In addition to the studies above, tar balls have been observed in a few laboratory studies using electron microscopy. Tar balls have also been observed often in field-collected samples of biomass burning smoke using electron microscopy. Studies show their number fractions increase during the first 5 hours of aging but decrease at longer times. As discussed above, we do not know if tar balls were present in our field samples and if tar balls are extracted well by our solvent system (methanol-water). If tar balls were not extracted well by our solvent system, then our results indicate the existence of a type of highly viscous non-tar ball BBOA.