Grantee Research Project Results
2003 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, 2002 through July 31, 2003
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:
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 temperature-programmed thermal desorption (TPTD), and then apply the method to: (1) a variety of single organic compounds selected from classes found in primary and secondary aerosols; (2) multicomponent aerosols composed of these same compounds to obtain activity coefficients; and (3) organic compounds in multicomponent 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 more strongly partition 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 a new 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 multicomponent particles generated using atomization techniques and smog chamber reactions followed by particle size selection using a differential mobility analyzer.
The first 2 years of this project focused on the development of the hardware and software, and an evaporation model to be used for measuring compound vapor pressures. A Labview-based software program for obtaining reproducible, linear temperature ramps for TPTD analysis was developed and used with a metal foil vaporizer to measure particle desorption profiles. A simple evaporation theory was applied to the results of these experiments to obtain vapor pressures and heats of vaporization for a series of mono- and di-carboxylic acids. The results were in good agreement with literature data available for these compounds, demonstrating the validity of the technique. A manuscript on the work was published in Analytical Chemistry. We subsequently improved the design of the particle vaporizer to reduce temperature gradients across the particle deposit to obtain the accurate temperature measurements (+/- 1°C) that are critical in the method. A micrometer-driven particle beam stop also was implemented to measure particle deposit size for use in method evaluation. As a result of further measurements, we also modified the evaporation theory being used to calculate evaporation rates. A new computational approach was developed to simulate evaporation profiles, and the new vaporizer, theory, and data analysis methods were 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 literature data available for these compounds.
In Year 3 of the project, we completed an evaluation of the method and theory by comparing desorption profiles for a number of compound classes and particle sizes with those obtained from computational simulations. The results have yielded significant new insights into the method and have led to what we believe is a relatively complete understanding of the data and the proper means for interpreting desorption profiles. This requires consideration of not only particle properties, but also the interaction of desorbing molecules with the vaporizer surface. These results now are being incorporated into a manuscript. We also have completed a series of evaporation measurements on internal mixtures of mono- and di-carboxylic acids. The results have been quite surprising and indicate that, for mixtures of solid compounds such as these, the volatile components can readily become trapped inside the particle matrix and not evaporate until the least volatile components evaporate. This suggests a mechanism by which volatile compounds (such as those incorporated during the cooling of a combustion plume or dissolved in an aqueous particle) might remain in particles under thermodynamically unfavorable conditions, and calls into question the extent to which standard gas-particle partitioning theory might work under such circumstances. A manuscript describing this work is in preparation and also will be presented at the upcoming annual meeting of the American Association for Aerosol Research.
Future Activities:
Future activities will focus on: (1) evaluating the utility of the method for liquid particles and those which melt before evaporation; (2) measuring vapor pressures and heats of vaporization for selected low-volatility multifunctional compounds containing hydroxyl, carbonyl, and acid groups; and (3) analyzing desorption profiles of secondary organic aerosol obtained in smog chamber experiments to obtain component vapor pressures.
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.