Development of a Thermal Desorption Mass Spectrometric Method for Measuring Vapor Pressures of Low-Volatility Organic Aerosol CompoundsEPA Grant Number: R828173
Title: Development of a Thermal Desorption Mass Spectrometric Method for Measuring Vapor Pressures of Low-Volatility Organic Aerosol Compounds
Investigators: Ziemann, Paul J.
Institution: University of California - Riverside
EPA Project Officer: Shapiro, Paul
Project Period: August 1, 2000 through July 31, 2002 (Extended to January 31, 2004)
Project Amount: $84,111
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Engineering and Environmental Chemistry
Atmospheric fine particles (diameter < 2.5 :m) are currently a major environmental concern because of their effects on human health, visibility, and global climate. A key process in organic aerosol formation is the combination of adsorption, absorption, and desorption reactions which control the partitioning between the gas and particle phases. Because compounds with lower vapor pressures tend to partition more strongly into the aerosol phase, knowledge of compound vapor pressures is critical for understanding and modeling atmospheric organic aerosol formation. Although various techniques have been employed for vapor pressure measurements, they are generally not well-suited to low-volatility organic compounds that form primary and secondary aerosol. In this project we plan to develop and employ a new temperature-programmed thermal desorption (TPTD) method for measuring vapor pressures and heats of vaporization of low-volatility organic compounds of atmospheric interest. The technique will employ a thermal desorption particle beam mass spectrometer (TDPBMS) we have developed for organic aerosol composition analysis. The objectives of the project are to develop the instrumentation, technique, and data analysis procedures for obtaining accurate, reproducible measurements of organic compound vapor pressures and heats of vaporization using TPTD, and then apply the method to: (1) a variety of single organic compounds selected from classes found in primary and secondary aerosols; (2) multi-component aerosols composed of these same compounds to obtain activity coefficients; and (3) organic compounds in multi-component aerosols created in smog chamber studies of secondary organic aerosol formation.
For TPTD analysis particles are sampled into a high-vacuum chamber as an aerodynamically focused beam, collected by impaction on a cryogenically-cooled surface, and then slowly desorbed and analyzed in a quadrupole mass spectrometer. This TPTD technique separates compounds according to volatility, so that mass spectra of single compounds can be obtained from mixtures, and vapor pressures and heats of vaporization can be determined from temperature-dependent desorption behavior. The method will be applied to monodisperse, single- and multi-component particles generated using atomization techniques and smog chamber reactions followed by particle size selection using a differential mobility analyzer.
Vapor pressures, heats of vaporization, and activity coefficients will be obtained for low-volatility aliphatic and aromatic aldehydes, alcohols, and mono- and di- carboxylic acids, and hydroperoxides, peroxides, and secondary ozonides which are found in primary and secondary aerosol. The results will provide a new database for developing methods for calculating these quantities based on molecular properties, and for modeling gas-particle partitioning of atmospheric organic compounds.