Final Report: Aerosol Partitioning and Heterogeneous Chemistry

EPA Grant Number: R826767
Title: Aerosol Partitioning and Heterogeneous Chemistry
Investigators: Miller, Roger E. , Hauser, Cindy DeForest
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1998 through September 30, 2001
Project Amount: $338,749
RFA: Air Pollution Chemistry and Physics (1998) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Engineering and Environmental Chemistry

Objective:

The main objective of this research project was to develop spectroscopic methods to characterize gas-particle systems under ambient conditions, and to apply these to better understand the nature of atmospheric particulates. This research specifically addressed the problem of characterizing and monitoring the fine fraction of atmospheric aerosols (particulate matter of diameters 2.5 µm or less, PM2.5) that are linked to health effects and are now subject to regulation. The semivolatile nature of these particles makes their detailed characterization in terms of composition and chemistry difficult, because all sampling methods tend to perturb the delicate equilibrium that exists between the gas and particle phases.

Summary/Accomplishments (Outputs/Outcomes):

During this research project, a variety of spectroscopic methods were developed to characterize the physical and chemical properties of laboratory-generated aerosols composed of organic species. The ability of spectroscopic methods to provide not only qualitative, but also quantitative information about organic compounds in a noninvasive matter, under atmospherically relevant pressure and temperature conditions, makes them an appealing class of techniques to be employed in aerosol characterization. As these methods are better suited to gas-phase analysis, the aerosols generated in these studies are evaporated prior to spectroscopic analysis. Although the spectroscopic techniques developed here were able to provide information about the composition of laboratory-generated aerosols, the detection limits were too high to study heterogeneous chemical reactions under atmospheric conditions. To expand these studies to include such processes, an aerosol time-of-flight mass spectrometer (ATOFMS) was used.

The first method was developed to qualitatively and quantitatively determine the composition of multicomponent organic aerosols using continuous heating and conventional Fourier transform infrared (FT-IR) spectroscopy. The second approach explores the pulsed heating of aqueous and organic aerosols using time-resolved techniques, which include visible light scattering and Step-Scan Fourier Transform Infrared (S2FT-IR) spectroscopy. Finally, the composition determination and pulsed-heating studies were extended to incorporate the compositional and kinetic exploration of the heterogeneous chemical reactions of organic aerosols with ozone using pulsed heating, visible-ultraviolet (VUV) ionization, and mass spectrometric analysis.

Composition Determination of MultiComponent Organic Aerosols by Online FT-IR Spectroscopy. FT-IR spectroscopy has been successfully applied to the compositional analysis of gas-phase mixtures, as the mixture spectrum is simply a linear combination of the spectra for the individual species. Using a two-stage heating design, we initially evaporated aerosols composed of an alkane, aromatic derivative, and ester. Comparing the experimentally determined spectra to a linear combination of the reference spectra for each species demonstrates the ability of the method to determine the composition of the aerosols both qualitatively and quantitatively. In the instance of an aerosol containing multiple species with similar functional groups, the method is unable to distinguish among the species. We are, however, able to determine the relative ratios of functional groups present using representative averaged spectra for each functional group. Analysis of aerosolized diesel fuel has demonstrated the application of the method to characterize an unknown mixture in terms of functional groups present and their relative ratios, in comparison with compositions reported in the literature. We also have investigated the strength of the technique for detecting changes in the aerosol by subtracting the initial aerosol spectrum from the final and analyzing the different spectrum. This method has a potential application for online monitoring of multicomponent organic aerosols generated in high-load environments such as aerosolized fuel from aircraft or industrial processes.

Time-Resolved Studies of the Interactions Between Pulsed Lasers and Aerosols. The combination of continuous heating and conventional FT-IR provided excellent qualitative and quantitative information about the composition of organic aerosols. The relatively long residence time (3 seconds) of the particles in the heating region could, however, pose a problem in the study of condensed phase or heterogeneous reactions, which may occur with reaction times on the order of seconds, and be temperature sensitive. If the particle heating could instead be instantaneous, and the resulting vapor is probed immediately, the FT-IR spectrum would essentially be a snapshot of the particle composition. The focus of this work was to combine pulsed laser heating S2FT-IR, which provides the requisite time resolution to measure vapor resulting from a short, high-energy pulse. Interpreting the time-resolved vapor transients provided by the FT-IR method, however, requires some understanding of the microphysics involved in the interaction of high-intensity laser beams with micron-sized aerosols.

High-speed microscope photography has been used to study the pulsed-laser vaporization of single-liquid droplets. These optical methods are only appropriate for larger droplets, with radii of tens to hundreds of microns. Droplet evaporation and shattering have been observed in this way and much of the theoretical literature was developed to interpret the results from these experiments. For smaller particles, it becomes necessary to monitor collective optical and/or acoustic phenomena. Among these techniques, transient laser transmission and/or scattering have been widely used to probe the temporal behavior of small particles. There is still, however, much about these phenomena that is poorly understood, particularly on the shortest time scales corresponding to the msec-long CO2 laser pulse.

In the present study, laser vaporization of micron-sized particles is studied by analyzing the resulting vapor plume using S2FT-IR. A laser is used to heat the liquid particles and the resulting vapor plume is monitored directly, based on the intensities of the corresponding gas-phase spectral features. This approach is combined with light-scattering methods that provide important information concerning the time dependence of the particle diameter, which in turn can be used to determine the heating regimes applicable to these spectroscopic experiments.

Preliminary two-color light-scattering experiments were conducted to investigate the effects of the CO2 laser on the aerosol stream. If the infrared studies were to be successful, a large fraction of the aerosol must be vaporized when heating. Furthermore, due to the conflicting theoretical and experimental work available on droplet heating by lasers as a function of laser power and particle composition, we needed to understand the microphysics associated with our particular system. The two-color scattering experiments revealed that the aerosol particles shatter with approximately 75 percent of the mass concentration converted to the vapor-phase independent of the composition, providing a vapor plume sufficient for FT-IR analysis. Those experiments also showed that: (1) the secondary aerosol particles evaporate as a function of laser power; (2) the shattered secondary particles of water and formamide re-equilibrate secondary processes such as recondensation or volume refill and begin to dominate on slightly different time scales: 250 and 150 µsec, respectively; and (3) the mass vapor loading by the secondary particle evaporation is greater for water than for formamide.

Although results obtained in the two-color scattering experiments were very informative, quantitative determination of the particle's radial temporal profile is challenged by requirements inherent in the method. These include assumed knowledge of the composition and temperature for refractive index determination and a lognormal profile of known distribution for the aerosols at all points in the analysis. These issues are addressed in an angular-resolved scattering experiment, as each angular scattering profile is unique to a given radius, distribution, and refractive index. Matching of experimental profiles to Mie calculated profiles requires that all of these parameters be accurately assessed. Angular-resolved light scattering experiments reveal that the refractive index of the particles changes at short times due to heating, and that the width of the particle distribution increases after the pulsed-heating event. The refractive index is shown to rebound as the particle cools through evaporation, while the width of the distribution continues to increase.

Because the particle sizes determined from the extinction experiments mainly correspond to sufficiently long times so that cooling of the particles already has occurred, the use of the room temperature refractive indices does not introduce significant error. However, the assumption that the width of the distribution does not change significantly could present a problem, particularly at long times where the resulting error in the particle size could be as much as 50 percent. Nevertheless, as short times, where we estimated the postshattering particle size, these effects are less important. In conclusion, the extent of vaporization upon shattering is high. We estimate an upper limit of approximately 75 percent mass loss into the vapor phase, ignoring the (presumably small) fraction of condensed-phase mass distributed among the very fine particles. To obtain independent verification of this vapor loss, we conducted transient infrared gas-phase absorption measurements under essentially the same conditions, using the S2FT-IR spectrometer.

Water and formamide aerosols interacting with the pulsed-CO2 laser were analyzed by TR-S2FT-IR. Transient absorption spectra of water aerosols evaporated by the CO2 laser show an increase in the sharp vapor-absorption bands, which correlate with the background equilibrium vapor-absorption structures, indicating vapor plume development. Also present in the spectrum is what appears to be a broadband decrease in extinction predominantly at higher wavenumbers due to a decrease in scattering of the infrared light by the particles, which would be observed in the shattering of large particles into smaller ones. The temporal behavior of the vapor plume can be monitored by determining the area of representative vapor absorption features as a function of time. The immediate rise in peak area of the vapor feature is indicative of a high fraction of evaporation during the shattering event resulting in a vapor plume. One quantitative interpretation of the trace at longer times is complicated by a convolution of the effects of shock wave/thermal expansion and continued mass vapor loading. The vapor pressure profile does not appear to display an immediate decrease in vapor pressure after the shattering event as would be observed if a shock wave/thermal expansion effect was the dominant mechanism. The continued increase in vapor pressure indicates instead that the presence of additional mass vapor loading of the observation region via evaporation of the secondary particles formed during the shattering event may be the dominant mechanism for water aerosols at times up to 170 µsec after the laser pulse.

Because the real focus of this technique development is on the analysis of volatile and semivolatile organic species, we also have studied formamide as a model organic compound. The absorption features of the vapor and liquid phases of formamide are clearly separated ( = 70 cm-1), making it straightforward to individually monitor them. Transient absorption spectra of formamide aerosols interacting with the pulsed-CO2 laser show an increase in absorbance in the region around 1,750 cm-1 at 25 µsec post-pulse as the result of an increase in the vapor concentration due to the vapor plume from the shattered aerosols. As the mass vapor loading by evaporation of the secondary particles is negligible for formamide aerosols, the long-time observed vapor pressure is dominated by the thermal expansion of the hot vapor as well as the presence of a shock wave in particle shattering and the vapor pressure decreases. Similar to water aerosols, the presence of the vapor plume is an immediate effect. Unlike water, however, the formamide vapor pressure decreases immediately following the shattering event due to the dominant effect of the shock wave/thermal expansion in the absence of continuous mass vapor loading.

We also studied the extent of vaporization of the aerosols upon shattering and the competition of evaporation of the secondary aerosols with thermal expansion as a function of CO2 laser power. Analysis of the vapor transients, arising from the evaporation of formamide aerosols by the CO2 laser, indicates an increase in vaporization with increasing laser fluence, as expected. After formation of the initial vapor plume, the integrated area of the vapor feature decreases. This is due to the dominant effect of thermal expansion. A decrease in the rate of decay as the laser power decreases was observed in these studies. This is consistent with the concept of thermal expansion, where an increase in laser power would result in an increase in temperature, which would in turn increase the rate of thermal expansion and the rate of decay of the observed vapor pressure.

Additional exploration of the applicability of this method to the analysis of multicomponent aerosols is planned. The information obtained by this combination of techniques enhances our understanding of the complicated phenomena involved in the interaction of high-energy pulsed laser beams and aerosols. The data presented here should provide excellent tests for theoretical methods designed to model the interaction of high-energy laser beams with particles, with the objective of analyzing the particle composition.

Reactive Uptake of Ozone by Oleic Acid Aerosol Particles: Application of Single-Particle Mass Spectrometry to Heterogeneous Reaction Kinetics. The ultimate objective in the method development presented thus far was the determination of particle composition for the purpose of studying chemical reactions in or on the surface of organic particles. In atmospheric chemistry, the combination of gas-surface interfacial mass transport, reactions at liquid and solid surfaces, transport into and reactions in liquid droplets is described as heterogeneous chemistry. Such chemical processing of atmospheric aerosols affects the gas-phase chemistry of the troposphere as well as the composition of the particulate fraction. Reactions at the surface, for example, can provide additional sources and sinks for many trace gases important in global climate modeling. Indeed, some reactions, which proceed slowly in the gas phase, can be catalyzed through surface reactions. Considering the particulate fraction, heterogeneous chemical reactions, which change the composition of the particle, may affect their impact on human health, climate, and visibility.

In these studies, an entrained aerosol flow tube is employed to measure the heterogeneous reaction of ozone with organic aerosols. The work presented above was undertaken for the purpose of studying these reactions. The pulsed-heating approach is advantageous for vaporizing particles without affecting their composition. Although the S2FT-IR technique likely would provide important information about the functionality of ozonolysis products, the sensitivity was not sufficient for quantitative studies of such reactions. It also was determined that the need to repeat the experiment at each step would make kinetic analysis difficult, as many exposure times under constant conditions must be sampled for a given decay. The determination of the composition of the aerosols as they underwent reaction in the flow tube was accomplished using a combination of pulsed-laser heating and VUV ionization with mass spectrometric analysis. Presented here are the preliminary results of studies aimed at determining the products formed during the ozonolysis of alkenes and measuring the reactive uptake of ozone by oleic acid aerosols.

The uptake appears to depend on the size of the particle with gamma, ranging from 3.9 x 10-3 to 0.5 x 10-3 for particles ranging in radius from 700 nm to 2.45 µm. An estimate of 5 x 10-3 on the upper limit of gamma is made and it is suggested that the decrease in the observed uptake with increasing particle size is a consequence of the reaction being limited by the diffusion of oleic acid within the particle. Solutions to the coupled partial differential equations describing simultaneous diffusion and reaction of both O3 and oleic acid are obtained numerically. These solutions suggest that to adequately describe the observed reaction rate, oleic acid must diffuse within the particle approximately three orders of magnitude slower than is predicted by the measured oleic acid self-diffusion constant. It is proposed that this oleic acid diffusion-limited uptake is a consequence of the reaction with O3. Possible implications that this has for the role of particle morphology in the reaction of gas-phase species with atmospheric aerosols are discussed.

Individual particles found in the atmosphere often contain many different species including alkanes, alkenes, alcohols, and carboxylic acids. The rate at which a particle of this nature takes up a reactive species such as O3 will depend on the composition of the particle as well as the rate of reaction of O3 with each of the component species. However, in assessing the significance of the magnitude of the reactive uptake coefficient measured in these experiments, it is useful to consider the lifetime (with respect to reaction with O3) of a pure oleic acid particle in the atmosphere. With a value of gamma = 5 x 10-3 (the upper limit from this work), and [O3] = 100 ppb, 99 percent of a 50-nm particle of pure oleic acid would react in only 1 minute. However, oleic acid has been measured from atmospheric particles, suggesting that oleic acid in aerosols has a much longer lifetime.

The apparent discrepancy between the calculated lifetime and the observation of oleic acid in particles could be attributed to the diffusion of oleic acid. The calculation assumes that oleic acid diffusion within the particle is fast and therefore, does not limit the uptake. However, we have seen that under conditions in which the oleic acid diffusion is significantly reduced (from self-diffusion), uptake is much slower. This discrepancy indicates that not only particle composition, but also particle morphology can play a role in the uptake of gas-phase species. If oleic acid exists in a particle with other species that effectively inhibit diffusion within the particle, uptake would be reduced. Additionally, the location of the oleic acid within each particle could affect uptake, because oleic acid near the surface reacts much more quickly than it would if it were deep within the particle. A particle with a very porous structure may trap oleic acid more efficiently than a liquid droplet, for example, and therefore significantly reduce O3 uptake.

In addition to affecting the composition and properties of particles in the atmosphere, the reaction of O3 with organic particle constituents also may affect the lifetime of O3 in the troposphere. The rate at which an aerosol removes a gas-phase species through reactive uptake can be estimated. Using a value of gamma = 5 x 10-3 (the upper limit estimated from this work) and a representative aerosol content of 1,500 particles cm-3 with an average radius of 25 nm, this rate is calculated to be 5 x 10-6 sec-1, corresponding to an O3 lifetime (with respect to reaction with the aerosol) of 2 days. The lifetime with respect to photochemical loss, on the other hand, varies from a couple of days to a few hundred days depending on season and latitude. Thus, organic species such as oleic acid in particles could have an impact on the atmospheric lifetime of O3.


Journal Articles on this Report : 5 Displayed | Download in RIS Format

Other project views: All 18 publications 5 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article DeForest CL, Qian J, Miller RE. Composition determination of multicomponent organic aerosols by on-line FT-IR spectroscopy. Applied Spectroscopy 2002;56(11):1429-1435. R826767 (2000)
R826767 (Final)
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  • Abstract: OSA-Abstract
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  • Journal Article DeForest CL, Qian J, Miller RE. Time-resolved studies of the interactions between pulsed lasers and aerosols. Applied Optics 2002;41(27):5804-5813. R826767 (1999)
    R826767 (2000)
    R826767 (Final)
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  • Journal Article Niedziela RF, Norman ML, DeForest CL, Miller RE, Worsnop DR. A temperature-and composition-dependent study of H2SO4 aerosol optical constants using Fourier transform and tunable diode laser infrared spectroscopy. Journal of Physical Chemistry A 1999;103(40):8030-8040. R826767 (Final)
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  • Journal Article Smith GD, Woods III E, DeForest CL, Baer T, Miller RE. Reactive uptake of ozone by oleic acid aerosol particles:application of single-particle mass spectrometry to heterogeneous reaction kinetics. Journal of Physical Chemistry A 2002;106(35):8085-8095. R826767 (Final)
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  • Journal Article Smith GD, Woods III E, Baer T, Miller RE. Aerosol uptake described by numerical solution of the diffusion-reaction equations in the particle. The Journal of Physical Chemistry A 2003;107(45):9582-9587. R826767 (Final)
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  • Supplemental Keywords:

    atmosphere, absorption, chemical transport, bioavailability, chemicals, toxics, organics, environmental chemistry, analytical, measurement methods, urban areas., Scientific Discipline, Air, Ecology, Environmental Chemistry, Engineering, Chemistry, & Physics, ambient aerosol, fate and transport, Fourier Transform Infrared measurement, gas/particle partitioning, risk assessment, atmospheric particles, air modeling, spectroscopic studies, aerosol partitioning, air sampling, spectroscopy, diode laser, PM2.5, real time monitoring

    Progress and Final Reports:

    Original Abstract
  • 1999 Progress Report
  • 2000 Progress Report