The use of STIM and PESA to measure profiles of aerosol mass and hydrogen content, respectively, across Mylar rotating drums impactor samples

Abstract

A method has been developed for measuring profiles of aerosol mass on thin (480 μg/cm2) Apiezon-L coated Mylar films employed in rotating drum aerosol impactor samplers using the ion beam analysis technique scanning transmission ion microscopy (STIM). The greased Mylar films are excellent impaction substrates and possess excellent uniformity in projected density, making them an ideal substrate for STIM analysis. The uniformity in projected density of a film enables STIM with a 3 MeV proton beam to produce profiles of aerosol mass with an accuracy of better than 90% and a mass sensitivity approaching 10 μg/cm2. Further, we have extended proton elastic scattering analysis (PESA) to the same films, achieving measurement of an organic surrogate. Although the films contain ∼20 μg/cm2 hydrogen, the spatial uniformity in film hydrogen content enables PESA with a 3 MeV proton beam to produce profiles of hydrogen arising solely from the aerosols with an accuracy to within ± 1 μg/cm2 and a mass sensitivity of ∼1 μg/cm2. These measurements when combined with synchrotron-x-ray fluorescence (S-XRF) measurements on the same film allow mass closure, sum of species versus measured mass, a key quality assurance protocol, to be approached. All 3 techniques were applied to very fine and ultra-fine particles collected in Fresno, CA, November, 2000 by slotted DRUM samplers. Temporal resolution in the resulting profiles was ≤6 h. The dramatic changes in composition versus size and time, and new types of elemental correlations unseen in PM2.5 filters, will be major assets in correlating aerosols and health impacts, visibility degradation, and the effects of aerosols on climate.