AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Effect of Aerosol Volatility on the Sizing Accuracy of Differential Mobility Analyzers
ANDREY KHLYSTOV, Research Triangle Institute
Abstract Number: 335 Working Group: Instrumentation and Methods
Abstract Differential Mobility Analyzers (DMAs), also called electrostatic classifiers, are widely used to measure aerosol size distribution, aerosol hygroscopic growth and kinetics of particle evaporation and condensation, and to produce monodisperse aerosol for calibrating other instruments such as Aerosol Mass Spectrometers. The DMAs classify aerosol particles according to their mobility in an electric field. A well-defined relationship exists between the size of transmitted particles, their charge and the electrical field in the DMA. This relationship assumes that particles do not change their size during passage through the classifier. However, particles containing volatile or hygroscopic compounds could evaporate or grow inside the DMA, if there are differences in temperature, relative humidity and / or gaseous concentrations between the sampled air and the conditions in the DMA. The differences in the gaseous concentrations could also be induced by losses of the volatile species, such as nitric acid, to the walls of the DMA or in the tubing used to supply the sheath air to the DMA. Such particle changes inside the DMA could lead to sizing errors.
I will present a simple theoretical treatment of the processes occurring in the DMA when measuring semi-volatile particles. The predictions of the theory will be compared to measurements of laboratory-generated ammonium nitrate aerosol using TSI long DMAs. Ammonium nitrate is a major semi-volatile component of ambient aerosol, especially in industrialized areas. It is also the main component that is used for calibration of Aerosol Mass Spectrometers and Aerosol Chemical Speciation Monitors (ARI). One of its gaseous dissociation products - nitric acid - is a very reactive component and can be readily removed from the gas upon contact with the walls of the sampling tubing and the instrument itself. The magnitude of the sizing errors when measuring ammonium nitrate and other semi-volatile compounds will be discussed.