The collection characteristics of a small deposit area low pressure impactor (SDI) were studied in order to employ the impactor for size distribution measurements of carbonaceous matter. In this work, the SDI was calibrated for soft and porous quartz substrate material in a series of laboratory experiments. The collection efficiency curves were measured by using monodisperse dioctyl seba-cate particles and by applying two different detection methods. One method was based on the detection of current carried by charged test particles, and the other measured number concentrations of particles in bipolar charge equilibrium by two condensation particle counters. Concerning the particle size corresponding to a 50 collection efficiency (D_(50)), significant shifts toward smaller particle sizes were found for the quartz fiber substrates compared with the flat plates. Also the shapes of the collection efficiency curves differed considerably: quartz substrate gave less steep curves than plain impaction plates. The new calibration was applied to field data from urban and rural sites. Compared with the original calibration of the SDI, the new calibration changed the measured size distributions of organic and elemental carbon. In addition, a reasonable size-segregated mass closure was achieved by combining data from thermal-optical analysis and ion-chromatography.
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