Look Within: Intraplume Differences on Smoke Aerosol Aging Driven by Concentration Gradients

The evolution of organic aerosol (OA) composition and aerosol size distributions within smoke plumes are uncertain due to variability in the rates of OA evaporation/condensation and coagulation within a plume. It remains unclear how the evolution varies across different parts of individual plumes. We use a large eddy simulation model coupled with aerosol‐microphysics and radiation models to simulate the Williams Flats fire sampled during the Fire Influence on Regional to Global Environments and Air Quality field campaign. At aircraft altitude, the model captures observed aerosol changes through 4 hr of aging. The model evolution of primary OA (POA), oxidized POA (OPOA), and secondary OA (SOA) shows that >90% of the SOA formation occurs before the first transect (∼40 min of aging). Lidar observations and the model show a significant amount of smoke in the planetary boundary layer (PBL) and free troposphere (FT) with the model having equal amounts of smoke in the PBL and FT. Due to faster initial dilution, PBL concentrations are more than a factor of two lower than the FT concentrations, resulting in slower coagulational growth in the PBL. A 20 K temperature decrease with height in the PBL influences faster POA evaporation near the surface, while net OA evaporation in the FT is driven by continued dilution after the first aircraft transect. Net OA condensation in the PBL after the first transect is the result of areas with higher OH concentration leading to OPOA formation. Our results motivate the need for systematic observations of the vertical gradients of aerosol size and composition within smoke plumes. Plain Language Summary Wildfires are an important source of aerosol particles to the atmosphere. These aerosol particles are important for climate and human health. A 2019 field campaign flew an aircraft through wildfire smoke plumes in the western United States to take measurements of gasses and aerosol particles in the plume. We use these measurements and a high‐resolution model to study the fine scale details of the evolution of the aerosol particles in the plume. In our case study, we find that there are differences in the evolution within the plume as a function of height due to temperature and concentration effects in the plume. These fine scale details have implications for large‐scale air quality and climate models, which cannot resolve these plumes explicitly.