Investigation of particle and vapor wall-loss effects on controlled wood-smoke smog-chamber experiments

摘要

Smog chambers are extensively used to study processes that drive gas andparticle evolution in the atmosphere. A limitation of these experiments isthat particles and gas-phase species may be lost to chamber walls on shortertimescales than the timescales of the atmospheric processes being studied inthe chamber experiments. These particle and vapor wall losses have beeninvestigated in recent studies of secondary organic aerosol (SOA) formation,but they have not been systematically investigated in experiments of primaryemissions from combustion. The semi-volatile nature of combustion emissions(e.g. from wood smoke) may complicate the behavior of particle and vaporwall deposition in the chamber over the course of the experiments due to thecompetition between gas/particle and gas/wall partitioning. Losses of vaporsto the walls may impact particle evaporation in these experiments, andpotential precursors for SOA formation from combustion may be lost to thewalls, causing underestimations of aerosol yields. Here, we conductsimulations to determine how particle and gas-phase wall losses contributedto the observed evolution of the aerosol during experiments in the thirdFire Lab At Missoula Experiment (FLAME III). We use the TwO-Moment AerosolSectional (TOMAS) microphysics algorithm coupled with the organic volatilitybasis set (VBS) and wall-loss formulations to examine the predicted extentof particle and vapor wall losses. We limit the scope of our study to thedark periods in the chamber before photo-oxidation to simplify the aerosolsystem for this initial study.Our model simulations suggest that over one-third of the initialparticle-phase organic mass (41 %) was lost during the experiments, andover half of this particle-organic mass loss was from direct particle wallloss (65 % of the loss) with the remainder from evaporation of theparticles driven by vapor losses to the walls (35 % of the loss). Weperform a series of sensitivity tests to understand uncertainties in oursimulations. Uncertainty in the initial wood-smoke volatility distributioncontributes 18 % uncertainty to the final particle-organic mass remainingin the chamber (relative to base-assumption simulation). We show that thetotal mass loss may depend on the effective saturation concentration of vaporwith respect to the walls as these values currently vary widely in theliterature. The details of smoke dilution during the filling of smog chambersmay influence the mass loss to the walls, and a dilution of ~ 25:1during the experiments increased particle-organic mass loss by 33 %compared to a simulation where we assume the particles and vapors areinitially in equilibrium in the chamber. Finally, we discuss how our findingsmay influence interpretations of emission factors and SOA production inwood-smoke smog-chamber experiments.