Aerodynamic diameter

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The aerodynamic diameter , as an abstract variable for describing the behavior of a gas-borne particle, is an auxiliary variable in particle measurement technology .

definition

The aerodynamic diameter is defined as the diameter of a spherical particle with a density of 1 g / cm 3 , which has the same sinking speed as the particle under consideration. In the standardization , it is often mentioned that the rate of descent of the particle under consideration is related to still air. With the aid of the aerodynamic diameter, not only the geometric size of the particle is taken into account, but also its density and shape, since spherical particles can only be assumed when condensing from the gas phase. For particles with an aerodynamic diameter smaller than 0.5 µm, the standard DIN ISO 7708: 1996-01 recommends the use of the particle diffusion diameter, which describes the diameter of a sphere with the same diffusion coefficient as the particle in question. This restriction must be made because Stokes' law is based on the model concept that the relative velocity of a fluid at the edge of a particle is zero. However, this assumption does not apply to particles that are not significantly larger than the mean free path of the surrounding gas.

meaning

Occupational and environmental protection

The aerodynamic diameter is used, among other things, in the conventions on inhalable , thoracic and respirable fractions . The fine dust particle size classes for PM 10 and PM 2.5 are also defined via the aerodynamic diameter. Aerodynamic diameters are measured using time-of-flight spectrometers or electrical low-pressure impactors (ELPI).

pharmacy

From a medical point of view, the aerodynamic diameter of particles is important for inhalation therapy, since only their optimal size ensures that they remain at the desired site of action (see below). Different impactors can be used to measure inhalants; in addition to the Andersen cascade impactor , the next-generation impactor (NGI) is preferred today.

MMAD

Size distributions of the aerodynamic diameter are often parameterized using the Mass Median Aerodynamic Diameter (MMAD), i.e. the median mass-related aerodynamic diameter. The MMAD is therefore that diameter at which particles smaller or larger than this value each contribute 50% of the total mass and thus a measure of the average size of a particle.

The MMAD plays a major role in inhalation therapy. While particles with an MMAD of 10 μm are already deposited (impact) in the throat due to their inertia, particles between 0.1 μm and 1.0 μm are too light and are exhaled again due to diffusion processes caused by Brownian molecular movement. Particle sizes between 1-5 μm sediment / deposit in the lungs (respiratory system).

Individual evidence

  1. ↑ Sheldon Kay Friedlander : Smoke, Dust, and Haze - Fundamentals of Aerosol Dynamics . John Wiley & Sons, New York 1977, ISBN 0-471-01468-0 , p. 164.
  2. ↑ a b c d DIN ISO 7708: 1996-01 Air quality - Definition of particle size distributions for health-related airborne dust sampling (ISO 7708: 1995). Beuth Verlag, Berlin, p. 3.
  3. ↑ a b VDI 3867 sheet 1: 2009-09 Measurement of particles in the outside air - Determination of the particle number concentration and number size distribution of aerosols - Basics (Measurement of particulate matter in ambient air - Determination of the particle number concentration and number size distribution of aerosols - Fundamentals ). Beuth Verlag, Berlin. P. 3.
  4. ↑ James H. Vincent: Aerosol Sampling - Science, Standards, Instrumentation and Applications . John Wiley & Sons, Chichester 2007, ISBN 978-0-470-02725-7 , p. 27.
  5. ↑ James H. Vincent: Aerosol Sampling - Science, Standards, Instrumentation and Applications . P. 4.
  6. ↑ Sheldon Kay Friedlander: Smoke, Dust, and Haze - Fundamentals of Aerosol Dynamics . P. 6.
  7. ↑ a b DIN EN ISO 23210: 2009-12 Emissions from stationary sources - Determination of the mass concentration of PM10 / PM2.5 in exhaust gas - Measurement at low concentrations with impactors (ISO 23210: 2009); German version EN ISO 23210: 2009. Beuth Verlag, Berlin. P. 6.
  8. ^ William C. Hinds: Aerosol Technology . John Wiley & Sons, New York 1982, ISBN 0-471-08726-2 , p. 43.
  9. ↑ VDI 3867 Part 5: 2013-11 Measurement of particles in the outside air - Determination of the particle number concentration and number size distribution of aerosols - Time-of-flight spectrometer (Measurement of particles in ambient air - Determination of the particle number concentration and particle size distribution of aerosols - Time-of- flight spectrometer). Beuth Verlag, Berlin. P. 3 .
  10. ↑ VDI 3867 Sheet 6: 2012-12 Measurement of particles in the outside air - Determination of the particle number concentration and number size distribution of aerosols - Electrical low pressure impactor (ELPI) (Measurement of particles in ambient air - Determination of the particle number concentration and number size distribution of aerosols - Electrical Low Pressure Impactor (ELPI)). Beuth Verlag, Berlin. P. 3.
  11. ↑ European Pharmacopoeia 9.0, Volume I, Chapter 2.9.18 “Preparations for inhalation: Aerodynamic assessment of fine particles”. EDQM Council of Europe.
  12. ^ Gerhard Scheuch, Martin J. Kohlhaeufl, Peter Brand, Rüdiger Siekmeier: Clinical perspectives on pulmonary systemic and macromolecular delivery . In: Advanced Drug Delivery Reviews (=  Challenges and Innovations in Effective Pulmonary Systemic and Macromolecular Drug Delivery ). tape 58 , no. 9-10 , October 31, 2006, pp. 996–1008 , doi : 10.1016 / j.addr.2006.07.009 ( sciencedirect.com [accessed December 22, 2016]).