SMPS - DMPS (Mobility Particle Size Spectrometer)

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Principle of a Mobility Particle Size Spectrometer (SMPS or DMPS)

First, aerosol particles larger than the size range to be investigated are removed by a pre-impactor and dried to a relative humidity below 40%. Afterwards, the aerosol particles are brought to a thermal charge equilibrium using a bipolar charger (or traditionally named neutralizer). In a DMA, charged aerosol particles are separated by means of their electrical mobility. The electrical mobility mainly depends on the gas viscosity, number of charges, and the particle size and shape (Stokes’ equivalent diameter). The smaller the particle diameter and the higher the number of charges, the larger is the particle’s electrical mobility.

A DMA is usually built as a cylindrical capacitor. Two flows enter the DMA, the particle-free sheath air and the aerosol flow containing charged particles. By knowing the dimensions of the DMA, one can calculate the voltage between the electrodes to transport particles with a certain electrical mobility to a small slit in the centre rod of the capacitor. A flow carrying particles with nearly the same electrical mobility is then sucked through this small slit. The number concentration of this sample aerosol (particles with the same electrical mobility) is finally counted in a CPC (Condensation Particle Counter). By scanning the voltage step-wise (Differential Mobility Particle Sizer, DMPS) or continuously (Scanning Mobility Particle Sizer, SMPS) through the entire electrical particle mobility range to be investigated, the mobility distribution of is measured.

By knowing the cut-off diameter of the pre-impactor, the bipolar charge distribution (e.g. Wiedensohler, 1988; Note: corrections for two approximation coefficients for positively charged particles are published in Baron and Willeke), the DMA-transfer function (Birmili et al., 1997), and the CPC counting efficiency function (e.g. Wiedensohler et al., 1997; Hermann et al., 2007), the particle number size distribution can be calculated from the measured mobility distribution (Hoppel, 1978).

However, the quality of the measurements performed with such mobility spectrometers depends mainly on the stability of the aerosol and sheath air flow and the performance of the CPC. A change in the sheath air flow corresponds to a shift in the selected particle mobility, while fluctuations in the aerosol flow impact directly the measured number concentrations (see examples in the appendix). Important is also the measure of the actual ambient pressure for possible size and concentration corrections afterwards. For the voltage calculation, a pressure near the actual pressure has to be assumed in case the actual pressure cannot be used in real time. The CPC’s size dependent detection efficiency curve must be known, particularly for the measurements near its cut-off diameter, and its long-term stability ensured.

Furthermore, for measurements of atmospheric aerosol, the relative humidity of the aerosol influences the particle size. Since soluble particle material takes up water, the size of some particles can be doubled if the relative humidity reaches values above 90% depending on the actual chemical composition. For comparability between data sets, the philosophy of size distribution measurements of atmospheric aerosol is thus to keep the relative humidity (RH) below 40%. Below this RH, the water take up of the aerosol particles is not significant anymore in terms of diameter change (Swietlicki et al., 2008).

Recommendation of mobility size spectrometers for measurements of atmospheric aerosols

A schematic sketch of our recommended closed-loop-based mobility size spectrometer is given below in Figure 1. The proposed set-up includes dryers for aerosol flow and sheath air, a heat exchanger, particle filters, and sensors for aerosol and sheath air flow rate, relative humidity and temperature of aerosol flow and sheath air, and absolute pressure. In the following a detailed list of standardized system parameters is provided. All recommended system parameters should be recorded and stored with at least the same time resolution as measured size distributions.

- The aerosol size distribution should be measured for “dry conditions” (<40% RH). The aerosol flow has to be dried e.g. by a membrane (e.g. Nafion) or an aerosol-friendly diffusion drier. A dry aerosol flow in the neutralizer is needed to ensure a correct bipolar charging process. Furthermore, particle losses in the drier should be characterized and accounted for in the data analysis. The RH of the aerosol flow should be monitored by a calibrated humidity sensor with an accuracy better than 5% RH in the range 10-80% RH.

- The aerosol flow has to be monitored e.g. by a calibrated pressure transducer. The aerosol flow should not deviate systematically more than 5% on daily average. Furthermore, it has to be checked manually at each service occasion.

- The sheath air flow rate has to be dried as well by a membrane (e.g. nafion) or a silica gel diffusion dryer (<40% RH). The RH in the sheath air flow rate determines the equilibrium particle size during sizing. The RH of the sheath air has to be monitored by a calibrated humidity sensor. The RH probe should be installed directly at the excess air outlet or at the sheath air entrance of the DMA, because it is important to measure at equal temperature and pressure as in the DMA.

- The sheath air flow rate should be monitored e.g. by a calibrated pressure transducer or mass flow meter. Pressure transducers should be installed directly at the excess air outlet of the DMA in order to assure that the volumetric flow rate is equal as in the sizing column. Mass flow meters can be installed anywhere in the sheath flow loop, as long as the reading is converted into a volumetric flow rate at the temperature and pressure in the DMA. The mean sheath air flow has to be constant with a maximum deviation of 2%. This criterion can be either met by a critical orifice/pump set-up or by a PID controlled blower. In case of a closed loop technique for the sheath air flow, a heat exchanger has to be used. The closed loop system contains two HEPA filters or filters with similar performance (one before entering and one after leaving the DMA). In case of a critical orifice/pump set-up, the absolute pressure downstream of the critical orifice should be monitored to ensure critical flow conditions (pressure downstream lower than ½ of the upstream pressure).

- DMA temperature and pressure in the system have to be monitored. The pressure measurement should be made in the sample flow, which is at equal pressure as the sizing column.

- Raw data of the mobility distribution (particle concentrations) have to be stored as function of nominal particle size (equivalent Stokes diameter for singly charged particles). The reason for this requirement is that also reference inversion routines (calculation of the number size distribution from the mobility distribution) should be able to employ the raw data set. The DMA dimensions should be given in the meta-data.

Download User Manual

User manual TROPOS-SMPS(German)

User manual TROPOS-SMPS(English)

References

  • Schladitz, A., Merkel, M., Bastian, S., Birmili, W., Weinhold, K., Löschau, G., and Wiedensohler, A.: A concept of an automated function control for ambient aerosol measurements using mobility particle size spectrometers, Atmos. Meas. Tech. Discuss., 6, 10551-10570, doi:10.5194/amtd-6-10551-2013, 2013. Download from AMTD
  • Wiedensohler, A., Birmili, W., Nowak, A., Sonntag, A., Weinhold, K. et al.: Mobility particle size spectrometers: harmonization of technical standards and data structure to facilitate high quality long-term observations of atmospheric particle number size distributions, Atmos. Meas. Tech., 5, 657-685, 2012. Download from AMT

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