Indoor particle age, a new concept for improving the accuracy of estimating indoor airborne SVOC concentrations, and applications

To appropriately assess exposure to indoor semi-volatile organic compounds (SVOCs), an accurate relationship considering the non-equilibrium sorption behavior between their gas-phase concentration (Cg) and particle-phase concentration (Csp) is necessary. The reasons are: (1) the equilibrium model frequently used in previous studies may result in significant errors because the time that particles stay in the indoor air is often much shorter than the critical time for Cg-Csp equilibrium; (2) several dynamic models developed previously took the mean residence time of indoor airborne particles as the time duration of the mass transfer between SVOCs and particles in the indoor environments, which may obviously reduce their accuracy because residence time varies among individual particles, and the Cg-Csp relationship and residence time are not linearly correlated. Therefore, we present a new concept, indoor particle age (the time that airborne particles have spent since entering the room), and develop a “η-indoor particle age method”: the Csp/Cg value can be obtained by multiplying the equilibrium result by a modification factor, η (which is a function of indoor particle age, particle size distribution and the critical time for Cg-Csp equilibrium). The η-indoor particle age method can reduce the relative error of the existing methods by 30% to more than 1000%, which is preliminarily supported by the experimental results in the literature.