Formation and evolution of Tar Balls from Northwestern US wildfires

Biomass burning is a major source of light-absorbing black and brown carbonaceous particles. Brown carbon is 10 a poorly characterized mixture that includes tar balls (TBs), a type of carbonaceous particle apparently unique to biomass burning. Here we describe the first atmospheric observations of the formation and evolution of TBs from forest fires. Aerosol particles were collected on TEM grids during aircraft transects at various downwind distances from the Colockum Tarp wildland fire. TB mass fractions, derived from TEM and in-situ measurements, increased from <1% near the fire to 3145 % downwind, with little change in TB diameter. Single-scattering albedo determined from scattering and absorption 15 measurements increased slightly with downwind distance. Similar TEM and SSA results were observed sampling multiple wildfires. Mie calculations are consistent with weak light absorbance by TBs (m=1.56 – 0.02i) but not consistent with orderof-magnitude stronger absorption observed in different settings. The field-derived TB mass fractions reported here indicate that this particle type should be accounted for in biomass-burn emission inventories.