ANALYTICAL SCIENCES JANUARY 2004, VOL The Japan Society for Analytical Chemistry

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1 2004 The Japan Society for Analytical Chemistry 213 Atmospheric Concentrations of Sulfur Dioxide, Nitrogen Oxides, Ammonia, Hydrogen Chloride, Nitric Acid, Formic and Acetic Acids in the South of Vietnam Measured by the Passive Sampling Method Notes Tran Thi Ngoc LAN,* 1 Rokuro NISHIMURA,* 2 Yoshio TSUJINO,* 3 Kiyoshi IMAMURA,* 3 Munehiro WARASHINA,* 4 Nguyen Thai HOANG,* 1 and Yasuaki MAEDA* 2 *1 Vietnam National University, 227 Nguyen Van Cu, Dist. 5, HoChiMinh, Vietnam *2 Osaka Prefecture University, 1-1 Gakuen-cho, Sakai , Osaka, Japan *3 Environmental Pollution Control Center of Osaka Prefectural Government, Nakamichi, Higashinari-ku, Osaka , Japan *4 Osaka City Institute of Public Health and Environmental Science, , Japan A passive sampling method was applied to measure the concentrations of air pollutants, such as sulfur dioxide, nitrogen oxides, ammonia, hydrogen chloride, and nitric, formic and acetic acids, in the ambient air at four test sites in the southern region of Vietnam. The monthly averages and the average concentrations of air pollutants during the period from July 2001 to September 2002 are reported here. The concentrations of air pollutants varied widely, depending on the test sites and the sampling periods. The average concentrations of sulfur dioxide in the air during the period from July 2001 to September 2002 at the four test sites were ppb, and those of nitrogen dioxide were ppb. The concentrations of nitrogen monoxide and hydrogen chloride were very low at all of the test sites. The observed concentrations of all of the above-mentioned pollutants were lower than those of the Vietnamese standards of air pollutants. (Received September 17, 2003; Accepted November 25, 2003) Vietnam has changed to a new economic policy since the end of 1980s. Rapid urbanization, industrial expansion and the boom of private transport means are major factors, which have increased air pollution in big cities. HoChiMinh is the biggest industrial and commercial center in Vietnam with a population of more than 7 million people. Although industrial enterprises must locate in the industrial parks outside of the city, many of them still remain inside the city due to historical situations. Thus, the monitoring of the air quality and the management of pollution sources are urgent issues. In our present research, four test sites at HoChiMinh, Bien Hoa, Vung Tau and My Tho in the southern Vietnam were chosen for monitoring the air quality. HoChiMinh is the biggest industrial and commercial center in Vietnam. Bien Hoa is an urban site located about 30 km north-east from HoChiMinh City. My Tho is a rural area located about 70 km south-west from HoChiMinh. Vung Tau is a marine town in the southeast about 120 km from HoChiMinh. MyTho and Vung Tau have no heavy industry. The passive sampling method was employed for measurements of atmospheric pollution gases owing to the low cost and low technical demand as well as expediency to widen the monitoring in many polluted locations. Although there are various passive samplers marketed especially for the analyses of ambient nitrogen oxides, they have some disadvantages, such as high influence of wind velocity, moisture or recycling possibility. 1,2 Since the diffusion passive samplers were To whom correspondence should be addressed. ttnlan@hcmuns.edu.vn suggested to overcome such problems, great consideration has been paid to their uses for atmospheric monitoring. Diffusion passive samplers were developed by Swedish Environmental Research Institute, 3 Rupprecht & Patashnick Company (USA), 4 Laboratory of Aerology in Toulouse (France), 5 CSISRO (Australia), 6 and others. 7,8 In general, they work by the diffusion of contaminant molecules across a concentration gradient that can be defined by the Fick s First Law of Diffusion. Contaminant diffuses from the area of high concentration in the air to the area of low concentration on the passive sampler and is trapped on an impregnated filter at the end of the diffusion path. The impregnating reagent depends on the kinds of gases to be sampled, which can be chosen by each research group. As an example, a solution of a basic substance, like NaOH, triethanolamine, Na 2CO 3 or tetrachloromercurate, can be used for SO 2 sampling. For the sampling of NO 2, a solution of TEA or a solution of NaI and Na 2CO 3 was employed. A solution of citric or phosphoric acids was used for trapping NH 3. Although the designs of commercially available samplers are different, all of them are used to make a diffusion path and to prevent the penetration of particulate mater and water into the passive sampler. In the present research, the diffusion passive samplers developed by Warashina et al. were applied to monitor air pollutants. 1,2 The subjects to be monitored were the concentrations of sulfur dioxide (SO 2), nitrogen oxides (NO 2 and NO), ammonia (NH 3), hydrogen chloride (HCl), nitric acid (HNO 3), and low-aliphatic carboxylic acids, like formic (HCOOH) and acetic (CH 3COOH) acids. The research was started in July 2001 and has continued up to now.

2 214 ANALYTICAL SCIENCES JANUARY 2004, VOL. 20 Table 1 Filter types and the impregnating reagents for the collecting filters of the passive samplers Analyte Filter type Impregnating reagent SO 2 Cellulose 5% Na 2CO 3 aqueous solution NO 2 Cellulose 10% triethanolamine (TEA) solution in acetone NO x Cellulose 10% TEA, 3% 2-phenyl-4,4,5,5 tetramethyl-imidazonline-3-oxide-1-oxyl (PTIO) solution in acetone NH 3 Cellulose 10% citric acid aqueous solution HNO 3 Cellulose 1% NaCl, 5% glycerin aqueous solution HCl, HCOOH, CH 3COOH Silica fiber 10% TEA, 5% glycerin aqueous solution Fig. 1 Diffusion passive samplers: (A) for HNO 3, and HCl and low aliphatic carboxylic acids, (B) for SO 2, NO 2, NO x, and NH 3. 1, Case; 2, impregnated filter; 3, Teflon ring (A), spacer (B); 4, particle excluding mesh sheet (A), Teflon water-non-permeable filter (B); 5, cap; 6, whole passive sampler (φ = 55 mm, h = 7 mm for A type, and φ = 30 mm, h = 9 mm for B type). Experimental Determination of the concentrations of gases using passive samplers The benefit of using diffusion passive samplers for long-term exposure was verified by Warashina et al. 1 8 The concentrations of the gases in the air (C, ppb) were estimated from the amounts of the gases (A, µg) collected on the collecting filters and the exposure time (t, day). The relation between C and A is given by A C = K ppb, (1) t where K is the conversion coefficient (ppb day µg 1 ) of the passive samplers. According to Eq. (1), the conversion coefficients were estimated from the amounts of gases collected by the passive samplers and the average concentrations of the gases in the air measured for the same exposure periods by the filter pack method or continuous monitoring analyzers. Five sets of three identical passive samplers for different gases were exposed for five weeks (one set for one week) together with continuous and filter-pack monitoring of the gases at the Vietnam National University in HoChiMinh City. The amounts of gases collected by passive samplers were measured as the amounts of ions extracted from the collecting filters of the passive samplers, as described in the following section. An automatic ambient SO 2 analyzer (APSA-360; Horiba, Kyoto, Japan) and an ambient NO 2-NO x analyzer (GPH-74M; DKK, Tokyo, Japan) were used to measure the average concentrations of sulfur dioxide and nitrogen oxides. The average concentrations of SO 2, NH 3, HNO 3, HCl, HCOOH and CH 3COOH were measured by the filter pack method using a filter pack (NL-I-02, NILU, 47 mm diameter). 2 Statistics software SPSS 10.0 was used for regression analysis to obtain conversion coefficients. Preparation of the passive samplers Six different types of the diffusion passive samplers were Table 2 Extracting reagents, volume of the extracts, temperature for extraction and ions to be analyzed for the passive samplers Analyte Extracting reagent Volume/ ml Temperature Ionic form SO 2 0.3% H 2O C 2 SO 4 in water NO 2 and NO x Water C NO 2 NH 3 Water C + NH 4 HNO 3 Water C NO 3 HCl, HCOOH, and CH 3COOH Water 5 10 Room Cl, HCOO and CH 3COO made in our laboratory for monitoring SO 2, NO 2, NO x, NH 3, HNO 3, HCl and low-aliphatic carboxylic acids. The designs of the passive samplers are shown in Figs. 1-(A) and 1-(B). The diffusion passive samplers consisted of four main parts: a collecting filter impregnated with appropriate reagent, a polypropylene vessel that served as a container as well as diffusion part, a filter or mesh for preventing the penetration of particles and water, and a cap with open holes through which the ambient air containing pollutants diffused. The filter types and impregnating reagents for the collecting filters of the passive samplers are given in Table 1. Preparation of the passive samplers was as follows. The reusable parts of the passive samplers (vessels, caps, mesh, ring) were washed with deionized water many times, dried, and kept in sealed polyethylene bags until assembling. Before impregnating, silica fibers were heated at C for 3 h, while cellulose filters were washed twice with Milipore water in an ultrasonic bath for 15 min and dried in a vacuum to remove any contaminants. In the case of the passive samplers for SO 2, NO 2, NO x and NH 3, 70 µl of impregnating solutions were added to a paper filter, which was placed directly on a sampler body. Drying was carried out in a vacuum at 50 C. The assembling was done as fast as possible after drying. For the passive samplers of HNO 3, HCl, HCOOH and CH 3COOH, the filters were immersed twice in impregnating solutions. The impregnated filters were placed in a tight petri disk on sealed aluminum envelope. The filters were taken out one by one for assembling. Each passive sampler was kept in a sealed nylon bag in a fastener laminate envelope and sent to the observation site.

3 215 Table 3 Conversion coefficients and detection limits of the passive samplers Analyte SO 2 NO 2 NO x NH 3 HNO 3 HCl HCOOH CH 3COOH Conversion coefficient (ppb, day µg 1 ) Standard derivation Detection limit (ppb, day) Fig. 2 Relationship between the amounts of SO 2 and NO 2 collected by the passive samplers and their accumulative concentration measured by the automatic analyzers. Fig. 4 The monthly average SO 2 concentrations in the air at Fig. 3 Relationship between the amounts of HCOOH ( ) and CH 3COOH ( ) collected by passive samplers and the time of exposure. Sampling methods and analytical procedures The passive samplers were exposed to open air under a rain shelter for one month each at the four test sites from July 2001 until now. After exposure, the passive samplers were sent back to the laboratory for analysis. The collecting filters of the passive samplers were extracted by appropriated solutions in an ultrasonic bath for 15 min; the extracts were then analyzed by ion chromatography for the amounts of gases collected by the passive samplers. The extracting reagents, the volume of the extracts, the temperature for extraction and the ions to be analyzed are given in the Table 2. Results and Discussion Conversion coefficients of the passive samplers The relationship between the amounts of SO 2 and NO 2 collected by the passive samplers and the accumulative concentrations of the SO 2 and NO 2 in air measured by automatic analyzers for periods from 1 to 6 weeks are shown in Fig. 2. The amounts of SO 2 and NO 2 collected by passive samplers and the accumulative concentrations of SO 2 and NO 2 in air showed a good linear relationship. Figure 3 represents the amounts of HCOOH and CH 3COOH collected by passive samplers exposed to laboratory air for 1 8 weeks versus the exposure time. The conversion coefficient for each type of passive sampler was estimated from the amounts of gases collected by the passive samplers and their average concentrations in the air measured by the automatic analyzers (SO 2, NO 2 and NO x) or by the filter pack methods (NH 3, HNO 3, HCl, HCOOH, CH 3COOH). The conversion coefficients (K, ppb day µg 1 ), their standard derivations and the detection limits of the passive samplers are summarized in Table 3, where the detection limits were estimated based upon the detection limits of 0.05 mg/l for each analyte obtained by the ion chromatograph and the extraction volumes given in Table 2. The low standard derivations of the conversion coefficients (7 15%) for all types of the passive samplers and the good linear relationship between the amounts of gases collected by the passive samplers and the concentrations of the gases in air for long period suggested that the passive samplers could be used to measure the concentrations of gases in ambient air. Concentrations of SO 2, NO 2, NO x, NH 3, HNO 3, HCl, HCOOH and CH 3COOH in air at four test sites The monthly average concentrations of the gases (SO 2, NO 2, NH 3, HCOOH and CH 3COOH) are illustrated in Figs Figure 4 shows the monthly average SO 2 concentrations at the four test sites. The highest monthly average SO 2 concentration was observed in HoChiMinh City with a range from 13 to 21 ppb, which was in the range of 5 13 ppb in Bien Hoa with an increasing tendency in the year of The monthly average SO 2 concentrations at My Tho and Vung Tau were almost comparable in the range of 2 6 ppb. The monthly average concentrations of NO 2 are given in Fig. 5. A high monthly average NO 2 concentration of about 10 ppb was observed in HoChiMinh and Bien Hoa. However, the NO 2 level in Bien Hoa was lower than that in HoChiMinh. In My Tho and Vung Tau, the monthly average NO 2 concentrations were in the range of about 3 8 ppb. The higher concentrations

4 216 ANALYTICAL SCIENCES JANUARY 2004, VOL. 20 Fig. 5 The monthly average NO 2 concentrations in the air at Fig. 7 The monthly concentrations of formic acid in the air at Fig. 6 The monthly concentrations of NH 3 in the air at HoChiMinh ( ), Bien Hoa ( ), Vung Tau ( ) and My Tho ( ). Fig. 8 The monthly rainfalls at HoChiMinh ( ) and VungTau ( ), and wind velocities at HoChiMinh ( ) and VungTau ( ). of SO 2 and NO 2 observed in HoChiMinh and Bien Hoa, compared to those in My Tho and Vung Tau, might have been caused by heavy industry and heavy traffic in those cities. The increasing tendencies of sulfur dioxide and nitrogen dioxide in BienHoa might have been related to the rapid expansion of industrial parks outside BienHoa City and moving enterprises from HoChiMinh to industrial parks. Figure 6 shows the monthly average concentrations of NH 3 at the four test sites. The highest monthly average concentration in the range of ppb was observed in Bien Hoa, while the lowest one, in the range of 8 20 ppb, was in Vung Tau. The monthly average NH 3 concentrations at HoChiMinh and My Tho were comparable and varied from ppb. However, the variation in the NH 3 concentrations at HoChiMinh, My Tho and Bien Hoa was not very large. In addition, it was found that the monthly average NH 3 concentrations increased during the period from September 2001 to November, and then decreased until April According to the observed monthly average concentrations of HNO 3 in the air, the four test sites could be divided into two groups: (1) HoChiMinh with the highest HNO 3 concentrations of about 0.8 to 1.6 ppb, and (2) My Tho, Vung Tau and Bien Hoa with the low HNO 3 concentrations of ppb. The concentration of HCl was lower than 0.2 ppb at all of the test sites. Figure 7 shows the monthly average concentrations of gaseous HCOOH at the four test sites. The site and time dependence of monthly average concentrations of gaseous CH 3COOH at the four test sites showed similar appearance like that of gaseous HCOOH. However, the concentration of gaseous HCOOH at most of the test site was little higher than that of gaseous CH 3COOH. Moreover, the monthly average concentrations of these low-aliphatic carboxylic acids were very different, depending on the site and seasons. The concentrations of gaseous HCOOH and CH 3COOH at the four sites were arranged in the following order: MyTho > HoChiMinh > Vung Tau > Bien Hoa. The monthly average concentrations of HCOOH and CH 3COOH at HoChiMinh, Vung Tau and Bien Hoa were lower than 10 ppb for HCOOH and 8 ppb for CH 3COOH. In addition, a large variation in the monthly average concentrations of HCOOH and CH 3COOH was observed, especially at the rural site of My Tho. Abnormally high monthly average concentrations of HCOOH and CH 3COOH were observed at MyTho during the period from October 2001 to December 2001 and in February and May compared with those of other months at My Tho and those at other sites. A seasonal variation in the concentrations of gases was observed. In total, the concentrations of the gases began to increase in August, reached a peak in November or December, and then decreased until January or February. During the period from March to June, they were more or less stable. Warashina observed similar phenomena for the NO 2 concentrations in Hong Kong, Shanghai (China), and Teagu and Taegon (Korea), 1 where the NO 2 concentrations were higher in the autumn. The observed seasonal variation in the concentrations of gases at the four test sites in the southern Vietnam likely depended on the local meteorological factors, like rainfalls and wind velocities, which are shown in Fig. 8. During the first half of the rainy season, a high volume of precipitation improved the air quality. During the second half of the rainy season, rains became less and the wind velocity

5 217 Table 4 Average concentrations (ppb) of SO 2, NO 2, NO, NH 3, HNO 3, HCl, HCOOH and CH 3COOH for the period from July 2001 to September 2002 SO 2 NO 2 NO NH 3 HNO 3 HCl HCOOH CH 3COOH HoChiMinh Standard deviation MyTho Standard deviation VungTau Standard deviation BienHoa Standard deviation decreased, and thus the air pollution level increased. From January, the wind velocity increased and winds blew off pollutants to distance areas, which resulted in a gradual decrease of the air pollution. Conclusion The average concentrations of air pollutants during the period from July 2001 to September 2002 are summarized in Table 4. From Table 4, the average concentrations of NO, HCl and HNO 3 were low at all of the test sites. The average concentrations of SO 2, NO 2, NH 3, HCOOH and CH 3COOH changed widely depending on the test sites. According to the pollution level, the sites are classified as follows: HoChiMinh and Bien Hoa are polluted cities, and Vung Tau and My Tho are clean areas. The average concentration of SO 2 during the period from July 2001 to September 2002 in HoChiMinh City was 16.9 ppb (43.5 µg/m 3 ), which was higher than the annual concentrations of SO 2 in Hanoi (capital of Vietnam) given in our previous report. 9,10 According to WHO air quality standards, the guideline values of the annual concentrations for combined exposure to SO 2 and SPM must be less than and 70 µg/m 3, respectively. Based on the observed concentrations of SO 2 in HoChiMinh observed in the present research and the PM 10 concentrations in HoChiMinh ( µg/m 3, depending on the sampling sites) observed in previous our research, 9,10 we can conclude that the air quality in HoChiMinh city is quite poor. If no emergent air pollution control/management is adapted, the rapid industrial development and the boom of private traffic means in HoChiMinh City would lead to a dramatic deterioration of the air quality in the near future. Gaseous pollutants of ammonia, hydrogen chloride, and nitric and low-carboxylic acids were measured for the first time in Vietnam, by using the passive sampling techniques, which allowed us to measure various air pollutants at four test sites. The passive samplers were small, light, re-usable, soundless and cost-efficient. All of these advantages of the passive samplers are certainly very important and useful for air pollution monitoring in the developing countries, like Vietnam. of Science. We highly appreciate Osaka City Institute of Public Health and Environmental Science, Environmental Pollution Control Center of Osaka Prefecture, College of Engineering of Osaka Prefecture University, and Department of Physical Chemistry of Vietnam National University and South Meteorological Center of Vietnam for their help in sampling and analyses. References 1. M. Warashina, M. Tanaka, Y. Tsujino, T. Mizoguchi, S. Hatakeyama, and Y. Maeda, Water, Air, Soil Pollut., 2001, 130, Y. Tsujino, K. Ohira, Y. Maeda, M. Matsumoto, Y. E. Yoo, and X. D. Zhang, The Proceeding of 2001 International Conference on The Effects of Acid Deposition on Cultural Properties and Materials in East Asia, 2001, Passive sampler for SO 2 and NO 2, 4. Rupprecht & Patashnick Co., Inc., High-Quality Passive Sampling of NO, NO 2, NO x, SO 2 and O 3 in Indoor or Outdoor Environments, 5. Measurements, 6. CSIRO Atmospheric Research, Passive gas samplers, 7. Ambient concentrations of pollutants in urban areas, 8. Radiello Model 3310 Passive Sampling System, 9. Y. Maeda, K. Imamura, T. T. N. Lan, N. T. P. Thoa, V. D. Nam, and P. H. Viet, The Proceeding of General Seminar on Environmental Science and Technology Issues Related to Urban and Coastal Zone Development, Osaka, Japan, 2001, N. T. P. Thao, Y. Maeda, T. D. Hoan, T. Q. Trung, Y. Tsujino, K. Imamura, N. T. Truc, P. H. Long, N. Q. Trung, and T. T. Nguyen, The Proceeding of General Seminar on Environmental Science and Technology Issues Related to Urban and Coastal Zone Development, Osaka, Japan, 2001, 11. Acknowledgements This research was supported by the Japan Society for Promotion