Biomass burning is an important source to the atmosphere of carbonaceous particulate matter that impacts air quality, climate, and human health. The semivolatile nature of directlyemitted organic particulate matter can result in particle evaporation as smoke plumes dilute. Further, oxidation of emitted and volatilized precursors can lead to additional formation of secondary organic aerosol (SOA) in the atmosphere. These processes are not fully understood, hindering efforts to quantify the impacts of prescribed and wildfires.

Experiments conducted in laboratory smog chambers are extensively used to study processes that drive gas and particle evolution in the atmosphere, with the findings then applied in air quality models. However, a limitation of smog chamber experiments is that particles and gas-phase species may be lost to chamber surfaces, biasing the conclusions. The objectives of this study, all of which were met, were: (1) To conduct detailed calculations of the phase partitioning behavior of fresh smoke emissions, including consideration of the evolving aerosol size distribution, with a view toward understanding conditions under which SOA precursors become available for oxidation; (2) To use the same framework to compute the potential phase partitioning behavior of oxidation products, to improve understanding of their potential impacts on PM2.5 concentrations; and (3) To apply our findings to improve understanding of how phase partitioning and its timescales may lead to potential artifacts in smog chamber experiments that seek to quantify the SOA-formation potential of biomass burning emissions.

We conducted simulations to determine how particle and gas-phase wall losses contributed to the observed evolution of the aerosol during experiments in the third Fire Lab At Missoula Experiment (FLAME III), using a modified version of the TwO-Moment Aerosol Sectional (TOMAS) microphysics algorithm coupled with the organic volatility basis set (VBS) and wall-loss formulations. By fitting to laboratory data, we could constrain many of the uncertain model parameters. Our first study was limited to simulating the dark periods in the chamber before photo-oxidation when only physical processes were active. Our model simulations suggested that over one-third of the initial particle-phase organic mass was lost during this portion of the experiments, and one-third of this loss arose from evaporation of the particles driven by vapor losses to the walls. Our second set of simulations considered the photooxidation stages of the FLAME III chamber experiments, and were able to reproduce the observed mean OA mass enhancement (the ratio of final to initial OA mass, corrected for particle-phase wall losses) of 1.7 across all experiments. The mean OA enhancement increased to over 3 when vapor wall losses were turned off, indicating that the presence of the chamber walls reduced SOA formation by almost a factor of 2. These results were robust across the ranges of uncertainties in the key model assumptions.

In ambient plumes, the plume dilution rate impacts partitioning between the gas and particle phases, which may impact the potential for SOA to form as well as the rate of SOA formation. Applying our model to plume evolution scenarios showed that the rapid dilution of smoke from small prescribed burns drives evaporation of organic vapors from the particle phase, leading to more effective SOA formation than in plumes emitted by large, intense wildfires. Emissions from the latter dilute more slowly, suppressing OA evaporation and subsequent SOA formation in the near field. Our results highlight that dilution and wall effects are likely contributors to past inconsistent observations and conclusions regarding the production of particulate matter from biomass burning emissions, and must be properly accounted for in future studies.