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Gas chromatographic analysis and aerosol mass spectrometer measurement of diesel exhaust particles composition
Talanta 70 (2006) 178–181
Gas chromatographic analysis and aerosol mass spectrometer measurement of diesel exhaust particles composition Kenichi Akiyama ∗ Japan Automobile Research Institute, Karima, Tsukuba-shi Ibaraki 305-0822, Japan Received 31 October 2005; received in revised form 8 February 2006; accepted 13 February 2006 Available online 27 March 2006
Abstract Aerosol particles have important effects on human health, climate, regional visibility, and the deposition of acidic and toxic substances. The aerosols also have significant pharmaceutical and industrial applications. Trials of gas chromatographic analysis of extracts composition of diesel exhaust particles and aerosol mass spectrometer measurement of diesel exhaust particles composition are introduced in this paper. Usually, organic fraction of automotive exhaust particles are concentrated to 1 mL by Kuderna-Danish concentrator after extracted into dichloromethane by soxhlet extraction. Then, these extracts are analyzed by GC/MS. In the extracts from the diesel exhaust particles, there are over several thousands of components, for example paraffinic hydrocarbons, aromatics, oxygenates and other hydrocarbons. © 2006 Elsevier B.V. All rights reserved. Keywords: Gas chromatography; Diesel exhaust; Aerosol; Ultra fine particles; Fuel; Lubricant oil; Speciation; Aerosol mass spectrometer; Semi-volatile
1. Introduction Atmospheric aerosols and particulate matter (PM) from a wide variety of emission sources are receiving increasing attention because of their influence on human health [1], visibility, acid deposition, and global climate [2]. These measurements are conventionally performed by recording PM mass. The ambient standards are written in terms of mass concentrations, and emission regulations are based on mass. However, in order to understand better the nature of the mobile source contribution to ambient PM, many research groups are currently extending their investigations to include speciation of particles in automotive exhaust. Progress in understanding and mitigating these problems is limited by the ability of existing instruments to provide real-time, size-resolved, quantitative measurements of aerosol mass and chemical composition [3]. A number of measurement techniques possessing some of the required aerosol analysis capabilities have emerged recently. Real-time aerosol mass spectrometers aiming to provide information on chemical composition of particle ensembles or individual particles. Most
∗
Tel.: +81 298 56 1111; fax: +81 298 56 1134. E-mail address: [email protected].
0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.02.044
of these instruments also provide information on particle size. A recent review of aerosol measurements by McMurry [3] states that “these mass spectrometers are, arguably, the most significant development in aerosol measurement in the past 20 years. Trials of gas chromatographic analysis of extracts composition of diesel exhaust particles and aerosol mass spectrometer measurement of diesel exhaust particles composition are introduced in this paper. 2. Materials and methods 2.1. Gas chromatographic analysis Organic fraction of automotive exhaust particles are concentrated to 1 mL by Kuderna-Danish concentrator after extracted into dichloromethane (special grade) by 24 h soxhlet extraction [4]. These extracts are analyzed by GC/MS. GC separation was carried out using a Hewlett Packard 6890 GC System and mass detection was carried out using a Hewlett Packard 5973 MS System. Data were acquired and processed on a Hewlett Packard kayak computer using the ChemStation soft ware. DB-5MS (30 m × 0.25 mmI.D. × 0.5 m film thickness) was used for separation column. The oven temperature was held at 40 ◦ C for 3 min and then programmed at 20 ◦ C min−1 to 100 ◦ C, and at 8 ◦ C min−1 to 300 ◦ C, and at 2 ◦ C min−1 to 320 ◦ C
K. Akiyama / Talanta 70 (2006) 178–181
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Fig. 1. Schematic of the aerodyne aerosol mass spectrometer (AMS).
and hold. Flow rate of helium was 1 mL min−1 , splitless injector was 320 ◦ C and interface of MS was 280 ◦ C. 2.2. Aerosol mass spectrometer An aerosol mass spectrometer (AMS) developed at Aerodyne Research, which has been designed to provide real-time quantitative information on particle size-resolved mass loadings for volatile and semi-volatile chemical components present in/on ambient aerosol particles [5]. In its present configuration, the AMS cannot detect refractory aerosol components such as sea salt, soil dust, and elemental carbon. A schematic of the AMS is presented in Fig. 1. The AMS consists of three main parts: an aerosol inlet [6,7], a particle sizing chamber, and a particle composition detection section. The different sections are separated by small apertures and differentially pumped. A computational fluid dynamics simulation of the AMS inlet system shows nearly 100% transmission efficiency to the detector for particles in the aerodynamic diameter range 70–500 nm, and shows substantial transmission for particles in the 20–70 and 500–2.5 mm ranges for spherical particles. Irregularly shaped particles may have lower transmission efficiencies [5]. Size-dependent particle velocities created by expansion into vacuum are used to determine particle size through a particle time-of-flight measurement. The focused particle beam is modulated by a rotating wheel chopper operating at about 100 Hz. Time-resolved particle detection after a known flight distance gives the particle velocity from which the particle aerodynamic diameter is obtained. Detection is performed by directing the particle beam onto a resistively heated, roughened surface under high vacuum. On this surface, the volatile and semi-volatile components in/on the particles flash vaporize. The vaporization source is integrally coupled to an electron impact ionizer at the entrance of a quadrupole mass spectrometer. When the quadrupole is tuned to a representative m/z, bursts of ions are produced that are averaged to produce a size-resolved mass distribution.
Fig. 2. Typical TIC examples of diesel exhaust particle extract: (a) one of the typical TIC of diesel exhaust particle extract (type 1); (b) one of the typical TIC of diesel exhaust particle extract (type 2).
Fig. 3. Typical TIC of boiled diesel fuel and lubricant oil.
removed of low boiling point hydrocarbons) and lubricant oil. Composition of the extract shown in Fig. 2(a) is mixture of boiled fuel and lubricant oil, and composition resulted in Fig. 2(b) seems to mainly composed of boiled fuel and small amount of lubricant oil is exist in this extract. It is thought that this difference depends on type of engine and driving cycle. Extract resulted in Fig. 2(a) and (b) are named type 1 and type 2. There are many peaks on these chromatograms. Fig. 4 shows mass chromatogram of m/z = 99 in the type 1 extracts that shows the presence of compounds including C7 H15 + hydrocarbons. This mass chromatogram of m/z = 99 shows almost paraffin
3. Result and discussion 3.1. Gas chromatographic analysis Fig. 2 shows typical two types of total ion chromatogram (TIC)s of the extracts from diesel exhaust particulate matter. Fig. 3 shows typical TIC of boiled diesel fuel (diesel fuel
Fig. 4. Typical mass chromatogram (m/z = 99:C7 H15 + ) of diesel exhaust particle extract.
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K. Akiyama / Talanta 70 (2006) 178–181 Table 1 Speciation of test vehicle Diesel passenger car
Fig. 5. Typical mass chromatogram (m/z = 91: C7 H7 + ) of type 1 diesel exhaust particles extract.
Exhaust gas regulation Overall width Overall height Overall length Vehicle weight Seating capacity Gross vehicle weight Engine cylinder arrangement Engine total displacement Engine maximum torque Engine maximum power Exhaust gas measurement Millage
Year mm mm mm kg Number kg Number L kW/rpm N m/rpm km
1998 1800 1770 4655 1850 5 2125 4 2982 N125/3400 270/4400 OCC, EGR 37403
These ions corresponding to molecule ions of PAHs are generated not only PAHs but also another heavy components. These results show a part of extracted components in diesel exhaust particles. It is thought that diesel exhaust particles extract include several thousand of components. Fig. 6. Typical mass chromatogram (m/z = 191: Hopanes, m/z = 178, 202, 228:PAHs) of type 1 diesel exhaust particles extract.
hydrocarbons and a part of high boiling point area (right side of chromatogram) is not only paraffin hydrocarbons, include C7 alkyl hydrocarbons chain and some oxygenate compounds. Sharp and large peaks on this chromatogram is normal paraffin hydrocarbons for example octadecane, eicosane, etc. and small peaks are seems to be naphthenes and branched chain hydrocarbons. Fig. 5 shows mass chromatogram of m/z = 91 in the type 1 extract that indicated compounds including C7 H7 + hydrocarbons, which are due to 1-ring aromatic hydrocarbons and some oxygenated aromatics include C7 H7 + hydrocarbons. Fig. 6 shows mass chromatograms of m/z = 191, 178, 202 and 228, these mass chromatograms indicate hopanes (m/z = 191) and molecular ion of poly nuclear aromatic hydrocarbons (PAHs, m/z = 178, 202, 228) of type 1 extract. Hopanes are well known for tracer components of lubricant oil. Mass chromatogram of m/z = 178 is typical molecular ion of phenanthrene and anthrathene, mass chromatogram of m/z = 202 is typical molecular ion of fluoranthene and pyrene and mass chromatogram of m/z = 228 is typical molecular ion of Benzo[a]anthracene and chrysene.
3.2. Result of aerosol mass spectrometer measurement 3.2.1. Result of aerosol mass spectrometer measurement Fig. 7 shows typical measurement result of AMS. Fig. 7(a) described raw analytical result of automotive exhaust particles by AMS (the horizontal axis: flight time of particles, the vertical axis: intensity of signal of each m/z ion). Particles of diesel exhaust are concentrated and most of gas phase are removed in the aerodynamic lens. Remaining gas phase of diesel exhaust arrive at detector of the AMS first. Fig. 7(b) shows an example of particle semi-volatile chemical components size distribution of diesel exhaust, after conversion from flight time of particles to particle size and signal of each ions to weight. 3.2.2. Particle size dependent distribution of semi-volatile chemical components in the diesel exhaust Particle size dependent distribution of semi-volatile chemical components in the diesel exhaust was examined. In this test, the diesel vehicle of 3000 ml displacement (Table 1) and diesel fuel (S:28 ppm) were used. Measurement interval of the AMS was every 12 s. Fig. 8 shows typical particle size dependent
Fig. 7. Example of particle size conversion: (a) raw measurement result of particles by AMS; (b) calculated result of particle size dependent distribution.
K. Akiyama / Talanta 70 (2006) 178–181
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are some interference to m/z = 17 which is used for quantify ammonium, but m/z = 17 is not only coming from ammonium, but also another components such as water which gives m/z = 17 fragment ion. Fig. 9 shows typical particle size distribution of semi-volatile chemical components in the diesel exhaust at idling condition. In this result, organics are main components, and distributed highest particle size is at 30–40 nm. This pattern is perfectly different from particle composition of high load driving condition. 4. Conclusions
Fig. 8. Semi-volatile particles components in the diesel exhaust particles (high load driving condition).
When using gas chromatographic analysis, extracts from diesel exhaust particulate matter consist mainly of diesel fuel and lubricant oil. These ratio depended on type of engine and driving cycle. It is thought that diesel exhaust particles contain several thousand of components. The AMS has been applied to obtain the information on the size and chemical composition of semi-volatile species in automobile exhaust particles. Mass spectrum, mass concentration, and size and chemically resolved mass distribution data were obtained. The major components observed on the diesel exhaust particles were sulfate and organics when high load condition and idling, respectively. Acknowledgments
Fig. 9. Semi-volatile particle components in the particles (low load driving condition).
distribution of semi-volatile chemical components in the diesel exhaust at 2200 rpm and 6% gradient condition. In this result, sulfate and ammonium were distributed highest on 50 nm particle size, furthermore, there was another peaks of those species at about 30 nm particle size. In this result, main components of semi-volatile chemical components in the diesel exhaust at high load driving condition was sulfate. Feasibility of the present of sulfate in such amount is high, because concentration of sulfate is relatively high and there are a few interfering fragment ions for m/z = 48 and 64 coming from other components, although reliability of ammonium quantity is low, because there
The authors are grateful to Dr. Akio Shimono of Sanyu plant service co. and Dr. Douglas Worsnop of the Aerodyne research for the ongoing collaboration that has greatly helped the development and testing of the AMS. References [1] D.W. Dockery, et al., N. Engl. J. Med. 329–324 (1993) 1753–1759. [2] J.H. Seinfeld, et al., Atmospheric Chemistry and Physics: from Air Pollution to Climate Change, John Wiley, New York, 1998. [3] P.H. McMurry, Atmos. Environ. 34 (12–14) (2000) 1959–1999. [4] K. Akiyama, Jpn. Soc. Atmos. Environ. 35–36 (2000) A73–A84. [5] J.T. Jayne, et al., Aerosol Sci. Technol. 33 (1–2) (2000) 49–70. [6] B.Y.H. Liu, et al., Aerosol Sci. Technol. 22 (1995) 293–313; B.Y.H. Liu, et al., Aerosol Sci. Technol. 22 (1995) 314–324. [7] X. Zhang, et al., Aerosol Sci. Technol. 36 (2002) 617–631.
Gas chromatographic analysis and aerosol mass spectrometer measurement of diesel exhaust particles composition Kenichi Akiyama ∗ Japan Automobile Research Institute, Karima, Tsukuba-shi Ibaraki 305-0822, Japan Received 31 October 2005; received in revised form 8 February 2006; accepted 13 February 2006 Available online 27 March 2006
Abstract Aerosol particles have important effects on human health, climate, regional visibility, and the deposition of acidic and toxic substances. The aerosols also have significant pharmaceutical and industrial applications. Trials of gas chromatographic analysis of extracts composition of diesel exhaust particles and aerosol mass spectrometer measurement of diesel exhaust particles composition are introduced in this paper. Usually, organic fraction of automotive exhaust particles are concentrated to 1 mL by Kuderna-Danish concentrator after extracted into dichloromethane by soxhlet extraction. Then, these extracts are analyzed by GC/MS. In the extracts from the diesel exhaust particles, there are over several thousands of components, for example paraffinic hydrocarbons, aromatics, oxygenates and other hydrocarbons. © 2006 Elsevier B.V. All rights reserved. Keywords: Gas chromatography; Diesel exhaust; Aerosol; Ultra fine particles; Fuel; Lubricant oil; Speciation; Aerosol mass spectrometer; Semi-volatile
1. Introduction Atmospheric aerosols and particulate matter (PM) from a wide variety of emission sources are receiving increasing attention because of their influence on human health [1], visibility, acid deposition, and global climate [2]. These measurements are conventionally performed by recording PM mass. The ambient standards are written in terms of mass concentrations, and emission regulations are based on mass. However, in order to understand better the nature of the mobile source contribution to ambient PM, many research groups are currently extending their investigations to include speciation of particles in automotive exhaust. Progress in understanding and mitigating these problems is limited by the ability of existing instruments to provide real-time, size-resolved, quantitative measurements of aerosol mass and chemical composition [3]. A number of measurement techniques possessing some of the required aerosol analysis capabilities have emerged recently. Real-time aerosol mass spectrometers aiming to provide information on chemical composition of particle ensembles or individual particles. Most
∗
Tel.: +81 298 56 1111; fax: +81 298 56 1134. E-mail address: [email protected].
0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.02.044
of these instruments also provide information on particle size. A recent review of aerosol measurements by McMurry [3] states that “these mass spectrometers are, arguably, the most significant development in aerosol measurement in the past 20 years. Trials of gas chromatographic analysis of extracts composition of diesel exhaust particles and aerosol mass spectrometer measurement of diesel exhaust particles composition are introduced in this paper. 2. Materials and methods 2.1. Gas chromatographic analysis Organic fraction of automotive exhaust particles are concentrated to 1 mL by Kuderna-Danish concentrator after extracted into dichloromethane (special grade) by 24 h soxhlet extraction [4]. These extracts are analyzed by GC/MS. GC separation was carried out using a Hewlett Packard 6890 GC System and mass detection was carried out using a Hewlett Packard 5973 MS System. Data were acquired and processed on a Hewlett Packard kayak computer using the ChemStation soft ware. DB-5MS (30 m × 0.25 mmI.D. × 0.5 m film thickness) was used for separation column. The oven temperature was held at 40 ◦ C for 3 min and then programmed at 20 ◦ C min−1 to 100 ◦ C, and at 8 ◦ C min−1 to 300 ◦ C, and at 2 ◦ C min−1 to 320 ◦ C
K. Akiyama / Talanta 70 (2006) 178–181
179
Fig. 1. Schematic of the aerodyne aerosol mass spectrometer (AMS).
and hold. Flow rate of helium was 1 mL min−1 , splitless injector was 320 ◦ C and interface of MS was 280 ◦ C. 2.2. Aerosol mass spectrometer An aerosol mass spectrometer (AMS) developed at Aerodyne Research, which has been designed to provide real-time quantitative information on particle size-resolved mass loadings for volatile and semi-volatile chemical components present in/on ambient aerosol particles [5]. In its present configuration, the AMS cannot detect refractory aerosol components such as sea salt, soil dust, and elemental carbon. A schematic of the AMS is presented in Fig. 1. The AMS consists of three main parts: an aerosol inlet [6,7], a particle sizing chamber, and a particle composition detection section. The different sections are separated by small apertures and differentially pumped. A computational fluid dynamics simulation of the AMS inlet system shows nearly 100% transmission efficiency to the detector for particles in the aerodynamic diameter range 70–500 nm, and shows substantial transmission for particles in the 20–70 and 500–2.5 mm ranges for spherical particles. Irregularly shaped particles may have lower transmission efficiencies [5]. Size-dependent particle velocities created by expansion into vacuum are used to determine particle size through a particle time-of-flight measurement. The focused particle beam is modulated by a rotating wheel chopper operating at about 100 Hz. Time-resolved particle detection after a known flight distance gives the particle velocity from which the particle aerodynamic diameter is obtained. Detection is performed by directing the particle beam onto a resistively heated, roughened surface under high vacuum. On this surface, the volatile and semi-volatile components in/on the particles flash vaporize. The vaporization source is integrally coupled to an electron impact ionizer at the entrance of a quadrupole mass spectrometer. When the quadrupole is tuned to a representative m/z, bursts of ions are produced that are averaged to produce a size-resolved mass distribution.
Fig. 2. Typical TIC examples of diesel exhaust particle extract: (a) one of the typical TIC of diesel exhaust particle extract (type 1); (b) one of the typical TIC of diesel exhaust particle extract (type 2).
Fig. 3. Typical TIC of boiled diesel fuel and lubricant oil.
removed of low boiling point hydrocarbons) and lubricant oil. Composition of the extract shown in Fig. 2(a) is mixture of boiled fuel and lubricant oil, and composition resulted in Fig. 2(b) seems to mainly composed of boiled fuel and small amount of lubricant oil is exist in this extract. It is thought that this difference depends on type of engine and driving cycle. Extract resulted in Fig. 2(a) and (b) are named type 1 and type 2. There are many peaks on these chromatograms. Fig. 4 shows mass chromatogram of m/z = 99 in the type 1 extracts that shows the presence of compounds including C7 H15 + hydrocarbons. This mass chromatogram of m/z = 99 shows almost paraffin
3. Result and discussion 3.1. Gas chromatographic analysis Fig. 2 shows typical two types of total ion chromatogram (TIC)s of the extracts from diesel exhaust particulate matter. Fig. 3 shows typical TIC of boiled diesel fuel (diesel fuel
Fig. 4. Typical mass chromatogram (m/z = 99:C7 H15 + ) of diesel exhaust particle extract.
180
K. Akiyama / Talanta 70 (2006) 178–181 Table 1 Speciation of test vehicle Diesel passenger car
Fig. 5. Typical mass chromatogram (m/z = 91: C7 H7 + ) of type 1 diesel exhaust particles extract.
Exhaust gas regulation Overall width Overall height Overall length Vehicle weight Seating capacity Gross vehicle weight Engine cylinder arrangement Engine total displacement Engine maximum torque Engine maximum power Exhaust gas measurement Millage
Year mm mm mm kg Number kg Number L kW/rpm N m/rpm km
1998 1800 1770 4655 1850 5 2125 4 2982 N125/3400 270/4400 OCC, EGR 37403
These ions corresponding to molecule ions of PAHs are generated not only PAHs but also another heavy components. These results show a part of extracted components in diesel exhaust particles. It is thought that diesel exhaust particles extract include several thousand of components. Fig. 6. Typical mass chromatogram (m/z = 191: Hopanes, m/z = 178, 202, 228:PAHs) of type 1 diesel exhaust particles extract.
hydrocarbons and a part of high boiling point area (right side of chromatogram) is not only paraffin hydrocarbons, include C7 alkyl hydrocarbons chain and some oxygenate compounds. Sharp and large peaks on this chromatogram is normal paraffin hydrocarbons for example octadecane, eicosane, etc. and small peaks are seems to be naphthenes and branched chain hydrocarbons. Fig. 5 shows mass chromatogram of m/z = 91 in the type 1 extract that indicated compounds including C7 H7 + hydrocarbons, which are due to 1-ring aromatic hydrocarbons and some oxygenated aromatics include C7 H7 + hydrocarbons. Fig. 6 shows mass chromatograms of m/z = 191, 178, 202 and 228, these mass chromatograms indicate hopanes (m/z = 191) and molecular ion of poly nuclear aromatic hydrocarbons (PAHs, m/z = 178, 202, 228) of type 1 extract. Hopanes are well known for tracer components of lubricant oil. Mass chromatogram of m/z = 178 is typical molecular ion of phenanthrene and anthrathene, mass chromatogram of m/z = 202 is typical molecular ion of fluoranthene and pyrene and mass chromatogram of m/z = 228 is typical molecular ion of Benzo[a]anthracene and chrysene.
3.2. Result of aerosol mass spectrometer measurement 3.2.1. Result of aerosol mass spectrometer measurement Fig. 7 shows typical measurement result of AMS. Fig. 7(a) described raw analytical result of automotive exhaust particles by AMS (the horizontal axis: flight time of particles, the vertical axis: intensity of signal of each m/z ion). Particles of diesel exhaust are concentrated and most of gas phase are removed in the aerodynamic lens. Remaining gas phase of diesel exhaust arrive at detector of the AMS first. Fig. 7(b) shows an example of particle semi-volatile chemical components size distribution of diesel exhaust, after conversion from flight time of particles to particle size and signal of each ions to weight. 3.2.2. Particle size dependent distribution of semi-volatile chemical components in the diesel exhaust Particle size dependent distribution of semi-volatile chemical components in the diesel exhaust was examined. In this test, the diesel vehicle of 3000 ml displacement (Table 1) and diesel fuel (S:28 ppm) were used. Measurement interval of the AMS was every 12 s. Fig. 8 shows typical particle size dependent
Fig. 7. Example of particle size conversion: (a) raw measurement result of particles by AMS; (b) calculated result of particle size dependent distribution.
K. Akiyama / Talanta 70 (2006) 178–181
181
are some interference to m/z = 17 which is used for quantify ammonium, but m/z = 17 is not only coming from ammonium, but also another components such as water which gives m/z = 17 fragment ion. Fig. 9 shows typical particle size distribution of semi-volatile chemical components in the diesel exhaust at idling condition. In this result, organics are main components, and distributed highest particle size is at 30–40 nm. This pattern is perfectly different from particle composition of high load driving condition. 4. Conclusions
Fig. 8. Semi-volatile particles components in the diesel exhaust particles (high load driving condition).
When using gas chromatographic analysis, extracts from diesel exhaust particulate matter consist mainly of diesel fuel and lubricant oil. These ratio depended on type of engine and driving cycle. It is thought that diesel exhaust particles contain several thousand of components. The AMS has been applied to obtain the information on the size and chemical composition of semi-volatile species in automobile exhaust particles. Mass spectrum, mass concentration, and size and chemically resolved mass distribution data were obtained. The major components observed on the diesel exhaust particles were sulfate and organics when high load condition and idling, respectively. Acknowledgments
Fig. 9. Semi-volatile particle components in the particles (low load driving condition).
distribution of semi-volatile chemical components in the diesel exhaust at 2200 rpm and 6% gradient condition. In this result, sulfate and ammonium were distributed highest on 50 nm particle size, furthermore, there was another peaks of those species at about 30 nm particle size. In this result, main components of semi-volatile chemical components in the diesel exhaust at high load driving condition was sulfate. Feasibility of the present of sulfate in such amount is high, because concentration of sulfate is relatively high and there are a few interfering fragment ions for m/z = 48 and 64 coming from other components, although reliability of ammonium quantity is low, because there
The authors are grateful to Dr. Akio Shimono of Sanyu plant service co. and Dr. Douglas Worsnop of the Aerodyne research for the ongoing collaboration that has greatly helped the development and testing of the AMS. References [1] D.W. Dockery, et al., N. Engl. J. Med. 329–324 (1993) 1753–1759. [2] J.H. Seinfeld, et al., Atmospheric Chemistry and Physics: from Air Pollution to Climate Change, John Wiley, New York, 1998. [3] P.H. McMurry, Atmos. Environ. 34 (12–14) (2000) 1959–1999. [4] K. Akiyama, Jpn. Soc. Atmos. Environ. 35–36 (2000) A73–A84. [5] J.T. Jayne, et al., Aerosol Sci. Technol. 33 (1–2) (2000) 49–70. [6] B.Y.H. Liu, et al., Aerosol Sci. Technol. 22 (1995) 293–313; B.Y.H. Liu, et al., Aerosol Sci. Technol. 22 (1995) 314–324. [7] X. Zhang, et al., Aerosol Sci. Technol. 36 (2002) 617–631.