Spherical tarball particles form through rapid chemical and physical changes of organic matter in biomass-burning smoke

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.

Abstract

Biomass burning (BB) emits enormous amounts of aerosol particles and gases into the atmosphere and thereby significantly influences regional air quality and global climate. A dominant particle type from BB is spherical organic aerosol particles commonly referred to as tarballs. Currently, tarballs can only be identified, using microscopy, from their uniquely spherical shapes following impaction onto a grid. Despite their abundance and potential significance for climate, many unanswered questions related to their formation, emission inventory, removal processes, and optical properties still remain. Here, we report analysis that supports tarball formation in which primary organic particles undergo chemical and physical processing within ∼3 h of emission. Transmission electron microscopy analysis reveals that the number fractions of tarballs and the ratios of N and O relative to K, the latter a conserved tracer, increase with particle age and that the more-spherical particles on the substrates had higher ratios of N and O relative to K. Scanning transmission X-ray spectrometry and electron energy loss spectrometry analyses show that these chemical changes are accompanied by the formation of organic compounds that contain nitrogen and carboxylic acid. The results imply that the chemical changes increase the particle sphericity on the substrates, which correlates with particle surface tension and viscosity, and contribute to tarball formation during aging in BB smoke. These findings will enable models to better partition tarball contributions to BB radiative forcing and, in so doing, better help constrain radiative forcing models of BB events.