Grantee Research Project Results
2002 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, 2001 through July 31, 2002
Project Amount: $84,111
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Water , Air , Safer Chemicals , Land and Waste Management
Objective:
The objectives of this research 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 a new temperature-programmed thermal desorption (TPTD), and then applying 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. Atmospheric fine particles (diameter < 2.5 µm) currently are a major environmental concern because of their effects on human health, visibility, and global climate. A key feature of organic aerosol formation is the adsorption, absorption, and desorption reactions that control partitioning between the gas and particle phases. Because less volatile compounds tend to partition more strongly into the aerosol phase, knowledge of compound vapor pressures is critical for understanding and modeling 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 are developing and employing the TPTD method for measuring vapor pressures and heats of vaporization of low-volatility organic compounds of atmospheric interest. The technique employs a thermal desorption particle beam mass spectrometer (TDPBMS) we have developed for organic aerosol composition analysis.
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 desorbed and analyzed in a quadrupole mass spectrometer. The TPTD technique separates compounds according to volatility, so 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 is being 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.
During the first year of this project, we developed a Labview-based software program for obtaining reproducible, linear temperature ramps for TPTD analysis. We also developed an evaporation rate theory for calculating particle vapor pressures from particle desorption curves. This theory was applied to the results of experiments performed using the new temperature ramp to measure vapor pressures and heats of vaporization for a series of mono- and di-carboxylic acids. Measurements were made for a variety of temperature ramp rates, particle sizes, and deposit sizes. The results were in good agreement with the literature data available for these compounds, demonstrating the validity of the technique. A manuscript on the work was published in Analytical Chemistry.
During the second year of this project, we improved the design of the particle vaporizer. As a result of calculations and measurements, we became concerned that the original vaporizer design was leading to significant temperature gradients across the particle deposit. This would yield inaccurate temperature measurements, which is a critical quantity in the method. Computational and experimental studies were performed to redesign and optimize the vaporizer to reduce temperature gradients. This successfully was completed and a new vaporizer was constructed, which yields temperatures that are uniform and accurate to within less than one degree Celsius. A micrometer-driven particle beam stop also was implemented to measure particle deposit size for use in method evaluation. As a result of these measurements, we concluded that the theory being used to calculate evaporation rates needed to be modified, since one of the assumptions was incorrect. It had been assumed that particles deposited on the vaporizer surface would only evaporate from one face of a particle, the one facing away from the vaporizer surface. Molecules on the bottom of the particle could not evaporate, and molecules on the sides would impact adjacent particles and stick. In contrast to this, the beam deposit measurements indicated that the particle deposits were sufficiently disperse, and that evaporating molecules would impact few other particles and instead impact the vaporizer surface or escape. A new computational approach was developed to simulate evaporation profiles, and the new vaporizer, theory, and data analysis methods have been used to redetermine the vapor pressures and heats of vaporization of the same mono- and di-carboxylic acids. The results are in good agreement with the literature data available for these compounds.
Future Activities:
Because of the need to redesign and evaluate the vaporizer and develop new data analysis procedures, we were not able to complete the work originally planned for the second year. Our studies for the next year will therefore focus on: (1) evaluating the utility of the method for liquid particles and those which melt before evaporation, (2) investigating the use of the technique for measurements of activity coefficients by analyzing mixtures of compounds with different polarities, and (3) measuring vapor pressures and heats of vaporization for low-volatility alkanes, aldehydes, alcohols, aromatics, hydroperoxides, peroxides, and secondary ozonides, which are found in primary and secondary aerosol. As the database becomes sufficiently large, we will investigate the use of 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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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, Air, Scientific Discipline, Toxics, particulate matter, Environmental Chemistry, Engineering, Chemistry, & Physics, tropospheric ozone, VOCs, climate change, secondary ozonides, aerosol particles, heterogeneous catalysts, thermal extraction, air quality, aerosols, aldehydes, mass spectrometry, particle size, ambient aerosol, Global Climate Change, adsorbents, atmospheric models, human exposure, cryogenics, aerosol formation, fine particles, climate variations, measurement methods , air quality modeling, environmental monitoring, air pollution models, carboxylic acids, gas/particle partitioning, global scale, particulates, vapor phase, high vacuum chamber, PM, hydroperoxides, ambient air, peroxides, spectroscopic studies, air modeling, PM2.5, PM 2.5Progress 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.