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 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.

Black carbon aerosol reductions during COVID-19 confinement quantified by aircraft measurements over Europe

“…Due to the large contribution of anthropogenic BC emissions on our measurements and the high uncertainty in the size distribution for diameters below the SP2 detection limit, we do not fit the size distribution to a log-normal distribution. Furthermore for M BC , diameters below 100 nm do not play a significant role, regardless of their potentially dominating role for N BC and importance for the BC surface area (Seinfeld and Pandis, 2006;Pileci et al, 2021;Schwarz et al, 2008;Reddington et al, 2013).…”

Model evaluation of short-lived climate forcers for the Arctic Monitoring and Assessment Programme: a multi-species, multi-model study

“…There are various BC measurement methods exploiting different properties of BC and thus measuring different quantities (Petzold et al, 2013): elemental carbon (EC) determined by thermal and/or thermal-optical methods, equivalent BC (eBC) by optical absorption methods, and refractory BC (rBC) by incandescence methods. The different types of BC measurements (EC, eBC, and rBC) usually agree with each other within a factor of 2 (AMAP, 2022;Pileci et al, 2021). However, it has been shown that, as the aerosol ages, the complex state of mixing of BC particles causes eBC to increase relative to EC (Zanatta et al, 2018).…”

“…The Japanese measurements provide rBC (refractory BC). Pileci et al (2021) showed that rBC and eBC are linearly related; thus, in order to compare the observations to models, we converted rBC to eBC via a factor of 1.8 (eBC = 1.8 × rBC; Zanatta et al, 2018;Pileci et al, 2021).…”

Estimates of mass absorption cross sections of black carbon for filter-based absorption photometers in the Arctic

“…Measurements of M EC can depend on the temperature protocol and optical charring correction method used (e.g., Bond et al, 2013). Agreements within 10 % of M BC (SP2), M BC (COSMOS), and M EC were reported by Kondo et al (2011), whereas systematic differences between M BC (SP2) and M EC up to a factor of 2 were found by Pileci et al (2021). Although the difference between M BC (COSMOS) and M EC was generally lower than 5 ng m −3 at the Arctic site Barrow (Sinha et al, 2017), this difference can be important for pristine summer Arctic conditions (M BC (COSMOS) <20 ng m −3 ).…”