Comparison of co-located refractory black carbon (rBC) and elemental carbon (EC) mass concentration measurements during field campaigns at several European sites

Abstract

Abstract. The mass concentration of black carbon (BC) particles in the\natmosphere has traditionally been quantified with two methods: as elemental\ncarbon (EC) concentrations measured by thermal–optical analysis and as\nequivalent black carbon (eBC) concentrations when BC mass is derived from\nparticle light absorption coefficient measurements. Over the last decade,\nambient measurements of refractory black carbon (rBC) mass concentrations\nbased on laser-induced incandescence (LII) have become more common, mostly\ndue to the development of the Single Particle Soot Photometer (SP2)\ninstrument. In this work, EC and rBC mass concentration measurements from\nfield campaigns across several background European sites (Palaiseau,\nBologna, Cabauw and Melpitz) have been collated and examined to identify the\nsimilarities and differences between BC mass concentrations measured by the\ntwo techniques. All EC concentration measurements in PM 2.5 were\nperformed with the EUSAAR-2 thermal–optical protocol. All rBC\nconcentration measurements were performed with SP2 instruments calibrated with the same\ncalibration material as recommended in the literature. The observed values\nof median rBC-to-EC mass concentration ratios on the single-campaign level were\n0.53, 0.65, 0.97, 1.20 and 1.29, respectively, and the geometric standard\ndeviation (GSD) was 1.5 when considering all data points from all five\ncampaigns. This shows that substantial systematic bias between these two\nquantities occurred during some campaigns, which also contributes to the\nlarge overall GSD. Despite considerable variability in BC properties and\nsources across the whole dataset, it was not possible to clearly assign\nreasons for discrepancies to one or the other method, both known to have\ntheir own specific limitations and uncertainties. However, differences in\nthe particle size range covered by these two methods were identified as one\nlikely reason for discrepancies. Overall, the observed correlation between rBC and EC mass reveals a linear\nrelationship with a constant ratio, thus providing clear evidence that both\nmethods essentially quantify the same property of atmospheric aerosols,\nwhereas systematic differences in measured absolute values by up to a factor\nof 2 can occur. This finding for the level of agreement between two current\nstate-of-the-art techniques has important implications for studies based\non BC mass concentration measurements, for example for the interpretation of\nuncertainties in inferred BC mass absorption coefficient values, which are\nrequired for modeling the radiative forcing of BC. Homogeneity between BC\nmass determination techniques is also very important for moving towards a routine BC\nmass measurement for air quality regulations.