Laboratory Characterization of Modified Tapered Element Oscillating Microbalance Samplers

Abstract

Laboratory tests with generated aerosols were conducted to test the efficacy of two recent design modifications to the well-established tapered element oscillating microbalance (TEOM) continuous particulate matter (PM) mass monitor. The two systems tested were the sample equilibration system-equipped TEOM monitor operating at 30°C, which uses a Nafion dryer as part of the sample inlet, and the differential TEOM monitor, which adds a switched electrostatic precipitator and uses a self-referencing algorithm to determine "true PM mass." Test aerosols included ammonium sulfate, ammonium nitrate, sodium chloride, copper (II) sulfate, and mixed aerosols. Aerosols were generated with an atomizer or a vibrating orifice generator and were equilibrated in a 450-L slow flow chamber before being sampled. Relative humidity in the chamber was varied between 10 and 90%, and step changes in humidity were executed while generating aerosol to test the response of the instruments. The sample equilibration system-equipped TEOM monitor does reduce, but not totally eliminate, the sensitivity of the TEOM mass monitor to changes in humidity. The differential TEOM monitor gives every indication of being a very robust technique for the continuous real-time measurement of ambient aerosol mass, even in the presence of semi-volatile particles and condensable gases. INTRODUCTIONThe tapered element oscillating microbalance (TEOM) mass monitor uses a hollow oscillating microbalance to detect and report changes in mass caused by the passage of ambient air through a replaceable filter. 1 The monitor provides a continuous measure of collected mass. The purpose of this work is to understand how to better measure the atmospheric particulate matter (PM) to which people are exposed in the ambient environment. Continuous measurements of PM are desirable for a number of reasons, including (1) the capture of short, but intense, pollution events; (2) hourly mass concentration data are required for use in PM with an aerodynamic diameter less than or equal to 2.5 m (PM 2.5 ) mapping projects; and (3) more highly time resolved data are required for model evaluation. While data with higher time resolution answer some of the objections to the 24-hr integrated filters collected in the U.S. Environmental Protection Agency (EPA) Federal Reference Method (FRM) network, the TEOM monitor still uses a filter for particle collection and is subject to the same artifact and interference problems of other filter-based PM collection methods. These limitations as they relate to the FRM for PM 2.5 2 are discussed and evaluated by Eatough et al. 3 The research described here consists of a thorough laboratory evaluation of modifications to the TEOM monitor. The modifications were made to address the limitations of the basic method and primarily involve treatment of the sample before it reaches the oscillating microbalance. The sample equilibration system (SES)-equipped TEOM monitor 4 was developed to mitigate interferences from water vapor, so extensive laboratory te...

Evaluation of Fine Particle Number Concentrations in CMAQ

“…Average ratios of measured to simulated number concentrations were 1.15 (mass), 1.43 (volume), 3.95 (surface area), and 29.34 (number) ( Table 3). Note that PM 2.5 mass measured by TEOM contains small amounts of water, although studies suggest that the amount of water would be negligible as a Nafian drier is used (Eatough et al 2003;Gong and Demerjian 1995;Meyer et al 2000;Schwab et al 2004). Considering the possibility of water in the particles, the true ratio of observed mass to modeled mass would be slightly less than 1.15.…”

Introducing the concept of Potential Aerosol Mass (PAM)

“…The concern is that semi-volatile organics may be lost from the TEOM (Schwab et al, 2004;Long et al, 2005;Schwab et al, 2006;Wilson et al, 2006), even when operated at 30 • C. If significant semi-volatile mass were being lost on the TEOM in the PAM measurements, then a large negative mass signal calculated by Eq. (2) would be observed after the UV lights were turned off.…”

New York State Urban and Rural Measurements of Continuous PM2.5 Mass by FDMS, TEOM, and BAM

“…The instrument is described by Grover et al 15 The basic differential measurement approach is presented by Patashnick et al 16 Descriptions and applications of the differential TEOM system using an electrostatic precipitator (ESP) to remove particles from the sample flow stream are presented in a number of recent papers. 8,[17][18][19] The FDMS TEOM is similar in principle to the ESP differential TEOM, with the major hardware changes being the addition of a switching valve and the replacement of the ESP module with a chilled Pallflex filter for the creation of the particle-free sample flow stream. The FDMS has been designated as an equivalent measurement method for PM 2.5 by the state of California.…”