Evolution of Filters for Aerosol Chemical and Physical Characterization

JUDITH CHOW, Xiaoliang Wang, John Watson, Desert Research Institute

     Abstract Number: 15
     Working Group: History of Aerosol Science

Abstract
Air has been drawn through porous materials for centuries to remove contaminants from inhaled air and more recently to obtain particles for subsequent analyses. Early characterization methods subjected the aerosol deposits to microscopic analysis and to measures of filter darkening as an indicator of particle concentrations. Weighing the filter before and after sampling became the standard method for determining aerosol mass loadings, and a wide range of chemical and physical analyses methods were subsequently developed and applied. These methods revealed limitations of the filter materials with respect to sampling efficiencies as a function of particle size, degree of penetration into the filter material, adsorption of gaseous pollutants and water vapor, excessive blank levels for the desired particle compositions, optical properties, loading capacities, durability, and cost. Filters that have been used for aerosol sampling include: 1) membrane (also termed “porous membrane”) filters (i.e., PTFE, polypropylene, PVC, nylon, silver, and mixed cellulose esters [MCE]); 2) capillary pore filters (i.e., polycarbonate Nuclepore); and 3) fibrous filters (i.e., cellulose-fiber, pure and mixed quartz-fiber, Teflon-coated glass-fiber, and glass-fiber). These filter types have undergone a variety of tests over the past 60 years for pairing with different sampling and analysis methods. Several of them are currently being evaluated for additional analytical methods that have not yet been applied to ambient aerosol analyses. The presentation intends to: 1) describe materials and structures of these filter types and how they evolved over time; 2) summarize past tests on filter collection efficiencies; 3) discuss potential atmospheric artifacts; 4) examine effects of particle deposit inhomogeneities; 5) tabulate filter costs and availability; and 6) identify future prospects for aerosol filter sampling to protect public health and identify pollution sources.