Cellulose Filter High-Volume Cascade Impactor Aerosol Collection Efficiency. A Technical Note Herman Severing’, Mehul J. Dave, Patric G. McCoy, and Keith Walther College of Environmental and Applied Sciences, Governors State University, Park Forest South, 111. 60466
Ambient aerosol collection efficiency by 40-cfm cascade impactors using cellulosic filters was found to differ markedly from efficiencies reported when using glass fiber filters. Simultaneous five-stage impactor cellulose and glass fiber filter samples of ambient aerosol were analyzed by optical and electron microscopy. The ratio of the 50% cutoff diameters for cellulosic filters to that for glass fiber filters was found to be approximately one-half. Cellulosic filters are known to be low in trace metal content; this fact is confirmed for both Misco P810-A and Whatman 41 filters. The consequences of reduced 50% cutoff diameters on trace metal sampling and analysis when using cellulosic filters are discussed. Glass fiber has been the standard filter type used with high-volume samplers for atmospheric aerosol mass concentration measurements. As interest in the chemical composition of aerosol increased, many alternative filter materials were assessed for their trace metal impurity levels as well as for impurity levels of other constituents ( I , 2 ) . Results of these assessments have shown that cellulose and membrane filters have substantially lower trace metal impurity levels than do glass fiber filters. In using these alternative filter materials with cascade impactors one must identify collection efficiency curves rather than assuming the manufacturer’s glass fiber filter collection efficiency curves. The primary purpose of this study was to quantitatively ascertain the collection efficiency of cellulose filters used in the 40-cfm high-volume cascade impactor. The impactor considered is the 235 Sierra slotted, five-stage impactor (hereafter, 235). A secondary purpose was to compare cellulose filter efficiency with glass fiber filter efficiency when using the 235 impactor. Willeke ( 3 ) sampled a known size aerosol using the 235 impactor and Gelman type A glass fiber filters. Collection efficiency curves were determined by optically sizing and counting aerosol collected on each impactor stage as well as on the backup. The 50%cutoff diameters were found to be 7.2 pm for stage 1,3.0 pm for stage 2,1.5 pm for stage 3,0.9 pm for stage 4, and 0.6 pm for stage 5. These are often stated as the ambient aerosol “50% cutoffs” for the impactor stages regardless of filter type. In considering an alternative filter material for trace metal sampling and analysis, two factors are of importance. First, the material must have substantially lower impurity levels than those reported for glass fiber. Second, the new material should have high retention of the 50.1 to 0.3 pm diameter aerosol for use as a backup filter since several trace metals are found in these smaller size ranges. For example, Skogerboe et al. ( 4 )showed that 260% of atmospheric aerosol P b and Cd are found on the
