Cascade Epiphaniometer: An instrument for aerosol “Fuchs” surface area size distribution measurements

• “Fuchs” surface of a particle is involved in interactions with the surrounding gas.
• A Cascade Epiphaniometer (CEPI) was thoroughly calibrated and evaluated.
• CEPI is developed for real time surface area size distribution measurements.
• SMPS provides the reference “Fuchs” surface area for spherical particles.
• CEPI response agreed well with the calculated surface area, over a wide size range.

The measurement of aerosol particle surface area is of high importance to environmental and health studies. Up to now, real-time measurements of the total “Fuchs” surface area of aerosol particles have been achieved among other instruments by the Epiphaniometer. This work demonstrates how a new calibration and set-up employing a Göttingen on-line α-impactor combined with a 227Ac radioactive source was performed, so that the impactor can be operated as a “Cascade Epiphaniometer (CEPI)” for real-time “Fuchs” surface area size distribution measurements. The CEPI consists of six-stages with Passivated-Implanted-Planar-Silicon (PIPS) detectors attached to each impactor stage and the back-up filter, with the cut-off sizes ranging from 53 nm (lower stage) to 2 μm (upper stage). The system was thoroughly tested and evaluated in the laboratory using monodisperse and polydisperse aerosols (PSL, DEHS, NaCl, (NH4)2SO4) of known properties. Parallel measurements by means of a Scanning Mobility Particle Sizing system (SMPS) provided a reference “Fuchs” surface area size distribution, for spherical aerosol particles. Results allowed the introduction of a size independent calibration factor for the CEPI signal to be expressed as a surface area concentration. An inversion technique is employed in order to reconstruct the continuous size distribution from the CEPI signal. The direct measurement of “Fuchs” surface area size distribution provided by CEPI compared well with that calculated by the SMPS within a mean uncertainty of 12%. The performance of CEPI in measuring the ambient aerosol “Fuchs” surface area size distribution was also investigated. During parallel ambient measurements, general agreement was observed between the CEPI “Fuchs” surface area and the calculated surface distribution from SMPS data. CEPI can be considered as an alternative method to the Electrical Low Pressure Impactor (ELPI) for the measurement of the “Fuchs” surface area size distribution, especially in cases of low aerosol concentration.