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A proper quantitative understanding of the dynamic interaction between gas-phase semivolatile organic compounds (SVOCs) and airborne particles is important for human exposure assessment and risk evaluation. Questions regarding how to properly address gas/particle interactions have introduced uncertainty when predicting SVOC concentrations and assessing exposures to these compounds. In this study, we have developed a dimensionless description for the dynamic interaction between SVOCs and organic particles. A better criterion to judge whether the internal resistance (diffusion in and out of aerosols) is negligible compared with the external one (from bulk air to aerosol surfaces) is presented. The analysis is applicable regardless of the phase state of particles (either liquid or amorphous semisolid/solid). It is found that for both porous and nonporous particles, the internal resistance can be reasonably neglected for particles with diameters between 0.01 and 10 μm if the particulate organic matter is in the liquid phase. A lumped description therefore can be applied to determine, with greater accuracy than in previous studies, the timescale required to attain gas/particle equilibrium for such particles. The instantaneous equilibrium assumption is found to be reasonable for relatively volatile species such as pyrene, but not for the less volatile species such as di-(2-ethylhexyl)-phthalate (DEHP). ForDEHPand 2.5 μm diameter particles, the instantaneous gas/particle equilibrium assumption can cause a two orders of magnitude error in the estimation of the gas-phase concentration and a factor of two error in the estimation of the particle-phase concentration. Copyright © American Association for Aerosol Research.
Liu et al. (Sat,) studied this question.