A Novel Inversion Method to Determine the Mass Distribution of Non-Refractory Coatings on Refractory Black Carbon using a Centrifugal Particle Mass Analyzer and Single Particle Soot Photometer

A novel inversion method is presented, which derives the two variable number distribution for black carbon aerosol, using a coupled centrifugal particle mass analyzer (CPMA) and single particle soot photometer (SP2). The CPMA classifies all particles by their mass-to-charge ratio, and the SP2 detects the mass of refractive black carbon (rBC) in each individual particle. The results of the inversion are the simultaneous number distributions of both rBC mass and total particle mass. Using the distribution, coating distribution on a population of rBC particles can be identified visually. Furthermore, the distribution can be integrated to find one variable mass and number concentration distributions as a function of total or rBC particle mass. These capabilities were demonstrated via smog chamber experiments, where an organic (non-rBC) coating was grown onto uncoated rBC aerosol over several hours via photo-oxidation of p-xylene. The particle distributions were constructed using the inversion over a range of 1–60 fg of total particle mass. As the non-rBC coating thickness increased over time, a shift in the number distribution towards higher total mass was observed. At the end of the experiment, uncoated rBC was injected into the chamber, and the distribution was clearly resolved using the inversion. The CPMA-SP2 method offers several advantages over “SP2 only” methods namely, i) coating mass information can be obtained over a wider range of total particle mass, ii) total particle mass is measured directly, and, iii) it does not make core-shell morphology assumptions.