Abstract View
Dilution and Photooxidation Driven Processes Explain the Evolution of Organic Aerosol in Wildfire Plumes
ALI AKHERATI, Charles He, Lauren A. Garofalo, Anna Hodshire, Delphine K. Farmer, Sonia Kreidenweis, Wade Permar, Lu Hu, Emily Fischer, Coty Jen, Allen Goldstein, Teresa Campos, Mike Reeves, Darin Toohey, Jeffrey R. Pierce, Shantanu Jathar, Colorado State University
Abstract Number: 202
Working Group: Wildfire Aerosols
Abstract
Wildfires are an important atmospheric source of primary organic aerosol (POA) and precursors for secondary organic aerosol (SOA). However, there are large uncertainties surrounding the emissions and physicochemical processes that control the transformation, evolution, and properties of POA and SOA in large wildfire plumes. In this work, we develop a plume version of a state-of-the-science model to simulate the dilution, oxidation chemistry, thermodynamic properties, and microphysics of organic aerosol (OA) in wildfire smoke. The model is applied to study the in-plume OA in four large wildfire smoke plumes intercepted during an aircraft-based field campaign in summer 2018 in the western US. Based on estimates of dilution and oxidant concentrations before the aircraft first intercepted the plumes, we simulate the OA evolution from very close to the fire to several hours downwind. Our model results and sensitivity simulations suggest that dilution-driven evaporation of POA and simultaneous photochemical production of SOA are likely to explain the observed evolution in OA mass and composition with physical age. In addition, we show that the rapid chemical transformation within the first hour after emission is driven by higher-than-ambient OH concentrations (3×106-107 molecules cm-3) and the slower evolution over the next several hours is a result of lower-than-ambient OH concentrations (<106 molecules cm-3) and depleted SOA precursors. Model predictions indicate that the OA measured several hours downwind of the fire is still dominated by POA but with an oxidized POA and SOA fraction that varies between 20% and 50% of the total OA. Semivolatile, heterocyclic, and oxygenated aromatic compounds, in that order, contribute substantially to SOA formation. This modeling work is a step forward in bridging laboratory and field observations of wildfire smoke aerosol.