Influence of airborne particles on convective mass transfer of SVOCs on flat surfaces: Novel insight and estimation formula

摘要

Impact of airborne particles on convective mass transfer coefficient (h_m) of semivolatile organic compounds (SVOCs) at flat surfaces has been investigated by assuming gas-particle equilibrium partition. In realistic indoor environments, however, this equilibrium partition might not be attained because the time required to reach equilibrium could be longer than residence time of airborne particles. No quantitative result of influence of particles on h_m under a non-equilibrium state has been given yet. A mass transfer model was used in this study to quantify this influence for the laminar air flow. The influence, calculated as ratio of h_m with particles present to h_m with particles absent, ε, was found to significantly depend on three dimensionless parameters: Bi_p, R, and C_p ~∗(∞). Bi_p represents the ratio of mass transfer rate between gas- and particle-phases SVOCs to mass transfer rate of gas-phase SVOCs transporting across the concentration boundary layer, R is the ratio of SVOC mass in particle-phase to that in gas-phase at equilibrium state, and C_p ~∗(∞) characterizes non-equilibrium degree of particle-phase SVOCs in the bulk air (higher non-equilibrium degree as it deviates further away from one, as one corresponds to equilibrium). At equilibrium state (C_p ~∗(∞) = 1), the presence of particles enhances h_m (ε > 1) to the same extent for source surfaces and sink ones. At non-equilibrium state, ε varies linearly as C_p ~∗(∞) changes (C_p ~∗(∞) > 1 for source surface, and C_p ~∗(∞) < 1 for sink surface). The presence of particles always enhances h_m for SVOC source surfaces. In contrast, it can either enhance (ε > 1) or weaken (0 < ε < 1) h_m for sink surfaces, depending on non-equilibrium degree between gas-phase SVOCs and particles (characterized by C_p ~∗(∞)). An empirical formula relating ε with these three parameters was obtained. Good agreement between predictions by the formula and experimental results in the literature moderately supported its reliability. Results of this study could be useful in better estimating indoor SVOC concentrations, as well as assessing and controlling human exposure to indoor SVOCs.