Environ. Sci. Technol. 1996, 30, 2667-2680
Real-Time Measurement of Correlated Size and Composition Profiles of Individual Atmospheric Aerosol Particles CHRISTOPHER A. NOBLE AND KIMBERLY A. PRATHER* Department of Chemistry, University of California, Riverside, California 92521
In this paper, the unique real-time measurement capabilities of aerosol time-of-flight mass spectrometry (ATOFMS) for characterizing atmospheric aerosol particles are demonstrated. ATOFMS is used to obtain the aerodynamic size and chemical composition of individual aerosol particles sampled directly into the instrument from outdoors. Such measurements are made in-situ by combining a unique dual-laser aerodynamic particle sizing system to size and track individual particles through the instrument and laser desorption/ionization time-of-flight mass spectrometry to obtain correlated single particle composition data. At typical ambient concentrations, the size and chemical composition of 50-100 particles per minute can be measured (up to 600 per minute at high particle concentrations). Presented here for the first time are compositionally resolved particle size distributions of ambient aerosol particles, showing definitive size/composition correlations. A goal of these studies is to ultimately couple data obtained from the dynamic monitoring of individual particles in atmospheric systems with that obtained using conventional ambient aerosol sampling to assist in sorting out complex field data on atmospheric processes.
Introduction Atmospheric aerosols are receiving increased attention by governmental (1), industrial (2), and scientific (3) communities due to recent studies that report a correlation between high levels of particulate pollution and adverse health effects (4-7). Both natural and anthropogenic sources contribute to atmospheric aerosols with concentrations reaching up to 108 particles/cm3 in some polluted regions (8). In order to enhance our understanding of the role of aerosols in atmospheric processes and to identify aerosol sources, the ability to characterize ambient particles in real time is of primary importance. Many important atmospheric processes are predicted to occur heterogeneously (i.e., gas/particle interactions). * Corresponding author fax:
[email protected].
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1996 American Chemical Society
The focus of the majority of studies probing these processes has been on heterogeneous reactions that occur in the stratosphere on ice particles, which are postulated to lead to depletion of the ozone layer (9-12). In contrast, relatively little experimental data exist regarding heterogeneous tropospheric reaction processes. In order to develop a better understanding of such reactions, it is imperative to use analytical techniques that allow accurate characterization of both gas and particle species co-existing under the same ambient conditions. Significant variations in concentrations of gaseous atmospheric species such as ozone and NOx can occur in minutes or hours during sampling. Many analysis techniques exist for measuring real-time gas-phase species concentration variations occurring on these time scales. In contrast, the majority of chemical analysis methods for particles rely on filter collection, which requires particle sampling for durations that preclude the measurement of hourly or diurnal time-resolved changes. Furthermore, offline analysis techniques cannot measure many chemicals originally present on particles that have been lost due to evaporation, condensation, or reaction between sampling and analysis (13-17). As a result, off-line approaches often cannot provide accurate information on the original particulate sample as it existed in the atmosphere. Source identification is a major goal of aerosol characterization studies. Specific chemical compounds associated with certain size modes allow sampled particles to be traced back to a single emission source. With the goal of providing such information, the focus of many ambient studies involves the determination of specific chemical functional groups or chemical markers in various particle size ranges (13, 18-22). In these studies, size information is obtained when particles are first collected using an impactor that separates particles by aerodynamic size. Subsequent laboratory analysis of particles collected on individual filters yields size-segregated composition information. Such approaches have been used to obtain the majority of what is known regarding particle size and composition correlations. However, by grouping particles of a given size range together for analysis, this type of characterization precludes the identification of multiple classes of particles within the same size mode. For example, particles collected in a given size range (i.e.,
