1999 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, 1998 through September 30, 1999
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:These studies focus on the development of the in situ spectroscopic methods necessary to characterize gas-particle systems under ambient conditions and the application of these methods 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 now subject to regulation. The semivolatile nature of these particles makes their detailed characterization difficult, because all sampling methods tend to perturb the delicate equilibrium that exists between the gas and particle phases.
During the 1998?1999 funding period, the application of time-resolved infrared spectroscopy to the in situ real-time analysis of laboratory-generated aqueous and organic aerosols, specifically formamide, was demonstrated. Interfacing of the step-scan fourier transform-infrared (FT-IR) with a pulsed CO2 laser and optically coupled external aerosol sampling cell has been completed. Aerosol flows through the sample cell, where it is heated by the CO2 laser, and the response is probed in the nano- to microsecond time scale before and after passage of the laser pulse by the FT-IR. Preliminary light scattering studies of the evaporation process reveal primary particle shattering with a high fraction of vaporization followed by secondary particle evaporation, the extent of which is a function of pulse intensity and particle composition. The composition and behavior of the vapor plume formed during the shattering event is further probed using the time-resolved step-scan FT-IR spectroscopic method. The combined time-resolution and compound-identification capabilities of this technique confirm the results of the light-scattering studies regarding the evaporative process while providing qualitative and potential quantitative analysis of otherwise elusive organic species in a noninvasive, nondestructive manner.
The aerosols are analyzed at room temperature and pressure, so as not to disturb the delicate gas-particle equilibrium, until they are heated to the point of shattering on time scales much faster than diffusion processes. This technique has the capability of providing not only composition information, but also the vapor-phase kinetics of the evaporation process. A more rigorous approach to the data processing also could yield particle kinetic information through the observed change in scattering. Furthermore, a rapid high-energy pulse, which has the ability to instantaneously shatter and evaporate a large fraction of the particle in a nonselective fashion, could prove useful in determining fractional composition of organic mixtures. The combination of composition and gas/particle interaction data for particles relevant to the fine fraction will be important in the predictive modeling of the reactivity of these species in the atmosphere and accurate evaluation of results from epidemiological studies.
One of the first priorities is improving the sensitivity of the step-scan system for the detection of very small changes. Improving the sensitivity of this system could permit detection of low levels of organic compounds found in real atmospheric aerosols such as soot.
Further studies under consideration involving soot strive to take advantage of the time resolution of the system in the study of aerosol heterogeneous chemistry. The soot particles will be coated with formamide or other organic compounds of interest and vaporized by the CO2 laser. As the analysis is performed under atmospheric temperature and pressure, not only can partitioning be evaluated, so can the potential for heterogeneous nucleation or condensation, including an uncoated seed particle in the aerosol stream to be vaporized, which also might be useful in the demonstration of competing factors in uptake phenomena.
The setup and parameterization of the tunable diode laser (TDL) system discussed in the proposal also is planned for the 1999?2000 funding period. The groundwork laid out in the two-color scattering and step-scan experiments is invaluable in maximizing the sensitivity and single laser shot data acquisition advantages provided by the TDL for enhancing the ability to probe the heterogeneous chemistry of atmospheric aerosols.
Journal Articles on this Report : 1 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. Time-resolved studies of the interactions between pulsed lasers and aerosols. Applied Optics 2002;41(27):5804-5813.||