Time-resolved aerosol collector for CCSEM/EDX single-particle analysis

An automated Time-Resolved Aerosol Collector (TRAC) has been developed for sequential sampling of field-collected aerosols for laboratory-based Computer Controlled Scanning Electron Microscopy/Energy Dispersed X-ray (CCSEM/EDX) single-particle analysis. The collector is optimized for the use of grid-supported 50 nm carbon films as deposition substrates. The carbon films have low enough X-ray background to permit EDX analysis down to 0.1-0.2 mum particles, including detection of low-Z elements: C, N, and O. The TRAC provides unattended sampling onto a set of 151 individual grids, at sequential time intervals as short as 1 min. After collection, the samples are sealed and refrigerated pending analysis. The utility of the TRAC-CCSEM/EDX approach is exemplified using the aerosol samples collected during the Texas 2000 Air Quality Study (August 15-September 15, 2000). We are able to follow the time evolution in the relative contribution of nonvolatile particles such as ammonium sulfate, mineral dust, sea salt, and carbonaceous in the aerosol makeup. The results show, among other things, the diurnal cycles in appearance of fine carbonaceous and ammonium sulfate particles and substantial mixing/coating of mineral particles with ammonium sulfates.