New indices to evaluate volatile organic compound sorption capacity of building materials (RP-1321)

The material to air equilibrium partition coefficient (K_e) is often used to represent sorption capacity of building materials. However, it does not represent the sorption dynamics (i.e., the sink effect) of volatile organic compounds (VOCs) inside a porous material, which depends not only on the partition, but also on the in-material diffusion rate and convective mass transfer rate through the boundary layer. Based on fundamental mass transfer theory for VOC sorption by building materials, this paper proposes VOC sorption mass (M(t)) and sorption saturation degree (SSD) as new evaluation indices for sorption capacity and dynamics of building materials under given constant inlet concentration. It is found that SSD can be characterized by dimensionless sorption mass (m*), which is a function of dimensionless air change rate (N*), dimensionless mass capacity (Θ), and Fourier number for mass transfer (Fo_m). Two cases, one with constant inlet VOC concentration and the other a hypothetical case under a no-ventilation condition, are simulated to illustrate material sorption capacity. This evaluation method can clarify the difference between air saturation state and material saturation state and would be useful for modeling the impact of material sorption on indoor air quality.