A two-dimensional laminar flow model for thermodenuders applied to vapor pressure measurements

Abstract

Thermodenuders (TD) have been used to quantify the volatility of aerosol species, frequently with the aid of modeling. Here we present a two-dimensional model of flow, heat transfer, and aerosol dynamics that is fast, yet includes spatial resolution of the complete aerosol size distribution. We first demonstrate the utility of the model by interpreting nonequilibrium TD measurement data previously reported in the literature. It is shown that the thermogram (temperature vs.mass fraction remaining) curve is remarkably insensitive to radial variations in temperature and vapor concentration under typical conditions. Therefore, the discrepancies among vapor pressure estimates determined in TD studies are unlikely to be due to oversimplified flow models, but are instead likely due to faulty assumptions concerning evaporation kinetics. We then show that the best-fit range for the parameters that dictate equilibrium partitioning (saturation vapor pressure at a reference temperature and enthalpy of vaporization) can also be obtained by fitting nonequilibrium TD data using a three-parameter model that accounts for mass transfer limitations (by also fitting the evaporation coefficient). The degree of agreement between experiments and model simulations are examined for two dicarboxylic acids using the model developed in this study. The best-fit parameters were within the uncertainty range previously found using an 'equilibrated' TD approach for butanedioic acid, whereas significantly bettermodel-experiment agreementwas obtained for a much lower value of enthalpy of vaporization than previously reported for hexanedioic acid.Copyright © American Association for Aerosol Research.