The epiphaniometer, a new device for continuous aerosol monitoring

J. Aerosol Sei., Vol. 19, No. 7, pp. 931 - 934, 1 9 8 8 Printed in Great Britain

0021-8502/88$3.00. + 0.00 Pergamon Press pie

THE EPIPHANIOMETER, A NEW DEVICE FOR CONTINUOUS AEROSOL MONITORING U. Baltensperger, H.W. G~iggeler, D.T. Jost Paul Scherrer Institute, CH-5303 Wtirenlingen, Switzerland

For studies of nuclear decay properties of short-lived isotopes so-called gas-jet systems are commonly used for transportation of the nuclides from an accelerator or a reactor to a counting device. With this technique, products recoiling from a target are thermalized in a carder gas which is saturated with aerosol panicles (mostly sodium or potassium chloride, Herrmann and Trautmann, 1982). In a continuous mode, this aerosol is pumped through a capillary to transport the panicles bearing the activity from the target chamber to the investigation site. This technique can also be applied to on-line monitoring of aerosols of any kind (Baltensperger et al., 1986, G~iggeler et al., 1986). A schematic diagram of the device is shown in Fig. 1. Artificial or environmental aerosols are pumped through a chamber containing a nTAc source (activity 2.104 Bq). By consecutive radioactive decay 2ZTAcproduces 2°7pb via the following decay chain (SeelmannEggebert et al., 1981, (1~) and (00 mean decay by [3- and ct-emission): 227Ac (TI/2 = 21.8 y) ([3) --~ 227Th (Tu2 = 18.7 d) (a)--~ 223Ra (Tit2 = 11.4 d) (ot)---~ 219Rn (TI/2 = 3.96 s) (a) ~ 215po (Tit2 = 1.8 ms) (or) -o 211pb O'v2 = 36.1 min) (~)--~ 2nBi (TI~= 2.17 rain) (tx)-~ 2°7Tl(T1~ = 4.77 rain) (~)--~ 2°7pb. -

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Fig. 1. Schematic diagram of the Epiphaniometer (for details see text). The noble gas isotope 219Rn escapes by emanation from the source, diffuses over a distance of only a few cm and then decays further to 2UPb via a very short-lived intermediate. If an aerosol is pumped through the vessel, the 2~Xpb atoms attach to the aerosol particles which are transported through a capillary (1.5 mm i.d., 1 m length) acting as a diffusion barrier for non-attached lead atoms. For a typical gas flow rate of 1 1/min the average residence time of an aerosol panicle in the container is 2 min. In the capillary, the Bernoulli force confines the particle flow near the capillary axis, where they are transported with high efficiency. At the end of the capillary, the particles and with them the attached lead atoms are deposited on a filter (nuclepore filter, pore size 0.4 I.tm, supported by a glass fiber filter and a steel frit). This assures that no activity can escape from the closed system to the environment. The deposited activity of 211pb on the filter is measured continuously by an annular surface barrier detector via the a-activity of its daughter nuclide 2UBi having a half-life of 2.2 min. Since the detector is facing the deposition spot it is only sensitive to the uppermost layer of the particles deposited. This allows counting the decaying 21tBi nuclides without loss of 0t-resolution even for very long counting periods with high amounts of several mg of material deposited onto the filter.

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Since m-spectroscopy is used, our set-up can also be used to measure the natural radon and thoron concentration in the air by means of their decay products. After each measurement windows are set to sum up the counts for 211Bi and for radon (214po) and thoron (212po). The calculated values for the windows are stored together with a time mark, the integrated gas flow, and the temperature in a non-volatile memory. The storage capacity is sufficient for a 161-day measurement if half-hour counting periods are used. The spectrum can also be sent to a serial port for off-line processing. Constant gas flow (0.25 to 2.5 1/min) is maintained by a mass flow controller. A battery operated version of the device consumes only 8 W, including counting and data processing. Due to the relatively short half-life of 211pb of 36 min, the device allows continuous monitoring of aerosols without changing or transporting the filter. Short fluctuations of aerosol concentrations in the minute-range are not detected because of the "damping constant" inherent in the system by the half-life of 211pb. Therefore, counting periods of half an hour were found optimal for aerosol monitoring in field studies. This means that in laboratory studies the system has to operate several half-lives of 211pb (usually about 2 hours) in order to reach the so-called saturation activity A 0 of 2Upb. However, since A 0 is connected to the activity A measured at any time before saturation is reached according to A = A0.(1-e-~'t) (1) with t = 1.92.10 .2 min -1 (for 2Upb) and t being the time between start and stop counting, A 0 can therefore also be deduced from short counting periods. However, one has to take into account that the daughter nuclide 211Bi which is measured has to be in equilibrium with 2npb. This takes about 10 rain. The attachment process was investigated on a silver aerosol. A detailed description of the experiments is given by G~iggeler et al. (1988). It was found that the activity of the actinium source together with its daughter nuclides acts as a perfect aerosol and 211pb neutralizer, and image forces have not to be taken into account when calculating attachment coefficients. The system was further calibrated with monodisperse polystyrene latex particles (G~iggeler et al., 1988). The latex particles were generated by a special pneumatic nebulizer system. Particle concentrations were simultaneously determined by a laser particle counter constructed at the NC Laboratory (Hailer et al., 1979), with a measuring range of 0.07 to 5 ~tm. Six different latex particle diameters between 0.091 and 2.84 ~tm were used. Figure 2 shows the measured equilibrium activity of 2UPb normalized by the particle concentration as a function of the diameter of the latex particles. The result is in line with the theory for the collision rate of atoms with aerosol particles developed by Fuchs (1964) (see e.g. Seinfeld, 1986). Thus, at small aerodynamic diameters (<100 nm) the Epiphaniometer signal is proportional to d 2, whereas at large aerodynamic diameters (>3 ~tm) the signal is proportional to d. Therefore, for a polydisperse aerosol the obtained signal is the integral of the differential products dN - dS, with N = particle concentration and S = "Fuchs surface" = n - d x, x varying between 1 and 2, depending on particle diameter. We therefore named the device EPIPHANIOMETER (greek epiphania = surface of a body).

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Fig. 2. Measured equilibrium activity of 211pb normalized by the particle concentration as a function of the diameter of the polystyrene latex particles. Also shown is the calculated curve (relative units) for the coagulation coefficient of 2Hpb with aerosol particles (from Seinfeld, 1986). The Epiphaniometer signal was constant within 20 % over the whole range of the flow rate investigated (0.3 - 2.5 I/min). Thus, since the adsorption coefficient over the corresponding residence times (0.8 to 7 rain) is constant, the smaller particle throughput at lower flow rates is compensated by the longer particle residence time in the 211pb container. Penetration through the nuclepore filter (pore size 0.4 I.tm) was determined at a gas velocity of 5.3 cm/s with latex particles (0.091 and 0.125 ktm in diameter) as well as with residual particles in the water. Maximum penetration was found at an aerodynamic diameter of about 0.1 I.tm and was less or equal to 0.1%. The Epiphaniometer can also be applied to monitor environmental aerosols. In this case, the signal is most sensitive for particles from the accumulation mode, since the Fuchs surface distribution

The epiphanlometer

933

peaks in this region (Seinfeld, 1986). Due to the high sensitivity the Epiphaniometer is able to measure aerosol concentrations of less than 100 ng/ms with gas flow rates as low as 1 l/rain. Extrapolating from the calibration experiment with latex particles, the detection limit (defined as 100 counts in a half-hour measurement interval) corresponds e.g. for 0.806-~tm particles to a concentration of about 0.2 cm 3 or 60 ng/m 3. Therefore, aerosol monitoring is possible also at high-alpine sites such as the Jungfraujoch (3540 m a.s.1.), with mean aerosol concentrations of about 5 pg/m 3 (BUS, 1987). Figure 3 shows the result of a simultaneous measuring period on Jungfraujoch and on Weissfluhjoch, Davos (2540 m a.s.1.) from 16 February to 22 March 1988. The ratio between the highest and the lowest half-hour value during this period for e.g. the Jungfraujoch is 2400. This proves the high dynamic range which is needed for monitoring at such sites, where clean background and highly contaminated air can be found, depending on the meteorological conditions. 90000

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Fig. 3. Epiphaniometer signal from Jungfraujoch (3540 m a.s.l., dashed line) and from Weissfluhjoch Davos (2540 m a.s.l., solid line) for a measuring period from 16 February to 22 March 1988 (half-hour integrals). Figure 3 also shows that always when the data on Jungfraujoch rise to higher than background values, the data on Weissfluhjoch rise also. This is not true for the reverse case. A tentative interpretation for this phenomenon is that at least in wintertime high values on Jungfraujoch are correlated with a large-scale air pollution, whereas on Weissfluhjoch also local air pollution can be detected. However, more experiments are needed to confirm this statement. In August 1988, experiments were started on Colle Gnifetti, Monte Rosa, at 4450 m a.s.l., to investigate if the Epiphaniometer, in combination with a solar panel, is suited for aerosol monitoring even at an extremely high-alpine background station• Furthermore, due to its high dynamic range, the Epiphaniometer can b¢ used for emission characterization, for example space heating or diesel emissions. Moreover, the device can be used for monitoring of indoor air quality, with simultaneous determination of aerosol, radon and thoron concentrations. On the other hand, the Epiphaniometer is a useful tool for laboratory investigations, for example characterization of artificial aerosols or examination of sticking probabilities as a function of chemical composition of the surface of the aerosol particles. Acknowledgements We thank A. Schmidt-Ott, Solid State Physics Laboratory, ETH-Hrnggerberg, Ztirich, and P. Hailer, NC Laboratory, Spiez, for their most valuable cooperation. The possibilities to perform aerosol measurements including valuable technical support and surveillance of the Epiphaniometer on Jungfraujoch (K. and M. Kocher) and on Weissfluhjoch (mainly E. Beck) are highly appreciated.

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R,fca'~nces Baltcnsperger, U., Jost, D. and G/lggeler, H. (1986) in Aerosols, Formation and Reactivity, W. Schikarski, H.J. Fissan, S.K. Friedlander (Editors), Proc. 2nd Int. Aerosol Conf., Berlin, Pergamon, Oxford, pp. 495-498. BUS (1987) L0ftbelastung 198(i, Schriftenreihe Umweltschutz Nr. 67, Bundesamt f~ir Umweltschutz (Editor), Bern. Fuchs, N.A. (1964)Mechanics of Aerosols, Pergamon, New York. Gliggeler, H.W., Baltensperger, U. and Jost, D.T. (1986) patents pending: CH 3763/86, 6020DE. Gliggeler, H.W., Baltensperger, U., Emmenegger, M., Jost, D.T., Schmidt-Ott, A., Hailer, P. and Hofmann, M. (1988) submitted to J. Aerosol Sci. Hailer, P., Hofmann, M. and Bumbovic, B. (1979) Proc. 2nd European Symposium on Particles Characterization, NUrnberg, pp. 191-196. Herrmann, G. and Trautmann, N. (1982) Annu. Rev. Nucl. Part. Sci. 32, 117-147. I.ederer, C.M. and Shirley, V.S. (1978) Table of Isotopes, 7th ed., John Wiley, New York. Schmidt-Ott, A. (1988) J. Aerosol $¢i, 19, in press. Seelmann-Eggebert, W., Pfennig, G., Miinzel, H. and Klewe-Nebius, H., (1981) Karlsruher Nuklidkarte, 5. Auflage, Kernforschungszentrum Karlsruhe (Editor). Seinfeld, J.H. (1986) Atmospheric Chemistry and Physics of Air Pollution, John Wiley, New York, 1986.