Atmospheric tar balls : aged primary droplets from biomass 1 burning ?

10 Atmospheric tar balls are particles of special morphology and composition that are fairly abundant 11 in the plumes of biomass smoke. These particles form a specific subset of brown carbon (BrC) 12 which has been shown to play a significant role in atmospheric shortwave absorption and thus 13 climate forcing. Here we suggest that tar balls are produced by the direct emission of liquid tar 14 droplets followed by heat transformation upon biomass burning. For the first time in atmospheric 15 chemistry we generated tar ball particles from liquid tar obtained previously by dry distillation of 16 wood in an all-glass apparatus in the laboratory with the total exclusion of flame processes. The 17 particles were perfectly spherical with a mean optical diameter of 300 nm, refractory, externally 18 mixed, and homogeneous in the contrast of the TEM images. They lacked any graphene-like 19 microstructure and exhibited a mean carbon-to-oxygen ratio of 10. All of the observed 20 characteristics of laboratory-generated particles were very similar to those reported for atmospheric 21 tar ball particles in the literature, strongly supporting our hypothesis regarding the formation 22 mechanism of atmospheric tar ball particles. 23