Design and Validation of a 6-Volatility Tandem Differential Mobility Analyzer (VTDMA)

Abstract

A volatility tandem differential mobility analyzer (VTDMA) was developed to allow fast field measurement of the volatile fraction of atmospheric aerosol particles in the particle size range 20–500 nm. In this VTDMA the volatile compounds are evaporated by heating the aerosol to a temperature between 25°C and 300°C. The heating unit is equipped with six symmetric columns kept at different temperatures that allow the heating temperature to be rapidly changed so that a higher temporal resolution can be achieved compared to a regular VTDMA. This work first focuses on the design and calibration of the heating units for the conditioning of a selected aerosol sample while minimizing sample losses due to thermophoresis and diffusion. The design was based on the modeling of the profiles of temperature and velocity and the behavior of a monodisperse aerosol in the heating units, using Computational Fluid Dynamics (CFD) Flow Modeling Software. This allowed for initial estimations of the heater dimensions and also calculation of the minimum length of heating tube needed to completely evaporate the aerosol particles at high temperature with sufficient residence time, as well as to cool the aerosol sample down to ambient temperature. Next, the aerosol heating rate and aerosol deposition losses within the flow tube were estimated, and re-condensation of volatilized compounds evaluated. Then the VTDMA was calibrated and tested in the laboratory to determine the transfer efficiency, and finally, atmospheric aerosols were analyzed, with the first results presented here. This work emphasizes the need for better standardization of thermo-desorbing units for atmospheric aerosol studies.

*Now at: Laboratoire de Chimie et Environnement, Université de Provence.

Acknowledgments

The authors would like to acknowledge the financial support of the CNRS National Program for Atmospheric Chemistry (PNCA), ADEME and Ministère de l'Ecologie et du dévelopement durable under the PRIMEQUAL program, the scientific council of region Auvergne. P. Villani acknowledges financial support from CNRS and region Auvergne under the BDI program. We acknowledge the support of the European Community—Research Infrastructure Action under the FP6 “Structuring the European Research Area” Programme EUSAAR contract (No. RII3-CT-2006-026140).

Notes

*Now at: Laboratoire de Chimie et Environnement, Université de Provence.