Single Particle Soot Photometer intercomparison at the AIDA chamber
Authors: Laborde M. , Schnaiter M. , Linke C. , Saathoff H. , Naumann K.-H. , Möhler O. , Berlenz S. , Wagner U. , Taylor J. W. , Liu D. , Flynn M. , Allan J. D. , Coe H. , Heimerl K. , Dahlkötter F. , Weinzierl B. , Wollny A. G. , Zanatta M. , Cozic J. , Laj P. , Hitzenberger R. , Schwarz J. P. , Gysel M. 
Published: May 2012

Abstract. Soot particles, consisting of black carbon (BC), organic carbon (OC), inorganic salts, and trace elements, are emitted into the atmosphere during incomplete combustion. Accurate measurements of atmospheric BC are important as BC particles cause adverse health effects and impact the climate. Unfortunately, the accurate measurement of the properties and mass concentrations of BC particles remains difficult. The Single Particle Soot Photometer (SP2) can contribute to improving this situation by measuring the mass of refractory BC in individual particles as well as its mixing state. Here, the results of the first detailed SP2 intercomparison, involving 6 SP2s from 6 different research groups, are presented, including the most evolved data products that can presently be calculated from SP2 measurements. It was shown that a detection efficiency of almost 100% down to 1 fg BC per particle can readily be achieved, and that this limit can be pushed down to ~0.3 fg BC with optimal SP2 setup. Number and mass size distributions of BC cores agreed within ±5% and ±10%, respectively, in between the SP2s, with larger deviations in the range below 1 fg BC. The accuracy of the SP2's mass concentration measurement depends on the calibration material chosen. The SP2 has previously been shown to be equally sensitive to fullerene soot and ambient BC from sources where fossil fuel were dominant and less sensitive to fullerene soot than to Aquadag. Fullerene soot was therefore chosen as the standard calibration material by the SP2 user community, however many datasets rely solely on Aquadag calibration measurements. The difference in SP2 sensitivity was found to be almost equal (fullerene soot to Aquadag response ratio of ~0.75 at 8.9 fg BC) for all SP2s. This allows the calculation of a fullerene soot equivalent calibration curve from a measured Aquadag calibration, when no fullerene soot calibration is available. It could be shown that this approach works well for all SP2s over the mass range of 1–10 fg. This range is suitable for typical BC mass distributions in the ambient air far from sources. The number size distribution of purely scattering particles optically measured by the 6 SP2s also agreed within 15%. Measurements of the thickness of non-refractory coatings (i.e. product from α-pinene ozonolysis) on the BC particles, relying on BC mass, optical size and on an additional particle position measurement, compared also well (within ±17%). The estimated coating thickness values were consistent with thermo-optical analysis of OC and EC content, though absolutely accurate values cannot be expected given all the assumptions that have to be made regarding refractive index, particle morphology etc. This study showed that the SP2 provides accurate and reproducible data but also that high data quality is only achieved if the SP2 is carefully tuned and calibrated.