Characterization of a thermodenuder-particle beam mass spectrometer system for the study of organic aerosol volatility and composition

Abstract

This paper describes the development and evaluation of a method for measuring the vapor pressure distribution and volatility-dependent mass spectrum of organic aerosol particles using a thermodenuder-particle beam mass spectrometer. The method is well suited for use with the widely used Aerodyne Aerosol Mass Spectrometer (AMS) and other quantitative aerosol mass spectrometers. The data that can be obtained are valuable for modeling organic gas-particle partitioning and for gaining improved composition information from aerosol mass spectra. The method is based on an empirically determined relationship between the thermodenuder temperature at which 50% of the organic aerosol mass evaporates (<i>T</i>₅₀) and the organic component vapor pressure at 25&deg;C (<i>P</i>₂₅). This approach avoids the need for complex modeling of aerosol evaporation, which normally requires detailed information on aerosol composition and physical properties. <i>T</i>₅₀ was measured for a variety of monodisperse, single-component organic aerosols with known <i>P</i>₂₅ values and the results used to create a log<i>P</i>₂₅ vs. <i>T</i>₅₀ calibration curve. Experiments and simulations were used to estimate the uncertainties in <i>P</i>₂₅ introduced by variations in particle size and mass concentration as well as mixing with other components. A vapor pressure distribution and volatility-dependent mass spectrum were then measured for laboratory-generated secondary organic aerosol particles. Vaporization profiles from this method can easily be converted to a volatility basis set representation, which shows the distribution of mass vs. saturation concentration and the gas-particle partitioning of aerosol material. The experiments and simulations indicate that this method can be used to estimate organic aerosol component vapor pressures to within approximately an order of magnitude and that useful mass-spectral separation based on volatility can be achieved.