Passive diffusive flux chambers – a new method to quantify vapour intrusion into indoor air

Abstract

A new instrumental method (a passive flux chamber) to quantify emission rates of volatile organic compounds (VOCs) from natural ground surfaces and floors or pavements was developed and tested against the traditional emission isolation flux chamber (the dynamic flux chamber). Dynamic flux chambers have been used for some decades to measure diffusive mass flux of VOCs at contaminated sites thereby providing quantitative estimates of the contribution of contaminant vapour fluxes to indoor air concentrations for human health risk assessments. The new method described here measures diffusive mass flux from surfaces utilising a high uptake rate passive absorptive sampling tube placed within a chamber to capture the mass molecular flux. The passive flux chamber was developed to provide a technically simpler and more cost effective means of quantifying vapour intrusion rates into buildings where the dominant pathway for VOC intrusion through floors is molecular diffusion rather than pressure driven advective flows through floor gaps and cracks. The passive flux chamber operation is based on the principal of molecular diffusion and the random movement of molecules in the gaseous phase. The efficiency of the passive chamber in capturing the total mass flux was tested by measuring comparative concentrations in adjacent identical chambers, one fitted with and the other without an absorptive sampling tube. For chambers fitted with sampling tubes the internal chamber concentrations of VOCs were on average 85% lower than for co-located chambers not fitted with the absorption tubes, demonstrating a high rate of capture of VOC flux into the chamber and a close and satisfactory approximation of mass flux. Eighteen field comparisons of surface fluxes measured by the passive and dynamic flux chamber methods showed that on average the passive chambers produced flux rates a factor of two greater than the dynamic flux chambers.