Grantee Research Project Results
Final 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 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 an important environmental concern because of their potential impact on human health and their role in global chemistry and climate. A significant fraction of the mass of these particles is organic matter, which is generated by combustion and by gas-to-particle conversion. Key processes in secondary aerosol formation are the adsorption, absorption, and desorption reactions that control the partitioning of organic compounds between the gas and particle phases. The driving force for the process is proportional to the saturation ratio of the vapor, so compounds with lower vapor pressures tend to partition more strongly into the aerosol phase. The same process controls the gas-particle redistribution of primary organic compounds from combustion sources, including toxic species. Knowledge of compound vapor pressures is therefore critical for understanding and modeling atmospheric organic aerosol formation and the distribution of organic components between the gas and particle phases.
In this research project, we developed a new temperature-programmed thermal desorption (TPTD) method for measuring vapor pressures and heats of sublimation/vaporization of low-volatility organic compounds and applied it to the analysis of various classes of organic compounds commonly found in atmospheric aerosol particles. The technique employed a thermal desorption particle beam mass spectrometer (TDPBMS) that we developed previously for organic aerosol composition analysis. The objectives of the research project were to: (1) develop the instrumentation, experimental techniques, and data analysis procedures for obtaining accurate, reproducible measurements of organic compound vapor pressures and heats of sublimation/vaporization using TDPBMS; (2) measure vapor pressures and heats of sublimation/vaporization of organic compounds in single-component aerosols for a variety of compound classes typically found in primary and secondary aerosols; (3) measure vapor pressures and heats of sublimation/vaporization of organic compounds in multicomponent aerosols for a variety of compound classes typically found in primary and secondary aerosols; and (4) measure vapor pressures and heats of sublimation/vaporization of organic compounds using multicomponent secondary organic aerosols created in an environmental chamber.
Summary/Accomplishments (Outputs/Outcomes):
Essentially all of the objectives have been met. In this research project, we have developed a new method for measuring the vapor pressures and heats of sublimation/vaporization of low-volatility compounds commonly found in atmospheric organic aerosol particles. The method employs a TDPBMS that we developed previously. Particles are sampled into the TDPBMS and focused into a beam using aerodynamic lenses, collected by impaction on a cooled vaporizer surface, and then vaporized using a 2ºC/minute ramp controlled by Labview software written for this purpose. The vapors are ionized by electrons and the ions are analyzed in a quadrupole mass spectrometer. We refer to this analysis as TPTD. The resulting desorption profile is modeled using a theory based on the evaporation of spherical particles into vacuum to obtain the compound vapor pressures, which are fit to the Clausius-Clapeyron equation to determine the heat of vaporization/sublimation.
We have designed and evaluated a new vaporizer consisting of a copper rod with a V-shaped notch coated by a high-temperature, non-stick polymer (Kisscote) film. This design reduces thermal gradients across the vaporizer to less than 1ºC, virtually eliminating measurement errors due to temperature uncertainties. The Kisscote film dramatically reduces adsorption of evaporating molecules onto the vaporizer surface that can lead to long tails on the desorption profiles that reduce the accuracy of measurements. In addition to these hardware improvements, we developed a theory for determining vapor pressures from the evaporation profiles. The theory is based on the kinetic theory of gases and is appropriate for evaporation into a vacuum of size-selected spherical particles distributed in a disperse deposit on a nonstick surface. It has been used to simulate desorption profiles as another means of testing its validity. The final version of the vaporizer and theory are significant improvements over those used in our initial studies.
The effects of particle properties and experimental parameters, including the ion fragmentation pattern, particle size and shape, evaporation coefficient, temperature ramp rate, and particle deposit size, on the measurements were evaluated and used to develop a procedure for obtaining accurate and reproducible results. The vaporizer, method, and theory have been tested by comparing the vapor pressures and heats of sublimation measured for pure C13-C22 monocarboxylic and C4-C12 dicarboxylic acid particles with those determined from tandem differential mobility analyzer (TDMA) and effusion measurements. The agreement is very good, and the data exhibit odd-even variations in vapor pressure and heat of sublimation that have been seen previously. The observation of these trends is an indication of the high precision of the TPTD method. These results not only verified the accuracy of the method but also provided new data on some monocarboxylic and dicarboxylic acids that had not been analyzed previously. These compounds are normally emitted to the atmosphere directly from combustion sources and also are formed in situ thorough photochemical reactions.
Upon establishing the validity of the TPTD method, it was used to measure the vapor pressures and heats of sublimation of a series of substituted monocarboxylic and dicarboxylic acids. Original plans were to analyze simple alkanes, aliphatic alcohols, aldehydes, and ketones, but those that are available commercially were either too volatile to form particles or their profiles exhibited such large tails because of sticking that they could not be analyzed using the TPTD method. High molecular weight alkyl nitrates were not commercially available. The effects of functional groups other than carboxyl and their positions on the molecule were therefore evaluated by measuring the vapor pressures of substituted monocarboxylic and dicarboxylic acids, for which a few compounds could be purchased. The compounds analyzed included hydroxy-monocarboxylic acid and oxo-dicarboxylic acid isomers. Although the addition of a carbonyl or hydroxyl group to the parent carboxylic acid usually reduced the compound vapor pressure by up to two orders of magnitude, in some cases the vapor pressure unexpectedly increased. The results of those projects demonstrated the high sensitivity of vapor pressure to small changes in compound crystal structures.
The analyses described above involved solid compounds whose melting points are well above the temperatures at which they evaporate. We also were interested in determining the extent to which the TPTD method could be used to measure the vapor pressures of liquid particles. This would greatly expand the range of compounds accessible to this technique and also allow for studies of mixtures of various compounds in a liquid organic matrix. The concern in analyzing liquid particles using the TPTD method was that the shape of particles deposited on the vaporizer would be very different from a sphere, so that the theory used to calculate vapor pressure no longer would be valid. The manufacturer’s literature on Kisscote, however, suggested that liquid organic particles deposited on the vaporizer might bead up on the surface and maintain a nearly spherical shape. The compounds chosen for analysis were oleic acid, an unsaturated monocarboxylic acid, and dioctyl phthalate (DOP), an ester. The vapor pressures measured for these compounds were within at least a factor of two of literature data, indicating that the TPTD method could be used for the analysis of organic liquids.
The methods developed for measuring the vapor pressures of compounds present in pure solid and liquid particles also were applied to compounds in mixtures. This made possible an evaluation of the effect of particle matrices on vapor pressures and estimates of activity coefficients (i.e., a = Pin solution/Ppure). Experiments usually were performed with particles composed of a 10:90 binary mixture so that one component could be considered as the solute (minor component) and the other the matrix (major component). Oleic acid and DOP were used as liquid matrices, monocarboxylic and dicarboxylic acids were used as solid matrices, and unsubstituted and substituted dicarboxylic acids were used as solutes. In all cases, the desorption profile of the matrix component (oleic acid or DOP) is monomodal and essentially the same in the mixture and in pure form, indicating that the solute has little effect on the behavior of the matrix. The desorption profiles of the pure solute compounds are either monomodal or have an additional shoulder, whereas the profiles for the solute in the mixture are eithermonomodal or bimodal, depending on the particular compound. The results are indicative of a number of different particle structures: (1) two-phase systems consisting of pure solute and pure matrix; (2) two-phase systems consisting of pure solute and a solution of the solute in the matrix; and (3) one-phase systems consisting of the solute dissolved in the matrix. There also are some slight variations on these general results. The most important observation from the studies of binary mixtures, however, was that when the solute is dissolved in the liquid or solid matrix, it usually evaporates with the matrix regardless of its volatility in the pure state. The intermolecular bonding between pure solute molecules that normally determines the volatility apparently is replaced by bonding between solute and matrix molecules that causes all components to have the same vapor pressure. Depending on if the pure dicarboxylic acid is more or less volatile than the matrix, its volatility is either decreased or increased by dissolution into the matrix. This effect can lead to a remarkably large range of activity coefficients from approximately 0.003 to 150. As the vapor pressure of the solute in solution is that of the matrix, the activity coefficient only depends on the vapor pressure of the pure compound.
In addition to measuring activity coefficients for compounds in simple binary mixtures, samples also were analyzed for dicarboxylic acids present in secondary organic aerosol formed from reactions of simple cyclic alkenes with O3 in an environmental chamber. This provided a more complex matrix that more closely resembles ambient conditions. In these experiments, the range of values of the activity coefficients was only 1.5-5.5, orders of magnitude narrower than for the binary mixtures. These results suggest that activity coefficients should be measured in complex matrices in which the structural order is disrupted. Otherwise, values may be extremely sensitive to the nature of the matrix. It also appears that in the ambient atmosphere, compounds are likely to have vapor pressures close to those of the pure compound as long as the crystal structure of the pure compound does not have a significant effect on its vapor pressure.
The vapor pressures also were measured for a number of compounds that we identified in secondary organic aerosol formed in environmental chamber reactions of alkenes with hydroxyl and nitrate radicals. These compounds were not available from commercial suppliers and included multifunctional compounds containing carbonyl, hydroxyl, and nitrate groups. The data are the first on any compounds of this type.
Conclusions:
The research performed during this project has led to the development of a new method for measuring the vapor pressures and heats of sublimation/vaporization of low-volatility organic compounds commonly found in atmospheric aerosol particles. The method has been evaluated thoroughly and shown to yield accurate and reproducible data. The method is much easier to use than the few others currently available and also has the advantage that it can be used to measure vapor pressures in realistic, complex mixtures. By analyzing compounds within homologous series, data were obtained that can be used to develop correlations for estimating vapor pressures and heats of sublimation/vaporization of similar compounds. By analyzing compounds from different classes, information has been gained on the effects of specific functional groups on volatility. In addition to performing measurements on pure monocarboxylic, hydroxy-monocarboxylic, dicarboxylic, and oxo-dicarboxylic acids, measurements were made on multifunctional secondary organic aerosol compounds formed in environmental chamber reactions. These compounds contained functional groups including carbonyl, hydroxyl, and nitrate, were not commercially available, and were not amenable to analysis by other techniques. Therefore, this study provides the first vapor pressure data on a number of important classes of atmospheric aerosol compounds. The effect of an organic matrix on compound vapor pressures has been investigated by comparing results of measurements made on single-component particles with those made on the same compounds in various binary mixtures with liquid and solid organics and also in secondary organic aerosol matrices. The activity coefficients determined from these measurements provide valuable insight into the nature of the molecular interactions inside particles and their effect on compound volatility. The database on compound vapor pressures, heats of sublimation/vaporization, and activity coefficients developed in this research project will be useful for researchers developing computational methods for calculating these quantities from molecular properties, as data for atmospherically relevant organic compounds are lacking. The data obtained during this research project, as well as those calculated using computational schemes developed with the aid of these data, will be used by atmospheric researchers modeling gas-particle partitioning in secondary organic aerosol formation and gas-particle redistribution of primary organic compounds, such as those from combustion sources. Such models can be included as modules in airshed models, which can in turn be used to estimate the effects of human activities, including pollution control strategies, on air quality and its potential impact on human health, global chemistry, and climate.
This project also has supported and formed the basis of the doctoral research of Sulekha Chattopadhyay, a student in the Environmental Sciences Graduate Program at the University of California, Riverside. Sulekha performed all of the research contained in this report. The results will be published in her Ph.D. dissertation and in a series of four to five peer-reviewed scientific journal articles.
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:
particulates, PM2.5, semi-volatile organics, adsorption, absorption, gas-particle partitioning, measurement methods, environmental chemistry, climate change, atmospheric models, air quality monitoring, ambient aerosol particles, carboxylic acids, particulates,, 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.