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This paper presents a theoretical and experimental investigation ofthermodenuders that addresses two controversial issues: (1) equilibration timescales and (2) the need for an activated carbon (AC) denuder in the coolingsection. We describe a plug flow model for transport phenomena in a TD, whichcan be used to simulate the rate of vapor build-up in the gas phase and thecorresponding change in particle size distribution. Model simulations werefound to have excellent agreement with experiments performed with pure andmixed dicarboxylic acid aerosols. Both simulations and experiments showedthat the aerosols approached equilibrium within reasonable residence times(15 s–30 s) for aerosol concentrations and size distributions typical forlaboratory measurements, and that volatility studies at sufficiently highaerosol loadings, therefore, need not resort to kinetic models for inferenceof thermodynamic properties. However, for size distributions relevant forambient aerosols, equilibration time scales were much larger than residencetimes available with current TD designs. We have also performed dimensionalanalysis on the problem of equilibration in TDs, and derived a dimensionlessequilibration parameter which can be used to determine the residence timeneeded for an aerosol of given size distribution and kinetic properties toapproach equilibrium. It is also shown theoretically and empirically thataerosol volatility has no effect on the equilibration time scales. Modelsimulations and experiments showed that with aerosol size distributionsrelevant to both ambient and laboratory measurements re-condensation in thecooling section, with and without an AC denuder, was negligible. Thus, thereis no significant benefit in using an AC denuder in the cooling section. Dueto the risk of stripping volatile material from the aerosol, the use of ACdenuders in the cooling section should be avoided. Finally, we present arationale for why Δ<i>C</i> is the proper measure of volatility, whileusing mass fraction remaining (MFR) can be misleading

On transport phenomena and equilibration time scales in thermodenuders