Examination of Method 201A cyclone catch fractions indicates that as much as 98% of the mass of material present in the PM2.5 fraction is due to particles with measured (physical) diameters greater than 2.5µm. Possible explanations for this phenomenon include contamination of the samples, differences in aerodynamic and physical diameter, and inherent properties of Method 201A. Exploration of these possible explanations indicates that 1) contamination is an issue for low catch weight samples and extraordinary care must be taken to ensure that contaminants are not introduced; 2) the difference between aerodynamic diameter and physical diameter makes no significant contribution to the predominance of large particles in the PM2.5 fraction; and 3) the presence of large particles in the PM2.5 fraction (up to 60% of the mass in a model system) is inherent in the method and due to the relatively wide collection efficiency curves of cyclones.
a. SEM Examination of PM 10 and PM 2.5 Filters
b. Physical vs. Aerodynamic Diameter
c. Contamination of PM 10 and PM 2.5 Filters
d. Inherent Issues in Method 201A Collection Efficiency
Tim Vander Wood is one of the executive directors at MVA Scientific Consultants. Tim is one of the pioneers in introducing electron microscopy particle sizing to the stack testing industry and he is very active in the stack testing professional organizations such as SES and CAPCA. He has been helping stack testing clients solve particle sizing and contamination issues for the past 20 years.