HOW DO WE DO IT?
What is the behind our technology and how do we tell you the mass of an object just by looking at it?
First, we need to distinguish our precise detection methods by the size of particles you want to measure. Is it either the larger particles you are interested in, meaning about 200 nanometers up to 35 microns, or is it the “sub-micron” range, the ultrafine particles below 100 nanometers down to less than 1 nanometer?
If the micron range is what you are interested in, then the optically measuring laser aerosol spectrometers are the instruments of your choice.
If you are more interested in nanoparticles in your research, , then a mix of sophisticated technologies comes into operation: Differential Mobility Analyzers (DMA), Faraday Cup Electrometers (FCE), Condensation Particle Counters (CPC) are the instruments of choice for this task.
Also a combination of instruments for the nanometer and micron-sized aerosol is possible – resulting in “Wide Range Aerosol Spectrometers” or shortly called WRAS.
Optical Aerosol Spectrometer
The sample air is drawn into the measuring chamber directly from the aerosol inlet or through the application-specific sample air collector. In the measurement cell, the accelerated aerosol is focused and guided into the optical volume passing through the laser beam at its highest intensity.
The laser being a round beam transforms into a flat band by an arrangement of sophisticated aligned optics, creating a reproducible and very precisely defined optical volume. Once the particle is inside the optical volume, the laser beam is scattered on the particle.. Each scattered signal pulse is counted by a photo diode and due to the different particle sizes, this pulse varies in its intensity. By combining the number of pulses and intensities of those, the instrument measures a particle numberand size distribution.
This distribution forms the basis for further calculations. A sophisticated and well-proven data inversion algorithm converts the particle number-size distribution into mass fractions PM10, PM4 and PM2.5. Here, after years of empirical studies, the company GRIMM has been the pioneer in developing the first optically measuring PM monitor worldwide, with the count-to-mass conversion even receiving approval as federal equivalent method (FEM).
After detection, the aerosols are filtered on a removeable PTFE filter that collects all particles . A share of the now purified air is led back into the optical measurement chamber constantly flushing the optical components. This process keeps the optics clean and is the reason why GRIMM laser aerosol spectrometers do not need any auto-zero function.
Differential Mobility Analyzer (DMA)
A DMA classifies aerosol particles according to their electrical mobility. Consequently, the particles need to carry at least one elementary charge. For this, various aerosol chargers or neutralizers are used, that involve an ionizing radiation to produce a large number of positive and negative ions, also called bipolar ionic atmosphere. GRIMM offers bipolar neutralizers based on the radioactive nuclides Am-241 or Ni-63, on a dielectric barrier discharge plasma and on soft x-ray radiation. All latter mentioned neutralizers have one in common: by interaction of the aerosol with the ions generated in the neutralizer, a well-defined stationary state charge distribution is established on the aerosol that was theoretically described by Fuchs (1963); a parametrized version is given by Wiedensohler (1988).
The most commonly used DMAs are based on a cylindrical electrode design, consisting of a center electrode on high voltage potential and an outer electrode on ground potential. GRIMM DMAs are based on the Vienna-type design (Reischl 1991, Winklmayr et al., 1991), well-known for its high resolution-power and low diffusional particle losses. When the aerosols enter the classification region of the DMA, the aerosol flow is merged with the particle free sheath air flow and the charged particles drift in the electric field between the electrodes towards an annular exit slit in the center electrode. At fixed geometry parameters (radii and length of the DMA) and operating conditions (air flows), there exists a unique correlation between the adjusted voltage on the center electrode and the electrical mobility of the particles entering the exit slit of the classification channel. The classified particles are subsequently detected by a condensation particle counter (CPC) or by a Faraday cup electrometer (FCE).
The recorded raw data of a DMA measurement forms a number-mobility distribution. It can be readily converted into a number-size distribution under the assumption that the size distribution of the aerosol does not quickly change during the measurement time and that the particle size distribution is limited within the measurement range of the instrument. For the correct data inversion, the knowledge of the charging state of the aerosol is crucial.
Typically, DMAs are operated in the size range between 1-1000nm. GRIMM offers DMAs with 3 different lengths of 15, 88 and 350mm, optimized for different particle size ranges to accommodate a variety of experimental needs.
Faraday Cup Electrometer (FCE)
An FCE is an aerosol detector that measures the electric current of a flow of charged aerosols. It consists of a particle filter that is mounted electrically insulated within a Faraday Cup. The charged aerosols that are collected on the particle filter change the potential of the cup which is compensated by in- or outflowing electrons. This current of electrons is typically in the femtoampere-range and is measured with a high sensitivity electrometer circuit with an input resistance of about 1TΩ. If the aerosol inlet flow rate and the charging state of the aerosols are known, the measured electrical current can be converted into a number concentration.
The main advantage of FCEs is that their particle detection efficiency is virtually independent from particle size. Also, it is often useful to use FCEs as detector behind a DMA, since all classified particles are electrically charged.
Condensation Particle Counter (CPC)
A CPC is a detector that enlargers aerosols by heterogeneous condensation of different vapors of various working fluids to sizes that are optically detectable. In the literature, expansion type, mixing type and thermally diffusive, laminar flow CPCs are described; all establishing a vapor supersaturation by different methods. The GRIMM CPC models are all thermally diffusive, laminar flow CPC where the aerosol flow is saturated with butanol vapor in the so-called saturator. In the following section, the condenser, the aerosol flow is cooled to trigger a supersaturation of the butanol vapor to condense onto the surface of the aerosol particles, thereby enlarging them in size. Subsequently, the particles grown into the micron size range, can be readily counted in an optical cell. CPCs are unmatched aerosol detectors when it comes to very low number concentrations: single particle counting is possible. Also, the aerosol does not necessarily need to be electrically charged.
Fuchs, N.A. (1963). On the stationary charge distribution on aerosol particles in a bipolar ionic atmosphere. Geofis. Pura Appl. 56, p. 185
Reischl, G.P. (1991). Measurement of Ambient Aerosols by the Differential Mobility Analyzer Method: Concepts and Realization criteria for the Size Range between 2 and 500 nm. Aerosol Science and Technology, 14, 5
Wiedensohler, A. (1988). An approximation of the bipolar charger distribution for particles in the submicron size range. J. Aerosol. Sci., 19, 3, 387-389
Winklmayr, W., Reischl, G.P., Lindner, A.O. and Berner A. (1991). A New Electromobility Spectrometer for the Measurement of Aerosol Size Distribution in the Size Range from 1 to 1000 nm. Journal of Aerosol Science, 22, 289