(445d) Estimation of Vapor Wall-Loss in Smog Chamber Experiments

Vapor wall-loss has been shown to be an important process in smog chamber studies, especially in yield experiments where organic aerosol (OA) concentrations can be underestimated due to semi-volatile vapor losses to chamber walls. Vapor wall-losses are also important, but largely unaccounted for, in active dilution experiments because an evaporating particle-phase component may never be able to fully reach equilibrium if the corresponding vapor-phase component is continually lost to the chamber walls. This study presents results that account for and estimate vapor wall-loss occurring during dilution experiments of secondary OA (SOA) generated from the ozonolysis of α-pinene and cyclohexene.

In this study, we generated SOA in a main chamber and then isothermally diluted the SOA close to ambient concentrations in a smaller dilution chamber, where the SOA evaporation over time was monitored. After correction for particle wall-losses in the dilution chamber, we generated areograms, particle mass fraction remaining as a function of time in the dilution chamber, by dividing the mass concentration at any given time by the initial mass concentration in the dilution chamber. We then used a dynamic mass transfer model coupled with an error minimization approach to estimate the SOA volatility distributions. However, the model was updated in this work to account for and estimate vapor wall-losses occurring in the dilution chamber. The model results indicated that the bulk vapor wall-loss constant for this chamber and the SOA species studied was ~0.3 h^-1 and, when compared to model results that did not account for vapor wall-losses, significantly reduced the uncertainty in the volatility distributions and corresponding properties. This work signifies the importance of accounting for vapor wall-losses not only in yield experiments, but also in dilution experiments.