 mg m 3. When all sampling sites were considered, on average, PM2.5 constituted 62% of the PM10 mass. On the average, three major soluble inorganic species (sulfate, nitrate, and ammonium) accounted for 42.2 and 35.7% of the PM2.5 and PM10 mass concentrations, respectively, and 90.0 and 80.6%, respectively, of the total dissolved ionic concentrations measured. The size distributions of the water-soluble ion species in this study were calculated based on the concentration ratio of PM2.5 to PM10. Sodium and chloride (sea salt particle components) were found to be associated with fine particles (PM2.5), and had average PM2.5/PM10 concentration ratios of 0.72 and 0.59, respectively. In this study, the average chloride residual rate decreased from 0.81 for PM10 to 0.67 for PM2.5, indicating the replacing the particulate chloride occurred. Sulfate and ammonium were found to be primarily in fine particle mode (PM2.5) and had the average PM2.5/ PM10 concentration ratios of 0.82 and 0.79. Nitrate had an average PM2.5/PM10 concentration ratio of 0.55. SOR and NOR were used to investigate the degree of atmospheric conversion of both SO2 to SO42 and NOx to
J.J. Lin / Environment International 28 (2002) 55–61
NO3 . The average SORs were 0.25 and 0.29 for PM2.5 and PM10, respectively. The distribution of NOR values between sampling sites was similar to those of SOR. The average NORs were 0.07 and 0.12 for PM2.5 and PM10, respectively. The high SOR and NOR values obtained in this study suggest that secondary formation of SO42 from SO2 and NO3 from NOx occurs in the urban atmosphere. The results obtained in this study indicate that further studies regarding the origin and composition of the primary and secondary particles are needed to construct reduction plans for emission sources control and to improve ambient air quality.
Acknowledgments This work was supported by the Environmental Protection Bureau of the Kaohsiung City Government, Taiwan, ROC. The support is gratefully acknowledged.
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