chemical composition of biomass burning particles as they evolve in the atmosphere. A Soot Particle Aerosol Mass Spectrometer (SP-AMS) and a Single Particle Soot Photometer (SP2) were used for the chemical characterization of the particles. An Aethalometer and a green and a blue photoacoustic extinctiometer (PAX) were used for the study of the aerosol optical properties. As smoke aged, exposed to UV light, ozone or OH radicals, organic material condensed on the preexisting particles. This coating led to an increase of the absorption of the black carbon-containing particles by as much as a factor of two. The absorption enhancement of biomass burning particles due to their coating with aromatic secondary organic aerosol (SOA) was also studied. The resulting absorption enhancement was determined mainly by the changes in the SOA mass concentration and not the changes of its oxygen to carbon ratio. The measured absorption enhancements were consistent with the predictions of Mie theory assuming core-shell morphology for the aged particles.