Occurrence, abundance, and formation of atmospheric tarballs from a wide range of wildfires in the western US

Abstract. Biomass burning emits large numbers of organic aerosol particles, a subset of which are called tarballs (TBs). TBs possess spherical morphology and unique physical, chemical, and optical properties. They are recognized as brown carbon aerosol particles, thereby having implications for climate through the absorption of solar radiation. Aerosol particles were collected from wildfire and agricultural fire smoke sampled by the NASA DC-8 aircraft during the FIREX-AQ campaign in the western US from July to September 2019. The current study developed an image analysis method applying deep learning to distinguish TBs from other round particles that deformed on the substrate, based on their morphological characteristics in the transmission electron microscopy images. This study detected 4567 TBs with mostly <10 h downwind from the emissions and measured their compositions, abundance, sizes, and mixing states. The number fraction, mass fraction, and concentration of TBs from all wildfire smoke were 10 % ± 1 %, 10 % ± 2 %, and 10.1 ± 4.6 µg m^-3, respectively. As the samples aged from emission up to 5 h, the TB number fractions roughly increased from 5 % to 15 %, indicating that TBs are processed primary particles. In more aged samples, the fraction decreased possibly due to dilution and removal. We also showed TBs within pyrocumulonimbus (PyroCb) activity and various TB mixing states. This study reveals the abundances and physical and chemical properties of a wide range of TBs from various biomass-burning events and enhances the knowledge of TB emissions, which contributes to the evaluation of the climate impact of TBs.