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Photoelectric charging and detection of ultrafine particles
Afmospheric Environment Printed
m Great
Vol. 17. No. 3, pp. 655-657,
ooo44981/83/03065543
1983
a
Britam
103.0010
1983 Pergamon
Press Ltd.
SHORT COMMUNICATION PHOTOELECTRIC CHARGING AND DETECTION OF ULTRAFINE PARTICLES (First received 29 March
1982 and received for publication 1 September 1982)
Abstract-Ultrafine aerosol particles can be photoelectrically charged by irradiation with u.v.-light of photon energy below the threshold for 0, and NO, production. A concentration of some hundred particles crnm3 of diameter < 20nm was determined in suburban air by measuring the product of concentration n (cm- “) and mean charge per particle 4. The relative concentration of the chargeable particles is high during rain periods and low during sunny periods. A consistent maximum is observed when the base of the morning inversion reaches the hill-ton measurina site. The particles appear to be organic in nature with photoelectrically active species on their surfaces.
1.
Continuous measurements of atmospheric aerosol particles were made on a roof at a suburban hill-top site during July-August 1981 (see Figs 1 and 3 for a map of the site). Supporting measurements of a typical pollution aerosol were made in an underground garage. The concentration of aerosol particles was also measured with a commercial electrical aerosol analyser (EAA).
INTRODUCTION
Photoelectric charging is an elegant method for measuring the mobility and the concentration of aerosols. It is particularly sensitive to very small particles, for which no other method for charging exists. It is also a convenient way to obtain some insight into the state of the surface of small particles (Schmidt-Ott and Federer, 1981).Ideally, one would like to choose high energy photons for effective photoionization of particles. However, formation of gases like 0, and NO, occurring below 240nm, which can lead to gas-toparticle conversion, has to be avoided. This is why a low pressure Hg-discharge was chosen as a light source which delivers photons of 254nm. The question was whether the atmosphere contained particles of sufficiently low work function Wto be charged in this small energy window and if so, how their number-concentration changes with varying meteorological conditions. It is thought that particles with low Ware especially important in atmospheric chemistry due to their high catalytic efficiency. The ultimate goal would be to develop an instrument which photoelectrically charges the atmospheric particles at different wavelengths of the light source (to discriminate between particles of different chemical surface composition) and then measures their sizedistribution by determining their charges and mobilities. Preliminary experiments using an instrument distinguishing two mobility classes are reported in this note.
3. RESULTS AND DISCUSSION
2. MEASUREMENT PRINCIPLE The apparatus to measure the chargeable particles consists of an electrostatic filter to remove precharged particles (Langevin ions, with typical concentrations of 800cme3 and 250e3 in the inversion and during rain, respectively), a photoelectric charging section and a section in which the electrical mobilities of the particles and their total charge are determined (Burtscher et cl., 1982). The Hg-lamp charges the particles positively according to their photoelectric threshold. An electrostatic filter subsequently precipitates the more mobile particles and the remaining large ones are collected in a mechanical filter. The number of charged particles per unit volume n is connected with the electrical current i generated in both filters according to uq = i/Q,
(1)
where Q is the air flow-rate through the instrument (normally 200-300cm3 s-r) and q is the mean charge per particle. The particles typically acquire between one and five elementary charges.
The charge-concentration product nq of the photoelectrically chargeable particles with high mobility (diameter d < 20nm), during a typical summer day is shown in Fig. 2. The most salient features are (1) the relatively high chargeconcentration product during the light rain period, when the concentration of particles measured by EAA tended toward zero, (2) the consistently large increase of nq after sunrise when the haze layer reached our measuring site and (3) the lower-than-average charge-concentration product during cloudless, hot weather when the EAA measurements were highest. These results are indeed surprising. Our laboratory measurements (Burtscher et al., 1982)indicate that particles with a diameter d = 10 nm, produced by smoke of burning organics (especially in an incomplete oxidation), are charged very efficiently by low energy photons of 4.9 eV. Garage-aerosol in the size range from 50 to lOOnm, for instance, showed a charging efficiency of 65-85 “/,. Chemical analyses* of the chargeable garage particles and those that could not be charged gave the same results (alkanes and unsaturated hydrocarbons). This indicates that all particles consist of a photoelectrically inactive core and that the chargeability is determined by a photoelectrically active surface species with negligible mass which may also influence other properties of the particles such as a catalytic activity in smog formation, nucleation capability or toxicity. In the light of these results we suspect that the photoelectrically chargeable particles measured during rain and at the base of the inversion layer are hydrophobic organic particles of small diameter (_ 10nm) which cannot be activated as condensation nuclei at the saturations prevailing in the atmosphere. Mixed particles which form the visible haze droplets in the morning inversion desorb the water from the hydrophobic portion of the particles when the sun starts to heat the top layers
of the haze. Hence
* We would like to thank spectroscopic analysis. 655
J. Muheim
the number
for
the
of
mass
Short communication
656
Fig. 1. Map of the area of Zurich showing the measuring site on the ETH campus Honggerberg.
FILTER
CURRENT [lO-s A]
ing high energy photons revealed that aerosols generated in light-indu~d gas-to-particle reactions in the presence of water vapor, cannot be charged photoelectrically with the Hg low-pressure lamp. It is, thus, presumed that during the hot, sunny afternoons, smog-particles are produced in sufficient numbers to remove the chargeable particles by coagulation. A second possible explanation for the low charge-concentration product of chargeable particles after about 1100 h in Fig, 2 is the t~nsformation of their surfaces by phot~hemi~l processes or their action as growth centers for condensible vapors.
S”” m?Cl
-3
cl
4
12
8
16
20
24 TIME
3. Aug. 81
[h] 4.
Fig. 2. Typical daily variation of the chargecon~nt~tion product nq of particles, or fitter current according to Equation (l), in the high mobility class. e is the unit electronic charge. chargeable particles increases markedly. The reduction of their charge-concentration product during the sunny periods, when the pollution levels (as evidenced by the EAAmeasurements) are high, seems to be due to the removal of the small chargeable particles by coagulation with nonchargeable ones, which seem to be mainly liquid. Separate experiments using a Xe high-pressure arc produc-
2 r
700
CONCLUSIONS
Suburban air usuaily contains several hundred ultrafine particles cm- 3 with su~cient~y low work function to be charged by a Hg Iow-pressure discharge (40 W). Their relative charge-concentration product is high during rain periods and low during sunny periods. A consistent maximum was observed after sunrise when the morning inversion, manifested by a visible haze layer, started to break upand the hazelayer reached our hill-top site. By comparing these measurements with those from garage air and other aerosols from combustion of organic material, we suspect that the particles which can be efficiently charged, are organic in origin with traces of photoelectrically active material on their surface.
PROFILE
A-B
L.5m &xl zj = 400
0
1
2
3 HORIZONTAL
L
5 DISTANCE
6 [km]
Fig. 3. Profile A-B in map of Fig. I.
7
6
Short communication This work was supported in part by the Branco WeissFonds. Atmospheric Physics ETH 8093 Ziirich Switzerland Solid State Physics ETH 8093 Ziirich Switzerland
657 REFERENCES
B. FEDERER Burtscher H., Scherrer L., Siegmann H. C., Schmidt-Ott A. and Federer B. (1982) Probing aerosols by photoelectric charging. .I. appl. Phys. 53, 3787-3791. H. BURTSCHER Schmidt-Ott A. and Federer B. (1981) Photoelectron emisA. SCHMIDT-OTT sion from small particles suspended in a gas. Surfnce Sci. H. C. SIEGMANN 106. 538-543.
m Great
Vol. 17. No. 3, pp. 655-657,
ooo44981/83/03065543
1983
a
Britam
103.0010
1983 Pergamon
Press Ltd.
SHORT COMMUNICATION PHOTOELECTRIC CHARGING AND DETECTION OF ULTRAFINE PARTICLES (First received 29 March
1982 and received for publication 1 September 1982)
Abstract-Ultrafine aerosol particles can be photoelectrically charged by irradiation with u.v.-light of photon energy below the threshold for 0, and NO, production. A concentration of some hundred particles crnm3 of diameter < 20nm was determined in suburban air by measuring the product of concentration n (cm- “) and mean charge per particle 4. The relative concentration of the chargeable particles is high during rain periods and low during sunny periods. A consistent maximum is observed when the base of the morning inversion reaches the hill-ton measurina site. The particles appear to be organic in nature with photoelectrically active species on their surfaces.
1.
Continuous measurements of atmospheric aerosol particles were made on a roof at a suburban hill-top site during July-August 1981 (see Figs 1 and 3 for a map of the site). Supporting measurements of a typical pollution aerosol were made in an underground garage. The concentration of aerosol particles was also measured with a commercial electrical aerosol analyser (EAA).
INTRODUCTION
Photoelectric charging is an elegant method for measuring the mobility and the concentration of aerosols. It is particularly sensitive to very small particles, for which no other method for charging exists. It is also a convenient way to obtain some insight into the state of the surface of small particles (Schmidt-Ott and Federer, 1981).Ideally, one would like to choose high energy photons for effective photoionization of particles. However, formation of gases like 0, and NO, occurring below 240nm, which can lead to gas-toparticle conversion, has to be avoided. This is why a low pressure Hg-discharge was chosen as a light source which delivers photons of 254nm. The question was whether the atmosphere contained particles of sufficiently low work function Wto be charged in this small energy window and if so, how their number-concentration changes with varying meteorological conditions. It is thought that particles with low Ware especially important in atmospheric chemistry due to their high catalytic efficiency. The ultimate goal would be to develop an instrument which photoelectrically charges the atmospheric particles at different wavelengths of the light source (to discriminate between particles of different chemical surface composition) and then measures their sizedistribution by determining their charges and mobilities. Preliminary experiments using an instrument distinguishing two mobility classes are reported in this note.
3. RESULTS AND DISCUSSION
2. MEASUREMENT PRINCIPLE The apparatus to measure the chargeable particles consists of an electrostatic filter to remove precharged particles (Langevin ions, with typical concentrations of 800cme3 and 250e3 in the inversion and during rain, respectively), a photoelectric charging section and a section in which the electrical mobilities of the particles and their total charge are determined (Burtscher et cl., 1982). The Hg-lamp charges the particles positively according to their photoelectric threshold. An electrostatic filter subsequently precipitates the more mobile particles and the remaining large ones are collected in a mechanical filter. The number of charged particles per unit volume n is connected with the electrical current i generated in both filters according to uq = i/Q,
(1)
where Q is the air flow-rate through the instrument (normally 200-300cm3 s-r) and q is the mean charge per particle. The particles typically acquire between one and five elementary charges.
The charge-concentration product nq of the photoelectrically chargeable particles with high mobility (diameter d < 20nm), during a typical summer day is shown in Fig. 2. The most salient features are (1) the relatively high chargeconcentration product during the light rain period, when the concentration of particles measured by EAA tended toward zero, (2) the consistently large increase of nq after sunrise when the haze layer reached our measuring site and (3) the lower-than-average charge-concentration product during cloudless, hot weather when the EAA measurements were highest. These results are indeed surprising. Our laboratory measurements (Burtscher et al., 1982)indicate that particles with a diameter d = 10 nm, produced by smoke of burning organics (especially in an incomplete oxidation), are charged very efficiently by low energy photons of 4.9 eV. Garage-aerosol in the size range from 50 to lOOnm, for instance, showed a charging efficiency of 65-85 “/,. Chemical analyses* of the chargeable garage particles and those that could not be charged gave the same results (alkanes and unsaturated hydrocarbons). This indicates that all particles consist of a photoelectrically inactive core and that the chargeability is determined by a photoelectrically active surface species with negligible mass which may also influence other properties of the particles such as a catalytic activity in smog formation, nucleation capability or toxicity. In the light of these results we suspect that the photoelectrically chargeable particles measured during rain and at the base of the inversion layer are hydrophobic organic particles of small diameter (_ 10nm) which cannot be activated as condensation nuclei at the saturations prevailing in the atmosphere. Mixed particles which form the visible haze droplets in the morning inversion desorb the water from the hydrophobic portion of the particles when the sun starts to heat the top layers
of the haze. Hence
* We would like to thank spectroscopic analysis. 655
J. Muheim
the number
for
the
of
mass
Short communication
656
Fig. 1. Map of the area of Zurich showing the measuring site on the ETH campus Honggerberg.
FILTER
CURRENT [lO-s A]
ing high energy photons revealed that aerosols generated in light-indu~d gas-to-particle reactions in the presence of water vapor, cannot be charged photoelectrically with the Hg low-pressure lamp. It is, thus, presumed that during the hot, sunny afternoons, smog-particles are produced in sufficient numbers to remove the chargeable particles by coagulation. A second possible explanation for the low charge-concentration product of chargeable particles after about 1100 h in Fig, 2 is the t~nsformation of their surfaces by phot~hemi~l processes or their action as growth centers for condensible vapors.
S”” m?Cl
-3
cl
4
12
8
16
20
24 TIME
3. Aug. 81
[h] 4.
Fig. 2. Typical daily variation of the chargecon~nt~tion product nq of particles, or fitter current according to Equation (l), in the high mobility class. e is the unit electronic charge. chargeable particles increases markedly. The reduction of their charge-concentration product during the sunny periods, when the pollution levels (as evidenced by the EAAmeasurements) are high, seems to be due to the removal of the small chargeable particles by coagulation with nonchargeable ones, which seem to be mainly liquid. Separate experiments using a Xe high-pressure arc produc-
2 r
700
CONCLUSIONS
Suburban air usuaily contains several hundred ultrafine particles cm- 3 with su~cient~y low work function to be charged by a Hg Iow-pressure discharge (40 W). Their relative charge-concentration product is high during rain periods and low during sunny periods. A consistent maximum was observed after sunrise when the morning inversion, manifested by a visible haze layer, started to break upand the hazelayer reached our hill-top site. By comparing these measurements with those from garage air and other aerosols from combustion of organic material, we suspect that the particles which can be efficiently charged, are organic in origin with traces of photoelectrically active material on their surface.
PROFILE
A-B
L.5m &xl zj = 400
0
1
2
3 HORIZONTAL
L
5 DISTANCE
6 [km]
Fig. 3. Profile A-B in map of Fig. I.
7
6
Short communication This work was supported in part by the Branco WeissFonds. Atmospheric Physics ETH 8093 Ziirich Switzerland Solid State Physics ETH 8093 Ziirich Switzerland
657 REFERENCES
B. FEDERER Burtscher H., Scherrer L., Siegmann H. C., Schmidt-Ott A. and Federer B. (1982) Probing aerosols by photoelectric charging. .I. appl. Phys. 53, 3787-3791. H. BURTSCHER Schmidt-Ott A. and Federer B. (1981) Photoelectron emisA. SCHMIDT-OTT sion from small particles suspended in a gas. Surfnce Sci. H. C. SIEGMANN 106. 538-543.