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Platinum concentrations in ambient aerosol at a coastal site in South China
ARTICLE IN PRESS
Atmospheric Environment 39 (2005) 2625–2630 www.elsevier.com/locate/atmosenv
Platinum concentrations in ambient aerosol at a coastal site in South China S.F. Kan, P.A. Tanner Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, SAR, PR China Received 10 November 2004; received in revised form 30 December 2004; accepted 11 January 2005
Abstract With the burgeoning vehicle ownership in China, the emissions of certain trace pollutants are becoming more widespread and of more concern. We present results from the first measurements of platinum in airborne total suspended particulate matter (TSP) at a suburban (CityU) and a roadside (MK) sampling site in southern China at Hong Kong. TSP samples were taken with high volume air samplers at these sites with low and high traffic density, respectively, on a 2-day basis and were analyzed by inductively coupled plasma—dynamic reaction cell—mass spectrometry (ICP-DRC-MS). The results are similar from the use of two different isotopes: 194 and 195. The 48-h average Pt concentrations were 8.8 pg m3 at CityU and 24.1 pg m3 at MK, being higher at the site with greater traffic density, and were in the range from 6 to 38 pg m3. The airborne Pt was found to be significantly correlated with vehicle tracer species (Zr and Ce) as well as with NO and NOx. The ratio Ce/Zr was the same at both sites whereas the ratios Pt/ Zr and Pt/Ce were about 20% greater at the MK site, showing the different mobility of Pt, attributed to chemical and/ or particle size effects. r 2005 Elsevier Ltd. All rights reserved. Keywords: Platinum; Particulate matter; Catalytic converter; Vehicle emissions
1. Introduction As at 2003, there are 0.36 million petrol and 6000 diesel vehicles equipped with catalytic converters in Hong Kong, corresponding to about 97% and 5% of the respective total numbers. Concomitant with the widespread use of catalytic converters in vehicles, since about 0.08% of a two-way catalyst comprises Pt, the ambient Pt concentrations are now of concern because of their adverse health effects. Although total suspended particulate matter (TSP) is routinely monitored in Hong Corresponding author. Tel.: +852 2788 7840; fax: +852 2788 7406. E-mail address: [email protected] (P.A. Tanner).
Kong and elsewhere for trace elements there are various limitations in the analytical methods for the determinand Pt. The first literature about Pt in airborne particulate matter appeared in 1975. Soluble Pt (halogenated Pt) has been considered to be more toxic than Pt compounds with low solubility. Recently, an extensive review of Pt group elements in the environment and their health risks was presented (Ravindra et al., 2004). Before the introduction of automobile catalysts, Pt was not detected in air samples in USA and Europe (Johnson et al., 1975, 1976; Ko¨nig et al., 1992). Zereini et al. (2001) studied Pt concentrations in airborne particulate matter (PM) in Germany and found there was 46-fold increase in Pt concentrations from 1988 to 1999. Seventy-five percent of Pt was found to occur in
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.01.014
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S.F. Kan, P.A. Tanner / Atmospheric Environment 39 (2005) 2625–2630
association with PM40.2 mm aerodynamic diameter and 10% of Pt was found to be soluble in 0.1 M HCl (Zereini et al., 2001). Besides, Go´mez et al. (2001) monitored Pt concentrations for 1 year (from 1998 to 1999) at seven sampling sites in Spain and found no seasonal change. In addition, the Pt distribution in airborne PM was studied by two different samplers, a wide-range aerosol classifier collector (10–65.3 mm) and a seven-stage PM-10 cascade impactor (o 0.39–9 mm). Pt was found to be in a wide range of particle sizes, indicating a rather inhomogenous distribution of Pt in airborne PM. The inductively coupled plasma mass spectrometric (ICP-MS) trace determination of Pt is strongly hampered by the isobaric interferences of HfO+ and HfOH+. Due to their simplicity, empirical corrections have been widely used in the literature to correct for the former (Rauch et al., 2001; Nischwitz et al., 2003; Djingova et al., 2003). We have recently pointed out the problems with such corrections—in particular, the fact that measurements of Hf are also beset by isobaric interferences. We overcame such problems by removing the interferences by dynamic reaction cell (DRC) technology using ammonia gas (Kan and Tanner, 2004). Bocca et al. (2003) and Kanitsar et al. (2003) have used sector-field ICP-MS for platinum determination but in low-resolution mode (mass resolution 300) and they corrected interferences by using a membrane desolvation unit and isotope dilution. In this study, ambient Pt concentrations were measured at urban and suburban sampling sites from January 2004 to March 2004 on a 2 day basis. Two different Pt isotopes, 194 and 195 were selected for analysis and intercomparison. The concentrations of SO2, NOx, NO2, NO, CO, as well as physical parameters, were simultaneously studied and correlated with the measured Pt concentrations. In addition, the relationships between the ambient concentrations of two vehicle tracer species (Ce and Zr) and the Pt concentrations at both sites were measured. Finally, the Pt concentrations in Hong Kong have been compared with those at other locations.
Hong Kong Environmental Protection Department (HK EPD) roadside monitoring station next to a busy road with annual average daily traffic of 40,000; 221180 N, 1141100 E; see http://www.epd-asg.gov.hk/english/backgd/Mong_Kok.php) from 27 January to 5 March 2004. The flow rate was ca. 1 m3 min1 and the sampling duration was 48 h. No other sources of Pt at these sites, other than vehicles, are known. After sampling, the filter paper was immediately cut into small pieces and divided into four sections, of which two served as a replicate. Each section was digested in 20 cm3 of aqua regia using a microwave oven (CEM Corporation, Matthews, USA) at 630 W power for 30 min, according to US EPA method 3051. After cooling, 5 cm3 of aqua regia was added and the sample was mineralized again for 30 min. Then, the digested sample was filtered and the two sub-samples were mixed together and preconcentrated by a rotary evaporator at 80 1C to about 5 cm3. Finally, the sample was diluted to 10 cm3 with milli-Q-water. Ce and Zr were analyzed by ICP-MS using the standard mode after suitable (ca. 100 times) sample dilution. Real-time ambient pollutant gas measurements at the MK site were kindly provided by the HK EPD, who operate a rigorous quality assurance program for their measurements (EPD, 2003). 2.2. DRC technology The DRC technology enables the rejection of polyatomic species with the same mass to charge ratio as Pt, notably the HfO+ species, by the charge-transfer reaction with NH3 gas. An Elan 6100 DRC-ICP-MS instrument (Perkin-Elmer, Beaconsfield, UK) fitted with a peristaltic pump, Meinhard-type nebulizer, quartz concentric spray chamber, nickel skimmer and sample cone was employed for the instrumental analyses. NH3 reaction gas (99.999% purity, Hong Kong Special Gas) was employed. The instrumental conditions of cell gas flow rate and Rpq value were 1.05 cm3 min1 and 0.75. Other parameter settings were as given for our previous study of Pt in road dust (Kan and Tanner, 2004) dust and are listed in Table 1.
2. Methodology 2.3. Internal standard and quality control 2.1. Sample collection and digestion 193
As pointed out above, Pt is present in a wide range of particle sizes in air, so that we collected TSP by a high volume air sampler (Andersen Samplers, Inc) using a 800 1000 quartz fiber filter at the roof (51 m PD, Hong Kong grid location 221210 N, 1141100 E) of City University of Hong Kong (CityU: suburban site, about 20 m from a light-traffic road; see http://www.cityu.edu.hk/ cityu/visitors/location.htm) and at Mong Kok (MK: a
Ir(NH3)02 (formed in the reaction cell from Ir+ standard solution spiked into samples and standards) was selected as the internal standard for Pt analysis in the DRC mode. The certified reference materials (CRM) of NIST 2557 for autocatalyst and BCR 723 for road dust were used for method validation with the respective recoveries being 93% using 194Pt and 94% for the 195Pt isotope for NIST 2557; and for BCR 723: 95% for 194Pt and 79% for 195Pt.
ARTICLE IN PRESS S.F. Kan, P.A. Tanner / Atmospheric Environment 39 (2005) 2625–2630
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Table 1 Parameter settings and the instrumental operating conditions of the ICP-DRC-MS
Table 2 Pt concentrations (pg m3) measured in TSP at CityU and MK stations in Hong Kong using two different Pt isotopes
Parameter (unit)
Value
Date in 2004
Nebulizer gas flow (NGF) (dm3 min1) Auxiliary gas flow (dm3 min1) Plasma gas flow (dm3 min1) Lens voltage (V) ICP RF power (W) Analog stage voltage (V) Pulse stage voltage (V) Std quadrupole rod offset (QRO) Std cell rod offset (CRO) Discriminator threshold Std cell path voltage (CPV) (V) Rpa Rpq DRC mode (NGF) (dm3 min1) DRC mode QRO DRC mode CRO DRC mode CPV (V) Cell gas flow (cm3 min1)
0.9–1.2 1.20 15.00 8–10 1100–1300 1850 1175 0.00 11.00 70.00 31.00 0.00 a
0.9–1.2 8.00 2.00 33.00
Sampling site
Pt conc. in TSP (pg m3) 194
195
Pt
Pt
27 Jan
MK CityU
23.0 7.4
23.6 7.5
11 Feb
MK CityU
37.4 8.9
38.2 9.3
23 Feb
MK CityU
23.2 7.9
24.5 8.4
25 Feb
MK CityU
22.8 8.7
23.1 8.8
27 Feb
MK CityU
13.5 5.9
13.7 6.5
1 Mar
MK CityU
28.9 7.7
29.2 7.9
3 Mar
MK CityU
17.3 10.8
17.7 11.1
5 Mar
MK CityU
24.9 12.4
25.4 12.6
Average Average
MK CityU
23.9 8.7
24.4 9.0
a
QRO is the voltage applied to analyzer quadrupole; CRO is the voltage applied to the quadrupole rods in the DRC; CPV is the voltage applied to apertures at either end of the DRC and the prefilter before the analyzer quadrupole; Rpa is a high mass cut off that controls the dc voltage applied to the quadrupole rods; Rpq is the low mass cutoff that controls the RF frequency applied to the quadrupole rods; Std is the standard mode. a Need to be optimized.
The method LOD (expressed as three times the standard deviation of the sample blank solution) for Pt in ambient TSP, using the Pt isotopes 194 and 195, under the standard mode is 1–1.2 pg m3 whereas it is 0.5–0.7 pg m3 in the DRC mode. The coefficients of variation (¼ 100(standard deviation)/mean) of measurements for 194Pt and 195Pt in the samples were 8.4% and 7.8%, respectively.
3. Results and discussion 3.1. Pt concentrations in ambient TSP Table 2 summarizes the Pt concentrations (measured using 194Pt and 195Pt) in ambient TSP collected at the CityU and MK stations. The Pt concentrations at MK were in the range from 14 to 38 pg m3, with an average value of 24 pg m3. The Pt concentrations at rooftop level at suburban CityU were in the range from 6 to 12 pg m3, with an average value of 9 pg m3. As expected, Pt concentrations at MK roadside station are higher than that at CityU station, but as Zereini et al. (2001) noted, Pt-containing particles are not restricted to deposition in the immediate vicinity of
roads. Compared with the mean values for Pt at each site, the concentrations at MK showed a wider variability (from 0.55 to 1.56 of the mean) than those at CityU (from 0.69 to 1.37 of the mean value). The ratio of the concentrations of one determinand measured on the same day at the two sites MK/CityU were 2.7 for Pt and 2.3 for Ce and Zr, showing a greater decrease for Pt on going from the roadside to the suburban site. The Pt concentrations measured at both sites in this study are well below the guidance values of 15–150 ng m3 (Zereini et al., 2001; Merget and Rosner, 2001). Previous studies have shown that Pt concentrations are strongly influenced by a variety of different factors, such as traffic volume on a particular roadway, the number of vehicles equipped with catalytic converters, the vehicle types and associated emission amounts (e.g. city or highway driving), the particle size of airborne PM and meteorological conditions (Ko¨nig et al., 1992; Helmers et al., 1998; Go´mez et al., 2001). It can be concluded that since the ambient Pt concentrations in TSP in Hong Kong are at least several orders of magnitude lower than the critical values there is no evidence to suggest that at present they pose a health risk such as allergic reactions to the general public.
ARTICLE IN PRESS S.F. Kan, P.A. Tanner / Atmospheric Environment 39 (2005) 2625–2630
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3.2. Correlation of Pt concentrations with other measurements
Tracer species conc. (µg m-3)
30 Ce 20
10 Zr 0 0
10
20
30
40
-3
(a)
Pt conc. (pg m )
Gas conc. (µg m-3)
500 NOx
400 300 200 100 0 10
NO 15
(b)
20 25 30 Pt conc. (pg m-3)
35
40
Fig. 1. Plots of Pt concentrations (measured as 194Pt) in TSP against the vehicle tracer species (Ce and Zr) at both sites (a) and with NO and NOx at MK station (b). The regression lines are drawn as a guide to the eye.
The autocatalyst washcoat contains zirconium and cerium oxides: the first of these functions as a stabilizer and the second as an oxygen reservoir. Ce is also a fuel additive. Fig. 1(a) shows the correlations of Pt concentrations with the two vehicle tracer species (Ce and Zr) at both sites. Significant correlations were observed between the concentrations of Ce (R2 ¼ 0:62; Po0.05; N ¼ 16) or Zr (R2 ¼ 0:75; Po0.05; N ¼ 16), and those of Pt. As Rauch et al. (2000) pointed out, the composition of autocatalysts has changed considerably since their introduction in 1975 and it also varies from one manufacturer to another. A typical SEM/EDX analysis of a fresh washcoat of a three-way catalytic converter gave the mass ratios Ce/Pt 10.3 and Zr/Pt 7.0 (Palacios et al., 2000). Go´mez et al. (2001) found average ratios Ce/Pt and Zr/Pt of 10 and 16 in exhaust fumes, but ratios of 363 and 802 in airborne PM10 samples. These authors commented that although a high Pt content in PM10 is found together with high Ce and Zr concentrations showing that the source of these pollutants is traffic-related, factors such as the mobility and particle-sizes of these species may be important in determining these ratios. Furthermore, Ce and Zr are also present in other (e.g. Pd) types of catalyst. Our present results indicate a constant ratio of Ce/Zr (1.79) at both sites, whereas the ratio of Pt/Ce and Pt/Zr is about 20% greater at MK. The Ce/Pt ratio is 714 at CityU and 605 at MK, whereas the Zr/Pt ratio is 400 at CityU and 337 at MK. Thus the transport of Pt differs
Table 3 Concentrations of Pt in PM (pg m3) at various locations City (country)
Year
Particle size
Pt conc.
Reference
California (USA) California (USA) Tsukuba (Japan) Offenbach (Germany) Dortmund (Germany) Munich (Germany) Munich (Germany)
1975 1976 1983 1988 1991 1993 1993 1995 1995 1998 1998 1998 1998 1999 2002 2002 2002 2004 2004
TSP TSP TSP TSP 0.5–8 mm TSP PM10 PM10 PM10 PM10 PM10 PM10 TSP PM10 PM10 2–10 mm 2–10 mm TSP TSP
o0.05 o0.06 o0.18a 3 o5.1 7.3 7.3 21.5 13.6 12.8 17.8 12.7 147 o19 4.3 8 1.4 24 9
Johnson et al. (1975) Johnson et al. (1976) Mukai et al. (1990) Zereini et al. (2001) Alt et al. (1993) Schierl and Fruhmann (1996) Schierl (2000) Schierl (2000) Dietl et al. (2000) Go´mez et al. (2001) Bocca et al. (2003) Petrucci et al. (2000) Zereini et al. (2001) Rauch et al. (2001) Kanitsar et al. (2003) Za´ray et al. (2004) Za´ray et al. (2004) This study, roadside This study, suburban
Munich (Germany) Madrid (Spain) Rome (Italy) Rome (Italy) Frankfurt (Germany) Go¨teborg (Sweden) Vienna (Austria) Vienna (Austria) Budapest (Hungary) Hong Kong (China) Hong Kong (China) a
mg g1.
ARTICLE IN PRESS S.F. Kan, P.A. Tanner / Atmospheric Environment 39 (2005) 2625–2630
from these other two species (due to chemical and/or particle size effects), and/or there are additional sources of Ce and Zr at the CityU site. However the latter postulate is unlikely since the ratio of Ce/Zr is the same at both sites. Concentrations of the gases SO2, NOx, NO2, NO, CO at MK, as well as the synoptic wind speed, were averaged over the sampling periods and correlated with the measured Pt concentrations. Only NOx (R2 ¼ 0:52; P ¼ o0:05; N ¼ 8) and NO (R2 ¼ 0:72; P ¼ o0:05; N ¼ 8) were significantly correlated with Pt concentrations (Fig. 1(b)), which can be attributed to their vehicles emissions source. The average concentrations of NO and NOx over the sampling period at MK were 350 and 157 mg m3, respectively.
4. Comparison of ambient Pt concentrations with those in other countries Limited studies are available for comparison, with only one previous Asian work performed in Japan in 1983 (Mukai et al., 1990). From Table 3 it is evident that Pt concentrations in the atmosphere have increased in the last 20 years by more than two orders of magnitude in other countries. From our study, Pt concentrations in airborne TSP in Hong Kong are found to slightly higher than those in recent studies in urban environments of some European countries except for the very high values for Pt in roadside air at Frankfurt in 1998, measured by adsorptive voltammetry. In all 3.25 million vehicles were manufactured in China in 2002 and with the projected rapid growth in the numbers of new vehicles it is important to carefully monitor the concentrations of Pt in urban environments in future.
Acknowledgements Financial assistance for this work from City University Research Grant 9360099 is gratefully acknowledged. We thank the Hong Kong EPD for providing monitoring data for gases at MK station.
References Alt, F., Bambauer, A., Hoppstock, K., Mergler, B., To¨lg, G., 1993. Determination of whole contents, particle size distribution and soluble platinum. Fresenius Journal of Analytical Chemistry 346, 693–696. Bocca, B., Petrucci, F., Alimonti, A., Caroli, S., 2003. Trafficrelated platinum and rhodium concentrations in the atmosphere of Rome. Journal of Environmental Monitoring 5, 563–568.
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Dietl, C., Laschka, D., Wa¨ber, M., Peichl, L., 2000. Biomonitoring of platinum emissions from motor vehicles. In: Zereini, F., Alt, F. (Eds.), Anthropogenic Platinum-group Elements Emissions. Their impact on Man and Environment. Springer, Berlin, pp. 65–71. Djingova, R., Heidenreich, H., Kovacheva, P., Markert, B., 2003. On the determination of platinum group elements in environmental materials by inductively coupled plasma mass spectrometry and microwave digestion. Analytica Chimica Acta 489, 245–251. EPD, 2003. Hong Kong Environmental Protection Department. Air Quality in Hong Kong, 2003. Government Printer, Hong Kong. Go´mez, B., Go´mez, M., Sanchez, J.L., Ferna´ndez, R., Palacios, M.A., 2001. Platinum and rhodium distribution in airborne particulate matter and road dust. The Science of the Total Environment 269, 131–144. Helmers, E., Schwarzer, M., Schuster, M., 1998. Comparison of palladium and platinum in environmental matrices: palladium pollution by automobile emission? Environmental Science and Pollution Research 5, 44–50. Johnson, D.E., Tillery, B., Prevost, R.J., 1975. Levels of palladium, platinum and lead in populations of Southern California. Environmental Health Perspectives 12, 27–33. Johnson, D.E., Prevost, R.J., Tillery, J.B., Caman, D.E., Hosenfeld, J.M., 1976. Baseline levels of platinum and palladium in human tissue. Report: EPA/600/1-76/019, Southwest Research Institution. Kan, S.F., Tanner, P.A., 2004. Determination of platinum in roadside dust samples by dynamic reaction cell—inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry 19, 639–643. Kanitsar, K., Koellensperger, G., Hann, S., Limbeck, A., Puxbaum, H., Stingeder, G., 2003. Determination of Pt, Pd and Rh by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) in size-classified urban aerosol samples. Journal of Analytical Atomic Spectrometry 18, 239–246. Ko¨nig, H.P., Hertel, R.F., Koch, W., Rosner, G., 1992. Determination of platinum emissions from a three-way catalyst-equipped gasoline engine. Atmospheric Environment 26, 741–745. Merget, R., Rosner, G., 2001. Evaluation of the health risk of platinum group metals emitted from automotive catalytic converters. The Science of the Total Environment 270, 165–173. Mukai, H., Ambe, Y., Morita, M., 1990. Flow injection inductively coupled plasma mass spectrometry for the determination of platinum in airborne particulate matter. Journal of Analytical Atomic Spectrometry 5, 75–80. Nischwitz, V., Michalke, B., Kettrup, A., 2003. Speciation of Pt(II) and Pt(IV) in spiked extracts from road dust using online liquid chromatography-inductively coupled plasma mass spectrometry. Journal of Chromatography A 1016, 223–234. Palacios, M.A., Go´mez, M.M., Moldovan, M., Morrison, G., Rauch, S., McLeod, C., Ma, R., Laserna, J., Lucena, P., Caroli, S., Alimonti, A., Petrucci, F., Bocca, B., Schramel, P., Lustig, S., Zischka, M., Wass, U., Stenbom, B., Luna, M., Saenz, J.C., Santamarı´ a, J., Torrens, J.M., 2000. Platinum group elements: quantification in collected exhaust
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fumes and studies of catalyst surfaces. The Science of the Total Environment 257, 1–15. Petrucci, F., Bocca, B., Alimonti, A., Caroli, S., 2000. Determination of Pd, Pt and Rh in airborne particulate and road dust by high-resolution ICP-MS: a preliminary investigation of the emission from automotive catalysts in the urban area of Rome. Journal of Analytical Atomic Spectrometry 15, 525–528. Rauch, S., Morrison, G.M., Motelica-Heino, M., Donard, O.F.X., Muris, M., 2000. Elemental association and fingerprinting of traffic-related metals in road sediments. Environmental Science and Technology 34, 3119–3123. Rauch, S., Lu, M., Morrison, G.M., 2001. Heterogeneity of platinum group metals in airborne particles. Environmental Science and Technology 35, 595–599.
Ravindra, K., Bencs, L., van Grieken, R.V., 2004. Platinum group elements in the environment and their health risk. The Science of the Total Environment 318, 1–43. Schierl, R., 2000. Environmental monitoring of platinum in air and urine. Microchemical Journal 67, 245–248. Schierl, R., Fruhmann, G., 1996. Airborne platinum concentrations in Munich city buses. The Science of the Total Environment 182, 21–23. Za´ray, G., O´va´ri, M., Salma, I., Steffan, I., Zeiner, M., Caroli, S., 2004. Determination of platinum in urine and airborne particulate matter from Budapest and Vienna. Microchemical Journal 76, 31–34. Zereini, F., Wiseman, C., Alt, F., Messerchmidt, J., Mu¨ller, J., Urban, H., 2001. Platinum and Rhodium concentrations in airborne particulate matter in Germany from 1988–1998. Environmental Science and Technology 35, 1996–2000.
Atmospheric Environment 39 (2005) 2625–2630 www.elsevier.com/locate/atmosenv
Platinum concentrations in ambient aerosol at a coastal site in South China S.F. Kan, P.A. Tanner Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, SAR, PR China Received 10 November 2004; received in revised form 30 December 2004; accepted 11 January 2005
Abstract With the burgeoning vehicle ownership in China, the emissions of certain trace pollutants are becoming more widespread and of more concern. We present results from the first measurements of platinum in airborne total suspended particulate matter (TSP) at a suburban (CityU) and a roadside (MK) sampling site in southern China at Hong Kong. TSP samples were taken with high volume air samplers at these sites with low and high traffic density, respectively, on a 2-day basis and were analyzed by inductively coupled plasma—dynamic reaction cell—mass spectrometry (ICP-DRC-MS). The results are similar from the use of two different isotopes: 194 and 195. The 48-h average Pt concentrations were 8.8 pg m3 at CityU and 24.1 pg m3 at MK, being higher at the site with greater traffic density, and were in the range from 6 to 38 pg m3. The airborne Pt was found to be significantly correlated with vehicle tracer species (Zr and Ce) as well as with NO and NOx. The ratio Ce/Zr was the same at both sites whereas the ratios Pt/ Zr and Pt/Ce were about 20% greater at the MK site, showing the different mobility of Pt, attributed to chemical and/ or particle size effects. r 2005 Elsevier Ltd. All rights reserved. Keywords: Platinum; Particulate matter; Catalytic converter; Vehicle emissions
1. Introduction As at 2003, there are 0.36 million petrol and 6000 diesel vehicles equipped with catalytic converters in Hong Kong, corresponding to about 97% and 5% of the respective total numbers. Concomitant with the widespread use of catalytic converters in vehicles, since about 0.08% of a two-way catalyst comprises Pt, the ambient Pt concentrations are now of concern because of their adverse health effects. Although total suspended particulate matter (TSP) is routinely monitored in Hong Corresponding author. Tel.: +852 2788 7840; fax: +852 2788 7406. E-mail address: [email protected] (P.A. Tanner).
Kong and elsewhere for trace elements there are various limitations in the analytical methods for the determinand Pt. The first literature about Pt in airborne particulate matter appeared in 1975. Soluble Pt (halogenated Pt) has been considered to be more toxic than Pt compounds with low solubility. Recently, an extensive review of Pt group elements in the environment and their health risks was presented (Ravindra et al., 2004). Before the introduction of automobile catalysts, Pt was not detected in air samples in USA and Europe (Johnson et al., 1975, 1976; Ko¨nig et al., 1992). Zereini et al. (2001) studied Pt concentrations in airborne particulate matter (PM) in Germany and found there was 46-fold increase in Pt concentrations from 1988 to 1999. Seventy-five percent of Pt was found to occur in
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.01.014
ARTICLE IN PRESS 2626
S.F. Kan, P.A. Tanner / Atmospheric Environment 39 (2005) 2625–2630
association with PM40.2 mm aerodynamic diameter and 10% of Pt was found to be soluble in 0.1 M HCl (Zereini et al., 2001). Besides, Go´mez et al. (2001) monitored Pt concentrations for 1 year (from 1998 to 1999) at seven sampling sites in Spain and found no seasonal change. In addition, the Pt distribution in airborne PM was studied by two different samplers, a wide-range aerosol classifier collector (10–65.3 mm) and a seven-stage PM-10 cascade impactor (o 0.39–9 mm). Pt was found to be in a wide range of particle sizes, indicating a rather inhomogenous distribution of Pt in airborne PM. The inductively coupled plasma mass spectrometric (ICP-MS) trace determination of Pt is strongly hampered by the isobaric interferences of HfO+ and HfOH+. Due to their simplicity, empirical corrections have been widely used in the literature to correct for the former (Rauch et al., 2001; Nischwitz et al., 2003; Djingova et al., 2003). We have recently pointed out the problems with such corrections—in particular, the fact that measurements of Hf are also beset by isobaric interferences. We overcame such problems by removing the interferences by dynamic reaction cell (DRC) technology using ammonia gas (Kan and Tanner, 2004). Bocca et al. (2003) and Kanitsar et al. (2003) have used sector-field ICP-MS for platinum determination but in low-resolution mode (mass resolution 300) and they corrected interferences by using a membrane desolvation unit and isotope dilution. In this study, ambient Pt concentrations were measured at urban and suburban sampling sites from January 2004 to March 2004 on a 2 day basis. Two different Pt isotopes, 194 and 195 were selected for analysis and intercomparison. The concentrations of SO2, NOx, NO2, NO, CO, as well as physical parameters, were simultaneously studied and correlated with the measured Pt concentrations. In addition, the relationships between the ambient concentrations of two vehicle tracer species (Ce and Zr) and the Pt concentrations at both sites were measured. Finally, the Pt concentrations in Hong Kong have been compared with those at other locations.
Hong Kong Environmental Protection Department (HK EPD) roadside monitoring station next to a busy road with annual average daily traffic of 40,000; 221180 N, 1141100 E; see http://www.epd-asg.gov.hk/english/backgd/Mong_Kok.php) from 27 January to 5 March 2004. The flow rate was ca. 1 m3 min1 and the sampling duration was 48 h. No other sources of Pt at these sites, other than vehicles, are known. After sampling, the filter paper was immediately cut into small pieces and divided into four sections, of which two served as a replicate. Each section was digested in 20 cm3 of aqua regia using a microwave oven (CEM Corporation, Matthews, USA) at 630 W power for 30 min, according to US EPA method 3051. After cooling, 5 cm3 of aqua regia was added and the sample was mineralized again for 30 min. Then, the digested sample was filtered and the two sub-samples were mixed together and preconcentrated by a rotary evaporator at 80 1C to about 5 cm3. Finally, the sample was diluted to 10 cm3 with milli-Q-water. Ce and Zr were analyzed by ICP-MS using the standard mode after suitable (ca. 100 times) sample dilution. Real-time ambient pollutant gas measurements at the MK site were kindly provided by the HK EPD, who operate a rigorous quality assurance program for their measurements (EPD, 2003). 2.2. DRC technology The DRC technology enables the rejection of polyatomic species with the same mass to charge ratio as Pt, notably the HfO+ species, by the charge-transfer reaction with NH3 gas. An Elan 6100 DRC-ICP-MS instrument (Perkin-Elmer, Beaconsfield, UK) fitted with a peristaltic pump, Meinhard-type nebulizer, quartz concentric spray chamber, nickel skimmer and sample cone was employed for the instrumental analyses. NH3 reaction gas (99.999% purity, Hong Kong Special Gas) was employed. The instrumental conditions of cell gas flow rate and Rpq value were 1.05 cm3 min1 and 0.75. Other parameter settings were as given for our previous study of Pt in road dust (Kan and Tanner, 2004) dust and are listed in Table 1.
2. Methodology 2.3. Internal standard and quality control 2.1. Sample collection and digestion 193
As pointed out above, Pt is present in a wide range of particle sizes in air, so that we collected TSP by a high volume air sampler (Andersen Samplers, Inc) using a 800 1000 quartz fiber filter at the roof (51 m PD, Hong Kong grid location 221210 N, 1141100 E) of City University of Hong Kong (CityU: suburban site, about 20 m from a light-traffic road; see http://www.cityu.edu.hk/ cityu/visitors/location.htm) and at Mong Kok (MK: a
Ir(NH3)02 (formed in the reaction cell from Ir+ standard solution spiked into samples and standards) was selected as the internal standard for Pt analysis in the DRC mode. The certified reference materials (CRM) of NIST 2557 for autocatalyst and BCR 723 for road dust were used for method validation with the respective recoveries being 93% using 194Pt and 94% for the 195Pt isotope for NIST 2557; and for BCR 723: 95% for 194Pt and 79% for 195Pt.
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Table 1 Parameter settings and the instrumental operating conditions of the ICP-DRC-MS
Table 2 Pt concentrations (pg m3) measured in TSP at CityU and MK stations in Hong Kong using two different Pt isotopes
Parameter (unit)
Value
Date in 2004
Nebulizer gas flow (NGF) (dm3 min1) Auxiliary gas flow (dm3 min1) Plasma gas flow (dm3 min1) Lens voltage (V) ICP RF power (W) Analog stage voltage (V) Pulse stage voltage (V) Std quadrupole rod offset (QRO) Std cell rod offset (CRO) Discriminator threshold Std cell path voltage (CPV) (V) Rpa Rpq DRC mode (NGF) (dm3 min1) DRC mode QRO DRC mode CRO DRC mode CPV (V) Cell gas flow (cm3 min1)
0.9–1.2 1.20 15.00 8–10 1100–1300 1850 1175 0.00 11.00 70.00 31.00 0.00 a
0.9–1.2 8.00 2.00 33.00
Sampling site
Pt conc. in TSP (pg m3) 194
195
Pt
Pt
27 Jan
MK CityU
23.0 7.4
23.6 7.5
11 Feb
MK CityU
37.4 8.9
38.2 9.3
23 Feb
MK CityU
23.2 7.9
24.5 8.4
25 Feb
MK CityU
22.8 8.7
23.1 8.8
27 Feb
MK CityU
13.5 5.9
13.7 6.5
1 Mar
MK CityU
28.9 7.7
29.2 7.9
3 Mar
MK CityU
17.3 10.8
17.7 11.1
5 Mar
MK CityU
24.9 12.4
25.4 12.6
Average Average
MK CityU
23.9 8.7
24.4 9.0
a
QRO is the voltage applied to analyzer quadrupole; CRO is the voltage applied to the quadrupole rods in the DRC; CPV is the voltage applied to apertures at either end of the DRC and the prefilter before the analyzer quadrupole; Rpa is a high mass cut off that controls the dc voltage applied to the quadrupole rods; Rpq is the low mass cutoff that controls the RF frequency applied to the quadrupole rods; Std is the standard mode. a Need to be optimized.
The method LOD (expressed as three times the standard deviation of the sample blank solution) for Pt in ambient TSP, using the Pt isotopes 194 and 195, under the standard mode is 1–1.2 pg m3 whereas it is 0.5–0.7 pg m3 in the DRC mode. The coefficients of variation (¼ 100(standard deviation)/mean) of measurements for 194Pt and 195Pt in the samples were 8.4% and 7.8%, respectively.
3. Results and discussion 3.1. Pt concentrations in ambient TSP Table 2 summarizes the Pt concentrations (measured using 194Pt and 195Pt) in ambient TSP collected at the CityU and MK stations. The Pt concentrations at MK were in the range from 14 to 38 pg m3, with an average value of 24 pg m3. The Pt concentrations at rooftop level at suburban CityU were in the range from 6 to 12 pg m3, with an average value of 9 pg m3. As expected, Pt concentrations at MK roadside station are higher than that at CityU station, but as Zereini et al. (2001) noted, Pt-containing particles are not restricted to deposition in the immediate vicinity of
roads. Compared with the mean values for Pt at each site, the concentrations at MK showed a wider variability (from 0.55 to 1.56 of the mean) than those at CityU (from 0.69 to 1.37 of the mean value). The ratio of the concentrations of one determinand measured on the same day at the two sites MK/CityU were 2.7 for Pt and 2.3 for Ce and Zr, showing a greater decrease for Pt on going from the roadside to the suburban site. The Pt concentrations measured at both sites in this study are well below the guidance values of 15–150 ng m3 (Zereini et al., 2001; Merget and Rosner, 2001). Previous studies have shown that Pt concentrations are strongly influenced by a variety of different factors, such as traffic volume on a particular roadway, the number of vehicles equipped with catalytic converters, the vehicle types and associated emission amounts (e.g. city or highway driving), the particle size of airborne PM and meteorological conditions (Ko¨nig et al., 1992; Helmers et al., 1998; Go´mez et al., 2001). It can be concluded that since the ambient Pt concentrations in TSP in Hong Kong are at least several orders of magnitude lower than the critical values there is no evidence to suggest that at present they pose a health risk such as allergic reactions to the general public.
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3.2. Correlation of Pt concentrations with other measurements
Tracer species conc. (µg m-3)
30 Ce 20
10 Zr 0 0
10
20
30
40
-3
(a)
Pt conc. (pg m )
Gas conc. (µg m-3)
500 NOx
400 300 200 100 0 10
NO 15
(b)
20 25 30 Pt conc. (pg m-3)
35
40
Fig. 1. Plots of Pt concentrations (measured as 194Pt) in TSP against the vehicle tracer species (Ce and Zr) at both sites (a) and with NO and NOx at MK station (b). The regression lines are drawn as a guide to the eye.
The autocatalyst washcoat contains zirconium and cerium oxides: the first of these functions as a stabilizer and the second as an oxygen reservoir. Ce is also a fuel additive. Fig. 1(a) shows the correlations of Pt concentrations with the two vehicle tracer species (Ce and Zr) at both sites. Significant correlations were observed between the concentrations of Ce (R2 ¼ 0:62; Po0.05; N ¼ 16) or Zr (R2 ¼ 0:75; Po0.05; N ¼ 16), and those of Pt. As Rauch et al. (2000) pointed out, the composition of autocatalysts has changed considerably since their introduction in 1975 and it also varies from one manufacturer to another. A typical SEM/EDX analysis of a fresh washcoat of a three-way catalytic converter gave the mass ratios Ce/Pt 10.3 and Zr/Pt 7.0 (Palacios et al., 2000). Go´mez et al. (2001) found average ratios Ce/Pt and Zr/Pt of 10 and 16 in exhaust fumes, but ratios of 363 and 802 in airborne PM10 samples. These authors commented that although a high Pt content in PM10 is found together with high Ce and Zr concentrations showing that the source of these pollutants is traffic-related, factors such as the mobility and particle-sizes of these species may be important in determining these ratios. Furthermore, Ce and Zr are also present in other (e.g. Pd) types of catalyst. Our present results indicate a constant ratio of Ce/Zr (1.79) at both sites, whereas the ratio of Pt/Ce and Pt/Zr is about 20% greater at MK. The Ce/Pt ratio is 714 at CityU and 605 at MK, whereas the Zr/Pt ratio is 400 at CityU and 337 at MK. Thus the transport of Pt differs
Table 3 Concentrations of Pt in PM (pg m3) at various locations City (country)
Year
Particle size
Pt conc.
Reference
California (USA) California (USA) Tsukuba (Japan) Offenbach (Germany) Dortmund (Germany) Munich (Germany) Munich (Germany)
1975 1976 1983 1988 1991 1993 1993 1995 1995 1998 1998 1998 1998 1999 2002 2002 2002 2004 2004
TSP TSP TSP TSP 0.5–8 mm TSP PM10 PM10 PM10 PM10 PM10 PM10 TSP PM10 PM10 2–10 mm 2–10 mm TSP TSP
o0.05 o0.06 o0.18a 3 o5.1 7.3 7.3 21.5 13.6 12.8 17.8 12.7 147 o19 4.3 8 1.4 24 9
Johnson et al. (1975) Johnson et al. (1976) Mukai et al. (1990) Zereini et al. (2001) Alt et al. (1993) Schierl and Fruhmann (1996) Schierl (2000) Schierl (2000) Dietl et al. (2000) Go´mez et al. (2001) Bocca et al. (2003) Petrucci et al. (2000) Zereini et al. (2001) Rauch et al. (2001) Kanitsar et al. (2003) Za´ray et al. (2004) Za´ray et al. (2004) This study, roadside This study, suburban
Munich (Germany) Madrid (Spain) Rome (Italy) Rome (Italy) Frankfurt (Germany) Go¨teborg (Sweden) Vienna (Austria) Vienna (Austria) Budapest (Hungary) Hong Kong (China) Hong Kong (China) a
mg g1.
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from these other two species (due to chemical and/or particle size effects), and/or there are additional sources of Ce and Zr at the CityU site. However the latter postulate is unlikely since the ratio of Ce/Zr is the same at both sites. Concentrations of the gases SO2, NOx, NO2, NO, CO at MK, as well as the synoptic wind speed, were averaged over the sampling periods and correlated with the measured Pt concentrations. Only NOx (R2 ¼ 0:52; P ¼ o0:05; N ¼ 8) and NO (R2 ¼ 0:72; P ¼ o0:05; N ¼ 8) were significantly correlated with Pt concentrations (Fig. 1(b)), which can be attributed to their vehicles emissions source. The average concentrations of NO and NOx over the sampling period at MK were 350 and 157 mg m3, respectively.
4. Comparison of ambient Pt concentrations with those in other countries Limited studies are available for comparison, with only one previous Asian work performed in Japan in 1983 (Mukai et al., 1990). From Table 3 it is evident that Pt concentrations in the atmosphere have increased in the last 20 years by more than two orders of magnitude in other countries. From our study, Pt concentrations in airborne TSP in Hong Kong are found to slightly higher than those in recent studies in urban environments of some European countries except for the very high values for Pt in roadside air at Frankfurt in 1998, measured by adsorptive voltammetry. In all 3.25 million vehicles were manufactured in China in 2002 and with the projected rapid growth in the numbers of new vehicles it is important to carefully monitor the concentrations of Pt in urban environments in future.
Acknowledgements Financial assistance for this work from City University Research Grant 9360099 is gratefully acknowledged. We thank the Hong Kong EPD for providing monitoring data for gases at MK station.
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