REDUCTION OF SAMPLING AND ANALYTICAL ERRORS FOR ELECTRON-MICROSCOPIC ANALYSIS OF ATMOSPHERIC AEROSOLS

Electron microscopy-energy dispersive spectroscopy (EM/EDS) can be use d to determine the elemental composition of individual particles. Howe ver, the accuracy with which atmospheric particle compositions can be quantitatively determined is not well understood. In this work we expl ore sources of sampling and analytical bias and methods of reducing bi as. Sulfuric acid [H2SO4] and ammonium sulfate [(NH4)(2)SO4] particles were collected on beryllium, silicon, and carbon substrates with simi lar deposition densities. While (NH4)(2)SO4 particles were observed on all substrates, H2SO4 and ammonia-treated H2SO4 particles could not b e found on beryllium substrates. Interactions between the substrate an d sulfuric acid particles are implicated. When measured with EM/EDS, H 2SO4 particles exposed to ammonia overnight were found having lower be am damage rates (0.000 +/- 0.002 fraction s(-1)) than those without an y treatment (0.023 +/- 0.006 fraction s(-1)). For laboratory-generated [C10H6(SO3Na)(2)] particles, the composition determined using the exp erimental k-factors evaluated from independent particle standards of s imilar composition and size shows an error less than 20% for ail const ituents, while greater than 78% errors were found when k-factors were calculated from the theory. This study suggests (1) that sulfate beam damage can be reduced by exposure of atmospheric particle samples to a mmonia before analysis, (2) that beryllium is not a suitable substrate for atmospheric particle analysis, and (3) calibration (k-factor dete rmination) using particle standards of similar size and composition to particles present in the atmosphere shows promise asa way of improvin g the accuracy of quantitative EM analysis. Copyright (C) 1996 Elsevie r Science Ltd