Chemical composition of tarballs and “charballs” formed by pyrolysis of wildfire-like organics

Biomass burning (BB) in wildfires and residential settings involves the pyrolytic depolymerization of cellulose and lignin to form primary BB organic-aerosol particles (BBOA) via bubbling or condensation. BBOA typically contains brown carbon (BrC), a complex mixture that absorbs light more strongly at shorter wavelengths. While low-volatility BBOA is known to be darker and possess a higher mass-specific light absorption (MAC) than bulk BBOA, previous studies typically determined volatility–MAC relationships by heating BBOA to achieve partial evaporation. For highly thermolabile BBOA, this process also induces carbonization.

Here, we investigate this carbonization process systematically by rapidly pyrolyzing laboratory-generated BBOA (~1 s) at temperatures representative of wildfire plumes and residential stoves: 150–450 °C. We show that this BBOA, initially comprising ≈1–3 kDa molecules, carbonizes into macromolecular, insoluble “tarballs” at moderate temperatures (100–300 °C) and into “charballs” (black-coloured nano-char spheres) at temperatures >350 °C. Although insoluble, this intermediate dark-BrC material was detectable by an aerosol mass spectrometer (HR-AMS). The HR-AMS exhibited a four-fold reduction in sensitivity after 250 °C treatment, despite a sample OC/TC of 0.8. Thus, the tarballs represent an intermediate material between organic and black carbon.

Above 350 °C, HR-AMS signals were negligible; charball quantification required the SP-AMS 1064-nm laser. The SP-AMS mass spectrum indicated that no refractory organics remained. It comprised only oxygenated ions (CO+, CO2+, H2O+), metals (Ca+, K+), and refractory black carbon (rBC) ions Cx+. The rBC fragmentation pattern was indistinguishable from that of mature soot at all heat-treatment temperatures. The O/C ratio inferred from the SP-AMS mass spectrum showed excellent agreement with energy dispersive X-ray spectroscopy (EDS) measurements. Overall, these results provide mechanistic insights into the composition of nano-char aerosols, while identifying a measurement gap where moderately carbonized tarballs are significantly under-measured by both HR-AMS and SP-AMS. Failure to account for this gap may result in a substantial underestimation of the particulate mass and radiative forcing of BBOA smoke.