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Two years of aerosol pollution monitoring in Singapore: a review
Nuclear Instruments and Methods in Physics Research B 150 (1999) 457±464
Two years of aerosol pollution monitoring in Singapore: a review I. Orlic *, X. Wen, T.H. Ng, S.M. Tang Department of Physics, Research Center for Nuclear Microscopy, National University of Singapore, Kent Ridge, Singapore 119260, Singapore
Abstract An aerosol sampling campaign was initiated more than two years ago in Singapore. The aim was to determine the average elemental concentrations in ®ne and coarse aerosol fractions as well as to identify major pollution sources and their impact. For that purpose, two air samplers were employed at two dierent sampling locations; one sampler was a ®ne particulate aerosol sampler (PM2.5) located at the vicinity of a major industrial area. The other was a stacked ®lter unit (SFU) sampler designed for collection of ®ne and coarse fractions (PM2.5 and PM10) and installed in the residential area. Samples were taken typically twice a week and in several occasions daily. During the period of two years more than 700 aerosol samples were collected and analyzed using PIXE and RBS techniques. All samples were analyzed for 18 elements ranging between Na, Mg, Al, etc. up to As and Pb. Large daily and seasonal variations were found for most of the elements. These variations are attributed mainly to meteorological changes, in particular changes in wind speed and direction. On several occasions, short term sampling was performed to identify ®ngerprints of major pollution sources such as road trac, re®neries, as well as the rain-forest ®res in neighboring countries. A summary of our ®ndings is presented and discussed. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Aerosols; Pollution; Metals; PM2.5; PM10; Biomass burning
1. Introduction A two-year sampling campaign was initiated in Singapore at the end of 1996 with the aim to determine average elemental concentrations of aerosols in Singapore as well as to identify major pollution sources and their impact. To achieve this, PIXE and micro-PIXE analytical techniques were used. Some of our results obtained by the broad beam PIXE analysis are already reported in
* Corresponding author. Tel.: +65-874-2962; fax: +65-7776126; e-mail: [email protected]
previous publications [1,2]. Similarly, some of our preliminary results obtained by micro-PIXE technique are reported in publications [3,4]. In this work an overview of results of bulk analysis is given. PIXE results are statistically evaluated using principal component analysis with the intention to identify major suspended aerosol particle sources. The results show that there are two major factors involved; natural, and anthropogenic. In natural factor marine and soil components were found to be the major contributors and in anthropogenic factor, sulfates, smoke, metals, road dust and vehicular components were identi®ed. The impact of these components varied a lot
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 1 0 5 3 - 2
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I. Orlic et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 457±464
depending on the sampling site and meteorological conditions. 2. Experimental 2.1. Sampling Aerosol samples have been collected twice weekly since Jan. 1996 at two sites: one site was the Civil Service College (CSC) located within the campus of the National University of Singapore (NUS) at Kent Ridge, and the other at the Anderson Junior College (AJC) located in the central part of Singapore ± see Fig. 1. The former site is in the West Coast area and is only a few kilometers southeast of the Jurong Industrial Estate. It is also northeast to a number of oil re®neries and a power station situated in a group of small islands o the West Coast of the Singapore Island. Although NUS is surrounded by an expressway and a few major roads with relatively heavy trac, the CSC building is up on a ridge surrounded by trees and not in the immediate vicinity of the expressway or any of these roads. The latter site is near the center of the Singapore Island and within one of the largest residential estates in Singapore. It is encircled by heavy-trac roads and highways. Other sampling sites, Nan Yang Technological University (NTU) and Raes Light House (RLH) were used only during a short-term sampling (one
month in 1996) to identify Ô®ngerprintÕ of major industrial pollutants located in Jurong Industrial Zone. The aerosol sampler used at CSC was a cyclone sampler manufactured by the Australian Nuclear Science and Technology Organization (ANSTO) designed to collect PM2.5 particulate matter. It was installed at the rooftop of the four-storey CSC building. To avoid overloading of ®lters the eective sampling time was reduced to a 12 h period per day (the sampler was switched on/o every alternate hour). The ¯ow-rate was maintained at approximately 22 l/min, which resulted with a typical sampling volume of approximately 16 m3 . The sampler used at AJC was a stacked ®lter unit (SFU) set to collect both PM10 as well as PM2.5 particulate matter. Two types of polycarbonate Nucleopore ®lters were used for the SFU; one with the pore size of 0.4 lm for collection of particles with Equivalent Aerodynamic Diameters (EADs) below 2.5 lm and the other with 8 lm pore size for particles with EADs between 2.5 and 10 lm. The sampler was equipped with the preimpaction stage to cut-o all particles with EAD > 10 lm. The sampler operated with a ¯owrate close to 15 l/min, hence drawing approximately 11 m3 per sampling session (day). The inlet of the SFU was installed at a height of 2 m above the ground level next to the College fence, which was approximately 40 m away from a busy road. During the entire sampling period sampling was on several occasions intensi®ed with the aim to identify ®ngerprint of individual sources such as road trac, industry, haze, etc. Results obtained are brie¯y outlined in this work. 2.2. Elemental analysis
Fig. 1. Map of Singapore with indicated sampling sites and the major industrial areas.
All samples were analyzed by means of PIXE and RBS techniques at the Research Center for the Nuclear Microscopy, NUS. The facility comprises the following major components: 2.5 MeV Van de Graa accelerator (operating at 2 MeV), Oxford Microbeams nuclear microscope end-stage and PC based Data Acquisition System. For the detection of X-rays, a LINK detector with a resolution of 150 eV at 5.9 keV and an active area of
I. Orlic et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 457±464
61 mm2 was used. Detailed description of the experimental setup used in this study can be found elsewhere [5]. 3. Results 3.1. Mean elemental concentrations Since December 1995, over 700 aerosol samples have been collected at the CSC and AJC sampling
459
locations. All samples were analyzed by means of PIXE and RBS techniques for concentrations of the following 18 elements: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Fe, Ni, Cu, Zn, Br, Pb. The concentrations of all elements were found to have great variations in time. This is caused mainly by the changes in the wind direction and speed and the fact that the major pollution sources in Singapore are localized and at the relatively short distance from each other and from the sampling locations.
Fig. 2. Top: Mean daily wind direction and speed during the entire sampling period ± Jan. 1996 to end of 1997. Full line on the top ®gure is a trend line to approximately indicate average wind direction. Middle: daily elemental concentrations of S, K and Si in the ®ne fraction of aerosols measured at CSC location. The bottom insert shows the total PM2.5 mass measured at the same location but only from May 1998 onwards. Note the signi®cant increase of S, K and PM2.5 mass during the last quarter of the 1997 when a severe forest ®res (referred to as the haze period) hit the SE Asian Region.
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For illustration, graphs of daily concentrations of S, K and Si measured at CSC location are shown in Fig. 2. All data points obtained during the entire sampling period are shown. On the top of the ®gure shown are mean daily wind directions and speeds. It is interesting to note that during the last two years a typical Monsoon pattern, characteristic for a tropical climate such as in Singapore, was disrupted. The rainy season characterized with the NE Monsoon (wintertime on the Northern Hemisphere) was much shorter than usually and started only in Feb. instead of in Dec. ± see Fig. 2. Throughout the rest of the 1996 and 1997-year, dry S-SE Monsoon dominated. The wind speed was generally low with average speed of approximately 1.1 m/s and it steadily decreased towards the last quarter of 1997 when the drought caused extensive forest ®res in Indo-
nesia and severe haze in the whole SE Asian region. The mean concentrations and the corresponding standard deviations of all measured elements obtained for the entire sampling periods are summarized in Tables 1 and 2. Concentrations measured for the CSC sampling location (the PM2.5 fraction) are given in Table 1 and for the AJC location in Table 2, separately for the ®ne and coarse fractions. All concentrations are given in lg/m3 separately for normal weather conditions (Jan. 1996±Aug. 1997) and for the period of haze (22 Aug.±7 Nov. 1997). PM2.5 mass was obtained by weighing ®lters before and after sample collection and it was measured only for the CSC location and for the second half of the 1997 year. Results are summarized in the following few sections.
Table 1 Mean elemental concentrations in PM2.5 fraction of aerosols sampled at CSC location (concentrations are given separately for the prehaze period (2 Jan. 1996±21 Aug. 1997) and for the haze period (22 Aug.±7 Nov. 1997). The total concentrations of all elements measured by PIXE is given in the second last row and the total PM2.5 mass in the last raw. All concentrations are expressed in lg/m3 . Concentration ratios between haze and non-haze periods are given in the last column) CSC PM2.5
Mean concentration (ng/m3 ) Pre-Haze (2 Jan. 96±21 Aug. 97)
Haze (22 Aug. 97±7 Nov. 97)
Na Mg Al Si P S Cl K Ca Ti V Mn Fe Ni Cu Zn Br Pb
212 287 52 53 84 56 173 102 27 32 2121 1023 166 536 332 238 116 70 7.6 5.9 18.5 11.5 7.2 6.9 105 64 5.0 3.3 5.2 5.7 43 39 15 19 29 23
87 50 40 13 100 42 201 78 104 59 5344 2062 343 233 1060 366 171 71 17.1 10.0 24.2 13.2 6.2 7.9 123 83 6.1 3.1 6.5 6.1 68 143 30 21 31 28
0.4 0.8 1.2 1.2 3.8 2.5 2.1 3.2 1.5 2.2 1.3 0.9 1.2 1.2 1.3 1.6 2.0 1.1
Total PM2.5
3521 1817 24957 10144
7763 2666 86243 48817
2.2 3.5
Ratio
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Table 2 Mean elemental concentrations in ®ne (PM2.5) and coarse fraction (PM10 minus PM2.5) of aerosols sampled at AJC location (concentrations are given separately for the pre-haze period and for the haze period. The total concentrations of all elements measured by PIXE are given in the last row. Concentration ratios between haze and non-haze periods are given in the last column) AJC (Fine)
Mean concentration (ng/m3 ) Pre-Haze (2 Jan. 96 ±21 Aug. 97)
Haze (22 Aug. 97 ±7 Nov. 97)
Na Mg Al Si P S Cl K Ca Ti V Mn Fe Ni Cu Zn Br Pb
207 304 188 313 231 313 278 299 153 310 1909 1010 105 138 315 187 82 43 4.4 5.2 8.5 9.5 5.2 5.8 115 69 7.1 6.3 26.6 55.0 41 33 17 11 42 39
40 50 84 31 192 68 240 86 112 68 3029 967 91 26 570 202 131 107 2.5 4.1 3.0 4.0 4.7 4.1 117 57 14 5 27 18 56 56 19 8 21 18
0.2 0.4 0.8 0.9 0.7 1.6 0.9 1.8 1.6 0.6 0.4 0.9 1.0 2.0 1.0 1.4 1.2 0.5
Total
373 2402
4753 956
1.3
Ratio
3.2. Normal weather conditions, ®ne fraction As expected, sulfur is the most dominant element in the ®ne fraction, with mean concentration of approximately 2000 ng/m3 at both sampling sites. Major sources of sulfur in Singapore are oil re®neries and oil-powered power plants, which are mainly located on the SW part of Singapore. Sulfur is followed by potassium with an average concentration slightly higher than 300 ng/m3 . Potassium is known as one of the major indicators of biomass burning processes. It is therefore likely that part of the measured concentration is coming from the local sources such as incinerators and part of it is probably due to a long-range transport from minor forest ®res in Indonesia. In spite of large temporal variations, concentrations of all elements show surprisingly good agreement at both sampling locations. Consequently, the overall mean values of the total ele-
AJC (Coarse)
Mean concentration (ng/m3 )
Ratio
Pre-Haze (2 Jan. 96 ±21 Aug. 97)
Haze (22 Aug. 97 ±7 Nov. 97)
Na Mg Al Si P S Cl K Ca Ti V Mn Fe Ni Cu Zn Br Pb
539 534 322 361 686 423 1217 600 142 194 677 322 1503 1372 269 113 765 406 37.0 20.3 3.4 6.4 6.7 9.0 491 231 4.7 5.9 28 27 39 48 43 19 27 31
269 197 209 80 722 279 1488 686 117 75 1147 456 1253 1131 444 185 1657 1454 30.3 28.7 1.4 4.3 8.7 3.9 644 297 11.8 5.8 32 23 38 34 35 11 12 10
0.5 0.6 1.1 1.2 0.8 1.7 0.8 1.6 2.2 0.8 0.4 1.3 1.3 2.5 1.1 1.0 0.8 0.5
Total
6801 2947
8118 2795
1.2
mental concentrations (TEC) of the ®ne fraction at both sampling sites were almost identical (3521 and 3732 ng/m3 , respectively). It is also important to note that the values given for TEC constitute only about 10±15% of the total PM2.5 suspended particulate matter. Fine soot particles containing mainly carbon with addition of H, O and N account for the other 90% of the total PM2.5 mass. 3.3. Normal weather conditions, coarse fraction Dominant elements on the coarse fraction are Al, Si, Fe, Ti and Na, Cl. These elements are all coming from natural sources, ®rst group from the soil erosion and the last two from the sea-spray. Sulfur and potassium are also present in this fraction but as our results of single particle analysis show [3,4] these elements are most likely present only as a thin coating on larger, soil and
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marine particles. Metals are present in almost identical concentrations as in the ®ne fraction. TEC of coarse fraction is 6801 ng/m3 , which is almost double the amount of the ®ne fraction. PM10 particulate matter was monitored only at AJC with the GENT sampler. 3.4. The period of haze During the haze period, which in Singapore started on 22 Aug. 1997 and lasted continuously till the mid-Nov., the average wind speed dropped to 0.3 m/s. For the same period, observed was almost threefold increase in concentrations of S, K and the total PM2.5 mass. It is also interesting to note that the PM2.5 mass, which was during the normal weather conditions found to be approximately 7 times higher than the TEC, was even 11 times higher during the haze period. Most of that mass is probably ®ne carbon soot. However, there was no signi®cant increase of the PM10 mass during the haze period. Concentrations of Cl and Br increased twofold as well. This is most likely a secondary eect caused by the missing sunlight. Namely, as Cl and Br are known to be highly volatile elements, they would normally evaporate from the particulate phase (due to the intense sunlight). However, during the haze period there was no direct sunlight for more than two months and these elements remained in the particulate phase. On the other hand, part of the noticed increase in concentrations of Cl and Br could also be due to forest ®res as these elements are often associated to biomass burning. Concentrations of P and Ti were slightly increased as well, but the changes are not considered signi®cant as the concentrations are close to detection limits. The other elements measured by PIXE did not change signi®cantly during the haze period. 3.5. Source apportionment To identify ®ngerprints of some major pollution sources in Singapore several sampling sessions were carried out at the proximity to the sources of interest. The sources that were of particular interest are industries located at the Jurong Industrial
Zone and the road trac. Characterization of the industrial sources was obtained during the S-SE Monsoon season (Sep.±Oct. 1996). Two identical ANSTO ®ne particulate samplers were employed; one positioned in the up-wind direction (RLH, see Fig. 1) and the other in down-wind direction (NTU) relative to the Industrial Zone. Sampling was carried out simultaneously at both sampling sites during the period of ®ve weeks. To characterize road trac, the same ®ne particulate sampler was placed into a very busy tunnel (Chin Swee Tunnel ± CST) and sampling was carried out during one week, four times a day. Results of the principal component analysis clearly show two major components: natural and anthropogenic. Within the natural component one can further identify soil (Al, Si, Fe, Ti, etc.) and sea-spray (Na, Mg, Cl) as major contributors. Within the anthropogenic component the following elements/groups of elements are dominant: sulfates (S), smoke (K), metals (Mn, Ni, Cu, Zn), construction site/road dust (Ca, K, Ti, Fe) and vehicular component (Pb, Br). Some of these elements are often associated to more than one source. For example, K, Ca, Ti, and Fe are often associated with both industrial and soil sources. To make fair apportionment of sources the mean elemental concentrations of sediment characteristic for Singapore obtained from one of our previous study [6] was used. According to that study it is estimated that the concentrations of K, Ca and Fe relative to Si in the soil are 6%, 6% and 12%, respectively. Knowing their relative concentrations, the following formulas were derived to calculate concentrations of all major sources: Natural sources: 1. Soil Al + Si + Si * 0.245 (where 0.245 is from 12% Fe, 6% K, 6% Ca and 0.5% Ti) 2. Marine Na + Mg + Cl Anthropogenic sources: 1. Sulfur S (only the concentration of pure S was taken into account), 2. Smoke K ÿ Si*0.06 (potassium minus Si attributed to the Soil component), 3. Metals V + Mn + Ni + Cu + Zn, 4. Construction site/ Road Dust P + Ca ÿ (Si * 0.06) + (Ti ÿ Si * 0.005) + (Fe ÿ Si*0.12), and 5. Vehicles Pb + Br.
I. Orlic et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 457±464
Please note that only concentrations of pure elements were taken into account and not their common chemical compounds (if corresponding compounds were used all concentrations would be 2±3 times higher). Results for RLH, NTU and CST are summarized in Table 3. Short-term measurements are compared with the long-term measurements obtained for the AJC and CSC (for normal conditions and the haze period). As expected, the most polluted air was found to be in the tunnel (CST) with very high concentrations of sulfur, metals and lead/bromine. Soil and a road dust were also 2±3 times higher than at the CSC location. However, concentrations of S and K were still not as high as during the haze period. On the other hand, the lowest concentrations were measured at the RLH where the concentrations of metals were found to be at least 15±20 times smaller than at the NTU site. This is expected as the RLH is far from the city and from all major industrial sources (and in the up-wind direction). However, the concentrations of S and K were still surpassingly high at this location. There are two possible explanations for that: 1. The highest contribution is probably coming from the local sources; sulfur from fossil fuel burning and potassium from incinerators. In spite of the dominant SE wind, these two elements diffuse to all directions during the night calms (to the South as well). In the morning, when the wind
463
pick-up, these ®ne aerosols simply drift back to Singapore, and to the RLH (it should be noted here that with the typical wind speed of 1 m/s blowing from 11:00 a.m. to 5:00 p.m. the air mass will drift with the wind only 20 km). 2. Part of the potassium and sulfur measured at the RLH is probably also coming from the minor forest ®res in the neighboring countries as these elements are often associated to the biomass burning processes. To get a more conclusive answer to this problem more simultaneous measurements should be carried out on larger distances from Singapore. 3.6. Size distribution Size distribution of air particulates was measured on several occasions during the haze period. The measurement was done by means of a sevenstage single ori®ce cascade impactor (PIXE Int.). A typical bimodal distribution was obtained. All anthropogenic elements (S, K, Pb, Br) were predominantly accumulated in the ®ne fraction (maximum at approximately 0.5 lm) while all natural components (Si, Ca, Ti, Fe) peaked at approximately 5 lm. Mean concentrations found on each stage of cascade impactor and derived from the three independent measurements are shown in Fig. 3. Only size distributions for two representative elements from each group are
Table 3 Average concentrations of several aerosol components identi®ed in samples collected at the CST, NTU, RLH, AJC and CSC (for details on sampling locations/periods see text. In this table, only concentrations of ®ne aerosols are given (PM2.5). Concentrations given for the AJC location are averaged over the two-year period (1996±1997) and for the CSC two sets of values are given, one for the 1996 and the other for the period of haze in the last quarter of 1997. All concentrations are given in ng/m3 ) Location
CST NTU RLH AJC CSC ± 96 CSC ± Haze
Natural components
Anthropogenic components
TEC
Seaspray (Na,Mg,Cl)
Soil (Al, Si,Ca,Fe, Ti)
Constr/ Road (P,Ca, Ti,Fe)
Fosil fuel (Sulfur)
Bio-Mass Smoke (K)
Inciner./ Cars (Mn,V, Ni,Cu,Zn)
Vehicles (Pb, Br)
557 308 593 487 565 497
1335 500 118 560 288 368
1523 627 54 287 198 389
4911 2888 1596 1907 2008 5827
476 321 233 333 285 1114
219 316 17 87 79 108
496 32 2 55 45 64
9518 4991 2767 3716 3468 8368
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I. Orlic et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 457±464
Fig. 3. Average elemental concentrations of S, K, Si and Fe on seven stages of cascade impactor measured during the haze period in 1997. A typical bimodal distribution was obtained with the anthropogenic elements accumulating in the sub-micron fraction and natural component at about 5 lm. The lowest stage with cut-o equivalent aerodynamic diameter of 0.1 lm is the backup ®lter (Bf). Error bars represent standard deviations from the three independent measurements. All concentrations are given in ng/m3 per stage.
shown: S, K from anthropogenic and Si and Fe from the natural component.
tions of 18 measured elements are obtained for two sampling locations and for the ®ne (PM2.5) and coarse (PM10) fractions. These values are compared with several short-term measurements that were conducted with the aim to identify ®ngerprints of several major pollutants; oil re®neries and road trac. Results were statistically evaluated using principal component analysis with the intention to identify major sources of the suspended aerosols. The results from bulk analysis shows that there are two major factors involved; one natural and the other anthropogenic. The impact of these components varies a lot depending on the sampling site and meteorological conditions. In the ®ne fraction anthropogenic component (mainly S, K, and metals) makes up to 70% of the TEC measured by PIXE while the other 30% belongs mainly to the natural components (soil, seaspray). In the coarse fraction the situation is inverse. During the haze period, the concentrations of sulfur, potassium and the PM2.5 were increased by factor 2±3. Acknowledgements Authors are indebted to the Ministry of the Environment, Singapore for their collaboration and assistance during the sampling as well as for willingness to share their data. Many thanks are also due to Mr. Chiam Sher Yi for his meticulous analysis of cascade impactor data and to Mr. T.F. Choo for operating the accelerator.
4. Conclusion PIXE and RBS analytical techniques were used to analyze more then 700 aerosol samples collected during a two-year period, 1996±1997. The main objective was to ®nd the average concentration of aerosols in Singapore and to identify major pollution sources and their impact. Our result shows that there are large temporal variations of concentrations of all measured elements. Such large changes are caused mainly by changes in meteorological conditions, in particular by wind direction and speed. Average concentra-
References [1] I. Orlic, B.W.F. Watt, S.M. Tang, Environ. Monitoring and Assessment 44 (1997) 455. [2] W.L. Bao, I. Orlic, S.M. Tang, Int. J. PIXE 5 (1995) 235. [3] I. Orlic, T. Osipowicz, F. Watt, S.M. Tang, Nucl. Instr. and Meth. B 104 (1995) 630. [4] I. Orlic, Nucl. Instr. and Meth. B 104 (1995) 602. [5] F. Watt, I. Orli, K.K. Loh, C.H. Sow, P. Thong, S.C. Liew, T. Osipowicz, T.F. Choo, S.M. Tang, Nucl. Instr. and Meth. B 85 (1994) 708. [6] S.M. Tang, I. Orlic, X.K. Wu, Nucl. Instr. and Meth. B 136±138 (1998) 1013.
Two years of aerosol pollution monitoring in Singapore: a review I. Orlic *, X. Wen, T.H. Ng, S.M. Tang Department of Physics, Research Center for Nuclear Microscopy, National University of Singapore, Kent Ridge, Singapore 119260, Singapore
Abstract An aerosol sampling campaign was initiated more than two years ago in Singapore. The aim was to determine the average elemental concentrations in ®ne and coarse aerosol fractions as well as to identify major pollution sources and their impact. For that purpose, two air samplers were employed at two dierent sampling locations; one sampler was a ®ne particulate aerosol sampler (PM2.5) located at the vicinity of a major industrial area. The other was a stacked ®lter unit (SFU) sampler designed for collection of ®ne and coarse fractions (PM2.5 and PM10) and installed in the residential area. Samples were taken typically twice a week and in several occasions daily. During the period of two years more than 700 aerosol samples were collected and analyzed using PIXE and RBS techniques. All samples were analyzed for 18 elements ranging between Na, Mg, Al, etc. up to As and Pb. Large daily and seasonal variations were found for most of the elements. These variations are attributed mainly to meteorological changes, in particular changes in wind speed and direction. On several occasions, short term sampling was performed to identify ®ngerprints of major pollution sources such as road trac, re®neries, as well as the rain-forest ®res in neighboring countries. A summary of our ®ndings is presented and discussed. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Aerosols; Pollution; Metals; PM2.5; PM10; Biomass burning
1. Introduction A two-year sampling campaign was initiated in Singapore at the end of 1996 with the aim to determine average elemental concentrations of aerosols in Singapore as well as to identify major pollution sources and their impact. To achieve this, PIXE and micro-PIXE analytical techniques were used. Some of our results obtained by the broad beam PIXE analysis are already reported in
* Corresponding author. Tel.: +65-874-2962; fax: +65-7776126; e-mail: [email protected]
previous publications [1,2]. Similarly, some of our preliminary results obtained by micro-PIXE technique are reported in publications [3,4]. In this work an overview of results of bulk analysis is given. PIXE results are statistically evaluated using principal component analysis with the intention to identify major suspended aerosol particle sources. The results show that there are two major factors involved; natural, and anthropogenic. In natural factor marine and soil components were found to be the major contributors and in anthropogenic factor, sulfates, smoke, metals, road dust and vehicular components were identi®ed. The impact of these components varied a lot
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 1 0 5 3 - 2
458
I. Orlic et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 457±464
depending on the sampling site and meteorological conditions. 2. Experimental 2.1. Sampling Aerosol samples have been collected twice weekly since Jan. 1996 at two sites: one site was the Civil Service College (CSC) located within the campus of the National University of Singapore (NUS) at Kent Ridge, and the other at the Anderson Junior College (AJC) located in the central part of Singapore ± see Fig. 1. The former site is in the West Coast area and is only a few kilometers southeast of the Jurong Industrial Estate. It is also northeast to a number of oil re®neries and a power station situated in a group of small islands o the West Coast of the Singapore Island. Although NUS is surrounded by an expressway and a few major roads with relatively heavy trac, the CSC building is up on a ridge surrounded by trees and not in the immediate vicinity of the expressway or any of these roads. The latter site is near the center of the Singapore Island and within one of the largest residential estates in Singapore. It is encircled by heavy-trac roads and highways. Other sampling sites, Nan Yang Technological University (NTU) and Raes Light House (RLH) were used only during a short-term sampling (one
month in 1996) to identify Ô®ngerprintÕ of major industrial pollutants located in Jurong Industrial Zone. The aerosol sampler used at CSC was a cyclone sampler manufactured by the Australian Nuclear Science and Technology Organization (ANSTO) designed to collect PM2.5 particulate matter. It was installed at the rooftop of the four-storey CSC building. To avoid overloading of ®lters the eective sampling time was reduced to a 12 h period per day (the sampler was switched on/o every alternate hour). The ¯ow-rate was maintained at approximately 22 l/min, which resulted with a typical sampling volume of approximately 16 m3 . The sampler used at AJC was a stacked ®lter unit (SFU) set to collect both PM10 as well as PM2.5 particulate matter. Two types of polycarbonate Nucleopore ®lters were used for the SFU; one with the pore size of 0.4 lm for collection of particles with Equivalent Aerodynamic Diameters (EADs) below 2.5 lm and the other with 8 lm pore size for particles with EADs between 2.5 and 10 lm. The sampler was equipped with the preimpaction stage to cut-o all particles with EAD > 10 lm. The sampler operated with a ¯owrate close to 15 l/min, hence drawing approximately 11 m3 per sampling session (day). The inlet of the SFU was installed at a height of 2 m above the ground level next to the College fence, which was approximately 40 m away from a busy road. During the entire sampling period sampling was on several occasions intensi®ed with the aim to identify ®ngerprint of individual sources such as road trac, industry, haze, etc. Results obtained are brie¯y outlined in this work. 2.2. Elemental analysis
Fig. 1. Map of Singapore with indicated sampling sites and the major industrial areas.
All samples were analyzed by means of PIXE and RBS techniques at the Research Center for the Nuclear Microscopy, NUS. The facility comprises the following major components: 2.5 MeV Van de Graa accelerator (operating at 2 MeV), Oxford Microbeams nuclear microscope end-stage and PC based Data Acquisition System. For the detection of X-rays, a LINK detector with a resolution of 150 eV at 5.9 keV and an active area of
I. Orlic et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 457±464
61 mm2 was used. Detailed description of the experimental setup used in this study can be found elsewhere [5]. 3. Results 3.1. Mean elemental concentrations Since December 1995, over 700 aerosol samples have been collected at the CSC and AJC sampling
459
locations. All samples were analyzed by means of PIXE and RBS techniques for concentrations of the following 18 elements: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Fe, Ni, Cu, Zn, Br, Pb. The concentrations of all elements were found to have great variations in time. This is caused mainly by the changes in the wind direction and speed and the fact that the major pollution sources in Singapore are localized and at the relatively short distance from each other and from the sampling locations.
Fig. 2. Top: Mean daily wind direction and speed during the entire sampling period ± Jan. 1996 to end of 1997. Full line on the top ®gure is a trend line to approximately indicate average wind direction. Middle: daily elemental concentrations of S, K and Si in the ®ne fraction of aerosols measured at CSC location. The bottom insert shows the total PM2.5 mass measured at the same location but only from May 1998 onwards. Note the signi®cant increase of S, K and PM2.5 mass during the last quarter of the 1997 when a severe forest ®res (referred to as the haze period) hit the SE Asian Region.
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For illustration, graphs of daily concentrations of S, K and Si measured at CSC location are shown in Fig. 2. All data points obtained during the entire sampling period are shown. On the top of the ®gure shown are mean daily wind directions and speeds. It is interesting to note that during the last two years a typical Monsoon pattern, characteristic for a tropical climate such as in Singapore, was disrupted. The rainy season characterized with the NE Monsoon (wintertime on the Northern Hemisphere) was much shorter than usually and started only in Feb. instead of in Dec. ± see Fig. 2. Throughout the rest of the 1996 and 1997-year, dry S-SE Monsoon dominated. The wind speed was generally low with average speed of approximately 1.1 m/s and it steadily decreased towards the last quarter of 1997 when the drought caused extensive forest ®res in Indo-
nesia and severe haze in the whole SE Asian region. The mean concentrations and the corresponding standard deviations of all measured elements obtained for the entire sampling periods are summarized in Tables 1 and 2. Concentrations measured for the CSC sampling location (the PM2.5 fraction) are given in Table 1 and for the AJC location in Table 2, separately for the ®ne and coarse fractions. All concentrations are given in lg/m3 separately for normal weather conditions (Jan. 1996±Aug. 1997) and for the period of haze (22 Aug.±7 Nov. 1997). PM2.5 mass was obtained by weighing ®lters before and after sample collection and it was measured only for the CSC location and for the second half of the 1997 year. Results are summarized in the following few sections.
Table 1 Mean elemental concentrations in PM2.5 fraction of aerosols sampled at CSC location (concentrations are given separately for the prehaze period (2 Jan. 1996±21 Aug. 1997) and for the haze period (22 Aug.±7 Nov. 1997). The total concentrations of all elements measured by PIXE is given in the second last row and the total PM2.5 mass in the last raw. All concentrations are expressed in lg/m3 . Concentration ratios between haze and non-haze periods are given in the last column) CSC PM2.5
Mean concentration (ng/m3 ) Pre-Haze (2 Jan. 96±21 Aug. 97)
Haze (22 Aug. 97±7 Nov. 97)
Na Mg Al Si P S Cl K Ca Ti V Mn Fe Ni Cu Zn Br Pb
212 287 52 53 84 56 173 102 27 32 2121 1023 166 536 332 238 116 70 7.6 5.9 18.5 11.5 7.2 6.9 105 64 5.0 3.3 5.2 5.7 43 39 15 19 29 23
87 50 40 13 100 42 201 78 104 59 5344 2062 343 233 1060 366 171 71 17.1 10.0 24.2 13.2 6.2 7.9 123 83 6.1 3.1 6.5 6.1 68 143 30 21 31 28
0.4 0.8 1.2 1.2 3.8 2.5 2.1 3.2 1.5 2.2 1.3 0.9 1.2 1.2 1.3 1.6 2.0 1.1
Total PM2.5
3521 1817 24957 10144
7763 2666 86243 48817
2.2 3.5
Ratio
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Table 2 Mean elemental concentrations in ®ne (PM2.5) and coarse fraction (PM10 minus PM2.5) of aerosols sampled at AJC location (concentrations are given separately for the pre-haze period and for the haze period. The total concentrations of all elements measured by PIXE are given in the last row. Concentration ratios between haze and non-haze periods are given in the last column) AJC (Fine)
Mean concentration (ng/m3 ) Pre-Haze (2 Jan. 96 ±21 Aug. 97)
Haze (22 Aug. 97 ±7 Nov. 97)
Na Mg Al Si P S Cl K Ca Ti V Mn Fe Ni Cu Zn Br Pb
207 304 188 313 231 313 278 299 153 310 1909 1010 105 138 315 187 82 43 4.4 5.2 8.5 9.5 5.2 5.8 115 69 7.1 6.3 26.6 55.0 41 33 17 11 42 39
40 50 84 31 192 68 240 86 112 68 3029 967 91 26 570 202 131 107 2.5 4.1 3.0 4.0 4.7 4.1 117 57 14 5 27 18 56 56 19 8 21 18
0.2 0.4 0.8 0.9 0.7 1.6 0.9 1.8 1.6 0.6 0.4 0.9 1.0 2.0 1.0 1.4 1.2 0.5
Total
373 2402
4753 956
1.3
Ratio
3.2. Normal weather conditions, ®ne fraction As expected, sulfur is the most dominant element in the ®ne fraction, with mean concentration of approximately 2000 ng/m3 at both sampling sites. Major sources of sulfur in Singapore are oil re®neries and oil-powered power plants, which are mainly located on the SW part of Singapore. Sulfur is followed by potassium with an average concentration slightly higher than 300 ng/m3 . Potassium is known as one of the major indicators of biomass burning processes. It is therefore likely that part of the measured concentration is coming from the local sources such as incinerators and part of it is probably due to a long-range transport from minor forest ®res in Indonesia. In spite of large temporal variations, concentrations of all elements show surprisingly good agreement at both sampling locations. Consequently, the overall mean values of the total ele-
AJC (Coarse)
Mean concentration (ng/m3 )
Ratio
Pre-Haze (2 Jan. 96 ±21 Aug. 97)
Haze (22 Aug. 97 ±7 Nov. 97)
Na Mg Al Si P S Cl K Ca Ti V Mn Fe Ni Cu Zn Br Pb
539 534 322 361 686 423 1217 600 142 194 677 322 1503 1372 269 113 765 406 37.0 20.3 3.4 6.4 6.7 9.0 491 231 4.7 5.9 28 27 39 48 43 19 27 31
269 197 209 80 722 279 1488 686 117 75 1147 456 1253 1131 444 185 1657 1454 30.3 28.7 1.4 4.3 8.7 3.9 644 297 11.8 5.8 32 23 38 34 35 11 12 10
0.5 0.6 1.1 1.2 0.8 1.7 0.8 1.6 2.2 0.8 0.4 1.3 1.3 2.5 1.1 1.0 0.8 0.5
Total
6801 2947
8118 2795
1.2
mental concentrations (TEC) of the ®ne fraction at both sampling sites were almost identical (3521 and 3732 ng/m3 , respectively). It is also important to note that the values given for TEC constitute only about 10±15% of the total PM2.5 suspended particulate matter. Fine soot particles containing mainly carbon with addition of H, O and N account for the other 90% of the total PM2.5 mass. 3.3. Normal weather conditions, coarse fraction Dominant elements on the coarse fraction are Al, Si, Fe, Ti and Na, Cl. These elements are all coming from natural sources, ®rst group from the soil erosion and the last two from the sea-spray. Sulfur and potassium are also present in this fraction but as our results of single particle analysis show [3,4] these elements are most likely present only as a thin coating on larger, soil and
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marine particles. Metals are present in almost identical concentrations as in the ®ne fraction. TEC of coarse fraction is 6801 ng/m3 , which is almost double the amount of the ®ne fraction. PM10 particulate matter was monitored only at AJC with the GENT sampler. 3.4. The period of haze During the haze period, which in Singapore started on 22 Aug. 1997 and lasted continuously till the mid-Nov., the average wind speed dropped to 0.3 m/s. For the same period, observed was almost threefold increase in concentrations of S, K and the total PM2.5 mass. It is also interesting to note that the PM2.5 mass, which was during the normal weather conditions found to be approximately 7 times higher than the TEC, was even 11 times higher during the haze period. Most of that mass is probably ®ne carbon soot. However, there was no signi®cant increase of the PM10 mass during the haze period. Concentrations of Cl and Br increased twofold as well. This is most likely a secondary eect caused by the missing sunlight. Namely, as Cl and Br are known to be highly volatile elements, they would normally evaporate from the particulate phase (due to the intense sunlight). However, during the haze period there was no direct sunlight for more than two months and these elements remained in the particulate phase. On the other hand, part of the noticed increase in concentrations of Cl and Br could also be due to forest ®res as these elements are often associated to biomass burning. Concentrations of P and Ti were slightly increased as well, but the changes are not considered signi®cant as the concentrations are close to detection limits. The other elements measured by PIXE did not change signi®cantly during the haze period. 3.5. Source apportionment To identify ®ngerprints of some major pollution sources in Singapore several sampling sessions were carried out at the proximity to the sources of interest. The sources that were of particular interest are industries located at the Jurong Industrial
Zone and the road trac. Characterization of the industrial sources was obtained during the S-SE Monsoon season (Sep.±Oct. 1996). Two identical ANSTO ®ne particulate samplers were employed; one positioned in the up-wind direction (RLH, see Fig. 1) and the other in down-wind direction (NTU) relative to the Industrial Zone. Sampling was carried out simultaneously at both sampling sites during the period of ®ve weeks. To characterize road trac, the same ®ne particulate sampler was placed into a very busy tunnel (Chin Swee Tunnel ± CST) and sampling was carried out during one week, four times a day. Results of the principal component analysis clearly show two major components: natural and anthropogenic. Within the natural component one can further identify soil (Al, Si, Fe, Ti, etc.) and sea-spray (Na, Mg, Cl) as major contributors. Within the anthropogenic component the following elements/groups of elements are dominant: sulfates (S), smoke (K), metals (Mn, Ni, Cu, Zn), construction site/road dust (Ca, K, Ti, Fe) and vehicular component (Pb, Br). Some of these elements are often associated to more than one source. For example, K, Ca, Ti, and Fe are often associated with both industrial and soil sources. To make fair apportionment of sources the mean elemental concentrations of sediment characteristic for Singapore obtained from one of our previous study [6] was used. According to that study it is estimated that the concentrations of K, Ca and Fe relative to Si in the soil are 6%, 6% and 12%, respectively. Knowing their relative concentrations, the following formulas were derived to calculate concentrations of all major sources: Natural sources: 1. Soil Al + Si + Si * 0.245 (where 0.245 is from 12% Fe, 6% K, 6% Ca and 0.5% Ti) 2. Marine Na + Mg + Cl Anthropogenic sources: 1. Sulfur S (only the concentration of pure S was taken into account), 2. Smoke K ÿ Si*0.06 (potassium minus Si attributed to the Soil component), 3. Metals V + Mn + Ni + Cu + Zn, 4. Construction site/ Road Dust P + Ca ÿ (Si * 0.06) + (Ti ÿ Si * 0.005) + (Fe ÿ Si*0.12), and 5. Vehicles Pb + Br.
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Please note that only concentrations of pure elements were taken into account and not their common chemical compounds (if corresponding compounds were used all concentrations would be 2±3 times higher). Results for RLH, NTU and CST are summarized in Table 3. Short-term measurements are compared with the long-term measurements obtained for the AJC and CSC (for normal conditions and the haze period). As expected, the most polluted air was found to be in the tunnel (CST) with very high concentrations of sulfur, metals and lead/bromine. Soil and a road dust were also 2±3 times higher than at the CSC location. However, concentrations of S and K were still not as high as during the haze period. On the other hand, the lowest concentrations were measured at the RLH where the concentrations of metals were found to be at least 15±20 times smaller than at the NTU site. This is expected as the RLH is far from the city and from all major industrial sources (and in the up-wind direction). However, the concentrations of S and K were still surpassingly high at this location. There are two possible explanations for that: 1. The highest contribution is probably coming from the local sources; sulfur from fossil fuel burning and potassium from incinerators. In spite of the dominant SE wind, these two elements diffuse to all directions during the night calms (to the South as well). In the morning, when the wind
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pick-up, these ®ne aerosols simply drift back to Singapore, and to the RLH (it should be noted here that with the typical wind speed of 1 m/s blowing from 11:00 a.m. to 5:00 p.m. the air mass will drift with the wind only 20 km). 2. Part of the potassium and sulfur measured at the RLH is probably also coming from the minor forest ®res in the neighboring countries as these elements are often associated to the biomass burning processes. To get a more conclusive answer to this problem more simultaneous measurements should be carried out on larger distances from Singapore. 3.6. Size distribution Size distribution of air particulates was measured on several occasions during the haze period. The measurement was done by means of a sevenstage single ori®ce cascade impactor (PIXE Int.). A typical bimodal distribution was obtained. All anthropogenic elements (S, K, Pb, Br) were predominantly accumulated in the ®ne fraction (maximum at approximately 0.5 lm) while all natural components (Si, Ca, Ti, Fe) peaked at approximately 5 lm. Mean concentrations found on each stage of cascade impactor and derived from the three independent measurements are shown in Fig. 3. Only size distributions for two representative elements from each group are
Table 3 Average concentrations of several aerosol components identi®ed in samples collected at the CST, NTU, RLH, AJC and CSC (for details on sampling locations/periods see text. In this table, only concentrations of ®ne aerosols are given (PM2.5). Concentrations given for the AJC location are averaged over the two-year period (1996±1997) and for the CSC two sets of values are given, one for the 1996 and the other for the period of haze in the last quarter of 1997. All concentrations are given in ng/m3 ) Location
CST NTU RLH AJC CSC ± 96 CSC ± Haze
Natural components
Anthropogenic components
TEC
Seaspray (Na,Mg,Cl)
Soil (Al, Si,Ca,Fe, Ti)
Constr/ Road (P,Ca, Ti,Fe)
Fosil fuel (Sulfur)
Bio-Mass Smoke (K)
Inciner./ Cars (Mn,V, Ni,Cu,Zn)
Vehicles (Pb, Br)
557 308 593 487 565 497
1335 500 118 560 288 368
1523 627 54 287 198 389
4911 2888 1596 1907 2008 5827
476 321 233 333 285 1114
219 316 17 87 79 108
496 32 2 55 45 64
9518 4991 2767 3716 3468 8368
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Fig. 3. Average elemental concentrations of S, K, Si and Fe on seven stages of cascade impactor measured during the haze period in 1997. A typical bimodal distribution was obtained with the anthropogenic elements accumulating in the sub-micron fraction and natural component at about 5 lm. The lowest stage with cut-o equivalent aerodynamic diameter of 0.1 lm is the backup ®lter (Bf). Error bars represent standard deviations from the three independent measurements. All concentrations are given in ng/m3 per stage.
shown: S, K from anthropogenic and Si and Fe from the natural component.
tions of 18 measured elements are obtained for two sampling locations and for the ®ne (PM2.5) and coarse (PM10) fractions. These values are compared with several short-term measurements that were conducted with the aim to identify ®ngerprints of several major pollutants; oil re®neries and road trac. Results were statistically evaluated using principal component analysis with the intention to identify major sources of the suspended aerosols. The results from bulk analysis shows that there are two major factors involved; one natural and the other anthropogenic. The impact of these components varies a lot depending on the sampling site and meteorological conditions. In the ®ne fraction anthropogenic component (mainly S, K, and metals) makes up to 70% of the TEC measured by PIXE while the other 30% belongs mainly to the natural components (soil, seaspray). In the coarse fraction the situation is inverse. During the haze period, the concentrations of sulfur, potassium and the PM2.5 were increased by factor 2±3. Acknowledgements Authors are indebted to the Ministry of the Environment, Singapore for their collaboration and assistance during the sampling as well as for willingness to share their data. Many thanks are also due to Mr. Chiam Sher Yi for his meticulous analysis of cascade impactor data and to Mr. T.F. Choo for operating the accelerator.
4. Conclusion PIXE and RBS analytical techniques were used to analyze more then 700 aerosol samples collected during a two-year period, 1996±1997. The main objective was to ®nd the average concentration of aerosols in Singapore and to identify major pollution sources and their impact. Our result shows that there are large temporal variations of concentrations of all measured elements. Such large changes are caused mainly by changes in meteorological conditions, in particular by wind direction and speed. Average concentra-
References [1] I. Orlic, B.W.F. Watt, S.M. Tang, Environ. Monitoring and Assessment 44 (1997) 455. [2] W.L. Bao, I. Orlic, S.M. Tang, Int. J. PIXE 5 (1995) 235. [3] I. Orlic, T. Osipowicz, F. Watt, S.M. Tang, Nucl. Instr. and Meth. B 104 (1995) 630. [4] I. Orlic, Nucl. Instr. and Meth. B 104 (1995) 602. [5] F. Watt, I. Orli, K.K. Loh, C.H. Sow, P. Thong, S.C. Liew, T. Osipowicz, T.F. Choo, S.M. Tang, Nucl. Instr. and Meth. B 85 (1994) 708. [6] S.M. Tang, I. Orlic, X.K. Wu, Nucl. Instr. and Meth. B 136±138 (1998) 1013.