Significance Wildfires emit large amounts of biomass-burning (BB) aerosol particles and contribute to regional and global climate. Moreover, BB emissions are expected to increase in coming decades as a result of climate change. Tarballs, spherical organic BB particles, are estimated to contribute up to ∼30% of the BB aerosol mass. However, uncertainty still exists as to how tarballs form and how they influence climate. Our observations show that tarballs form through a combination of chemical and physical changes of primary organic aerosols within the first hours following emission. The finding of tarball formation will improve assessments of BB particle evolution and of BB impacts on regional and global climate.