Arthur J Sedlacek — Brookhaven National Laboratory
Timothy B Onasch — Aerodyne Research, Inc.
Leonid Nichman — Boston College
Ernie R. Lewis — Brookhaven National Laboratory
Paul Davidovits — Boston College
Andrew Freedman — Aerodyne Research, Inc.
Leah R Williams — Aerodyne Research Inc
Category
Absorbing aerosol
Description
Upper panel: the number of detected rBC (downward triangles) and pure scattering (diamonds) particles as a function of SP2 laser power for nigrosin. Lower panel, left axis: ratio of SP2-derived rBC particle mass to original particle mass selected by the CPMA as a function of laser power for nigrosin (red circles) and carbon black (red squares); right axis: number fraction of BC particles to total number of particles detected by the SP2 for nigrosin (green circles) and carbon black (green squares) as a function of laser power.
Black carbon (BC) in the atmosphere continues to be a focus of research because its light-absorptive properties put it second only to CO2 as a warming agent of Earth's climate. The measurement of ambient BC has been aided greatly by the development of the Single Particle Soot Photometer (SP2) – an instrument that detects refractory black carbon (rBC) through laser-induced incandescence. Potential interference from other substances that can incandesce under 1064 nm illumination (e.g., some metals and minerals) is mitigated through the use of spectral bandpass filters (color temperature) to ensure that the SP2 remains highly selective to rBC. Here, we report on the detection of rBC that is produced through SP2 laser-induced charring (i.e., carbonization) of organic aerosol particles. Nigrosin and fulvic and humic acids – all non rBC-containing materials – were used as a surrogate for light absorbing organic aerosol. In all three cases, the particles either exhibited a fundamental absorption at the SP2 laser wavelength (i.e., nigrosin) or were induced to absorb by heating in a thermal denuder (i.e., fulvic and humic acids). Possessing the requisite absorption, particles undergo rapid heating in the SP2, which brings about charring, resulting in the particle being detected through incandescence and identified as rBC. The color temperature of the detected particles originating from charred materials is near that of carbon black, fullerene soot, and ethylene soot, indicating that the detected material is indeed rBC. Failure to properly account for this heretofore unidentified source of rBC will lead to an overestimate of rBC loadings, which could, in turn, impact aerosol radiative forcing model predictions. The discovery that the SP2 can induce charring in non-rBC particles that absorb light at 1064 nm (e.g., tar balls) suggests that this “artifact” can be turned around and used to detect some types brown carbon (BrC) by exploiting the dependence of rBC production on SP2 laser power that is exhibited by these particles.
--------------------
Sedlacek III, A. J., T. B. Onasch, L. Nichmon, E. Lewis, P. Davidovitz, A. Freedman, and L. Williams (2018) Formation of Refractory Black Carbon by SP2-Induced Charring of Organic Aerosol, Aerosol Res. Letter DOI: 10.1080/02786826.2018.1531107.
