2000 Progress Report: Aerosol Partitioning and Heterogeneous ChemistryEPA 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 Period Covered by this Report: October 1, 1999 through September 30, 2000
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:This project focuses on the development of 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 addresses 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 is now subject to regulation. The semi-volatile 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.
In the past year, we have: (1) modified the step-scan experiment to include increased sensitivity and the ability to quantitatively determine the composition of aerosols vaporized by a pulsed CO2 laser; (2) assembled an angular scattering experiment to promote characterization of the aerosol physics of the CO2?aerosol interaction; and (3) developed a continuous heating, conventional fourier transform-infrared (FT-IR) experiment for the qualitative and quantitative evaluation of the composition of multi-component organic aerosols. Each of these are addressed below.
Prior to proceeding with modifications to the step-scan experiment, we 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.
The utility of this method in analyzing aerosols undergoing heterogeneous chemical reactions would benefit from increases in detection sensitivity and quantitative capabilities. Modifications to improve detection sensitivity, through increases in the signal to noise ratio, include using a larger area detector element and improved collection optics. The configuration of the interaction region also was changed from a closed to open sample cell design to increase the flexibility of the system, while permitting essentially continuous analysis without sample back-up, thus increasing signal averaging capabilities. Modifications addressing quantitative analysis of the aerosol composition have included minimizing baseline effects due to changes in scattering of the infrared beam by removing the aerosol stream from the observation region, and focusing the CO2 laser spot more tightly to move from the shattering regime to that of explosive vaporization. Complete evaporation of the droplets will permit quantitative analysis of aerosol composition. These modifications are almost complete, and we anticipate using this method in the near future to probe the composition of aerosols undergoing heterogeneous chemical reactions.
In the past year, we have set up an angular resolved scattering experiment to continue our investigation of the interaction between the CO2 laser pulse and the aerosols. Although results obtained in the two-color scattering experiments discussed in last year's annual report have been 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 log normal profile of known distribution for the aerosols, at all points in the analysis. These issues are addressed in the 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. We currently are collecting time-resolved angular resolved scattering profiles of the interaction of the CO2 laser with water and formamide aerosols generated using a Meinhard nebulizer, which provides log normal distribution of aerosols. Characterization of the particle size, temperature, and refractive index of room temperature aerosols correlates well with that determined using an Aerodynamic Particle Sizer. We currently are interpreting the post-pulse aerosol profiles. Understanding this interaction will be critical in analyzing vapor transients collected in the step-scan studies.
Finally, the remainder of our time has been spent on the development of a continuous heating, conventional FT-IR method for the qualitative and quantitative determination of the composition of multi-component aerosols. 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 characterizing 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 difference spectrum. This method has potential application for online monitoring of multi-component organic aerosols generated in high-load environments, such as aerosolized fuel from aircraft or industrial processes.
Future Activities:In the upcoming year, in addition to the ongoing study of the interaction between the CO2 laser pulse and the aerosols, we will move into the final phase of the proposed experiments involving the study of heterogeneous chemical reactions. We currently are in the process of constructing the flow tube for these experiments. We then will use two methods to analyze the aerosols undergoing reaction in the flow tube. The first of these, upon completion of the aforementioned modifications, will be the step-scan method. We also plan to take advantage of a Time-of-Flight Mass Spectrometry (TOFMS) system developed in our laboratories, which uses a combination of droplet evaporation by CO2 laser and ultraviolet (UV) ionization to analyze single particles. We will begin with the study of gas-phase ozone reactions with organic aerosols. In these experiments, we will characterize the products formed, as well as investigate the kinetics of the reaction.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 18 publications||5 publications in selected types||All 5 journal articles|
||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.||
||DeForest CL, Qian J, Miller RE. Time-resolved studies of the interactions between pulsed lasers and aerosols. Applied Optics 2002;41(27):5804-5813.||