Diffusion-controlled reference material for VOC emissions testing: validation and application of a mass transfer model Zhe Liu 1 , Cynthia Howard-Reed 2 , Steven S. Cox 1 and John C. Little 1,* 1 Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA USA 2 National Institute of Standards and Technology, Gaithersburg, MD USA * Corresponding email: jcl@vt.edu Keywords: standard source, volatile organic compounds, model, validation 1 Introduction To reduce indoor exposure to harmful volatile organic compounds (VOCs), low VOC-emitting products are increasingly in demand. These are usually tested in emission chambers by independent laboratories, but very different profiles are often obtained for the same product tested in different laboratories. There is a compelling need for a reference emission source that can be used to evaluate and calibrate the testing procedures. We have developed a reference material by loading toluene into a polymethyl pentene (PMP) film, which mimics real building products and can be tested in typical emissions chambers (Cox et al., 2010). A unique advantage of this reference material is that the emission profile can be predicted accurately by a fundamental mass transfer model. The predicted emission profile therefore serves as a standard value for validating the measured results by different laboratories and evaluating the test performance. This paper presents the validation of the model and an example application of the model for diagnosing problems of emission testing procedures in a recent pilot inter-laboratory study (ILS). 2 Materials/Methods Figure 1 shows the configuration of the model predicting gas-phase concentration of VOC (y) emitted from the reference material (thickness is L and exposed area is A) in a chamber with Q V y y, Q C x=L =Ky A D x x=0 x=L C 0 Figure 1. Schematic representation of the model volume of V and ventilation rate of Q. The transient diffusion of the VOC within the slab is ( ) ( ) 2 2 , , x t x C D t t x C ∂ ∂ ⋅ = ∂ ∂ , (1) where C is the material-phase concentration in the slab and D is the diffusion coefficient of the VOC within the material. The bottom boundary condition assumes a zero mass flux through the base. The upper boundary condition is given by a mass balance on the VOC in the air, or () ( ) () t y Q x t x C D A V dt t dy L x ⋅ − ∂ ∂ ⋅ − = = , , (2) where y is assumed to be always in equilibrium with the material surface via a partition coefficient (K). This assumption is valid when convective mass transfer is fast compared to internal diffusion (Cox et al., 2010). Given a uniform initial concentration (C 0 ) in the material, there is an analytical solution of the equation set for calculating y (Cox et al., 2010). D and K are determined by microbalance sorption and desorption tests to be (3.6± 0.7)×10 -14 m 2 /s and 500 ± 30, respectively (Howard-Reed et al., 2011; Cox et al., 2010). When loading the material with toluene, an airstream with a known concentration of toluene is introduced, and the absorbed mass at the end of the loading process is divided by sample volume to get C 0 . Each loaded reference material film is wrapped in aluminum foil, placed in a sealed plastic bag, and shipped in a cooler on dry ice to the testing laboratories. Arriving at the laboratory, the reference material is retained in the original packaging and stored in a freezer at -20 o C. Then it is tested in a stainless steel chamber following the guidelines of ASTM D5116-2010 (ASTM, 2010) to measure the toluene concentration profile in the chamber.