PNNL

Abstract

Wildfires are significant sources of carbonaceous particles in the atmosphere. Given the dependence of atmospheric processes on particle physical and molecular properties, the interplay between particle size, phase state and chemical composition is investigated here for a 2021 Pacific Northwest wildfire event. Both micro-spectroscopy and high resolution mass spectrometry analyses highlight a remarkable similarity in wildfire particle compositions independent of both particle size (0.1 – 0.56 µm particle diameters) and day/night cycle influences. Microscopy techniques revealed similar phase states across the size distribution for periods of both day and night, with increases in liquid-like character for smaller particles. Finally, we apply an evaporation kinetics model on estimated volatility distribution from assigned molecular formulae, similarly revealing a slight increase in liquid-like character for smaller particles with no significant day/night dependency. The apparent lack of influence from the day/night cycle on chemical composition and phase state, which is otherwise commonly observed for different environments/sources, may be an intrinsic property of plumes heavily influenced by wildfires. This observation, combined with the lack of compositional dependencies for size-resolved wildfire particles, has substantial implications for wildfire particle optical properties, transport, and atmospheric models.