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Real-time characterization of atmospheric salt particles by surface ionization
.l. Aerosol Sci. Vol. 30, Suppl. I, pp. $77-$78, 1999 © 1999 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/99/$ - see front matter
Pergamon
REAL-TIME CHARACTERIZATION OF ATMOSPHERIC SALT PARTICLES BY SURFACE IONIZATION MAGNUS HAGSTROM and JAN B.C. PETTERSSON
Department of Chemistry, Physical Chemistry, GOteborg University, SE-412 96 G6teborg, Sweden E-mail: [email protected]; http://www.che.chalmers.se/-hagstrom
Keywords: Particle detection, individual particle, particle counting, alkali salt, sea salt, surface ionization, real-time measurement, atmospheric pressure Standard procedures for analyzing the chemical composition of atmospheric particles generally involve collecting samples on filters, for subsequent analysis in the lab. A number of methods based on mass spectrometry (MS) have also been devised (Johnston and Wexler, 1995; Wood and Prather, 1998), some of which can provide full elemental analysis of individual particles on a real-time basis. All operation of MS systems depend on a certain degree of vacuum, and the systems are rarely what you would call mobile. This will inevitably limit the applicability of MS systems, and there is ample use for other, less complicated, methods to provide contributions to the complete chemical picture of aerosol particles. The chemical composition of atmospheric salt particles can provide vital information about the origins and transport paths of the air masses containing the particles. The predominant constituent of freshly formed seasait particles will naturally be sodium chloride, but as the particles encounter gases containing sulfur dioxide and/or nitrogen oxides the chloride in the particles will gradually be replaced by sulfate and/or nitrate (Chameides and Stelson, 1992, Mclnnes et a/., 1994). The degree to which chloride depletion has occurred can illuminate the history of the individual particle. The surface ionization (SI) technique is a highly efficient and sensitive tool for detecting alkali metal compounds in ambient air at atmospheric pressure (Jiiglid et al., 1996) Due to their uniquely low ionization potentials, alkali atoms adsorbed on a hot metal surface will desorb in ionic form with almost 100% ionization efficiency. Our group has previously used the SI method for real-time measurements of the alkali vapor content in environments such a s combustion power plants (Davidsson et aL, 1999) as well as for kinetic measurements of alkali ion desorption (HagstrOm et.al, 1999). We have also utilized the SI technique for measuring the alkali metal content of individual salt particles in sizes down to 20 um diameter at atmospheric pressure. The SI detector mainly consists of a positively biased platinum placed close to an ion collector which is grounded through a current amplifier. A flow of ambient air is led through the detector at a typical flow rate of less than 1000 ml/minute. Upon being deposited on the heated Pt surface, the particles melt, partially or completely, and the salt dissociates. The alkali atoms desorb as ions while most other elements have ionization potentials significantly higher than the work function of Pt and will not be ionized. A burst of positive alkali ions is emitted from the surface and can be registered as a transient positive current at the collector. The integrated ion yield from each particle can give information about the particle size and the count rate can be used to determine the particle concentration with high time resolution.
$77
$78
Abstracts of the 1999 EuropeanAerosol Conference
The shape of the signal peak is observed to be strongly influenced by the chemical composition of the particles. Na2SO4 particles give current signals lasting on the order of hundreds of microseconds while signals from NaC1 particle detection last on the order of tens of milliseconds, a difference of two orders of magnitude. This distinction is believed to result from different melting and evaporation pattems and can readily be used to separate different salts and to evaluate their relative concentrations.
10.0
10.0, a)
Na2SO4
5.0
5
¢/) 2.5 0.0 0.0
b)
7.5
7.5
.
NaCI
0
~
2.5 0.0
0.2
0.4 0.6 Thine(ms)
0.8
0
25 50 Time(ms)
75
Fig. 1: Signal resulting from detecting: a) Na2SO 4 and b) NaCl particles in the surface ionization detector.
Through our cooperation with the Institute for applied environmental research (ITM) in Stockholm we have been able to participate with a surface ionization detector at two recent international meteorological field campaigns; STREAM 98 in Timmins, Canada, 1998, and INDOEX 99 in Male, Republic of Maldives, 1999. Some preliminary results from these campaigns will be presented to illustrate the measurement capabilities of the surface ionization technique.
References: Chameides, W.L. and Stelson, A.W., (1992) Aqueous-phase chemical processes in deliquescent sea-salt aerosols: A mechanism that couples the atmospheric cycles of S and sea-salt, J. Geophys. Res., 97, 2056520580. Davidsson, K.O., Engvall, K., Olsson, J.G., L6nn, B. and Pettersson, J.B.C., (1999) On-line alkali measurements in flue gas from combustion of coal and biofuels, 15th International Conference on Fluidized Bed Combustion, Savannah, Georgia, USA 1999. Hagstr6m, M., Engvall, K. and Pettersson, J.B.C., (1999) Desorption kinetics at atmospheric pressure: Alkali metal ion emission from hot platinum surfaces, Submitted to J. Phys. Chem. B. Johnston. M.V. and Wexler, A.S., (1995) MS of individual aerosol particles, Anal. Chem., 67, A721-A726 Jaglid, U., Olsson, J.G. and Pettersson, J.B.C., (1996) Detection of sodium and potassium salt particles using surface ionization at atmospheric pressure, J. Aerosol Sci. 27, 967-977. Mclnnes, L.M., Covert, D.S., Quinn, P.K. and Germani, M.S., (1994) Measurements of chloride depletion and sulfur enrichment in individual sea-salt particles collected from the remote marine boundary layer, J. Geophys. Res., 99, 8257-8268. Wood, S.H. and Prather, K.A., (1998) Time-of-flight mass spectrometry methods for real time analysis of individual aerosol particles, Trac - Trends in Analytical Chemistry, 17, 346-356.
Pergamon
REAL-TIME CHARACTERIZATION OF ATMOSPHERIC SALT PARTICLES BY SURFACE IONIZATION MAGNUS HAGSTROM and JAN B.C. PETTERSSON
Department of Chemistry, Physical Chemistry, GOteborg University, SE-412 96 G6teborg, Sweden E-mail: [email protected]; http://www.che.chalmers.se/-hagstrom
Keywords: Particle detection, individual particle, particle counting, alkali salt, sea salt, surface ionization, real-time measurement, atmospheric pressure Standard procedures for analyzing the chemical composition of atmospheric particles generally involve collecting samples on filters, for subsequent analysis in the lab. A number of methods based on mass spectrometry (MS) have also been devised (Johnston and Wexler, 1995; Wood and Prather, 1998), some of which can provide full elemental analysis of individual particles on a real-time basis. All operation of MS systems depend on a certain degree of vacuum, and the systems are rarely what you would call mobile. This will inevitably limit the applicability of MS systems, and there is ample use for other, less complicated, methods to provide contributions to the complete chemical picture of aerosol particles. The chemical composition of atmospheric salt particles can provide vital information about the origins and transport paths of the air masses containing the particles. The predominant constituent of freshly formed seasait particles will naturally be sodium chloride, but as the particles encounter gases containing sulfur dioxide and/or nitrogen oxides the chloride in the particles will gradually be replaced by sulfate and/or nitrate (Chameides and Stelson, 1992, Mclnnes et a/., 1994). The degree to which chloride depletion has occurred can illuminate the history of the individual particle. The surface ionization (SI) technique is a highly efficient and sensitive tool for detecting alkali metal compounds in ambient air at atmospheric pressure (Jiiglid et al., 1996) Due to their uniquely low ionization potentials, alkali atoms adsorbed on a hot metal surface will desorb in ionic form with almost 100% ionization efficiency. Our group has previously used the SI method for real-time measurements of the alkali vapor content in environments such a s combustion power plants (Davidsson et aL, 1999) as well as for kinetic measurements of alkali ion desorption (HagstrOm et.al, 1999). We have also utilized the SI technique for measuring the alkali metal content of individual salt particles in sizes down to 20 um diameter at atmospheric pressure. The SI detector mainly consists of a positively biased platinum placed close to an ion collector which is grounded through a current amplifier. A flow of ambient air is led through the detector at a typical flow rate of less than 1000 ml/minute. Upon being deposited on the heated Pt surface, the particles melt, partially or completely, and the salt dissociates. The alkali atoms desorb as ions while most other elements have ionization potentials significantly higher than the work function of Pt and will not be ionized. A burst of positive alkali ions is emitted from the surface and can be registered as a transient positive current at the collector. The integrated ion yield from each particle can give information about the particle size and the count rate can be used to determine the particle concentration with high time resolution.
$77
$78
Abstracts of the 1999 EuropeanAerosol Conference
The shape of the signal peak is observed to be strongly influenced by the chemical composition of the particles. Na2SO4 particles give current signals lasting on the order of hundreds of microseconds while signals from NaC1 particle detection last on the order of tens of milliseconds, a difference of two orders of magnitude. This distinction is believed to result from different melting and evaporation pattems and can readily be used to separate different salts and to evaluate their relative concentrations.
10.0
10.0, a)
Na2SO4
5.0
5
¢/) 2.5 0.0 0.0
b)
7.5
7.5
.
NaCI
0
~
2.5 0.0
0.2
0.4 0.6 Thine(ms)
0.8
0
25 50 Time(ms)
75
Fig. 1: Signal resulting from detecting: a) Na2SO 4 and b) NaCl particles in the surface ionization detector.
Through our cooperation with the Institute for applied environmental research (ITM) in Stockholm we have been able to participate with a surface ionization detector at two recent international meteorological field campaigns; STREAM 98 in Timmins, Canada, 1998, and INDOEX 99 in Male, Republic of Maldives, 1999. Some preliminary results from these campaigns will be presented to illustrate the measurement capabilities of the surface ionization technique.
References: Chameides, W.L. and Stelson, A.W., (1992) Aqueous-phase chemical processes in deliquescent sea-salt aerosols: A mechanism that couples the atmospheric cycles of S and sea-salt, J. Geophys. Res., 97, 2056520580. Davidsson, K.O., Engvall, K., Olsson, J.G., L6nn, B. and Pettersson, J.B.C., (1999) On-line alkali measurements in flue gas from combustion of coal and biofuels, 15th International Conference on Fluidized Bed Combustion, Savannah, Georgia, USA 1999. Hagstr6m, M., Engvall, K. and Pettersson, J.B.C., (1999) Desorption kinetics at atmospheric pressure: Alkali metal ion emission from hot platinum surfaces, Submitted to J. Phys. Chem. B. Johnston. M.V. and Wexler, A.S., (1995) MS of individual aerosol particles, Anal. Chem., 67, A721-A726 Jaglid, U., Olsson, J.G. and Pettersson, J.B.C., (1996) Detection of sodium and potassium salt particles using surface ionization at atmospheric pressure, J. Aerosol Sci. 27, 967-977. Mclnnes, L.M., Covert, D.S., Quinn, P.K. and Germani, M.S., (1994) Measurements of chloride depletion and sulfur enrichment in individual sea-salt particles collected from the remote marine boundary layer, J. Geophys. Res., 99, 8257-8268. Wood, S.H. and Prather, K.A., (1998) Time-of-flight mass spectrometry methods for real time analysis of individual aerosol particles, Trac - Trends in Analytical Chemistry, 17, 346-356.