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In recent years, scientific discussion has included the influence of thermodynamic conditions (e.g., temperature, relative humidity, and filter face velocity) on PM retention efficiency of filter-based samplers and monitors. Method-associated thermodynamic conditions can, in some instances, dramatically influence the presence of particle-bound water and other light-molecular-weight chemical components such as particulate nitrates and certain organic compounds. The measurement of fine particle mass presents a new challenge for all PM measurement methods, since a relatively greater fraction of the mass is semi-volatile.The tapered element oscillating microbalance (TEOM) continuous PM monitor is a U.S. Environmental Protection Agency (EPA) PM 10 equivalent method (EQPM-1090-079). Several hundred of these monitors are deployed throughout the United States. The TEOM monitor has the unique characteristic of providing direct PM mass measurement without the calibration uncertainty inherent in mass surrogate methods. In addition, it provides highprecision, near-real-time continuous data automatically. Much attention has been given to semi-volatile species retention of the TEOM method. IMPLICATIONSThousands of TEOM continuous PM monitors have been deployed throughout the world to provide regulatory and special study measurements of PM in ambient air. Some regions of the world, owing to their topography and pollution sources, are characterized by fine particle pollution that can contain an elevated semi-volatile mass fraction. In these areas, it is desirable to increase collection efficiency for this material by maintaining as low an operating temperature as practical while also removing unwanted particle-bound water. The new SES has been developed to allow conditioning of the PM sample stream to a lower humidity and temperature level than the conventional TEOM settings. It is a significant development because it can be applied easily to existing TEOM monitors.While using this monitor, it is desirable to maintain as low an operating temperature as practical and to remove unwanted particle-bound water. A new sample equilibration system (SES) has been developed to allow conditioning of the PM sample stream to a lower humidity and temperature level. The SES incorporates a special low-particle-loss Nafion dryer. This paper discusses the configuration and theory of the SES. Performance results include high time-resolved PM 2.5 data comparison between a 30 °C sample stream TEOM monitor with SES and a standard 50 °C TEOM monitor. In addition, 24-hr integrated data are compared with data collected using an EPA PM 2.5 Federal Reference Method (FRM)-type sampler. The SES is a significant development because it can be applied easily to existing TEOM monitors. INTRODUCTIONThe tapered element oscillating microbalance (TEOM) continuous PM monitor has been in use worldwide for the automatic, near-real-time measurement of PM in ambient air since 1988. It has been designated as a U.S. Environmental Protection Agency (EPA) PM 10 fed...

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