Distribution of transient formaldehyde concentration in confined small glass desiccators and its impact on emission rate measurement

The headspace method is used frequently for measuring the emissions rates of chemical compounds from building materials when operating labeling systems for these materials. A numerical investigation of transient formaldehyde concentration distribution in confined small glass desiccators was conducted in this study. To clarify the impact of the desiccator geometry on the formaldehyde emission and diffusion characteristics, three-dimensional (3D) laser-based measurement was first conducted to define the detailed inner geometries of the different commercial desiccators. A petri dish containing water was used as the sorbent for test specimens of formaldehyde emission sources produced inside the desiccators. Fundamental experiments concerning formaldehyde emission rates and sorption onto the sorbent were next performed using high- and low-emission types of building materials in the desiccators. Transient and steady numerical analyses were conducted for formaldehyde emissions from the building materials, molecular diffusion, and sorption onto the water sorbent. Transient analysis of the formaldehyde diffusion field over 24 h revealed that the total amount of dissolution of formaldehyde in the water was not significantly affected by the desiccator geometry in the building material of the inner diffusion control type. For evaporative/external diffusion emission-type, however, the formaldehyde emission rate was strongly affected by the desiccator shape. To obtain reproducible emission rate measurements in desiccators with different geometrical shapes, substantial adjustment is needed for the relative positions of the emission source and the sorbent in terms of the equivalent diffusion length, L_d.