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 a spherical morphology and unique physical, chemical, and optical properties. They are recognized as brown-carbon aerosol particles, influencing the climate through the absorption of solar radiation. Aerosol particles were collected from wildfire and agricultural-fire smoke sampled by NASA's DC-8 aircraft during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign in the western US from July to September 2019. This study developed an image analysis method using deep learning to distinguish TBs from other round particles that deformed on the substrate, based on the particles' morphological characteristics in transmission electron microscopy images. This study detected 4567 TBs, with most occurring < 10 h downwind of the emissions, and measured their compositions, abundance, sizes, and mixing states. The number fraction, mass fraction, and concentration of TBs in wildfire smoke corresponded to 10 ± 1 %, 10 ± 2 %, and 10.1 ± 4.6 µg m−3, respectively. As the smoke aged for up to 5 h after emission, the TB number fractions roughly increased from 5 % to 15 %, indicating that TBs are processed primary particles. We also observed TBs associated with pyrocumulonimbus (pyroCb) activity and various TB mixing states. This study reveals the abundance, as well as the physical and chemical properties, of a wide range of TBs from various biomass-burning events and enhances our knowledge of TB emissions, contributing to the evaluation of the climate impact of TBs.