Atmospheric tar balls from biomass burning in Mexico

Atmospheric tar balls (TBs) are spherical, organic aerosol particles that occur in smoke from biomass burning (BB). They absorb sunlight and thereby cause warming of the atmosphere. This study reports a transmission electron microscope (TEM) study of TBs from BB smoke samples collected within minutes to hours from emission in a tropical area of Mexico. Their spherical shapes as seen in both scanning electron microscope images and with electron tomography indicate that they were solid when collected. They consist of C and minor O, S, K, and N. The hygroscopic growth factor for our relatively fresh TBs is 1.09 ± 0.04 at a relative humidity of 100%. In samples <0.6 km and >1.6 km from the fire, an average of ∼1 and 14%, respectively, of particles with aerodynamic diameter from 50 to 300 nm consisted of TBs. For the latter, more aged samples, the total volume was roughly double that of soot, and their total calculated light absorption at a wavelength of 550 nm was between 74 and 96% that of soot, with the exact amount depending on the size, shape, and coating of the soot. In general, the TBs that we analyzed were similar to those from North America, southern Africa, and Europe in terms of size, external mixing, relative freedom of inclusions, and composition. This and previous studies show that TBs result from a range of biomass fuels. Their distribution from various regions across the globe, combined with their optical properties, suggests they have important effects on regional and perhaps global climate.