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: Hahn, Intaek
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 , Safer Chemicals
Description:The focus of these studies is on the development of the in situ spectroscopic methods necessary 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 mm or less: PM2.5) that are linked to health effects and now subject to regulation. The semi-volatile nature of these particles makes their detailed characterization difficult, since all sampling methods tend to perturb the delicate equilibrium that exists between the gas and particle phases.
Approach:The basic approach is to combine time-resolved Fourier Transform infrared (FTIR) and diode laser (TDL) spectroscopies with laser evaporation methods to determine the partitioning of semi-volatile species between the gas and particle phases. A pulsed CO2 laser, with adjustable output intensities, is being used to heat the cross-section of an aerosol stream flowing through a sample cell equipped with a White Cell optical configuration for enhanced detection. Concurrently, a step-scan FTIR or TDL system provides a measurement of gas phase concentrations on the nanosecond time scale as the particle evaporates. Through collaborations that are already in place with the Environmental Sciences Department at UNC, we will have the opportunity to interface these new methods with a smog chamber for quantitative comparison with more traditional methods. Ultimately, although not part of this proposal, it is our goal to apply these new methods to ambient particles. In the present grant period, we will evaluate the conditions under which the proposed methods could be used for real-time field measurements.
The experiments are laboratory based and focus on the characterization of the aerosols through the development of new in situ spectroscopic methods. Measurements as a function of laser fluence are beginning to provide information on (1) the composition of the particles, (2) the partitioning of various species between the two phases, (3) the rates of evaporation of the various species, which in turn will provide information on the layered structure of the particles and (4) the rates of re-equilibration after the heating cycle, providing insights into the uptake of semi-volatiles on short time scales.
Improvement in Risk Assessment or Risk Management: Reliable risk assessment with respect to particles requires better methods for their characterization and a better overall understanding of their properties. In the work proposed here we will address these two issues by first developing new in situ methods for monitoring the gas and particle phases and second by providing a data base of compositional and rate information that will improve our understanding of the complex processes associated with these semi-volatile systems.