Diffusion Sampling Method for Ambient Aerosol Size Discrimination with Chemical Composition Determination William H. Marlow* and Roger L. Tanner Department of Applied Sclence, Brookhaven National Laboratory, Upton, N. Y. 11973
A method for sampling aerosols is described by which particles below the optical scatterlng range (C0.3 pm dlameter) can be separated into size-related categories by dlffusion, and variations in the chemical composltlon of sulfur compounds in these partlcles as a function of size can be identified by fllter collection and subsequent analysis. Application of a battery of diffusion cells of appropriate length allows sampling of amblent aerosols for the purpose of chemical speciation of airborne sulfate as a function of particle size with time discrimination of 1 to 2 h. Representative data illustrating the dlffusion sampling method are discussed. Precision and accuracy of the chemical methods for sulfate speciation are f15% at amblent concentrations and the limit of detection of sulfate and strong acld are, respectively, 2 pg and 0.1 pequiv.
determining the fractional removal of particles from the original sample. The “collimated hole structure” type of diffusion battery used here (6) consists of a series of cylindrical stainless steel blocks containing numerous parallel, approximately cylindrical holes. The diffusion battery is operated under conditions (6) such that Equations 1and 2 describe the fractional penetration, fa, of particles of diffusion coefficient, D, through a cylindrical tube of length, 1, with a volume flow rate, q , through each tube (7):
+
f a = 0.819 exp(-3.657a) 0.097 exp(-22.3a) 0.032 exp (-57a) 0.027 exp(-123a) 0.025 exp(-7504
+
+
+
a = aDl/q
(1)
(2)
THEORY
If the aerosol stream is passed through a serial sequence of such diffusion cells, the resultant fractional penetration is the product of fractional penetrations through the individual cells according to Equation 1. By making aerosol measurements between the cells, the aerosol size distribution may be determined. Fractions of particles sampled according to diameter (as determined from their diffusion coefficients (7) at 293 K and with a molecular mean free path of 0.09 pm) have been calculated for the larger, serial Sinclair diffusion battery (6) and are presented in Figure 1. This paper reports the use of the diffusion battery as a sample processor in which a filter sample taken for chemical analysis replaces the CNC traditionally employed with a diffusion battery. From two or more simultaneous air samples, one collected directly from ambient and the others via the diffusion battery, the chemical content of the smaller size fraction of the aerosol (removed first by diffusion to the cell walls) can be inferred by subtracting the concentrations measured on the filter collecting particles from the diffusion-processedair from those measured on the filter collecting directly from ambient air. To avoid potential misinterpretation, we refer only to the particle size at 50% penetrationdefined as the diameter for which approximately 50% by number of the particles penetrate the battery-as is the practice for data reported from cascade impactors.
The method of diffusion processing of aerosols for filter sampling described here utilizes the diffusion battery (a “battery” of diffusion cells) which has conventionally been used for the determination of the numerical size distribution of suboptical sized particles (5).By utilizing highly collimated, uniform, porous structures for diffusion cells, a diffusion processing device applicable for field measurement of sizeclassified chemical composition of ambient aerosol particles in the size region
