Comparison of co-located refractory black carbon (rBC) and elemental carbon (EC) mass concentration measurements during field campaigns at several European sites
Authors: Pileci Rosaria E. , Modini Robin L. , Bertò Michele , Yuan Jinfeng , Corbin Joel C. , Marinoni Angela , Henzing Bas , Moerman Marcel M. , Putaud Jean P. , Spindler Gerald , Wehner Birgit , Müller Thomas , Tuch Thomas , Trentini Arianna , Zanatta Marco , Baltensperger Urs , Gysel-Beer Martin 
Published: February 2021

Abstract. The mass concentration of black carbon (BC) particles in the
atmosphere has traditionally been quantified with two methods: as elemental
carbon (EC) concentrations measured by thermal–optical analysis and as
equivalent black carbon (eBC) concentrations when BC mass is derived from
particle light absorption coefficient measurements. Over the last decade,
ambient measurements of refractory black carbon (rBC) mass concentrations
based on laser-induced incandescence (LII) have become more common, mostly
due to the development of the Single Particle Soot Photometer (SP2)
instrument. In this work, EC and rBC mass concentration measurements from
field campaigns across several background European sites (Palaiseau,
Bologna, Cabauw and Melpitz) have been collated and examined to identify the
similarities and differences between BC mass concentrations measured by the
two techniques. All EC concentration measurements in PM2.5 were
performed with the EUSAAR-2 thermal–optical protocol. All rBC
concentration measurements were performed with SP2 instruments calibrated with the same
calibration material as recommended in the literature. The observed values
of median rBC-to-EC mass concentration ratios on the single-campaign level were
0.53, 0.65, 0.97, 1.20 and 1.29, respectively, and the geometric standard
deviation (GSD) was 1.5 when considering all data points from all five
campaigns. This shows that substantial systematic bias between these two
quantities occurred during some campaigns, which also contributes to the
large overall GSD. Despite considerable variability in BC properties and
sources across the whole dataset, it was not possible to clearly assign
reasons for discrepancies to one or the other method, both known to have
their own specific limitations and uncertainties. However, differences in
the particle size range covered by these two methods were identified as one
likely reason for discrepancies. Overall, the observed correlation between rBC and EC mass reveals a linear
relationship with a constant ratio, thus providing clear evidence that both
methods essentially quantify the same property of atmospheric aerosols,
whereas systematic differences in measured absolute values by up to a factor
of 2 can occur. This finding for the level of agreement between two current
state-of-the-art techniques has important implications for studies based
on BC mass concentration measurements, for example for the interpretation of
uncertainties in inferred BC mass absorption coefficient values, which are
required for modeling the radiative forcing of BC. Homogeneity between BC
mass determination techniques is also very important for moving towards a routine BC
mass measurement for air quality regulations.