Tar Balls Observed in Wildfire Plumes are Weakly Absorbing Secondary Aerosol

Description

The Biomass Burning Observation Project (BBOP), a Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility-sponsored aircraft study, investigating the near-field evolution of biomass-burning (BB) aerosol particles during the summer and fall of 2013. Here we report our observations of tar balls (TBs) in the smoke emissions from the Colockum Tarps wildfire and describe the evolution of TBs as a function of the atmospheric transport time of the emission plume, the particulate mass fraction contribution of TBs, and the light absorption properties of TBs within the plume. Employing a Lagrangian sampling strategy, microscopy samples were collected for differing plume ages, and examined using transmission electron microscopy (TEM). The number fraction of TBs in the emission plume increased from less than 5% in samples collected close to the emission source to greater than 40% (>60% in some cases) ~3 hours downwind of the fire. This analysis suggests that TBs are derived from low-viscosity organic aerosol particles and are not a primary in origin as some laboratory studies suggest. We conducted experiments designed to evaluate the thermal stability of TBs and show that these particles are relatively resistant to thermal vaporization at 650ºC. By combining the TB/soot ratio from TEM analysis with the organic and refractory black carbon (rBC) mass loadings from the AMS (Aerosol Mass Spectrometer) and the SP2 (Single-Particle Soot Photometer), respectively, the TB mass fraction in the emission plume is estimated to be 20-30% in the downwind samples. This is the first time that the TB mass fraction has been derived from field measurements. An SSA derived from optical measurements is found to be consistent with Mie calculations that assume that TBs are weak absorbers, in contrast to reports from laboratory studies. A potential explanation for this discrepancy is that TBs generated in wildfires are exposed to a chemically rich environment where SOA production can result in an increase in very weak or non-absorbing organic material, thereby increasing the scattering contribution from these particles. By combining TEM with AMS and SP2 measurements to estimate TB mass fractions in wildland fire plumes, we show that the TB mass fractions may be an important and significant particle type to be accounted for in BB emission inventories.