Grantee Research Project Results
2001 Progress Report: Development of a Thermal Desorption Mass Spectrometric Method for Measuring Vapor Pressures of Low-Volatility Organic Aerosol Compounds
EPA Grant Number: R828173Title: 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: Hahn, Intaek
Project Period: August 1, 2000 through July 31, 2002 (Extended to January 31, 2004)
Project Period Covered by this Report: August 1, 2000 through July 31, 2001
Project Amount: $84,111
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Safer Chemicals
Objective:
Atmospheric fine particles (diameter < 2.5 µm) are a major environmental concern due to their effects on human health, visibility, and global climate. A key process in organic aerosol formation is the combination of absorption, adsorption, and desorption reactions, which control the partitioning between the gas and particle phases. Because compounds with lower vapor pressures tend to more strongly partition into the aerosol phase, knowledge of compound vapor pressures is critical to understand and model atmospheric organic aerosol formation. Although various techniques have been employed for vapor pressure measurements, they generally are 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 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.Progress Summary:
For vapor pressure measurements by TPTD, 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.
In the first year of this project, we developed a Labview-based software program to obtain highly reproducible, linear temperature ramps, for use in TPTD analysis. We also modified our original evaporation rate theory used to calculate particle vapor pressures from particle desorption curves. Previously, our theory assumed that the particle evaporated at the same rate as a single particle suspended in a vacuum, but later, we realized that particles deposited on the vaporizer surface will only evaporate from one face, the one facing away from the vaporizer surface. Molecules on the bottom of the particle cannot evaporate, and molecules on the sides will impact adjacent particles and stick. This consideration reduced the previously calculated evaporation rates by a factor of six. This theory was applied to the results of a series of careful experiments performed using the new temperature ramp to measure vapor pressures and heats of vaporization for C13-C18 monocarboxylic acids and C6-C8 dicarboxylic acids. Measurements were made for a variety of temperature ramp rates, particle sizes, and deposit sizes. The results are in excellent agreement with literature data available for these compounds, demonstrating the validity of the technique. A manuscript on the work recently was published in Analytical Chemistry. Since completion of the work, we have begun to perform experiments on liquid organic compounds and solid particles that melt prior to evaporation. The experiments are intended to determine the extent to which the method can be applied to compounds other than those that evaporate prior to melting. We also found a source for a software program that can be used to develop structure-activity relationships, which relate compound structure to quantities, such as vapor pressure and heat of vaporization.
Future Activities:
Our next studies will focus on completing our evaluation of the utility of the method for liquid particles and those which melt before evaporation. We also will investigate the use of the technique for measurements of activity coefficients by analyzing various mixtures of polar and nonpolar compounds. For these studies, we will need to develop a data analysis procedure for extracting desorption profiles from overlapping curves. When this work is complete, we will measure vapor pressures and heats of vaporization for a series of low-volatility alkanes, aliphatic aldehydes, alcohols, and mono- and di-carboxylic acids. Later, we will perform similar measurements on aromatic compounds, hydroperoxides, peroxides, and secondary ozonides that are found in primary and secondary aerosol. As the database becomes sufficiently large, we will investigate the use of the structure-activity software for developing quantitative relationships for calculating vapor pressures and heats of vaporization from compound structure.Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 6 publications | 1 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Chattopadhyay S, Tobias HJ, Ziemann PJ. A method for measuring vapor pressures of low-volatility organic aerosol compounds using a thermal desorption particle beam mass spectrometer. Analytical Chemistry 2001;73(16):3797-3803. |
R828173 (2001) R828173 (2002) R828173 (2003) R828173 (Final) R826235 (2000) |
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Supplemental Keywords:
chemicals, particulates, environmental chemistry, measurement methods, tropospheric., RFA, Scientific Discipline, Air, Toxics, particulate matter, Environmental Chemistry, climate change, VOCs, tropospheric ozone, Engineering, Chemistry, & Physics, air quality modeling, gas/particle partitioning, particle size, particulates, thermal extraction, aerosol formation, ambient aerosol, environmental monitoring, aldehydes, adsorbents, hydroperoxides, peroxides, aerosol particles, global scale, mass spectrometry, cryogenics, fine particles, PM 2.5, air modeling, ambient air, climate variations, spectroscopic studies, vapor phase, high vacuum chamber, air pollution models, human exposure, carboxylic acids, PM2.5, PM, measurement methods , air quality, atmospheric models, secondary ozonides, heterogeneous catalysts, aerosols, ambient aerosol particlesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.