Real-Time Size-Distributed Measurement of Aerosol Mass Concentration

 

Author

Amir Naqwi — MSP Corporation

Category

General Topics – Aerosol

Description

Figure 1: Schematic of the instrument and the preliminary results
A new instrument for airborne real-time measurement of aerosol mass distribution is designed and built. The core component of the instrument is a research-grade cascade impactor (MOUDI™ from MSP), which is modified to include a quartz-crystal microbalance (QCM) on each stage, in order to measure the collected aerosol mass in real time. The resulting device is referred to as QCM-MOUDI. A schematic of the instrument is shown in Figure 1. Air is sampled at a rate of 10.5 liters/min. A virtual impactor is used to separate and reject particles larger than 2.5 micron in aerodynamic diameter. Smaller particles are carried with the 10 liters/min major flow. A flow heater is included to warm up the cold inlet air from high altitude (with temperature as low as –40 °C) to the constant temperature of 25 °C, which is optimal for QCMs. Also, a moderate value of relative humidity (RH), i.e. 50-60%, is desired for reliable measurements with QCMs. The RH Conditioner, as shown in the schematic, consists of nafion tubes that enable addition or removal of moisture from the aerosol stream, so the RH of the outgoing flow is about 55%, regardless of the RH at the sampling inlet. The QCM-MOUDI consists of 8 impaction stages covering stage cut-points from 10 nm to 560 nm in aerodynamic diameter. The temperature and RH at the QCM-MOUDI inlet as well as the absolute pressure at each MOUDI stage are monitored during the operation. A 24V DC dry scroll pump is used to drive all the airflow in this instrument. Performances of the virtual impactor, the heater, and the RH Conditioner have been verified separately. Total particle loss through these components was less than 5%. The performance of the QCM-MOUDI was examined with polydisperse ammonium sulfate particles as well as particles from room air in the laboratory. Mass collected on each stage of the QCM-MOUDI was compared with the mass estimated using a differential mobility analyzer (DMA), whose voltage was scanned to cover the size range of 10-600 nm. Particle number concentrations from DMA were measured using a condensation particle counter and converted to the corresponding mass concentrations. Since the actual densities of room air particles were unknown, the normalized relative contribution of each MOUDI stage was used for comparison. As shown in Figure 1, QCM-MOUDI and DMA based mass measurements are generally in agreement. QCM-MOUDI is shown to be a promising instrument for airborne atmospheric aerosol measurements.