Dilution and photooxidation driven processes explain the evolution of organic aerosol in wildfire plumes

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 x 10(6)-10(7) molecules per cm(3)) and the slower evolution over the next several hours is a result of lower-than-ambient OH concentrations (<10(6) molecules per 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.

Automated summary

Wildfires are a significant source of organic aerosol (POA and SOA) in the atmosphere at regional and global scales. This study develops a kinetic model to simulate the behavior of organic aerosol in wildfire smoke and applies it to study smoke plumes intercepted during a field campaign in the western United States. The model suggests that the evolution of organic aerosol mass with physical age can be explained by dilution-driven evaporation of primary aerosol and simultaneous photochemical production of secondary aerosol.