Giant aerosol particles and pyrometeors emitted by western US wildfires: shape, occurrence, and transport

Wildfires have become much more frequent in recent decades and are posing an increasing threat to human health and the surrounding environment. Beside generating aerosol particles predominantly in the accumulation mode, wildfires also emit giant aerosol particles (> 20 µm) and pyrometeors (> 2 mm), whose occurrence and transport in the atmosphere are not yet fully understood. This knowledge gap must be addressed in order to improve our understanding of the possible effects of these particles on the climate and to advance the early detection and tracking of wildfires using weather radar.

The NOAA/NASA joint aircraft field campaign FIREX-AQ of 2019 conducted systematic measurements of trace gases and aerosol particles in wildfire smoke plumes. During the project, we observed smoke particles up to a nominal diameter of 6.2 mm with state-of-the-art open-path instruments (Cloud, Aerosol, and Precipitation Spectrometer and Precipitation Imaging Probe; both manufactured by Droplet Measurement Technologies, Longmont, CO, USA) aboard the NASA DC-8 research aircraft at various distances from the wildfire. In total, 194 smoke plume encounters (“transects”) were investigated from nine different wildfires with some measured on multiple days. In this study we discuss the shape, occurrence, and transport of particles larger than 0.1 mm emitted by western US wildfires in the near- to mid-field from the source.

Giant aerosol particles and pyrometeors were found in the vast majority of the transects examined, with younger smoke containing more of the very massive particles than 4-hour old smoke. In only 4% of cases where the smoke age was less than 2 hours particles larger than 0.1 mm were absent. The largest particles, measuring up to over 4 mm, were observed during transects in which the Modified Combustion Efficiency (MCE) indicates flaming combustion conditions. All observed particles larger than 0.1 mm were analyzed based on their shape. The results show that the larger the particles are, the more elongated their shape is with median aspect ratios (ratio of major to minor axis length) of 5.2 for particles larger than 2.6 mm. Furthermore, a case study was considered in which we attempt to reconstruct the observed settling of pyrometeors with a size of about 3.5 mm with theoretical calculations.