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Wildfires are an important atmospheric source of primary organic aerosol (POA) and precursors for secondary organic aerosol (SOA) at regional and global scales. 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. We develop a plume version of a kinetic 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 United States. 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 with physical age. The model, however, substantially underestimates the change in the oxygen-to-carbon ratio of the OA compared to measurements. In addition, we show that the rapid chemical transformation within the first hour after emission is driven by higher-than-ambient OH concentrations (3×10 6 -10 7 molecules cm -3 ) and the slower evolution over the next several hours is a result of lower-than-ambient OH concentrations (<10 6 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 SOA fraction that varies between 30% and 56% of the total OA. Semivolatile, heterocyclic, and oxygenated aromatic compounds, in that order, were found to contribute substantially (>90%) to SOA formation. Future work needs to focus on better understanding the dynamic evolution closer to the fire and resolving the rapid change in the oxidation state of OA with physical age. 

Abstract. Biomass burning emits vapors and aerosols into the atmosphere thatcan rapidly evolve as smoke plumes travel downwind and dilute, affectingclimate- and health-relevant properties of the smoke. To date, theory hasbeen unable to explain observed variability in smoke evolution. Here, we useobservational data from the Biomass BurningObservation Project (BBOP) field campaign and show that initial smokeorganic aerosol mass concentrations can help predict changes in smokeaerosol aging markers, number concentration, and number mean diameterbetween 40–262 nm. Because initial field measurements of plumes aregenerally >10 min downwind, smaller plumes will have alreadyundergone substantial dilution relative to larger plumes and have lowerconcentrations of smoke species at these observations closest to the fire.The extent to which dilution has occurred prior to the first observation isnot a directly measurable quantity. We show that initial observed plumeconcentrations can serve as a rough indicator of the extent of dilutionprior to the first measurement, which impacts photochemistry, aerosolevaporation, and coagulation. Cores of plumes have higher concentrationsthan edges. By segregating the observed plumes into cores and edges, we findevidence that particle aging, evaporation, and coagulation occurred beforethe first measurement. We further find that on the plume edges, the organicaerosol is more oxygenated, while a marker for primary biomass burningaerosol emissions has decreased in relative abundance compared to the plumecores. Finally, we attempt to decouple the roles of the initialconcentrations and physical age since emission by performing multivariatelinear regression of various aerosol properties (composition, size) on thesetwo factors.