Effective density of Aquadag and fullerene soot black carbon reference materials used for SP2 calibration

The mass and effective density of black carbon (BC) particles generated from aqueous suspensions of Aquadag and fullerene soot was measured and parametrized as a function of their mobility diameter. The measurements were made by two independent research groups by operating a differential mobility analyser (DMA) in series with an aerosol particle mass analyser (APM) or a Couette centrifugal particle mass analyser (CPMA). Consistent and reproducible results were found in this study for different production lots of Aquadag, indicating that the effective density of these particles is a stable quantity and largely unaffected by differences in aerosol generation procedures and suspension treatments. The effective density of fullerene soot particles from one production lot was also found to be stable and independent of suspension treatments. Some differences to previous literature data were observed for both Aquadag and fullerene soot at larger particle diameters. Knowledge of the exact relationship between mobility diameter and particle mass is of great importance, as DMAs are commonly used to size-select particles from BC reference materials for calibration of single particle soot photometers (SP2), which quantitatively detect the BC mass in single particles. Correspondence to: M. Gysel ([email protected]) 1 Motivation Black carbon, BC, is the main absorbing component of atmospheric aerosols and estimates show that it may be the second strongest contributor to global warming (Ramanathan and Carmichael, 2008). Traffic-related BC particles have also been shown to cause adverse health effects (e.g. Laden et al., 2000; Slama et al., 2007). Quantitative measurements of BC mass concentrations in atmospheric aerosols are most commonly done with optical, thermal, combined thermaloptical or photo-acoustic methods (Gelencsér, 2004). Recently the single particle soot photometer (SP2; Stephens et al., 2003) was introduced as an alternative method, using laser-induced incandescence (LII) for quantitative detection of refractory BC mass. All BC measurement methods have their advantages and disadvantages, though recent studies showed that consistent results can be obtained for BC measured by different methods, if appropriate calibrations, sampling techniques and data analysis procedures are applied (e.g. Kondo et al., 2011). The SP2 is a highly sensitive method for selective and quantitative detection of refractory BC in single particles. The peak height of the incandescence signal is nearly linearly dependent on the mass of refractory BC in the particle, largely independent of particle morphology, and without interference from non-refractory coatings (Schwarz et al., 2006; Slowik et al., 2007; Moteki and Kondo, 2007; Cross et al., 2010). Nevertheless, empirical calibration of the SP2 is still required to obtain absolute BC mass measurements. This is commonly done with BC reference materials such as Published by Copernicus Publications on behalf of the European Geosciences Union. 2852 M. Gysel et al.: Effective density of Aquadag and fullerene soot Aquadag, glassy carbon or fullerene soot. It has been shown that significant differences exist in the SP2’s sensitivity to different BC materials and that fullerene soot behaves most similarly to ambient BC in terms of SP2 response per unit mass of BC, refractive index and effective density (Moteki and Kondo, 2010; Moteki et al., 2010; Kondo et al., 2011; Laborde et al., 2011b). The calibration curve of the SP2 is obtained by recording the incandescence signal peak height for pure BC particles of known mass over the whole dynamic range of the SP2. Selecting particles by mass is ideally done with an aerosol particle mass analyser (APM) or a Couette centrifugal particle mass analyser (CPMA). However, most research groups operating an SP2 do not have access to such a device. Thus the calibration particles are often selected by their mobility diameter using a differential mobility analyser (DMA) instead. Unfortunately particles of many BC reference materials are not compact spheres (Moteki et al., 2009), instead possessing a fractal-like or other irregular shape. Thus, the effective density (ρeff), defined as