Real-Time Size-Distributed Measurement of Aerosol Mass Concentration
In atmospheric aerosol research, size-distributed aerosol samples are often collected with airborne instrumentation, but are not analyzed in any manner until after they are brought to a laboratory on the ground. The information thus obtained is averaged for all the locations covered during the mission. A novel technology is being developed in this project to constantly measure the size-distributed mass and hygroscopic behavior of aerosols during the flight while samples are collected. This project addresses the Department of Energys interest in measurement of airborne aerosol size distributions using instruments onboard aircrafts, and addresses a key need in understanding the role of aerosols in cloud formation. Cascade impactors are a reliable and traditional method for collecting size-distributed aerosol samples. Each stage of a cascade impactor collects aerosols of a certain narrow size range. MSPs MOUDI impactors are industry standard for research-grade impactor measurements. To address the present needs, this project has focused on incorporating a microbalance with rapid electronic measurement of the mass of collected particles on each stage of the cascade impactor. Further, we have sought microbalances that would be sensitive to moisture gain and loss by the collected sample, and measure aerosol hygroscopicity. In Phase I of this project we evaluated the feasibility of two different approaches to meet the above need. A MEMS device was tested as a microbalance, because of the anticipated high sensitivity of the device. A highly sensitive device would have allowed us to reduce the overall instrument size. However, the MEMS devices had typically an unstable baseline reading in the presence of an air flow (e.g., while sampling) and proved to be difficult to use. We also tested impedance-based quartz crystal microbalances (IQCM) on several stages of a cascade impactor. These IQCM sensors provided sub-nanogram resolution in measuring deposited aerosols. We found that the IQCM sensors provide a cleanable smooth surface for particle deposition and one that can be exposed to high and low-humidity air without affecting the sensor performance. This attribute is ideal for measuring moisture uptake and relative humidity values at which deliquescence and efflorescence happens. Further, we developed microelectronics that eliminates noise and enables digitizing signals as early as possible and processing them at the sensor site to determine the resonance frequency within a fraction of a second in a digital form. In Phase II, we plan to build two cascade impactors with real-time mass measuring ability on five stages covering the 10-nm to 600-nm size range. Incoming air will be heated to dry the aerosols and maintain low humidity condition. In a basic mode of operation, one of the impactors will measure the dry aerosol mass, whereas the flow to the second impactor will be humidified and dehumidified to detect the point at which deliquescence starts during the rising phase of the humidity, which would last long enough to measure the moisture uptake on each stage. The relative humidity for efflorescence will be determined during the falling phase of the humidity. These cascade impactors with the ability to measure real-time mass and hygroscopicity will be made flight-worthy and be available with a complete software package enabling a skilled user to operate the instrument and acquire the data. Commercial Applications and Other Benefits: Following a successful Phase II project, MSP will develop a similar product for this research market, such as MSPs current MOUDI customers. Further, a simpler and lower cost device will be developed for PM2.5 monitoring, aimed at the air quality compliance marketplace. In addition to providing the mass of fine particles (smaller than 2.5 micron), this device will have the unique ability to determine the hygroscopicity of the collected sample, enabling the user to estimate a particle-related visibility index, which is expected to become a part of future clean air standards, because it is now well-established that particulate matter is often responsible for atmospheric haze and poor visibility conditions.
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