Condensation particle counter

A condensation particle counter or condensation nucleus counter ( English Condensation Particle Counter , short CPC , or English Condensation Nucleus Counter , short CNC ) is an optical measuring instrument, with the small particles in the atmosphere ( aerosols ), with sizes of a few nanometers to micrometers can be detected. The physical principles on which the measurement process is based are scattered light measurement and the fact that in an atmosphere with supersaturated vapor, particles can act as condensation nuclei . To enable condensation, the particles that are too small for optical detection are enlarged by heterogeneous condensation of , for example, n-butanol or water on their surface. In contrast to fractionating processes, particle measurement with the condensation particle counter is one of the integrating processes.

Structure and functionality

The CPC consists of a saturator , a condenser and laser optics to capture the particles.

Satiator

In the saturator, the aerosol, consisting of particles and carrier gas (ambient air), is saturated with the vapor of the liquid used. The air flow is directed over a wet sponge, which is located in a housing (saturator block) and has an elevated temperature. The Mollier-hx diagram shows how much liquid vapor the air can absorb at what temperature. In principle, the higher the temperature, more steam can be absorbed by a gas. The most widespread use as an evaporation liquid have alcohols such. B. n-butanol or isopropanol .

Condenser

The temperature in the condenser is much lower than in the saturator. The saturated air flow is cooled in the condenser. The cooling is usually done by external cooling. The cooling reduces the amount of liquid that can be absorbed by the air. One speaks now of oversaturated air. The liquid, which can no longer be absorbed by the air, condenses on the surface of the particles and forms a shell around them. Due to the larger diameter, it is now possible to capture the particles in an optical system. The ratio of the speed of mass transport to the speed of heat transport is important for the function of the condenser.

An alternative design of the CPC manages without external cooling, in that the aerosol to be sampled is mixed in a mixing nozzle with a warm, steam-saturated, but particle-free steam flow. The mixture leads to oversaturation of the gas.

All processes have in common that in the ideal case sudden oversaturation leads to the formation of a monodisperse aerosol. In the real case, a size distribution can be expected.

Laser optics

A laser beam is reflected off a screen behind which a detector is located. When a sufficiently large particle passes through the laser beam, scattered light is created, which is registered by the detector behind the aperture. The number and strength of the events recorded in this way are a measure of the number of particles in the area of the laser beam. The particle count is carried out by means of pulse counting.

Areas of application

Depending on the embodiment, a condensation particle counter can cover a size range from approx. 5 µm down to 3 nm. The measuring range depends on which smallest particles can still be stimulated to condensation growth and which particles get into the measuring volume without loss.

CPCs are used to measure the concentration of very small particles in the air that may a. are of particular importance in two areas:

In medicine : Due to their small size, the measurable particles are respirable and penetrate into the alveoli, where they can cause respiratory diseases. Condensation particle counters can be used as so-called personal monitors.
In environmental physics and atmospheric chemistry : Aerosol particles have a significant influence on weather and climate. In the atmosphere, water is deposited on the particles and clouds form. In addition, the climate is influenced by the ability of the particles to absorb radiation and, in part, to release it back into space. This effect is known as the cold store effect.

Further possible uses of a CPC are in the area of filter tests and quality control of test aerosols .

If a CPC is coupled with a differential mobility analyzer (DMA), a measuring device for determining the particle size distribution of an aerosol is obtained, which is called a Scanning Mobility Particle Sizer (SMPS).

A disadvantage of the CPC is that information about the original particle size is lost due to the condensation process. In addition, if the number concentration of particles is too high, the available amount of steam may be too small, so that too small particles cannot be counted.

literature

Yung-Sung Cheng: Condensation Particle Counters . In: Aerosol Measurement: Principles, Techniques, and Applications (Ed .: P. Kulkarni, PA Baron, K. Willeke) . 3. Edition. John Wiley & Sons, Hoboken, New Jersey 2011, ISBN 978-0-470-38741-2 , pp. 381–392 ( limited preview in Google Book search).
VDI 3867 Sheet 2: 2008-02 Measurement of particles in the outside air; Characterization of test aerosols; Determination of particle number concentration and number size distribution; Condensation Particle Counter (CPC) (Measurement of particulate matter in ambient air; Methods for characterizing test aerosols; Determination of the particle number concentration and particle size distribution; Condensation Particle Counter (CPC)). Beuth Verlag, Berlin. ( Summary and table of contents online )

Individual evidence

↑ ^a ^b Leander Mölter, Patricia Keßler: Basics of particle size and particle number determination in the outside air with counting measuring methods. In: Hazardous substances - cleanliness. Air . 64, No. 7/8, 2004, ISSN 0949-8036 , pp. 319-323.
↑ DIN EN 1822-1: 2011-01 Particulate filter (EPA, HEPA and ULPA); Part 1: Classification, performance testing, labeling; German version EN 1822-1: 2009. Beuth Verlag, Berlin, p. 10.
↑ ^a ^b VDI 3867 sheet 2: 2008-02 Measurement of particles in the outside air; Characterization of test aerosols; Determination of particle number concentration and number size distribution; Condensation Particle Counter (CPC) (Measurement of particulate matter in ambient air; Methods for characterizing test aerosols; Determination of the particle number concentration and particle size distribution; Condensation Particle Counter (CPC)). Beuth Verlag, Berlin, p. 9.
↑ VDI 3867 sheet 2: 2008-02 Measurement of particles in the outside air; Characterization of test aerosols; Determination of particle number concentration and number size distribution; Condensation Particle Counter (CPC) (Measurement of particulate matter in ambient air; Methods for characterizing test aerosols; Determination of the particle number concentration and particle size distribution; Condensation Particle Counter (CPC)). Beuth Verlag, Berlin, p. 3.
↑ Christof Asbach, Ana Maria Todea: Personal exposure to ultrafine particles in everyday life. In: Hazardous substances - cleanliness. Air. 76, No. 9, 2016, ISSN 0949-8036 , pp. 315–321.

[q1-1] Leander Mölter, Patricia Keßler: Basics of particle size and particle number determination in the outside air with counting measuring methods. In: Hazardous substances - cleanliness. Air . 64, No. 7/8, 2004, ISSN 0949-8036 , pp. 319-323.

[2] DIN EN 1822-1: 2011-01 Particulate filter (EPA, HEPA and ULPA); Part 1: Classification, performance testing, labeling; German version EN 1822-1: 2009. Beuth Verlag, Berlin, p. 10.

[q2-3] VDI 3867 sheet 2: 2008-02 Measurement of particles in the outside air; Characterization of test aerosols; Determination of particle number concentration and number size distribution; Condensation Particle Counter (CPC) (Measurement of particulate matter in ambient air; Methods for characterizing test aerosols; Determination of the particle number concentration and particle size distribution; Condensation Particle Counter (CPC)). Beuth Verlag, Berlin, p. 9.

[4] VDI 3867 sheet 2: 2008-02 Measurement of particles in the outside air; Characterization of test aerosols; Determination of particle number concentration and number size distribution; Condensation Particle Counter (CPC) (Measurement of particulate matter in ambient air; Methods for characterizing test aerosols; Determination of the particle number concentration and particle size distribution; Condensation Particle Counter (CPC)). Beuth Verlag, Berlin, p. 3.

[5] Christof Asbach, Ana Maria Todea: Personal exposure to ultrafine particles in everyday life. In: Hazardous substances - cleanliness. Air. 76, No. 9, 2016, ISSN 0949-8036 , pp. 315–321.