Differential mobility analyzer

The differential mobility analyzer ( DMA , engl .: Differential Mobility Analyzer ) acts as a größenselektierender electrostatic precipitator , which - depending on applied by its geometrical dimensions and the (variable) electric voltage - from an aerosol stream only particles of a certain electric mobility to pass through. As a rule, a DMA has a cylindrical structure and, in addition to the aerosol inlet and outlet, requires an auxiliary gas flow and accordingly has additional connections.

Together with an electric charger (usually a neutralizer or a bipolar diffusion charger ), the DMA becomes an online classifier for aerosols according to electric mobility. If a device for determining the particle concentration is also connected downstream (e.g. condensation particle counter , electrometer , FCE ), a Differential Mobility Analyzing System (DMAS) is created, depending on the operating mode as a Differential Mobility Particle Sizer (DMPS) or as a Scanning Mobility Particle Sizer (SMPS). In the future CEN guideline for atmospheric measurements, these systems will generally be referred to as Mobility Particle Size Spectrometer (MPSS). This also applies to the guidelines of the WMO-GAW network ( World Meteorological Organization - Global Atmosphere ) and the European research infrastructure ACTRIS ( Aerosols, Clouds, and Trace gases Research InfraStructure ).

functionality

The DMA consists of a capacitor , which is usually cylindrical in order to avoid edge effects of the electric field . Electrically charged particles are introduced into its outer wall and carried along in a laminar flow . Particle-free air (or Hüllluftvolumenstrom Sheath -Volumenstrom) is guided along the inner electrode, which separates the particles from the inner cylindrical electrode. If an electrical voltage is now applied to the cylinder electrodes, the charged particles drift towards the inner or outer electrode, depending on their electrical polarity. Only the particles that reach a narrow annular gap on the inner electrode can be removed from the DMA and fed to a particle counter. The removed fraction of the particles has a certain electrical mobility that correlates with the applied voltage according to: ${\displaystyle Zp}$

${\displaystyle U={\frac {Q\cdot \ln \left({\frac {R_{\mathrm {a} }}{R_{\mathrm {i} }}}\right)}{2\cdot \pi \cdot L\cdot Zp}}}$

Here, the gas volume flow that is passed through the capacitor is the radius of the outer electrode, the radius of the inner electrode and the electrode length between the aerosol inlet and outlet. The electrical mobility is a function of the particle radius via Stokes law ${\displaystyle Q}$ ${\displaystyle R_{\mathrm {a} }}$ ${\displaystyle R_{\mathrm {i} }}$ ${\displaystyle L}$ ${\displaystyle D_{p}}$

${\displaystyle Zp=n\ e\ C_{c}\ /\ 3\ \pi \ \eta \ D_{p}}$

with the number of electrical charges per particle , elementary charge , Cunningham slip correction factor , dynamic viscosity of air (see also ISO 15900) ${\displaystyle n}$ ${\displaystyle e}$ ${\displaystyle C_{c}}$ ${\displaystyle \eta }$

DMPS, SMPS or MPSS

By scanning, i. H. Through the continuous increase in voltage, the classifier becomes a measuring device and an electrical mobility distribution of the charged aerosol is obtained. If the charge distribution on the particles is known, one can draw conclusions about the mobility distribution and thus indirectly about the size distribution of the particles by back calculation with the help of an inversion algorithm.

As a rule, the charge distribution of the aerosol to be analyzed is unknown and the particles must be charged in a defined manner in order to be able to apply the back calculation to the particle size. A neutralizer ( bipolar diffusion charger ) is often used for this purpose, which sets the aerosol into a charge equilibrium. Typically, the aerosol is guided past a weakly radioactive source (e.g. the krypton -85 isotope , ⁸⁵ Kr as beta emitter ) or other nuclides (e.g. Po210, Ni63 or Am241). The radiation primarily causes an ionization of the ambient gas, i. H. Both negative and positive charge carriers arise, which attach to the particles through diffusion . The result is a defined bipolar charge distribution ( Boltzmann charge distribution or the Fuchs charge distribution), which is made up of positive, negative and electrically neutral particles and is in a first approximation independent of the particle material. The term neutralizer is therefore initially misleading, since the particles are not discharged, but rather an externally neutral bipolar charge equilibrium is established. The disadvantage here is that a large proportion of the particles (uncharged or incorrect polarity) cannot be used for the analysis. Furthermore, the bipolar charge distribution is shifted towards more negatively charged particles. This is due to the higher mean electrical mobility of the negative air ions. Internationally, only the bipolar charge distribution according to Alfred Wiedensohler [1] is used, which is also described in the ISO 15900 standard.

In other methods for the defined charging of aerosol particles, a corona (locally limited partial discharge of the ambient gas) is used to attach the emitted charge carriers to the particles via field and diffusion charging. Since only charge carriers of a certain polarity are used here, the result is a unipolar charge distribution. The advantage of the unipolar charging of aerosol particles is that almost all particles can be used for the analysis, so that electrometers can also be used to determine the concentration ( Ultrafine Particle Monitor ). Disadvantages of the unipolar diffusion charger are that the charge distribution becomes very broad (many charge states) and some of the particles are deposited in the charging zone.

Calibration, maintenance and operation

A DMPS, SMPS or MPSS should be calibrated regularly. This includes the size-resolved calibration of the condensation particle counter, the particle size calibration with PSL particles (polystyrene latex) and the comparison of the particle size distribution with a reference system. [2]. Recommendations for the operation of atmospheric measurements are described in detail in [3].

literature

[1] Alfred Wiedensohler (1988). An approximation of the bipolar charge distribution for particles in the submicron size range. J. Aerosol Sci. 19, 387-389.

[2] Wiedensohler, A., A. Wiesner, K. Weinhold, W. Birmili, M. Hermann, M. Merkel, T. Müller, S. Pfeifer, A. Schmidt, T. Tuch, F. Velarde, P. Quincey, S. Seeger and A. Nowak (2018). Mobility Particle Size Spectrometers: Calibration Procedures and Measurement Uncertainties. Aerosol Science & Technology 52 (2), 146-164.

[3] Wiedensohler, A., W. Birmili, A. Nowak, A. Sonntag, K. Weinhold, M. Merkel, B. Wehner, T. Tuch, S. Pfeifer, M. Fiebig, AM Fjäraa, E. Asmi , K. Sellegri, H. Venzac, P. Villani, P. Laj, P. Aalto, JA Ogren, E. Swietlicki, P. Roldin, P. Williams, P. Quincey, C. Hüglin, R. Fierz-Schmidhauser, M. Gysel, E. Weingartner, F. Riccobono, S.Santos, C. Grüning, K. Faloon, D. Beddows, R. Harrison, C. Monahan, SG Jennings, CDO'Dowd, A. Marioni, H.- G. Horn, L. Keck, J. Jiang, J. Scheckman, PH McMurry, Z. Deng, CS Zhao, M. Moerman, B. Henzing, G. d. Leeuw, G. Löschau and S. Bastian (2012). Mobility Particle Size Spectrometers: Harmonization of Technical Standards and Data Structure to Facilitate High Quality Long-term Observations of Atmospheric Particle Number Size Distributions. AMT 5, 657-685.

See also

• Electrostatics