GV Scanning Mobility Particle Sizer

Instrument Description and Background:

Figure 1. The Electrostatic Classifier (tubing and filter cover removed for clarity).

The GV Scanning Mobility Particle Sizer (SMPS) measures the particle size distribution over the mobility diameter range of 3 to 500 nm (pressure-dependent, see discussion below). It consists of two components: an electrostatic classifier (EC) and a condensation particle counter (CPC). The EC samples aerosol-laden ambient air, dries the air to below 40% RH using a diffusion dryer, places a well-defined charge distribution on the particles, and then selects a narrow range of particle “mobility diameter” (approx. equal to cross-sectional area-to-charge ratio) using a differential mobility analyzer (DMA)[1]. The selected diameter is determined by a time-varying high voltage applied to the DMA; following this, particles are counted by the CPC. The total scan time and the number of counting intervals, the latter of which determines the number of diameter bins in the size distribution, are selected based on ambient particle concentrations and altitude. The raw data (particle counts over each counting interval as a function elapsed time during the linear diameter scan) is mathematically inverted during post-processing to obtain the particle size distribution.

Hardware:

The SMPS system consists of the following equipment:

  • Electrostatic Classifier (EC) mounted on a standard HIAPER shelf, using 115 VAC 60 Hz @ ~1 A. The EC using a generally licensed, sealed radioactive source (NRD, LLC model P-2021; material: Po210; activity: 370 MBq). The EC is pictured in Figure 1.
  • Condensation Particle Counter (CPC), a commercial instrument manufactured by TSI, Inc. At the present time we plan to use a model 3010 CPC for DC3 and SEAC4RS, which is the same instrument used during the DC3 test flights in May 2011. This CPC will be mounted next to the CN counter (PI = Dave Rogers/EOL) on a separate HIAPER shelf located in the same rack as the EC. The instrument uses n-butanol as the working fluid, which is completely enclosed within the instrument.
  • Vacuum pump, which is required for both the EC and the CPC. The current plan calls for locating this on a plate mounted on the floor next to the rack, with tubes connecting it to the EC and CPC. It will operate on an EOL-provided 28 VDC, which will be provided by a 120 V AC (400 Hz) – 28 VDC transformer located in the rack.

Configuration on GV:

The particle size range is dependent on altitude (inlet pressure), DMA voltage range, and the detection limits of the CPC. This relationship is plotted in Figure 2. For the model 3010 CPC the detection limit is 10 nm in diameter, which will be the minimum diameter in the distribution. The maximum diameter corresponds to a DMA voltage of 3000VDC, which is prevents corona discharge at the lowest expected pressures. Thus the diameter range for this instrument for DC3 and SEAC4RS will range from 10 – 70 nm at sea level to 10 – 155 nm at the highest expected altitudes. These ranges will overlap well with the GV UHSAS instrument (PI = Dave Rogers/EOL), which measures particle size distributions with a diameter range of 60 nm to 1 mm. The WCN and SMPS will share the same HIML inlet and sampling line, which will be valuable to compare total particle counts to the particle size distribution. The instrument can operate with only minor attention by in-flight personnel (e.g., turning on power). Key parameters can be viewed and changed remotely by operators on the ground if necessary.

Figure 2. GV SMPS particle size/voltage relationship.

Data:

The SMPS raw data are recorded on an on-board (PC/104) computer and critical data are also fed into the aircraft data system (ADS) using UDP so that they can be viewed from the ground using Aeros software. Since these critical data are fed into the ADS, a data back-up is automatically generated for each measurement. Flight parameters including inlet temperature, inlet pressure, relative humidity and altitude are read from the ADS (also using UDP) and are also saved. The combination of these stored parameters (including raw CPC particle counts, DMA voltage, volumetric sheath flow rate, temperature, and pressure) are used to calculate the final particle size distribution for each up/down scan combination. The SMPS control and data acquisition computer’s clock is synchronized with the ADS so that particle data accurately reflects the exact conditions of the rest of the measurements. During DC3 and SEAC4RS, a one-way diameter scan will be performed over 15 size bins every 60 seconds. Instrument conditions can be changed during flight to capture certain particle ranges or achieve faster scan times.

References:

  1. Flagan, R.C., Electrical mobility methods for submicrometer particle characterization, in Aerosol measurement: Principles, techniques, and applications, P. Kulkarni, P.A. Baron, and K. Willeke, Editors. 2011, John Wiley & Sons: Hoboken.

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