Mass spectrometry of refractory black carbon particles from six sources: carbon-cluster and oxygenated ions

Abstract. We discuss the major mass spectral features of different types of refractory carbonaceous particles, ionized after laser vaporization with an Aerodyne high-resolution soot-particle aerosol mass spectrometer (SP-AMS). The SP-AMS was operated with a switchable 1064 nm laser and a 600 °C thermal vaporizer, yielding respective measurements of the refractory and non-refractory particle components. Six samples were investigated, all of which were composed primarily of refractory material: fuel-rich and fuel-lean propane/air diffusion-flame combustion particles; graphite-spark-generated particles; a commercial fullerene-enriched soot; Regal Black, a commercial carbon black; and nascent aircraft-turbine combustion particles.
All samples exhibited a spectrum of carbon-cluster ions C_xⁿ⁺ in their refractory mass spectrum. Smaller clusters (x < 6) were found to dominate the C_xⁿ⁺ distribution. For fullerene soot, fuel-rich-flame particles and spark-generated particles, significant C_xⁿ⁺ clusters at x ≫ 6 were present, with significant contributions from multiply charged ions (n > 1). In all six cases, the ions C₁⁺ and C₃⁺ contributed over 60% to the total C₁+ intensity. Furthermore, the ratio of these major ions C₁⁺ / C₃⁺ could be used to predict whether significant C_xⁿ⁺ signals with x > 5 were present. When such signals were present, C₁⁺ / C₃⁺ was close to 1. When absent, C₁⁺ / C₃⁺ was < 0.8. This ratio may therefore serve as a proxy to distinguish between the two types of spectra in atmospheric SP-AMS measurements.
Significant refractory oxygenated ions such as CO⁺ and CO₂⁺ were also observed for all samples. We discuss these signals in detail for Regal Black, and describe their formation via decomposition of oxygenated moieties incorporated into the refractory carbon structure. These species may be of importance in atmospheric processes such as water uptake and heterogeneous chemistry.
If atmospherically stable, these oxidized species may be useful for distinguishing between different combustion sources. If unstable, they may provide a means to estimate the atmospheric age of an rBC sample. Future studies should attempt to establish which of these scenarios is more realistic.