Evaluation of Computer-Controlled Scanning Electron

Aerosol Science and Technology 34: 97– 107 (2001) ° c 2001 American Association for Aerosol Research Published by Taylor and Francis 0278-6826=01=$12.00 C .00

Evaluation of Computer-Controlled Scanning Electron Microscopy Applied to an Ambient Urban Aerosol Sample Yaacov Mamane,1 Robert Willis, 2 and Teri Conner 3 1 Environmental and Water Resources Engineering, Technion, Haifa, Israel 2 ManTech Environmental Technology, Inc., Research Triangle Park, North Carolina 3 U.S.

Environmental Protection Agency, National Exposure Research Laboratory, Research Triangle Park, North Carolina

Concerns about the environmental and public health effects of particulate matter (PM) have stimulated interest in analytical techniques capable of measuring the size and chemical composition of individual aerosol particles. Computer-controlled scanning electron microscopy (CCSEM) coupled with energy-dispersive X-ray analysis (EDX) allows automated analysis of particle size, chemistry, and particle classiŽ cation. In combination with manual SEM and bulk analytical techniques such as X-ray  uorescence, CCSEM can be a valuable tool for characterizing individual ambient particles and determining sources of ambient PM. The goal of this study was to examine several issues related to the quality and validity of CCSEM data. These included the stability of unattended CCSEM for multihour runs, the number of particles that must be analyzed in order to yield representative results, and errors associated with CCSEM. CCSEM was applied to the analysis of a 24 h ambient particle sample collected in Baltimore, MD. The coarse-fractionsample (PM10¡2:5 ) was collected with a dichotomous sampler on a polycarbonate Ž lter. A total of 2819 particles in 78 randomly selected Ž elds of view were analyzed by CCSEM during an unattended 8 h run. Particle diameter, aspect ratio, particle location, X-ray counts for 20 elements, and digital images of each particle and its Ž eld of view were stored. The average number of particles per Ž eld (N/F), average particle diameter (Dave ), average mass loading per Ž eld (Mave ), and average particle composition were calculated for subsets of the data and compared against results for the full data set in order to assess the stability of the CCSEM analysis over time and the number of particles needed to obtain representative results. These comparisons demonstrated excellent stability of CCSEM over the 8 h run. Physical properties (represented by N/F, Dave , and Mave ) of the sample were well characterized by analyzing approximately 360 particles. Chemical properties of the sample (average elemental composition and major chemical class abundances) converged to within a few percent of their Ž nal values after analyzing about 1000 particles. However, for many purposes several hundred particles may provide adequate characterization.Convergence of minor

Received 2 February 2000; accepted 5 June 2000. Address correspondenceto Robert Willis, ManTech Environmental Technology, Inc., P.O. Box 12313, Research Triangle Park, NC 277092313.

class abundances was limited by statistical  uctuations as the number of particles populating a class became very small. Manual review of the CCSEM data identiŽ ed errors associated with CCSEM due to missed particles, overlapping particles, contrast artifacts, sizing errors, and heterogeneous particles. Most errors could be corrected or eliminated during manual off-line review of the data or avoided by maintaining a proper particle loading on the Ž lter.

INTRODUCTION In July 1997, the U.S. Environmental Protection Agency (U.S. EPA) promulgated a new national ambient air quality standard (NAAQS) for particulate matter (PM) in response to recent studies showing an association between human morbidity/mortality and PM concentrations (Dockery and Pope 1994; Heath et al. 1995; Pope et al. 1995; Schwartz 1994). The new NAAQS is aimed at regulating ambient concentrations of PM10 and PM2:5 (particles with aerodynamic diameter