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A proper quantitative understanding of the dynamic interaction between gas-phase semivolatile organiccompounds (SVOCs) and airborne particles is important for human exposure assessment and risk valuation.Questions regarding how to properly address gas/particle interactions have introduced uncertainty whenpredicting SVOC concentrations and assessing exposures to these compounds. In this study, we have developed adimensionless description for the dynamic interaction between SVOCs and organic particles. A better criterion tojudge whether the internal resistance (diffusion in and out of aerosols) is negligible compared with the externalone (from bulk air to aerosol surfaces) is presented. The analysis is applicable regardless of the phase state ofparticles (either liquid or amorphous semisolid/solid). It is found that for both porous and nonporous particles, theinternal resistance can be reasonably neglected for particles with diameters between 0.01 and 10 μm if theparticulate organic matter is in the liquid phase. A lumped description therefore can be applied to determine, withgreater accuracy than in previous studies, the timescale required to attain gas/particle equilibrium for suchparticles. The instantaneous equilibrium assumption is found to be reasonable for relatively volatile species suchas pyrene, but not for the less volatile species such as di-(2-ethylhexyl)-phthalate (DEHP). For DEHP and 2.5 μmdiameter particles, the instantaneous gas/particle equilibrium assumption can cause a two orders of magnitudeerror in the estimation of the gas-phase concentration and a factor of two error in the estimation of theparticle-phase concentration.