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Simulation of the mobility spectrum of charged particles during bursts in atmospheric air
J AelvsoISci,
%1.3 I, Suppl. 1, pp. $700-$701. 2000
Pergamon www.elsevier.com/locate/jaerosci
Poster Session II. Atmospheric aerosols: physical properties SIM-ULATION OF TIIE MOBILITY SPECTRUM OF CHARGED PARTICLES DURING BURSTS IN ATMOSPHERIC AIR M. NOPPEL and U. HORRAK Institute of Environmental Physics, University of Tartu, Tartu, Estonia
KEYWORDS: Air Ions, Nanometer Particles, Mobility Spectrum, Simulation INTRODUCTION The mobility spectrum of air ions has been measured at Tahknse Observatory in Estonia (H~rrak et aL,1998). On the level of low background (the average concentration about 50 cm-3), high concentration bursts of intermediate air ions ( mobilities 0.05 - 0.5 cm2V-ls-J) occur occasionally. An example of these bursts is given in Figure by three consecutive hourly average mobility spectra measured in the aitemoon of October 20, 1994. This paper presents the preliminary results of the simulation of this event. CALCULATION OF A MOBILITY SPECTRUM The ion-induced nucleation was considered as a mechanism of the formation of intermediate ions. The relation between size and mobility of ions was described by the theory of Tammet (1995). The formed ions (charged nanometer particles) were taken to grow with constant rate by condensation. The Kelvin effect was ignored. The growth rates of ions were chosen by fitting the location of peak value in calculated spectra to the location of the peak v.alue in the observed spectra. The nucleation rate was fitted by equalising the sum of calculated fraction concentrations of ions with the sum of experimental fraction concentrations. The growing intermediate ions recombine with small ions and coagulate with pre-existing aerosol particles. During recombination the intermediate ions loose their charges and are not observable by a spectrometer any more. Coagulation with pre-existing aerosol takes also intermediate ions away from an observable spectrum. The experimental values of small ion concentration were used in recombination calculations. Due to the lack of experimental data about aerosol properties, the scavenging properties of pre-existing aerosol were described by the size spectrum typical for a rural site. Uncharged growing nanometer particles that escape coagulation with pre-existing aerosol particles, can reappear in the observable mobility spectrum when charged by small ions. Recombination and coagulation between intermediate ions were ignored, but recharging of uncharged intermediate ions was taken into account. Fuchs coagulation (Fuchs, 1964) and particles charging theories (Reischl et al., 1996) were used. RESULTS The calculated mobility spectra with some experimentally observed spectra in the form of fraction concentrations divided by the logarithm of the ratio of mobilities that correspond to the fraction boundaries of the spectrometers are presented in Figure. The calculated spectra reflect the basic features of the experimentally observed mobility spectra, but are flatter and broader. The depression in calculated spectra at the mobilities, smaller than 0.01 cm2V-ls -1 is less expressed than in measured spectra and also its locations is less changed. The depression is evoked by recombination of growing intermediate ions with small air ions. Uncharged particles are not observable by electrical spectrometers. The uncharging of charged nanometer particles is essentially more effective than charging of neutral particles. The rise in mobility spectra towards smaller mobilities after the depression minimum is a result of recharging of uncharged growing particles. The charging probability of uncharged particles is growing deeply with their size. The processes of recombination, coagulation with pre-existing aerosol particles, and condensational growth are competing in the formation of the modal structure of the mobility spectrum of charged nanometer particles. At the low condensation recombination removes particles from a mobility spectrum
$700
Abstracts of the 2000 EuropeanAerosol Conference
$701
and there are no modes in the mobility range of 0.5 - 0.01 cm2V-Is-t. If the scavenging by pre-existing aerosol particles is absent, the growth rate of intermediate ions larger than 0.05 nm.min -t is needed for 600 500 .."
400 ~ 300
•
200
".,,.,,•
° '~. °. ~..,"
/ /
\
/
100
0,001
0,01
0,1
1
Mobility k [cm2/(V s)]
Figure. The calculated (solid lines) and observed air ion mobility spectra. The observed spectra in ascending order are sequance of hourly average spectra measured at Tahkuse Observatory at 13:00-16:00 local time on October 20, 1994. the appearance of a mode. At this growth rate the mobility spectrum has a minimum at the mobility of about 0.04 cm2V-ms-I and then rises towards smaller mobilities due to recharging process. In case of aerosol scavenging used in the simulation of mobility spectra the growth rates larger than 0.2 nm.min-t are needed for the mode appearance. The coagulation effect with an aerosol used in the simulation can be approximately characterised by the scavenging rates (in cm-~s-I) of 9.5.10-5, 2.3.10TM, 6.5.10 -4 per unit concentration of ions with mobilities (in cm2WIs-~) 0.01, 0.03, 0.1, respectively. In case of absence of aerosol scaven~ng the growth rate larger than 0.5 nm.min-~ evokes the constantly rising spectrum in the above-mentioned mobility range. In case of model aerosol scavenging the growth rate of 0.7 nm.min-~ is needed. The model spectra in ascending order in Figure are characterised with growth rates of 0.25 run.rain -1, 0.38 nm.min-l, 0.47 nm.min-~, respectively, and it takes approximately 5 rain, 11 rain, 36 min to grow to the mobility of the modes, respectively. The nucleation rates that is needed to get the approximately total intermediate ion concentrations observed in measurements are (in ion pair per cm3.s) 0.76, 0.65, 0.5, respectively. The flatness of obtained spectra shows that in the simulation some essential physical aspects, which effect the formation 'of mobility spectrum, are missing. The deepness of the depression in observed spectra indicates that the rate of recharging in model calculations is probably overpredicted and the rate of recombination is under-predicted. ACKNOWLEDGEMENTS This research has been supported by the Estonian Science Foundation grants 3326 and 3903. REFERENCES HiSrrak, U., J. Salm and H. Tammet (1998) Bursts of intermediate ions in the atmospheric air, 3GR, 103, 13909-13915 Fuchs, N.A. (1964) The Mechanics of Aerosols, Pergamon Press, Oxford. Reischl, G.P., J.M. Makelti, R. Karch and J. Necid (1996) Bipolar charging of ultrafine particles in the size range below 10 nm, J. Aerosol Sci. 27, 931-949. Tammet, H. (1995) Size and mobility of nanometer particles, clusters and ions, J. Aerosol Sci. 26, 459-475.
%1.3 I, Suppl. 1, pp. $700-$701. 2000
Pergamon www.elsevier.com/locate/jaerosci
Poster Session II. Atmospheric aerosols: physical properties SIM-ULATION OF TIIE MOBILITY SPECTRUM OF CHARGED PARTICLES DURING BURSTS IN ATMOSPHERIC AIR M. NOPPEL and U. HORRAK Institute of Environmental Physics, University of Tartu, Tartu, Estonia
KEYWORDS: Air Ions, Nanometer Particles, Mobility Spectrum, Simulation INTRODUCTION The mobility spectrum of air ions has been measured at Tahknse Observatory in Estonia (H~rrak et aL,1998). On the level of low background (the average concentration about 50 cm-3), high concentration bursts of intermediate air ions ( mobilities 0.05 - 0.5 cm2V-ls-J) occur occasionally. An example of these bursts is given in Figure by three consecutive hourly average mobility spectra measured in the aitemoon of October 20, 1994. This paper presents the preliminary results of the simulation of this event. CALCULATION OF A MOBILITY SPECTRUM The ion-induced nucleation was considered as a mechanism of the formation of intermediate ions. The relation between size and mobility of ions was described by the theory of Tammet (1995). The formed ions (charged nanometer particles) were taken to grow with constant rate by condensation. The Kelvin effect was ignored. The growth rates of ions were chosen by fitting the location of peak value in calculated spectra to the location of the peak v.alue in the observed spectra. The nucleation rate was fitted by equalising the sum of calculated fraction concentrations of ions with the sum of experimental fraction concentrations. The growing intermediate ions recombine with small ions and coagulate with pre-existing aerosol particles. During recombination the intermediate ions loose their charges and are not observable by a spectrometer any more. Coagulation with pre-existing aerosol takes also intermediate ions away from an observable spectrum. The experimental values of small ion concentration were used in recombination calculations. Due to the lack of experimental data about aerosol properties, the scavenging properties of pre-existing aerosol were described by the size spectrum typical for a rural site. Uncharged growing nanometer particles that escape coagulation with pre-existing aerosol particles, can reappear in the observable mobility spectrum when charged by small ions. Recombination and coagulation between intermediate ions were ignored, but recharging of uncharged intermediate ions was taken into account. Fuchs coagulation (Fuchs, 1964) and particles charging theories (Reischl et al., 1996) were used. RESULTS The calculated mobility spectra with some experimentally observed spectra in the form of fraction concentrations divided by the logarithm of the ratio of mobilities that correspond to the fraction boundaries of the spectrometers are presented in Figure. The calculated spectra reflect the basic features of the experimentally observed mobility spectra, but are flatter and broader. The depression in calculated spectra at the mobilities, smaller than 0.01 cm2V-ls -1 is less expressed than in measured spectra and also its locations is less changed. The depression is evoked by recombination of growing intermediate ions with small air ions. Uncharged particles are not observable by electrical spectrometers. The uncharging of charged nanometer particles is essentially more effective than charging of neutral particles. The rise in mobility spectra towards smaller mobilities after the depression minimum is a result of recharging of uncharged growing particles. The charging probability of uncharged particles is growing deeply with their size. The processes of recombination, coagulation with pre-existing aerosol particles, and condensational growth are competing in the formation of the modal structure of the mobility spectrum of charged nanometer particles. At the low condensation recombination removes particles from a mobility spectrum
$700
Abstracts of the 2000 EuropeanAerosol Conference
$701
and there are no modes in the mobility range of 0.5 - 0.01 cm2V-Is-t. If the scavenging by pre-existing aerosol particles is absent, the growth rate of intermediate ions larger than 0.05 nm.min -t is needed for 600 500 .."
400 ~ 300
•
200
".,,.,,•
° '~. °. ~..,"
/ /
\
/
100
0,001
0,01
0,1
1
Mobility k [cm2/(V s)]
Figure. The calculated (solid lines) and observed air ion mobility spectra. The observed spectra in ascending order are sequance of hourly average spectra measured at Tahkuse Observatory at 13:00-16:00 local time on October 20, 1994. the appearance of a mode. At this growth rate the mobility spectrum has a minimum at the mobility of about 0.04 cm2V-ms-I and then rises towards smaller mobilities due to recharging process. In case of aerosol scavenging used in the simulation of mobility spectra the growth rates larger than 0.2 nm.min-t are needed for the mode appearance. The coagulation effect with an aerosol used in the simulation can be approximately characterised by the scavenging rates (in cm-~s-I) of 9.5.10-5, 2.3.10TM, 6.5.10 -4 per unit concentration of ions with mobilities (in cm2WIs-~) 0.01, 0.03, 0.1, respectively. In case of absence of aerosol scaven~ng the growth rate larger than 0.5 nm.min-~ evokes the constantly rising spectrum in the above-mentioned mobility range. In case of model aerosol scavenging the growth rate of 0.7 nm.min-~ is needed. The model spectra in ascending order in Figure are characterised with growth rates of 0.25 run.rain -1, 0.38 nm.min-l, 0.47 nm.min-~, respectively, and it takes approximately 5 rain, 11 rain, 36 min to grow to the mobility of the modes, respectively. The nucleation rates that is needed to get the approximately total intermediate ion concentrations observed in measurements are (in ion pair per cm3.s) 0.76, 0.65, 0.5, respectively. The flatness of obtained spectra shows that in the simulation some essential physical aspects, which effect the formation 'of mobility spectrum, are missing. The deepness of the depression in observed spectra indicates that the rate of recharging in model calculations is probably overpredicted and the rate of recombination is under-predicted. ACKNOWLEDGEMENTS This research has been supported by the Estonian Science Foundation grants 3326 and 3903. REFERENCES HiSrrak, U., J. Salm and H. Tammet (1998) Bursts of intermediate ions in the atmospheric air, 3GR, 103, 13909-13915 Fuchs, N.A. (1964) The Mechanics of Aerosols, Pergamon Press, Oxford. Reischl, G.P., J.M. Makelti, R. Karch and J. Necid (1996) Bipolar charging of ultrafine particles in the size range below 10 nm, J. Aerosol Sci. 27, 931-949. Tammet, H. (1995) Size and mobility of nanometer particles, clusters and ions, J. Aerosol Sci. 26, 459-475.