PM2.5 (that is the particulate matter with aerodynamic diameter lower than 2.5 ðmm) is becoming more and more studied because of its adverse health effects; the new European legislation is trying to lower the exposition limits.
Transmission electron microscopy can be a very suitable method for the physic-chemical characterization of PM2.5(1, 2 ), but sampling and specimen preparation techniques must be very accurate in order to conserve the morphology and the composition of the samples.
The aim of our investigation was to develop a method for the morphological and compositional characterization of airborne particulate by transmission electron microscopy.
The most critical part of the investigation was sampling the soot without introducing artifacts (3, 4); in order to get the largest amount of information, we used three different procedures and compared the results.
In on-line sampling the TEM copper grids were deposited on paper filters and the particulate was collected directly on them with short collecting times, without any manipulation; we had information on particles and aggregates and the material around.
In the section sampling the soot was collected directly on paper filters, then embedded into epoxy resin and cut; we studied the internal composition of the particles and the aggregates core.
In the deposition procedure the particulate collected on paper filter was diluted in isopropanol, solicited and deposited on copper grids; we studied particles and aggregates morphology in 2D.
The particulate was characterized using a Philips 420T (acc. voltage 120kV) and a Jeol 3010 (acc. voltage 300kV). Different types of particulate, from different sources and fuels, were examined.
All of them have the same basic structure (5) and are formed by elementary particles of C linked in aggregates (chains or ramified).
Dimensional ranges for primary particles and aggregates were determined by image analysis (6); the diameter for primary particles is 25-40 nm while the aggregate size is from 0.1 to 2 ðmm. Further investigations are still going on.
D. Berti, B. Stocchi e M.Buroni
source : E.N.I
References
[1] K.A. Berube and others, Physicochemical characterization of urban airborne particulate matter, RMS Proceedings, Vol.33/2 (1998).
[2] F.D. Pooley, Microscopy and the characterization of particulates, RMS Proceedings, Vol.33/2 (1998).
[3] C.M Megaridis and others, Morphological description of flame-generated materals, Combust. Sci. and Tech., Vol.71, pag. 95-109 (1990).
[4] U.O. Koilu e altri, Simultaneous measurements of soot volume fraction and particle size using thermophoretic sampling technique, Combustion and Flame, Vol.110, pag. 494-507 (1997).
[5] H.U. Franke, H. Tschoeke, Influence of fuels on the soot-particle geometry, 2nd International Colloquim on Fuels 1999, p.517-524 (1999).
[6] M.D. Tasic and others, Characterization of aerosols in the urban area of Belgrade using an automatic image analysis system, RMS Proceedings Vol.33/2 (1998).
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