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Abstract
Abstract. Atmospheric tar balls are particles of special morphology and composition that are fairly abundant in the plumes of biomass smoke. These particles form a specific subset of brown carbon (BrC) which has been shown to play a significant role in atmospheric shortwave absorption and, by extension, climate forcing. Here we suggest that tar balls are produced by the direct emission of liquid tar droplets followed by heat transformation upon biomass burning. For the first time in atmospheric chemistry we generated tar-ball particles from liquid tar obtained previously by dry distillation of wood in an all-glass apparatus in the laboratory with the total exclusion of flame processes. The particles were perfectly spherical with a mean optical diameter of 300 nm, refractory, externally mixed, and homogeneous in the contrast of the transmission electron microscopy (TEM) images. They lacked any graphene-like microstructure and exhibited a mean carbon-to-oxygen ratio of 10. All of the observed characteristics of laboratory-generated particles were very similar to those reported for atmospheric tar-ball particles in the literature, strongly supporting our hypothesis regarding the formation mechanism of atmospheric tar-ball particles.
Highlights
Light absorption by anthropogenic aerosol is getting increasingly important as carbonaceous particulates including black carbon (BC) become more predominant in the chemical composition of tropospheric aerosol
The deformed shape of the particles and the presence of a liquid phase indicated that the majority of these particles were liquid tar droplets of varying degree of viscosity
For the first time perfectly spherical carbonaceous particles very similar to atmospheric tar balls in all of their observed properties were produced in the laboratory with the total exclusion of flame processes
Summary
Light absorption by anthropogenic aerosol is getting increasingly important as carbonaceous particulates including black carbon (BC) become more predominant in the chemical composition of tropospheric aerosol. Some recent studies indicate that BC has become the second most potent climate forcing agent, accounting for as much as 60 % of the greenhouse absorption of excess carbon dioxide (Ramanathan and Carmichael, 2008). It should be noted, that unlike CO2 that absorbs only in the infrared spectral range, atmospheric BC directly absorbs sunlight (both incoming and reflected) over the entire solar spectrum with an exceptionally high specific efficiency (mass absorption coefficient > 5 m2 g−1 at 550 nm). Due to the combination of different factors the assessment of the global climate forcing of BC is loaded with very high uncertainty (best estimate +1.1 W m−2; 90 % uncertainty bounds of +0.27 W m−2 to +2.1 W m−2; Bond et al, 2013)
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